Practical Control Valve Sizing, Selection and Maintenance

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Presents
Practical Control Valve Sizing, Selection and
Maintenance
Revision 6.1
Dave Macdonald BSc(Eng)
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Contents
1 Introduction to Control Valves and Fluid Flow 1
1.1 Purpose of a control valve 2
1.2 Choked Flow Conditions (Critical Flow) 9
1.3 Typical control valve applications 12
1.4 Requirements of control valves 15
2 Types of Control Valves 19
2.1 Broad classification of control valves 20
2.2 Sliding stem valves 20
2.3 Rotary Valves 35
2.4 Other types of control valves 40
2.5 Control valve selection summary 42
2.6 Summary 46
3 Valve Sizing for Liquid Flow 47
3.1 Principles of the full sizing equation 48
3.2 Formulae for sizing control valves for Liquids 51
3.3 Practical example of Cv sizing calculation 52
3.4 Summary 54
4 Valve Sizing for Gas and Vapor Flow 55
4.1 Compressibility in gas and steam 55
4.2 Formulae for sizing control valves for gas and vapour service 56
4.3 Practical example of Cv sizing calculation 59
4.4 Notes on gas sizing and noise 62
4.5 Summary 62
5 Software Tools for Valve Sizing 65
5.1 Valve sizing software packages in the selection process 65
5.2 Examples of software packages 66
5.3 Software demonstration package 67
5.4
6 Inherent and Installed Flow Characteristics of Control Valves 73
6.1 Control valve characteristics 73
6.2 Inherent flow characteristics 74
6.3 Installed Characteristics 79
6.4 Process control loop 81
6.5 Process pressure ratio 84
6.6 Conclusion: Choosing the right characteristic 85

7 Actuators and Positioners 87
7.1 Principles of actuators for control valves 87
7.2 Issues of size, force and speed matched to valve type 88
7.3 Pneumatic actuators 88
7.4 Electric actuators 93
7.5 Hydraulic actuators for large valves 96
7.6 Selection guidelines for actuators 97
7.7 A benchset and stroking 98
7.8 The purpose of positioners 100
7.9 How positioners linearise installed characteristics 100
7.10 Smart positioners 102
7.11 Conclusion 104
8 Cavitation and Flashing in Control Valves 105
8.1 Cavitation phenomenon and problems 105
8.2 Flashing of liquids 109
8.3 Methods of reducing cavitation 109
8.4 Selection guide for cavitation applications 113
8.5 Disc Stack Technology 113
8.6 Conclusion 115
9 Noise and Noise Reduction Methods 117
9.1 Sources of control valve noise 117
9.2 Predicting control valve noise 118
9.3 Measures used to reduce control valve noise 120
9.4 Summary 123
10 Choosing Valve Materials of Construction 125
10.1 Overview of material selection issues 125
10.2 Typical materials of construction for body and valve trim 127
10.3 Materials for the valve body 127
10.4 Corrosion and erosion 130
10.5 Gasket materials 131
10.6 Valve trim materials 131
10.7 Problem of leaks from seals 133
10.8 Application of bellows seals for toxic material 135
10.9 Conclusion on valve materials 135
11 Key issues for Control Valve Maintenance 137
11.1 Scope of Maintenance Activities 137
11.2 Installation features 139
11.3 Key maintenance tasks 140
11.4 Detection of wear 143
11.5 Detection of back lash and stiction in the valve drive 143

11.6 Diagnostic tools and Smart Positioners 147
11.7 Summary 149
12 Principles of Pressure Relief Valves 151
13 Appendix A (FAQ) 189
14 Appendix B (Glossary of Valve and Related Terms) 205
15 Appendix C (Application case studies) 213
16 Practical Exercises 245
17 Answers to Practical Exercises 261

1
Introduction to Control Valves and Fluid
Flow
Control valves are the essential final elements used to control fluid flow and pressure conditions in
a vast range of industrial processes. The control valve industry is itself a vast enterprise whilst the
influence of control valves on the performance of high value processes worldwide is very much
larger. Hence it is a major responsibility on control and instrumentation engineers to deliver the
best possible control valve choices for every application they encounter.
The task of specifying and selecting the appropriate control valve for any given application
requires an understanding of the principles of:
• How fluid flow and pressure conditions determine what happens inside a control
valve.
• How control valves act to modify pressure and flow conditions in a process.
• What types of valves are commonly available
• How to determine the size and capacity requirements of a control valve for any given
application
• How actuators and positioners drive the control valve
• How the type of valve influences the costs
Selecting the right valve for the job requires that the engineer should be able to:
• Ensure that the process requirements are properly defined
• Calculate the required flow capacity over the operating range
• Determine any limiting or adverse conditions such as cavitation and noise and know
how to deal with these
• Know how to select the valve that will satisfy the constraints of price and
maintainability whilst providing good performance in the control of the process.
This manual is intended to provide an understanding of the key issues involved in the selection of
control valves for typical process industry applications. The training material should provide a
general background in the subject but it assumes that participants have a basic knowledge of
process industry equipment and terminologies.
To begin the training manual this chapter looks at the fundamental principles involved in the
control of fluid flow and it describes how the adjustment of flow capacity is typically used to
control pressure, flow, level and temperature in processes. We then outline the main performance
requirements that are expected from a control valve as an aid to selection. The following chapters
will then provide training guidance in each of the key subjects.

2
Control Valve Sizing, Selection and Maintenance
• Purpose of a control valve and how it works to regulate flow or pressure
• What happens inside the control valve
• Examples of process applications of control valves
• General performance requirements of control valves
• Training needs for sizing and selection
1.1 Purpose of a control valve
The purpose of a control valve is to provide the means of implementing or actuation of a control
strategy for a given process operation. Control valves are normally regarded as valves that provide
a continuously variable flow area for the purpose of regulating or adjusting the steady state running
conditions of a process. However the subject can be extended to include the specification and
selection of on-off control valves such as used for batch control processes or for sequentially
operated processes such as mixing or routing of fluids. Many instrument engineers also asked to be
responsible for the specification of pressure relief valves. These topics will also be considered
briefly in this text but the main emphasis in this training will be on the selection and sizing of
valves for continuously variable processes.
1.1.1 Definition of a control valve
A control valve is defined as a mechanical device that fits in a pipeline creating an externally
adjustable variable restriction.
P1 P2
Learning objectives
Pipe line flow depends on effective area x square root (P1 –P2)
Figure 1.1
Control valve adjusts the effective area of flow in the pipe
This throttles the flow for any given pressure drop or it raises the pressure drop for any given flow.
Typical process applications can be made based on this ability to change pressure drop or flow
capacity as will be seen in the next section. However, we must firstly understand how a typical
control valve actually creates a pressure drop by looking at the fundamentals of flow in a pipeline
and through a restricted area.
1.1.2 Turbulent and laminar flow in pipes
When a fluid is moving slowly through a pipe or if the fluid is very viscous, the individual particles
of the fluid effectively travel in layers at different speeds and the particles slide over each other,
creating a laminar flow pattern in a pipeline. As can be seen in Figure 1.2 the flow velocity profile
is sharply curved and much higher speeds at seen at the centre of a pipeline where there is no drag
effect from contact with the wall of the pipe.

Introduction to Control Valves and Fluid Flow 3
Figure 1.2
Laminar flow in a pipeline has a low energy-loss rate
At higher velocities high shear forces disturb the fluid flow pattern and the fluid particles start to
move in erratic paths, creating turbulent flow. This results in a much flatter flow velocity profile as
can be seen in Figure 1.3. The velocity gradient is small across the centre of the pipe but is high at
close proximity to the pipe wall.
Figure 1.3
Turbulent flow in a pipeline has a high energy-loss rate
The transition from laminar flow to turbulent flow can be predicted by the parameter known as the
Reynolds number (Re), which is given by the equation:
Re = V. D/ν
Where: V = flow velocity, d= nominal diameter, ν = kinematic viscosity
In a straight pipe the critical values for transition form laminar to turbulent flow is approximately
3000. When the flow is turbulent, part of the flow energy in the moving fluid is used to create
eddies which dissipate the energy as frictional heat and noise, leading to pressure losses in the
fluid.
1.1.3 Formation of vortices
A more drastic change in velocity profile with greater energy losses arises when a fluid passes
through a restrictor such as an orifice plate or a control valve opening. Downstream of a restriction
there is an abrupt increase in flow area where some of the fluid will be moving relatively slowly.
Into this there flows a high velocity jet from the orifice or valve, which will cause strong vortices
causing pressure losses and often creating noise if the fluid is a liquid since it is incompressible and
cannot absorb the forces.
1.1.4 Flow separation
Just after the point where a large increase in flow area occurs the unbalanced forces in the flowing
fluid can be sufficiently high to cause the fluid close to the surface of the restricting object to lose
all forward motion and even start to flow backwards. This is called the flow separation point and it
causes substantial energy losses at the exit of a control valve port. It is these energy losses along
with the vortices that contribute much of pressure difference created by a control valve in practice.
Figures 1.4 and 1.5 illustrate flow separation and vortices in butterfly and globe valve
configurations.

4
Vortices
Flow Separation
Figure 1.4
Flow separation effects in a butterfly valve.
Figure 1.5
Flow Separation
Flow Separation
Flow separation effects in a single seated globe valve.
1.1.5 Flow pulsation
One of the potential problems caused by vortex formation as described by Neles Jamesbury is that
if large vortices are formed they can cause excessive pressure losses and disturb the valve capacity.
Hence special measures have to be taken in high performance valves to reduce the size of vortices.
These involve flow path modifications to shape the flow paths and create “micro vortices”.
Understanding fluid dynamics and separation effects contributes to control valve design in high
performance applications particularly in high velocity applications when noise and vibration effects
become critical.
1.1.6 Principles of valve throttling processes
The following notes are applicable to incompressible fluid flow as applicable to liquids but these
can be extended to compressible flow of gases if expansion effects are taken into account. These
notes are intended to provide a basic understanding of what happens inside a control valve and
should serve as a foundation for understanding the valve sizing procedures we are going to study in
later chapters.
A control valve modifies the fluid flow rate in a process pipeline by providing a means to change
the effective cross sectional area at the valve. This in turn forces the fluid to increase its velocity as
passes through the restriction. Even though it slows down again after leaving the valve, some of the

energy in the fluid is dissipated through flow separation effects and frictional losses, leaving a
reduced pressure in the fluid downstream of the valve.
To display the general behaviour of flow through a control valve the valve is simplified to an
orifice in a pipeline as shown in Figure 1.6.
Figure 1.6
Vena contracta
Flow through an orifice showing vena contracta point of minimum area
Figure 1.6 shows the change in the cross-section area of the actual flow when the flow goes
through a control valve. In a control valve the flow is forced through the control valve orifice, or a
series of orifices, by the pressure difference across the valve. The actual flow area is smallest at a
point called vena contracta (Avc), the location of which is typically slightly downstream of the
valve orifice, but can be extended even into the downstream piping, depending on pressure
conditions across the valve, and on valve type and size.
It is important to understand how the pressure conditions change in the fluid as it passes through
the restriction and the vena contracta and then how the pressure partially recovers as the fluid
enters the downstream pipe area. The first point to note is that the velocity of the fluid must
increase as the flow area decreases. This is given by the continuity of flow equation:
V1 . A1. = V2 . A2
Where: V = mean velocity and A = flow area.
Subscript 1 refers to upstream conditions
Subscript 2 refer to down stream conditions
Hence we would expect to see that maximum velocity occurs at the vena contracta point.
Now to consider the pressure conditions we apply Bernoulli’s equation, which demonstrates the
balance between dynamic, static and hydrostatic pressure. Energy must be balanced each side of
the flow restriction so that:
(½ x ρ1 x V1
2) + (ρ1 x g x H1) + P1 = (½ x ρ2 x V2
2) + (ρ2 x g x H2) + P2 +Δ P
Where:
P = static pressure
ρ = density
Δ P = pressure loss (due to losses through the restrictor)
H = relative height
g = acceleration of gravity
The hydrostatic pressure is due to the relative height of fluid above the pipeline level (i.e. liquid
head) and is generally constant for a control valve so we can simplify the equation by making H1 =
H2.
The dynamic pressure component is (½ x ρ1 x V1
2) at the entry velocity, rising to (½ x ρ2 x V2
2) as
the fluid speed increases through the restriction. Due to the reduction in flow area a significant
increase in flow velocity has to occur to give equal amounts of flow through the valve inlet area
(Ain) and vena contracta area (Avc). The energy for this velocity change is taken from the valve

6
inlet pressure, which gives a typical pressure profile inside the valve. The velocity and the dynamic
pressure fall again as the velocity decreases after the vena contracta.
The static pressure P experiences the opposite effect and falls as velocity increases and then
recovers partially as velocity slows again after the vena contracta. This effect is called pressure
recovery but it can be seen that there is only a partial recovery due the pressure loss component, Δ
P.
The interchange of static and dynamic pressure can be seen clearly in Figure 1.7 where the pressure
profile is shown as the fluid passes through the restriction and the vena contracta. The sum of the
two pressures gives the total pressure energy in the system and shows the pressure loss developing
as the vena contracta point is reached.
Figure 1.7
Static + Dynamic Pressure: P + (½*ρ*V2)
Static Pressure: P
Dynamic Pressure: (½*ρ*V2)
Pressure Loss Δ P
Static and dynamic pressure profiles showing pressure loss
The pressure recovery after the Vena Contracta point depends on the valve style, and is represented
by valve pressure recovery factor (F L) as given in equation below. The closer the valve pressure
recovery factor (F L) is to 1.0, the lower the pressure recovery.
FL = √ (P1-P2)/ (P1-Pvc)
The dynamic pressure profile corresponds to a flow velocity profile so that we can also see what
happens to the fluid speed as it travels through a control valve. Figure 1.8 shows a simplified
pressure and velocity profile as a fluid passes through a basic single seat control valve. It can be
seen that the fluid reaches a high velocity at the vena contracta.

Figure 1.8
Static pressure and velocity profiles across a single seat control valve
We shall see later how the pressure profile is critical to the performance of the control valve
because the static pressure determines the point at which a liquid turns to vapor. Flashing will
occur if the pressure falls below the vapor pressure value and cavitation will result if condensing
occurs when the pressure rises again.
Figure 1.8 therefore represents the typical velocity and pressure profiles that we can expect through
a control valve. Now we need to outline the basic flow versus pressure relationship for the control
valve that arises from these characteristics.
1.1.7 Pressure to flow relationships
For sizing a control valve we are interested in knowing how much flow we can get through the
valve for any given opening of the valve and for any given pressure differential. Under normal low
flow conditions and provided no limiting factors are involved, the flow through the control valve as
derived from the Bernoulli equation is given by:
Q = Valve coefficient x √ (Δ P / ρ)
Where Q = the volumetric flow in the pipeline (= Area of pipe x mean velocity)
Δ P is the overall pressure drop across the valve and ρ is the fluid density
This relationship is simple if the liquid or gas conditions remain within their normal range without
a change of state or if the velocity of the gas does not reach a limiting value. Hence for a simple
liquid flow application the effective area for any control valve can be found by modeling and
experiments and it is then defined as the flow capacity coefficient Cv.
Hence we can show that the flow versus square root of pressure drop relationship for any valve is
given in the form shown in Figure 1.9 as a straight line with slope Cv.

8
Q = Valve Coefficient (Cv) x P
Figure 1.9
Flow
Q
Slope = Cv
P
ρ
Basic flow versus pressure drop relationship for a control valve (sub-critical flow)
1.1.8 Definition of the valve coefficient Cv
The flow coefficient, Cv, or its metric equivalent, Kv, has been adopted universally as a
comparative value for measuring the capacity of control valves. Cv is defined as the number of US
gallons/minute at 60°F that will flow through a control valve at a specified opening when a
pressure differential of 1 pound per square inch is applied.
The metric equivalent of Cv is Kv, which is defined as the amount of water that will flow in m3/hr
with a 1 bar pressure drop. Converting between the two coefficients is simply based on the
relationship:
Cv = 1.16 Kv
In its simplest form for a liquid the flow rate provided by any particular Cv is given by the basic
sizing equation:
Q = Cv√ (Δ P / SG)
Where SG is the specific gravity of the fluid referenced to water at 60°F and Q is the flow in US
Gallons per minute.
Hence a valve with a specified opening giving Cv =1 will pass 1 US gallon of water (at 60°F) per
minute if 1 psi pressure difference exists between the upstream and downstream points each side of
the valve. For the same pressure conditions if we increase the opening of the valve to create Cv =10
it will pass 10 US gallons/minute provide the pressure difference across the valve remains at 1 psi.
In metric terms:
Q = (1/1.16). Cv √ (Δ P/SG)
Where Q is in m3/hr and Δ P is in bars and SG =1 for water at 15°C.
Hence the same a valve with a specified opening giving Cv =1 will pass 0.862 m3/hr of water (at
15°C) if 1 bar pressure difference exists between the upstream and downstream points each side of
the valve.
These simplified equations allow us to see the principles of valve sizing. It should be clear that if
we know the pressure conditions and the SG of the fluid and we have the Cv of the valve at the
chosen opening we can predict the amount of flow that will pass.
Unfortunately it is not always as simple as this because there are many factors which will modify
the Cv values for the valve and there are limits to the flow velocities and pressure drops that the
valve can handle before we reach limiting conditions. The most significant limitations that we need
to understand at this point in the training are those associated with choked flow or critical flow as it
also known. Here is brief outline of the meaning and causes of choked flow.

1.2 Choked flow conditions (critical flow)
Choked flow is also known as critical flow and it occurs when an increase in pressure drop across
the valve no longer creates an increase in flow. In liquid applications the capacity of the valve is
severely limited if the pressure conditions for a liquid are low enough to cause flashing and
cavitation For gases and vapors the capacity is limited if the velocity reaches the sonic velocity
(Mach 1). To understand how these conditions occur we first need to look at the normal pressure to
flow relationship and then see how it changes when choked flow conditions occur.
As pressure differential increases the flow will reach a choked flow condition where no further
flow increase can be obtained. Figure 1.10 shows this effect for a liquid where choked flow
conditions occur when vapor formation occurs at the vena contracta point within the valve.
Non-choked flow Choked flow
Figure 1.10
Slope = Cv
P
Flow versus DP for liquid control valve showing choked flow.
Qmax
Flow
Q
Vapor formation in liquid flow is generally termed flashing and it results either in a vapor stream or
bubbles continuing downstream from the valve, if the bubbles condensed again the transient effect
is described as cavitation.
1.2.1 Cavitation
The pressure profile diagram in Figure 1.11 best illustrates how flashing and cavitation occur. As
static pressure falls on the approach to the vena contracta, it may fall below the vapor pressure of
the flowing liquid. As soon as this happens vapor bubbles will form in the liquid stream, with
resulting expansion and instability effects.

10
Cavitation
Figure 1.11
Pressure profiles for flashing and cavitation
In the diagram the bubbles so formed are collapsing again as the pressure rises after the vena
contracta and the fluids leave the valve as a liquid. This is cavitation, which can potentially damage
the internals of the valve. Figure 1.12 illustrates the same effect in the flow profile through a simple
valve.
Figure 1.12
Pressure profile for cavitation in a single seated globe valve
Static Pressure: P
P1
Vapour Pressure: Pv P2
Flashing
Bubbles form
Bubbles collapse

1.2.2 Flashing
Flashing in the control valve also describes the formation of vapor bubbles but if the downstream
pressure remains below the boiling point of the liquid, the bubbles will not condense and the flow
from the valve will be partially or fully in the vapor state. Again this effect severely chokes the
flow rate possible through the valve. Figure 1.13 illustrates this effect.
Figure 1.13
Pressure profile for flashing in a single seated globe valve
The problem in valve sizing work is determining when critical flow conditions apply, as we cannot
easily see how much the static pressure will fall within a particular valve; we can only see the
downstream pressure after recovery has occurred. In Chapter 3 we shall see how liquid sizing
equations are set up to determine flashing conditions and how the sizing equations are modified to
deal with this condition.
1.2.4 Choked flow in gas valves
Choked or critical flow also occurs in gas and vapor applications when the gas reaches sonic
velocity as it squeezes through the valve opening. Under these conditions the velocity of the gas
cannot be increased further and increasing the differential pressure will not itself increase the flow.
Figure 1.14 shows the capping of gas velocity at Mach 1.
However, in the case of gases and vapors the flow rate is still affected by the density of the gas at
the flowing conditions. Raising the inlet pressure P1 will increase flow and raising the flowing
temperature will reduce flow. These influences will be seen in the Gas sizing equations and
calculations in Chapter 4.

12
Figure 1.14
Pressure profile for gas at critical flow through a single seat globe valve
This completes our introduction the key features of flow through the control valve. Now we can
turn our attention to understanding how the control valve works within typical process control
applications.
1.3 Typical control valve applications
Typically the control valve is required to behave as a means of adjusting flow or pressure
conditions in a process plant or in an item of plant equipment such as a compressor. It is
fundamental to control valve sizing and selection that full consideration must always be given to
the overall performance requirements of the combined valve and process.
Some of the most commonly encountered applications are outlined here so that we can see what is
typically required for the sizing and selection process.
1.3.1 Flow control
A typical flow control loop has the control valve as its final element designed to provide a
controlled flow rate in the pipeline. Ideally the flow rate should change in a fixed proportion to the
control signal delivered from the flow controller system as depicted in Figure 1.15.
Figure 1.15
Typical flow control arrangement
FT
1
FIC
1
I/P
FC
Pressure
source
Pf
Destination
under
pressure
Control
signal
to
valve
Flow

In a typical process arrangement a fluid is supplied from a pressure source along a pipeline that has
a significant flow resistance upstream and there is also some downstream flow resistance. Flow
resistance is seen as the effect that pressure differential across a flow restriction rises with velocity
squared as we have seen for the valve.
1.3.2 Flow resistance model diagram
The pressure drops and flow resistance for the flow control application are depicted in Figure 1.16.
Here we see that the control valve must act as a variable resistance or pressure drop element that
modifies the total flow resistance of the line to change the flow to a desired value between zero and
maximum.
Max Flow
Min Flow
P1
P2
P2
P1
Upstream flow resistance Control Valve Downstream flow resistance
Ps P1 P2
Showing how (P1-P2) changes as Cv is increased
Figure 1.16
Pf
Pf
Pressure profile for gas at critical flow through a single seat globe valve
Ps
Pf
The upstream pressure of the valve, P1, is determined by the pressure at the source (Ps) minus the
pressure drop over the upstream pipeline. Similarly P2 is determined by the flow through the
pipeline downstream of the valve added to the pressure at the destination (Pf) which might be a
tank under pressure or an open ended pipe. As Figure 1.14 shows, when flow increases from
minimum (when the valve has a small opening) to the maximum (when the valve would be
typically 70% open) the value of P1 will fall as flow rises whilst P2 will rise. Hence the value of
DP available for driving flow through the valve falls substantially as the valve is opened.
It should be clear from this model that the specification of the flow and Cv requirements for the
valve must take into account the extremes encountered between minimum flow at high DP and
maximum flow at low DP. The valve is also required to tolerate the maximum shut off pressure
that can be delivered from the source and it may also be required to ensure a low level leakage
when it is shut.
It is also very important that the sensitivity or gain of the valve should be more or less constant
over the range of the controlled operation so that the feedback control loop may have a near
constant overall gain under all conditions. Failure to meet this requirement means that the control
loop may be sluggish at high flows and oversensitive at low flows leading to instability.
The overall sensitivity of the control valve opening versus the flow that is delivered in response to
a control signal is called the installed characteristic. We shall study this further in chapter 4 but it

14
should noted here that the valve characteristic provided by the manufacturer is always based on a
fixed pressure drop across the valve whilst the finally installed characteristic depends on the
combination of the process pressure characteristics and the valve characteristic.
1.3.3 Level control
In level control applications the control valve may operate in a similar mode to the flow control
situation and it provides flow in proportion to the level control deviation. In some level
applications the valve may simply be draining liquid from a tank. In this case the upstream pressure
may be variable due to the changing level in the tank. The downstream pressure may be constant or
in some cases the pipeline to a drain may create a siphon effect leading to a very low downstream
pressure with risk of flashing.
Temperature control by adjusting steam flow rate to a load
In a typical temperature control application, steam flow to a heat exchanger is modulated by a
steam control valve to maintain a consistent secondary fluid outlet temperature. This can be
achieved by using a control valve on the inlet to the primary side of the heat exchanger, as shown
in Figure 1.17.
Figure 1.17
Typical temperature control of a steam/water shell and tube heat exchanger
(Source: International site from Spirax Sarco)
In the typical installation example shown in Figure 1.17 the outlet hot water from a heat exchanger
is required to be controlled at 60°C. The cold-water inlet at 10°C may vary in temperature and
quantity, which presents a variable load to the system. Modulating the position of the steam valve
then controls the outlet temperature of the secondary fluid. A sensor on the secondary fluid outlet
monitors its temperature, and provides a signal for the controller. The controller compares the
actual temperature with the set temperature and, as a result, signals the actuator to adjust the
position of the control valve.

Level control by adjusting in flow or out flow from a tank
Figure 1.18
An adjustable on/off level control system (Source: International site from Spirax Sarco)
It consists of a controller and a capacitance probe, an on/off control valve and electric solenoid
valve shown in Figure 1.18. The system provides control valve open/closed control plus one alarm
point. Additional alarm points for high and low level indication are also available in the controller.
The on and off levels at which the valve operates can be adjusted through the controller functions.
A pneumatic electric solenoid operated valve gets the signal from the controller as per the level in
the reservoir.
Adjustable on/off level control allows the level settings to be altered without shutting down the
water outflow to process. This control can be used for most liquids, including those with low
conductivities.
This system can also be used in situations where the liquid surface is turbulent, and the in-built
electronics can be adjusted to prevent rapid on/off cycling of the pump (or valve).
1.4 Requirements of control valves
1.4.1 Wide range of types and sizes
A wide variety of types and sizes of control valves is needed to cater for a very wide range of
industries. Major valve families and types can be based on type of fluids used, for example:
• Liquids
• Gases
• Steam
• Slurries
Another major category of valve is produced for “hygienic designs” needed for food and drug
processing.
The control valve must have adequate capacity to meet process design requirements like pressure,
flow, temperature etc. with good control over the operating range. Consequently suppliers must
have available a reasonably wide range of sizes in each type to meet the demands.

16
1.4.2 Cost effective solutions
During the application and selection of control valve one of the important aspects is cost
effectiveness. At the same time it should not be in any case at the cost of performance. Hence the
cost of the valve versus line size or capacity needs to be well defined. For example, a butterfly
valve is cheaper than a globe valve for the same capacity when applied to larger sizes if it can
perform the pressure duty. Globe valves often produce the best control for higher pressures but are
very expensive as size increases. However it is important to note that very often a control valve will
have the required capacity in a size somewhat smaller than the pipeline size, hence costs will be
reduced if a smaller valve body is adapted to a larger line size by using pipeline reducers.
1.4.3 Flow in proportion to travel
To achieve control over the process the control valve is expected to have control over one or more
important process variables at any point of time. This need calls for an ability to allow fluid
through valve accurately in proportion to valve opening, repeatedly. In the operation of the valve
the valve stem movement is either linear or rotary. In other words we say the valve should have
ability to adjust flow in proportion to valve travel. This is always needed for stable feedback
control. To get the best performance the valve characteristics need to be matched to the process
characteristics.
1.4.4 Ability to close fully and provide good shut off
In operation where the control valve is used for on-off functions the effective closing during shut
off condition is very essential. In applications with corrosive, hazardous or expensive fluids the
ability of the valve to close fully and provide good shut off is very much needed.
1.4.5 Ability to withstand static pressure in the pipe without leakage
The control valves are subjected to pressure changes during the operation of full opening to
minimum opening. The control valve should have ability to withstand the static pressure in the pipe
without external /internal leakages.
1.4.6 Material of construction resistant to corrosion
The control valves are subjected to fluids, which are corrosive in nature. These fluids can have
chemical reactions on seals, valve body, glands and gaskets. In such applications the material used
for construction of the control valve must be resistant to the corrosion caused by the particular
fluid.
1.4.7 Internal resistance to erosion and corrosion
The internal parts of the valve, which are responsible for proper operation, are also vunerable to
corrosion due to the corrosive effect of the fluid. The material of the internal parts must be selected
so it is resistant to the action of the fluid. This is very important for consistent and desired service
from a control valve. In addition to the corrosive resistance as a selection criteria we have to
consider pressure abusive on the internal parts of the control valve. If the valve is subjected to high
pressure, operates with large pressure drops and frequent variations of pressures, then the material
of the internal parts of the control valve should be such that they can be treated to offer resistance
to erosion.
1.4.8 Dimensions standardized to fit mechanical standards of pipes and flanges
For an installation in the pipe line the valve body is either threaded or flanged at the inlet and outlet
connection points. To have installation and maintenance conformity there has to be a fixed relation

in pipe sizes with the mounting threads or flanges. The American Petroleum Institute (API) has set
down standards for pipes and flanges and these have been widely adopted. Schedule numbers
define the pressure rating of the piping and there are eleven Schedules ranging from the lowest at 5
through 10, 20, 30, 40, 60, 80, 100, 120, 140 to schedule No. 160. All pipes of particular size have
the same outside diameter but as the wall thickness increases with pressure rating, the inside
diameter is reduced.
The control valve pipe connection dimensions are specified to conform to the API standards for
pipes and flanges. In European standards the pipeline dimensions are expressed in metric sizes
according to the relevant DIN standards for pipes.
1.4.9 Designed to avoid excessive reaction forces on the moving parts such as
stems or shafts
As the size and duty of a control valve increases, the forces generated by the fluids passing through
it become large and in an unbalanced design these can place high stress on the valve stem or shaft.
Similarly the forces needed to close of the flow and withstand static pressure will also determine the
size and thrust requirements of the control valve actuator. Control valve designers seek to minimize
these forces by the internal design of the valve trim and the configuration of the valve seats. Lower
forces permit smoother operation and smaller cheaper actuators, which can be offset against the cost
of the valve.
1.4.10 Provided with actuators to deliver adequate forces to unstick, open smoothly
and reach full opening
In smaller sizes the operation of control valves can be manual. When the pipe size increases (2
inches and above) the energy required to over come sticking, opening and closing is very high.
Different types of actuators like pneumatic, electrical, hydraulic are used to provide this force.
Smooth and controlled operation of the valve is necessary to avoid jerks and water hammer effect.
With a positioner and controller combination, very smooth and continuous control between closed
position of the valve and full open position is achieved.
1.5 Training needs for sizing and selection
The training requirements for an instrument technician or engineer involved in sizing and selection
of control valves should concentrate on developing several important skills including the
following:
1) Having a basic understanding of valve capacity and how it can be calculated
2) Understanding how the control valve changes the process flow and pressure conditions in any
particular installation. This implies understanding the interaction between flow through the
valve and pressure drops in the piping system.
3) Knowing how to manipulate the basic valve sizing equations to arrive at the capacity versus
flow values for any given situation.
4) Knowing how to make use of valve sizing software to quickly explore sizing options and then
arrive at a sufficiently accurate solution to make the sizing decision.
5) Understand the causes and conditions of choked flow, cavitation and noise in outline
sufficiently to discuss and understand solutions offered by vendors. I.e it is not necessary to be
an expert in the field when specialist companies have the knowledge and experience in house.
6) Be aware of the principal types of control valve and the factors influencing the choice for an
application.
7) Be able to understand the meaning of the catalog data available from major suppliers of control
valves and be able to discuss selection choice with the vendor.
8) Be aware of the important roles of actuators and positioners in providing an integrated package
for final control of the process.

18
9) Be able to recognize the role of the installed control valve as part of the complete process
control loop.
10) Be aware of the critically important role of materials of construction and trim materials in the
long-term reliability of the control valve you select.
The training chapters in this manual together with the glossary, the appendix of frequently asked
questions and the application examples are all intended to promote the above skills.

2
Types of Control Valves
Control valves are manufactured in a wide range of types and sizes to cater for applications across
all industries. Our objective in this text is to recognize the key features of the most widely applied
types of control valve to support the task of choosing the right type of valve for each application.
For any particular application the engineer should be in a position to know the essential features
of the main types of valves and he or she will need to make a selection based on balancing good
operating performance in the application against costs. Costs should ideally be seen as life cycle
costs in which the capital cost and the life time servicing and repair costs are combined with what
may be a much larger cost item, this being the cost associated with energy losses in the process
under control.
It is important to recognize that poor control in the form of instability or inaccurate positioning of
the valve opening can have a very detrimental effect on the efficiency of a chemical or energy using
process. Further to this any application that uses a higher than necessary pressure drop to achieve
control is wasting energy than can amount to large amounts of money during the daily operations.
Other lifecycle operating costs we should keep in mind, considerations often of equal importance in
the selection process may include:
• Ability to shut off the flow completely and the degree of leakage through the valve
• Leakage potential to atmosphere through seals
• Accuracy of control required
• Availability of suitable trim sizes and options
• Overall size and weight of the installation with actuators
• Resistance to erosion and corrosion
• Noise and vibration potentials
• Hygienic design and cleaning ability
Learning objectives
• Recognize a classification of control valve types
• Know the key features of each type of control valve and how these may help to solve
problems.
• Develop outline principles for type selection

20
2.1 Broad classification of control valves
Figure 2.1 provides a simple classification of the most widely used types of control valves.
Gate
Pinch
Eccentric Plug
Butterfly
Figure 2.1
Types of control valves
• Valve Types
The first level of grouping of valve types distinguishes between linear and rotary movement of the
internal parts required to provide the variable opening required. One easy way to visualize this
difference is to look at Figure 2.2 where each of the main types of valve is shown in the same
pipeline.
Figure 2.2
Principal types of control valve
2.2 Sliding stem valves
These types use a plunger type action to adjust the flow area of the valve. This enables a very wide
range of plug and seat styles to be applied to achieve accurate and finely graded adjustment and
allows very robust guides to be made for resisting high pressure differentials.
The basic body styles are:
• Globe
• Cage
• Angle body
Linear
Rotary
Globe
Diaphragm
Ball
Plug valve Ball valve Butterfly
valve
Gate valve
Linear
actuation
Rotary actuation
Linear
actuation

Types of Control Valves 21
• Y pattern
• Split body
• Three way
• Single seated
• Double seated
The internal parts of a control valve in flowing contact with the fluid are collectively known as the
trim. These will typically include the plug, seats, stem, guides and bushings that support the stem
and the cage that may be provided to enclose the plug. Trim configurations are:
• Unbalanced
• Balanced
The guiding configurations are:
• Cage
• Post
• Top
• Top and bottom
• Stem
• Skirt
Valve trim designs are provided by most manufacturers to give three different flow characteristics
(see Chapter 5 for details):
• Equal percentage
• Linear
• Quick opening
The meaning of the above terms will be explained as we proceed through the following notes and
diagrams of the principal types of sliding stem valves. Firstly we summarize the principal types of
sliding stem valves with particular attention to the most widely used globe valve. Then we look
more closely at the subjects of guides and cages in the trim.
2.2.1 Globe valves
The globe is the most common type of body style for sliding-stem valves. The valve body often
resembles a globe divided across internally to provide two separated cavities. The valve orifice
allows the fluid to pass through the body and a plug and seat are arranged to throttle and shut off
the flow (Figure 2.3).

22
Figure 2.3
Single seated globe valve
Features
• This valve has a linear action with a plug moving up and down.
• This valve type is very versatile, able to handle high pressures and temperatures and
is made in a large variety of materials.
• Competitive on small sizes but becoming very expensive as the size increases
• Most common type of control valve used throughout world
• Well actuated with pneumatics though the forces become difficult to handle on high
pressures and large sizes unless pressure balanced
• Can handle very low flow rates
• Pressure drop across valve is high compared to all other valve types
• Sizes from 12 mm to 400 mm
• Ratings to ASME 4500#
• Seat leakage to ANSI IV - optionally ANSI V + VI
• Special trims available to handle high pressure drops
The globe body differs considerably depending on the trim used. The main components of the
valve trim are the plug and stem and the seat ring. The most widely used valve is the single-stage
orifice and plug assembly. Multi-stage orifice elements are usually found in trim designs to reduce
noise, erosion and cavitations.
Advantages
• Minimizes disassembly for maintenance
• Streamlined flow path with a minimum of parts and no irregular cavities
Disadvantages
• Leaking of the central joint due to thermal cycles or piping loads
• Valves cannot be welded in-line since the valve body is required to be split

Parts of a typical globe valve
• A body
Main pressure containing structure
Contains all of the valve's internal parts
Bonnet is connected to the body and provides
the containment of the fluid, gas or slurry
Threaded section of stem goes through a hole
with matching threads in bonnet
• Bonnet
Provides leakproof closure for the valve body
Screw-in bonnet
Union bonnet
Bolted Bonnet
Simplest bonnet, offering a
durable, pressure-tight seal
Suitable for applications requiring
frequent inspection or cleaning
Used for larger or higher
pressure applications
Contain packing which maintains seal between
bonnet and stem during valve cycling operations
Cross section of a globe valve
• A PLUG Closure member of the valve
& Connected to the stem
Balanced plugs
Have holes through plug
Unbalanced plugs
Solid. Used with smaller
valves or with low pressure
drops across the valve

24
• A STEM :
Serves as a connector from the
actuator to inside of the valve
and transmits actuation force
Must be very straight, or
have low runout
For actuator controlled valves
Smooth
Threaded
Surrounded by packing material to prevent
leaking material from the valve
Ends are threaded to allow connection to
plug and the actuator
For manual valves
Useful to guide the plug to the seat of the valve for a
good shutoff, substituting the guiding from the bonnet
• A CAGE
Part of the valve that surrounds the plug and
is located inside the body of the valve
Determines flow within the valve
Design and layout of the openings have a
large effect on flow of material
Held in place by pressure from fastening of the
bonnet to the top of the body
• A SEAT RING
Provides a stable, uniform and replaceable shut off surface
Beveled at seating surface to allow for some guiding
during final stages of closing the valve
• MATERIALS
Made of metallic alloys
Selection based on pressure, temperature,
controlled media properties
Corrosive and/or erosive process streams may require a
compromise in material selection or exotic alloys or body
coatings to minimize these material interactions and
extend the life of the valve or valve trim components.
Examples - Carbon steel alloys Hastelloy, Monel, Inconel etc.

2.2.2 Cage valves
Cage valves (Figure 2.4) use the principle of cage guiding, where the plug rides inside a cage. This
is quite common in most valves, because the bearing forces on the plug are near the fluid forces. As
the cage aligns the plug, the valve effectively self-aligns so that during assembly all the pieces fit
together. Correct alignment reduces the problems of side loads.
Cage guiding is not recommended when the fluid is highly viscous. Such fluids that are sticky or
gummy can also cause problems, as can fluids that contain solids. The problem is the possible build
up between the plug and cage, which can cause operational problems. This problem of build up is
also referred to as fouling. Fouling can cause a restriction of travel in the valve movement, or a
delayed response time to a control signal.
Block and bypass valves are used for assisting in the maintenance of valves. The need for block
and bypass valves is eliminated since cage valves are very rugged and have a good service life.
Whereas globe valves (also known as post-guided) are characterized by the shape and contours of
the valve plug, Cage valves are characterized by the shape of the cage window.
Cage valves are popular due to the variety of trim types available. Trim type may be selected for
various performances such as reducing cavitations (anti-cavitations trim), or for reducing noise.
Figure 2.4
Cage valves with clamped-in seat ring and characterized plug (courtesy of Valtek Inc).
Summary:
• Plug rides inside cage
• Valve self-aligns - during assembly, all pieces fit together
• Correct alignment - reduces problem of side loading
• Not recommended for highly viscous fluids
• Above can cause build-ups
Advantages
• No threaded joints
• Suitable for many trim types to be used
• Plug and cage designs allow different characteristics to be offered
• Easy to maintain
• Top entry
• No threaded joints to corrode

26
• Trouble-free when specified correctly
2.2.3 Split body valves
Split body valves provide streamlined flow and reduce the number of bolted joints (Figure 2.5).
These valves use one bolt to secure the valve with the seat ring clamped between the body halves.
Their original design and subsequent operation was for difficult flows with high viscosity. Fouling
is minimized due to the valves, simple streamlined construction.
Figure 2.5
Split body valve
Maintenance requires that in order to service the valve, the flange connections must be broken. The
advantages from simple valve design are outweighed by the concerns over line flange leakage after
maintenance.
Advantages
• Streamlined flow
• Minimum number of parts
• No irregular cavities
Disadvantages
• Leakage problems with central joint
• Inability to weld
• Maintenance complications
• Limitations on trim modifications
• Fewer of these valves are used today.

2.2.4 Angle valves
These valves can be likened to mounting a globe valve in an elbow. The exiting flow is 90 degrees
to the inlet flow. The obvious advantage is the elimination of an elbow, should one be required,
however the flow does make fewer turns as it passes through the body (Figure 2.6). The Angle
valve has little restriction on the out flow, so if flashing or cavitations occurs then it tends to do so
further downstream from the valve. This saves not only on the maintenance life of the valve, but
also minimizes any degradation in valve performance. Angle valves are limited in use and are
generally used for erosive applications requiring replaceable inserts on the out-flow piping.
Figure 2.6
Streamlined angle valve
2.2.5 Y-style valves
This style of valve has the operating components tilted at a 45-degree angle to the flow path. In
theory, the flow stream has fewer turns when fully open. In practice, they are mainly used for
drainage applications, operating at or near the closed position (Figure 2.7). When installed in
horizontal pipe, maintenance is impaired with the added difficulty of aligning and handling the
components. This is true for any extraction angle other than vertical.
Another inhibiting factor is that when installed with moving parts not vertical, the added side load
due to gravity increases wears with the need for more frequent maintenance.

28
Figure 2.7
Y-styled angle valve
2.2.6 Three-way valves
Three-way valves are a special type of double ported valve.
Two types of three-way valves available are mixing and diverting.
Mixing
The mixing valve has two inlets and one outlet (Figure 2.8).
This type of valve would be used for blending two fluids with the associated ratio control of the
mix.
Figure 2.8
Three-way valve for mixing
• Diverting
– One inlet
– Two outlets
• Used for switching or bypass operations
– Good for chilled water systems, to
prevent freezing in pipes

Diverting
The diverting valve has one inlet and two outlets (Figure 2.9). Diverting valves can be used for
switching or for bypass operations. The relative split provides the required controlled flow with one
outlet, while allowing a constant flow through the system with the other outlet. Such valves are
used in chilled water systems to prevent freezing in the pipes.
Figure 2.9
3-way valve for diverting
• Mixing
– Two inlets
– One outlet
• Used for blending two fluids
with an associated ratio
Operation
Because of the dual function of the plug, these valves are generally not pressure balanced, however
the stem forces required for operation are similar to single port valves.
2.2.7 Notes on valve seating styles
Single seated
Single seated valves are one form of globe valve that are very common and quite simple in design.
These valves have few internal parts. They are also smaller than double seated valves and provide
good shut off capability (Figure 2.10).
Maintenance is simplified due to easy access with top entry to the valve components. Because of
their widespread usage, they are available in a variety of trim configurations, and therefore a
greater range of flow characteristics are available. They also produce less vibration due to the
reduced plug mass.

30
• Form of globe valve
• Very common
• Quite simple
• Fewer internal parts
• Smaller than Double Seated valves
• Good shut-off
• Variety of trims
• Less vibration (smaller plug mass)
Figure 2.10
Single seated globe valve
Advantages
• Simple design
• Simplified maintenance
• Smaller and lighter
• Good shutoff
Disadvantages
• More complex designs required for balancing
Double seated
Globe valve bodies are available with double-seated designs (Figure 2.11). In this approach, there
are two plugs and two seats that operate within the valve body.
In a single seated valve, the forces of the flow stream can push against the plug, requiring greater
actuator force to operate the valve movement. Double seated valves use opposing forces from the
two plugs to minimize the actuator force required for control movement. Balancing is the term used
when the net force on the stem is minimized in this way.
These valves are not truly balanced. The result of the hydrostatic forces on the plugs may not be
zero due to the geometry and dynamics. They are therefore termed semi and dynamic forces when
sizing the actuator.
Shutoff is poor with the double-seated valve and is one of the downfalls with this type of
construction. Even though manufacturing tolerances may be tight, due to different forces on the
plugs it is not possible for both plugs to make contact at the same time.
Maintenance is increased with the added internal parts required. Also these valves tend to be quite
heavy and large. These valves have fewer advantages compared with the inherent disadvantages.
Although they can be found in older systems, they are seldom used in newer applications.

• 2 seats & s bodies
• With single seat, forces of flow push against plug,
• Here, opposing forces minimise actuator force to
Figure 2.11
requiring greater forces to operate
Double seated and double ported valve with diaphragm actuator
control movement
Advantages
• Reduced actuator force due to balancing
• Action easily changed (Direct/Reverse)
• High flow capacity
Disadvantages
• Poor shutoff
• Heavy and bulky
• More parts to service
• Only semi-balanced
2.2.8 Balanced valves
Balancing is the term used when the resultant force on a plug is neutral. This means that the plug is
neither forced up or down by the pressure of the flow stream. The advantage with balancing is that
the actuator force required for controlled movement is greatly reduced. This allows for smaller and
cheaper actuators. Balancing is applied to single-seated and double-seated valves in different ways.
Double-seated balancing
Double-seated valves were originally designed for balancing. These valves use opposing forces
from the two plugs to minimize the actuator force required for control of movement. That is, the
pressure of the flow stream acting on the upper plug is intended to cancel the pressure acting on the
lower plug.
The force on the upper plug is in the opposite direction to that on the lower plug and as such, the
result should be zero. However, because the plug sizes differ, the forces are not equal and the result
is an unbalanced force. Double-seated valves are actually semi-balanced.

32
Single-seated balancing
In a single seated valve, the forces of the flow stream can push against the plug, requiring greater
actuator force to operate the valve movement. To balance a single-seated valve, balancing holes are
added to equalize the pressure on both sides of the plug. This eliminates any unbalanced force on
the plug; however further seals are required for the extra leakage path between the plug and the
cage.
An unbalanced valve has better shut off capability because there is only the problem of leakage
between the seat and the plug. A balanced valve however, has a total leakage of the sum of the
following:
• Leakage between the seat and the plug
• Leakage between the plug and the cage
The seal for the seat and the plug is a closing seal. This is applicable to both balanced and
unbalanced valves. But the seal between the plug and the cage is a dynamic seal and applies to
balanced valves only. Being a dynamic seal, maintenance and service life need to be considered, as
do the operating conditions on the sealing material. Maintenance on unbalanced valves can involve
the machining of the valve seats to rectify leakage problems.
If leakage occurs with balanced valves, there are two types of seals responsible. It is not uncommon
for the seat to be machined to rectify the problem when in fact the cause of the leakage is due to the
trim. Although Teflon is limited by temperature compared to graphite, it does have better sealing
properties. Better sealing can be achieved by not over specifying operating temperature ratings.
2.2.9 Guiding
The control valve guide is used to support and position the valve plug over the full range of travel.
Various control valve-guiding designs are available and should be considered as they affect the
operating life and reliability of a valve. The guide provides the support for the valve plug. Any
forces on the plug are resisted by the guide. If the guide wears or fails then vibration can become a
problem. Under high bearing loads, the surface of the guide can break down causing increased
friction and impeding valve performance.
In choosing suitable guides:
• Use bearing materials with different hardness levels
• Avoid nickel and unhardened stainless steel
Types of guiding designs:
• Cage
• Stem
• Post
• Top
• Top and bottom
• Port
2.2.10 Cage guiding
The most common type of guiding is cage guiding (Figure 2.12). The plug moves within a cage
with little tolerance between the two. This design enables the loading on the plug to be supported
by the cage with a large bearing area between the two. Maintenance is reduced as the assembly is
simplified with the components self-aligning.

Figure 2.12
Cage guiding
2.2.11 Stem guiding
Stem guiding is a simple design where the stem itself is responsible for supporting and controlling
the plug (Figure 2.13). Limitations occur due to the stem's strength, as the support of the stem is
farther away from the load on the plug. Guiding performance is impaired but this type of valve is
cheaper to manufacture and maintain.
Figure 2.13
Stem Guiding
2.2.12 Post guiding
Post guiding is mostly used if there is a risk of fouling. The post is a section of the stem from the
plug that extends into the valve body. The post is smaller in diameter than the plug but larger than
the stem (Figure 2.14).

34
The post supports the plug from bearing loads, with the narrower stem providing positioning
control.
This type of guiding also helps keep the bearing surfaces out of the flow stream. This reduces the
buildup of fluid.
The two types of post guiding are:
• Top guided, when the post is above the plug, the valve is termed 'Top guided'.
• Top and bottom guided, when the plug is supported from above and below, or in
the case of some dual port valves, the valve is termed 'Top and bottom
guided'.
Figure 2.14
Post guiding
2.2.13 Port guided
Very seldom used, but still in existence, is the port guided valve. In this design, the port is used to
align and guide the plug. The port-guided design (Figure 2.15) also has a relatively small bearing
surface and has the same problems with fouling as with the cage-guided valves.
Figure 2.15
Port guiding

2.3 Rotary valves
In this major category of control valves the flow aperture is adjusted by a rotating shaft connected
to a disc or a hollowed out ball mounted in the body. The two types we shall consider here are the
butterfly valve and the ball valve. This category also includes the various types of vane devices that
are typically seen as dampers in combustion systems or low-pressure air flow installations such as
ventilation and air conditioning systems.
2.3.1 Butterfly valves
Standard butterfly valves are dampers that are shaped from discs, which rotate in the flow path to
regulate the rate of flow (Figure 2.16). The disc is quite narrow and occupies little space in the
pipeline. The shaft is centered on the axis of the pipeline and is in line with the seal. The disc pulls
away from the seal upon opening. This minimizes seal wear and reduces friction. Control of the
valve near the closed position can be difficult due to the breakout torque required to pull the valve
out of the seat.
The flow characteristics are essentially equal percentage, but the rotation is limited to about 60
degrees as the leading edges are hidden in the shaft area as the disc is rotated further. The Fishtail is
one modification of the disc that permits effective control out to 90 degrees of rotation.
Figure 2.16
Basic butterfly valve
In its simplest form the butterfly control valve can be made with a “swivel through” disc that does
not fully close of the flow, it simply provides a variable area restriction and still enables flow
control to be achieved. However to provide good shutoff when closed the valve requires either soft
seals around the circumference of the body or highly accurate metal seals with eccentric motion of
the disc such that it is pressed tightly to seal. The soft seals are made of an elastomer material such
as rubber or PTFE.

36
Features of butterfly valves
• The butterfly is a quarter turn rotary
valve consisting of a disk that turns
with a shaft.
• In the closed position the disk is
forced into a rubber seal that
deforms to create a tight seal
• The liner can be moulded to the
body or can be a loose replaceable
item.
• The body is normally wafer style that
is clamped between flanges. The
liner usually serves the purpose of
the line gaskets which then should
not be used.
• Sizes vary from 50mm to 3000mm
• Operating pressures are seldom above 1000 kPag
• These valves can be used on most applications where the pressures are low and
temperatures do not exceed 100°C except when metal-seated versions are used.
• Many liner materials are available to handle different corrosive fluids
• Butterfly valves are readily actuated by pneumatics and by electrical actuators
Figure 2.17
Table 2.1 shows three typical types of butterfly valves and their characteristics.
Resilient or soft-seated butterfly valves can achieve very high “bubble tight” shutoff. When fully
open they present a large flow aperture and hence have a high Cv for the given line size when
compare with a globe valve.
Table 2.1
Butterfly valves
Resilient butterfly valve Flexible rubber seat. Working pressure up to
1.6 Megapascals (MPa)/232 pounds per
square inch (PSI)
High performance butterfly valve Double eccentric in design. Working pressure
up to 5.0 MPa/725 PSI
Tricentric butterfly valve Metal seated design. Working pressure up to
10 MPA/1450 PSI
High performance butterfly valves
The high performance butterfly valve is a development from the conventional valve where the
rotation axis of the disc is offset from both the centerline of flow and the plane of the seal (Figure
2.18). This design produces a number of advantages, including better seal performance, lower
dynamic torque, and higher allowable pressure drops. The seal performance is improved because
the disc cams in and out of the seat, only contacting it at closure and so wear is reduced. As the disc
only approaches the seal from one side, the pressure drop across the valve can be used to provide a
pressure-assisted seal. This further improves performance.

Figure 2.18
• Otherwise known as ‘high
performance butterfly valves’
• Similar to the butterfly but usually
supplied with a metal seat
• Good for control
• Readily actuated by pneumatics
or other types
• Sizes from 50mm to 1200mm
• Pressures up to ASME 600# (
100 bar)
• Temperatures from cryogenic to
600 degC
• Become very competitive as size
increases
The modified shape and contour of the disc are used to reduce dynamic torque and drag. This also
permits higher-pressure drops. As the disc is never hidden behind the shaft, good control through
the 90 degrees of operation is possible with a linear characteristic.
Figure 2.19
Butterfly valve disc shapes developed for improved performance range
The high performance butterfly valve is gaining greater acceptance and use due to its increased
capability and the relatively high capacity to cost ratio.
Butterfly valve advantages
• Low cost and weight relative to globes as size increases
• High flow capacities
• Fire safe design
• Low stem leakage
Disadvantages
• Over sizing
• Pressure limitations
By over designing the capacity of a flow system, the result is either oversized valves, or correctly
sized valves in oversized pipes. The valves in either case cause pressure drops in the flow due to
the restriction. In the application of ball valves, the recovery of the pressure loss is good, but noise
and cavitations then may become a problem.

38
2.3.2 Ball valves
The ball valve is one of the most common types of rotary valves available. The valve is named
from the valve plug segment being a ball or sphere that rotates on an axis perpendicular to the flow
stream (Figure 2.20). Fully open to fully closed is performed by a 90 degree rotation of the plug
segment.
Figure 2.18
Features of the ball valve
The full-ball valve
The full ball valve (Figure 2.21) is shaped from a spherical segment with a cylindrical hole for the
flow of fluid. Among the various configurations, the 'floating' ball has two seals, which provide
bearing support to the ball segment. This does provide simplicity in the design, however the
friction levels are higher than conventional bearing designs, which can affect control performance.
Figure 2.21
Typical section through a top entry full ball valve
• Ball Valves
Durable and usually work to achieve perfect shutoff
• Body styles of ball valves
A full port ball valve has an oversized ball resulting in lower friction
loss. Flow is unrestricted, but the valve is larger.
A standard port ball valve less expensive, but has a smaller
ball and a correspondingly smaller port.

Ball valves have been extensively developed for high performance on-off duties and for high range
control valve duties. In shut-off duties the emphasis is on sealing designs that will provide tight
shutoff and yet have minimal torque for opening.
In control valve duties many types have been developed with segmented ball trims and V-ball trims
that are contoured to ensure that smooth equal percentage or linear characteristics can be achieved.
For high temperature applications metal-seated full-bore valves are available in which the ball and
stem have been cast into one piece to ensure no hysterisis can occur between the stem and the ball.
Features of soft-seated ball valves
• Simple in design and concept
• Ball with orifice through centre rotates through 90 degrees
• Line forces hold ball against downstream seat
• Seats usually made of PTFE or other elastomer
• No restriction in line when fully open so suitable for pigging
• Can be full bore (same as pipeline) or reduced bore
• Good for control
• Can be supplied in multi-port configurations
• Can be designed for high pressure when trunnion mounted
• Seat leakage to ANSI VI standard
• Can be actuated pneumatically or by other types of actuators
• Not good for slurries due to build up of material in body cavity
• Cannot handle high pressure drop applications
• Sizes from 6mm to 800mm
Features of metal-seated ball valves
• Used for high pressure and temperature applications where PTFE cannot be used
• Metal coating can be stellite, or a ceramic coating such as tungsten carbide
• With ceramic materials sealing can be to ANSI VI or tighter if ball and seat are
ground to high precision and lapped together
• This is the only valve that can be used for control applications on high temperature
slurries
The characterized ball valve
The full-ball valve was originally designed for ON-OFF control. Although modulation control is
possible, the flow characteristics can be difficult to work with. The opening between the ball and
the seal can be modified to provide different flow characteristics (Figure 2.22). The V-notch is one
example that produces a more gradual opening to give better range ability and throttling capability.
Most characterized ball valves are modified so that only a portion of the ball is used and these are
often called segmented ball valves. The edge of the partial ball can be shaped to obtain the desired
flow characteristics.
Figure 2.22
Using a segmented and characterized ball valve for modulating control

40
Various manufacturers promote their valves on the characteristics achieved by this design. Apart
from the V-notch, other designs can be U-notch or parabolic curve. Although favorable
characteristics may be achieved with the characterization of the ball, problems may occur due to
the reduced strength of the partial ball. Bending is one such problem, which occurs under operating
loads. Care also needs to be taken during installation as over tightening of the flange bolts can
damage the seals.
Ball valve advantages
• Low cost and weight relative to globes as size increases
• High flow capacities (2 to 3 times that of globe valves)
• Tight shutoff
• Fire safe design
• Low stem leakage
• Easily fitted with quarter turn actuators
Disadvantages
• Over sizing
• High cost in large sizes compared to butterfly valves.
2.4 Other types of control valves
In this chapter we have briefly outlined the main features of globe valves and rotary valves as
widely used in process control applications. Here are some brief notes on valves that are widely
used in mining applications where abrasive slurry mixtures and other difficult fluids are used
2.4.1 Diaphragm valves
Whilst generally used for isolation duties in water, steam and slurry applications, as well as in
corrosive fluid duties the diaphragm valve can be very effectively applied to modulating control by
the use of actuator and positioner system (Figure 2.23).
Figure 2.21
Basic diaphragm valve
Features
• Valve design is abrasion-resistant and non-clogging
• Elastomer lined versions highly resistant to aggressive chemicals

• Suitable for throttling and on/off service in applications ranging from water treatment
to chemical abrasion processes
• Operated manually, electrically, or pneumatically
• Top-entry design, allowing in-line maintenance
2.4.2 Pinch valves
Pinch valves (Figure 2.24) include any valve with a flexible elastomer body that can be pinched
closed, cutting off flow, using a mechanism or fluid pressure. Pinch valves are full bore, linear
action valves so they can be used in both an on/off manner or in a variable position or throttling
service.
Figure 2.24
Basic pinch valve
Some typical applications for pinch valves are medical, pharmaceutical, wastewater, slurries, pulp,
powder and pellets. They can effectively control the flow of both abrasives and corrosives, as there
is no contact between metal parts and the transport media. The merits of the pinch valve for control
duties are similar to those for the diaphragm valve.
Pinch valves may be closed either by manual means, or fluid actuation. Electromechanical closure
is effected by actuating a solenoid, which then lowers a bar or gate onto the sleeve, cutting off the
flow. With fluid actuated pinch valves, the pinching action is accomplished by air or hydraulic
pressure placed directly on the elastomer sleeve. The valve body acts as a built-in actuator,
eliminating costly hydraulic, pneumatic, or electric operators.
Pinch valves are used widely in medical, pharmaceutical and other sanitary applications. They
contain a number of design advantages allowing for cleanliness, excellent drainage, and ease of
cleaning. Most varieties are constructed so that the compression pressure is from the top only
allowing the valve to drain thoroughly in all positions except upside down. Additionally, many
have a straight-through design that allows for a high rate of flow with minimal turbulence. Both of
these features call for low air consumption, allowing the system to stay relatively closed, reducing
the introduction of airborne contaminants. Optional sterility features include end flange
configurations that connect flush with the transport tubing; and in situations where the tubing does
not connect flush, seals in both the valve and fittings to eliminate particle entrapment, and facilitate
in-line cleaning. Other advantages, not specifically related to sterile operation, include low
maintenance, low weight (due to the largely plastic body), and suitability for use in systems
requiring explosion-proof line closure.

42
While the design of pinch valves provides extensive advantages for use in sterile lines, and in
situations where product purity is a high priority; these same design features create some
disadvantages. Due to their elastomer bodies, pinch valves are not viable in situations where the
transport media is of a high temperature. They are also contraindicated for services that require
high-pressure flow, and for use with gases.
2.4.3 Rotary plug valve
The plug valve (Figure 2.25) is used primarily for on-off service and some throttling services. It
controls flow by means of a cylindrical or tapered plug with a hole in the centre that lines up with
the flow path of the valve to permit flow. A quarter turn in either direction blocks the flow path.
Figure 2.25
Rotary plug valve
Features of rotary plug valves:
• Similar to the ball valve but instead of a spherical ball there is a tapered plug with an
orifice through the centre
• The advantage of this is that the plug can be forced further into the seat if a leak
develops whereas a ball valve must be removed from the line for repair
• There is no body cavity and so this valve is more suitable for liquids that crystallize
• Most commonly used on chemical plants
• Can be fully PTFE lined
• Available in multi-port configuration for diverting or blending
• Lubricated plug valves can be used for very high pressures and for erosive
applications but require high maintenance
• Non lubricated valves available from 10mm to 250mm
• Lubricated plug valves available from 50mm to 800mm
2.5 Control valve selection summary
The technology of control valves has become highly specialized and each manufacturer has
considerably more detail to offer on the special measures taken in their designs to satisfy the needs
of their customers. You are therefore advised to continue studying particular aspects of any
application problem by referring to technical information notes available for the valve makers.
The following procedures are suggested as basic steps in the selection of typical control valves
for many processes.
Step 1: Ensure the best possible understanding of process performance requirements
Step 2: Decide the selection criteria. What is most important?
Step 3: Specify technical requirements

Step 4: Obtain proposals and sizing calculation checks from suppliers based on the best available
data.
Step 5: Evaluate your choice on the available offerings against your selection criteria.
The first step is to make sure that the duties to be performed by the valve are properly known and
their criticality for the process and the impact on the production costs of non performance or
failures is well defined. Where uncertainty exists over the technical conditions the valve will have
to be conservatively rated to allow an error margin.
The second step is to define the selection criteria that apply to your application. As previously
discussed the technical performance of the valve in a process control application may have the
highest priority in terms of its potential for efficiency and cost saving in the process. Accurate
control can produce great paybacks for investment in good controllability. In summary therefore
the selection criteria may be:
Best control performance
• Lowest initial purchase price
• Lowest lifetime cost (decide with or without considering the impact on process
accuracy)
• Longest time between maintenance. (Consider process downtime costs)
• Smallest size and weight
• Environmental and health impact (relates to leakage potentials in cases of toxic
fluids)
• Availability and delivery period (Control valves can be long lead items)
The basic technical requirements are most commonly defined by means of standardized valve
specification sheet, which will ask all the essential questions. The minimum requirements are
summarized here:
Specify technical requirements
• Maximum pressure and temperature
• Fluid types and properties that may impact performance, wear and corrosion
• Face to face limitations (these may be imposed by existing piping design)
• Allowable pressure drop at maximum flow requirement
• Leak rate permitted at shut off
• Size and weight restrictions
• End connection style/standard
• Material of construction and trim if known
• Type of actuation and the response to power failure.
In practice your company may also have a policy of standardization on control valves from specific
manufacturers because of their proven track record in terms of the product and service support.
Your maintenance workshop may have specialized in certain models for which the artisans are well
trained and for which spares are readily available. Clearly these factors will favor designs of
control valves, which have a very wide range of capabilities for your industry. Historically this
requirement has favored globe style valves for which a very wide range of trims be engineered to
meet most performance requirements in same basic package.
2.5.1 Type selection guide
Figure 2.26 shows a decision chart that is suggested for guidance on general-purpose applications.
This is further assisted by a rated table of suitability as suggested by an established valve
manufacturer. The reader is cautioned that there are so many special circumstances applicable to
many valve applications that the guide tables must be considered as very approximate first level
guides only.

44
NO YES
Is
Line >= 150mm?
Is P1>1000 kPa? NO YES
NO YES
Is T1>100oC?
NO YES
Is P1/P2 > 3?
Butterfly Globe
NO YES
Is Full
Bore OK?
Rotary Plug Eccentric Disk
Figure 2.26
Control valve selection decision chart for gases and liquids
Globe
The decision chart shown here basically favours the use of butterfly and other rotary valve styles
for line sizes above 150 mm unless there are high pressure-drop conditions. The alternative choice
to the globe style would also be to consider a characterized ball valve for any of the above
variations.
Table 2.2 may also provide some guidance on suitability of control valve type by reference to the
ranking values shown for each condition.
Table 2.2
Comparison of different control valve types
Application Globe Pinch/diaphragm Butterfly Disk/Rotary plug Ball
Controllability/turndown 1 3 1 1 1
High pressure: >30 bar 1 X X 2 1
High pressure drop
Δ P >0.5P1; P1 >10 bar
Not
e:
Slurry services 3 1 2 3 3
Cost <100 mm
Cost >100 mm
1
=
Anti-corrosion 2 1 1 2 2
Size & weight <100 mm
Size & weight <100 mm
Goo
1 X X 3 3
2
3
1
2
2
3
2
3
Temperature >100 ° C 1 X 3 1 2
Note: d; 2 = Average; 3 = Poor; X = Not suitable
2.5.2 Notes on seat leakage
3
1
3
1
3
2
2
1
2
1
2
3
Seat leakage may or may not be critical in an application but the degree of leakage should be
specified within the selection procedure. Seat leakage rates are defined on a sliding scale of
performance as defined by the ANSI standard: ANSI/FCI 70-2 or by the IEC standard IEC 60534
part 4.

Here is simplified summary of seat leakage rates by ANSI class codes:
• ANSI II.
0.5 % of rated Cv.
Used for pressure-balanced trims with metal seals.
• ANSI IV.
0.01 % of rated Cv.
Used for all standard valves with metal seals.
• ANSI V.
4 x 10-12 m3/hr per mm of orifice diameter per bar pressure drop
• ANSI VI.
Bubble tight.
Used for soft-seated valves where tight shut off is important.
Seat leakage rates are defined in the standards as a fraction of the rated Cv or as a function of the
orifice perimeter or seat length for particular ranges of closure pressure from the actuator. Testing
is then carried out with air or water as appropriate.
• All tests are carried out with air or water at 3.5 bar, except for Class V which is tested
at the maximum operating pressure drop
• It is important to know if a valve is likely to stay shut for long periods at a time. If so
and the differential pressure is high, a soft seat may be advisable in order to eliminate
micro-cavitation or wire drawing.
• A damper is a butterfly valve with no seat and a leakage rate of about 1 % of rated
Cv.
In practice you can evaluate the approximate level of leakage by calculation from the standard if
you know the valve CV for moderate leakage or if you have the seat dimensions for low leakage
classes (below).
>
Calculation of Seat Leakage rates.
A 50mm Globe valve has a Cv value of about 50
ANSI II = 0.5% ie Cv = 50*0.5/100 = 0.25 in this case
ANSI IV = 0.01% ie Cv = 50*0.01/100 = 0.005
ANSI V = 0.0003 * 41 * 3 * 60 ml/hr
= 2.2 milli-litres/hr ( 1 bubble in 4 mins )
ANSI VI = 18 milli-litres/hr ( 2 bubbles/min
)

46
2.6 Summary
This chapter has outlined the principal types of control valves seen in most industries but it is only
a fraction of a very wide subject. The end user must always be very clear on the operating
conditions and leakage limitations applicable to the valve before deciding on a particular type of
valve. The manufacturer’s recommendations must always be carefully considered since the life
cycle operating costs will depend on the durability of the valve under the given conditions.
Material of construction of the body, trim and seals will all be relevant to the ability of the valve to
perform and remain available to the plant, and the manufacturer has the specialist knowledge
somewhere in the organisation to confirm the suitability of these parts.
We have seen that globe valves and globe style valves are suitable for demanding duties and for
precision control in small sizes. Costs rise steeply for the globe valve with size but this may have to
be accepted if performance criteria such as high pressure and temperature conditions cannot be
satisfied by alternative cheaper styles.
Newer construction methods and materials have allowed rotary valve styles to achieve high
performance levels such they are likely to be highly suitable for most control applications above
the 150 mm size where globe style valves become expensive.

3
Valve Sizing for Liquid Flow
Valve sizing is the task of determining the required flow capacities of a control valve for a given
range of conditions followed by the selection of a suitably sized valve to handle the range with
adequate control. To be able work adequately on valve sizing it is necessary to have an
understanding of the fluid flow effects at work in a control valve as we have already outlined in
Chapter 1.
This chapter will proceed from the principles outlined in Chapter 1 to introduce and apply the
basic equations used for size and flow calculation of liquid flow for given process conditions. With
some familiarity with these equations it soon becomes routine to arrive at Cv values for the
operating range in question and from these it becomes possible to choose a suitable size of valve.
Valve sizing is to some extent an iterative process since the final sizing data depends itself on the
final choice of valve. Hence the following sequence, as shown in Figure 3.1, will normally take
place.
Decide valve style and estimated size
Prelim. Calculation of Cv
Check velocity and noise
Select best available size and materials
Sizing of actuator
Figure 3.1
Final Calculation of Cv
Outline of the valve sizing procedure
Confirm or revise
You will start with some idea of the valve style that you want since this will provide a first level of
data for the Cv calculation. You will need to know the approximate value of FL the pressure
recovery factor (or Cf the critical flow factor) for the type of valve you plan to use. For guidance
on this value please see the table of typical values in Table 3.1.

48
Table 3.1
Typical values for pressure recovery factor FL or Cf
Recovery Factor
FL or Cf
Std Globe Disk 600 Disk 900 Ball 900
FL 0.85 0.75 0.5 0.6
FL
2 0.72 0.56 0.25 0.36
Using the pressure recovery factor assumed for your choice of valve you will check for the flow
conditions to be sub-critical or critical and then use the relevant formula to calculate the required
Cvs for minimum and maximum flow conditions and select a suitable size of valve. You may wish to
adjust flowing conditions or valve size to find the most workable solution. Once the valve size has
been determined the final data for the chosen valve can be re-applied to the calculation to replace
any provisional data you have used.
With the valve size decided it is possible to check that flow velocities are in an acceptable range
and that noise levels from the valve will also be low enough to be acceptable.
Velocities for liquid flow should generally be kept below 6 m/sec. If the velocity or noise levels are
unacceptable then a change of valve size or configuration is required. Hence the calculation will
be need to be reworked with new data until a satisfactory set of conditions has been found.
The details provided in this chapter outline the manual calculation method so that the principles of
valve sizing calculations can be seen and understood. In practice such procedures can be
reproduced very effectively in software calculation packages which will have built in data tables
and guidance values. Using a software tool enables you to rapidly test the sizing options you are
considering so that the best available solution can be found rapidly and most effectively.
For our training purposes we shall proceed with the manual calculations for liquid flow, followed
by Chapter 4 on gas and steam flows. Then in Chapter 5 we shall introduce a commercial valve
sizing software tool that is readily available to all participants in the workshop. Using this tool we
shall be able to gain experience of trying out several worked examples shown in the manual.
Learning objectives
• Recognize the factors influencing valve capacity and see how they are expressed in a
general valve sizing equation
• Learn how to use a simplified equation for finding the approximate Cv of control
valves for liquid duties
3.1 Principles of the sizing equation
Before looking at the practical valve sizing equations it may be useful to note how the basic flow
capacity of a valve is defined. We saw in Chapter 1 that the Cv for a liquid is defined as the number
of US gallons per minute of water at 60°F that will flow when a pressure drop of 1 psi is applied
from upstream to downstream.
The Cv for any valve is therefore defined for a constant pressure drop situation when the valve is
installed in a pipeline of the same size as the valve as shown in Figure 3.2.

Valve sizing for liquid flow 49
Figure 3.2
Standardized control valve Installation for testing Cv
This illustrates the throttling process between upstream pressure P1 and downstream pressure P2 as
shown in Figure 3.1, where upstream pressure P1 is the pressure measured by a tap located two
pipe diameters upstream of upstream valve flange, and downstream pressure P2 is the pressure
measured by a tap located six pipe diameters downstream of downstream valve flange. (Per std.
IEC 605534/ISA S75)
We have already seen that if the conditions are ideal the equation for flow Q is
Q = Cv√( Δ P / SG)
The problem here is that the conditions in practice may be somewhat different from those used to
define the Cv for the valve. For example if the line size is greater than the valve size there will have
to be a conical pipe reducer section each side of the valve or there may be a line size change at the
valve. Consequently the pressure values used for the C measurement and flow conditions that were
present have to be corrected for the effect of the actual installation arrangement.
Similarly if the flowing velocity regime of the fluid is not turbulent as was assumed for the Cv
measurement this will affect the valve pressure drop to flow relationship. All of these factors have
to be taken into account in a general purpose flow equation that incorporates the basic CV which
has been obtained by experimental testing of the valve on the test rig as shown in Figure 3.1.
Consequently the full equation for Liquid flow (Q) in non-cavitating or non-flashing conditions is
given by equation (2) is as follows:
Q = N 1 x F P x F R x C V x P 1 - P 2 (2)
G r
Where Q = liquid volumetric flow rate(SI units : [m 3/h];US units:[gpm])
N 1 = 0.865 (SI units : [m 2/h];[bar A])
= 1.0 (US units : [gpm];[psi A])
F P = Piping geometry factor
F R = Reynold’s number factor
C V = Valve flow coefficient
P 1 = Upstream pressure (SI units : [bar A]; US units: [psi A])
P 2 = Downstream pressure (SI units : [bar A]; US units: [psi A])
G r = specific gravity (G r = ρ / ρ 0 = 1 for water at 15.6°C or at 60 °F)
ρ = fluid density (SI units : [kg/m3]; US units: [lb/ft3])
ρ 0 = Density of water at 15.6°C or at 60 °F
= 999 (SI units : [kg/m3];)
= 62.36 (US units: [lb/ft3])

Practical Control Valve Sizing, Selection and Maintenance

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