What Is A Control Valve Process Control Terminology Sliding-Stem Control Valve Terminology Rotary-Shaft Control Valve Terminology Control Valve Functions and Characteristics Terminology Other Process Control Terminology sliding stem control valve

1. Control Valve
By
Ashvani Kumar Shukla
Reliance

2. Index
 What Is A Control Valve
 Process Control Terminology
 Sliding-Stem Control Valve Terminology
 Rotary-Shaft Control Valve Terminology
 Control Valve Functions and Characteristics Terminology
 Other Process Control Terminology

3. What Is A Control Valve
 Control valves are valves used to control conditions such as flow, pressure, temperature, and liquid level by
fully or partially opening or closing in response to signals received from controllers that compare a "set
point" to a "process variable" whose value is provided by sensors that monitor changes in such conditions.
Control Valve is also termed as the Final Control Element.
 Process plants consist of hundreds, or even thousands, of control loops all networked together to produce
a product to be offered for sale. Each of these control loops is designed to keep some important process
variable such as pressure, flow, level, temperature, etc. within a required operating range to ensure the
quality of the end product. Each of these loops receives and internally creates disturbances that
detrimentally affect the process variable, and interaction from other loops in the network provides
disturbances that influence the process variable. To reduce the effect of these load disturbances, sensors
and transmitters collect information about the process variable and its relationship to some desired set
point. A controller then processes this information and decides what must be done to get the process
variable back to where it should be after a load disturbance occurs. When all the measuring, comparing,
and calculating are done, some type of final control element must implement the strategy selected by the
controller. The most common final control element in the process control industries is the control valve. The
control valve manipulates a flowing fluid, such as gas, steam, water, or chemical compounds, to
compensate for the load disturbance and keep the regulated process variable as close as possible to the
desired set point. Many people who talk about control valves or valves are really referring to a control valve
assembly. The control valve assembly typically consists of the valve body, the internal trim parts, an actuator
to provide the motive power to operate the valve, and a variety

4.  of additional valve accessories, which can include positioners, transducers, supply pressure regulators,
manual operators, snubbers, or limit switches. Other chapters of this handbook supply more detail
about each of these control valve assembly components. Whether it is called a valve, control valve or
a control valve assembly is not as important as recognizing that the control valve is a critical part of
the control loop. It is not accurate to say that the control valve is the most important part of the loop.
It is useful to think of a control loop as an instrumentation chain. Like any other chain, the whole
chain is only as good as its weakest link. It is important to ensure that the control valve is not the
weakest link.

5. Process Control Terminology
 Accessory: A device that is mounted on the actuator to complement the actuator’s function and make
it a complete operating unit. Examples include positioners, supply pressure regulators, solenoids, and
limit switches.
 Actuator*: A pneumatic, hydraulic, or electrically powered device that supplies force and motion to
open or close a valve.
 Actuator Assembly: An actuator, including all the pertinent accessories that make it a complete
operating unit.
 Backlash: The general name given to a form of dead band that results from a temporary discontinuity
between the input and output of a device when the input of the device changes direction. Slack, or
looseness of a mechanical connection is a typical example.
 Capacity* (Valve): The rate of flow through a valve under stated conditions. Closed Loop: The
interconnection of process control components such that information regarding the process variable
is continuously fed back to the controller set point to provide continuous, automatic corrections to
the process variable.
 Controller: A device that operates automatically by use of some established algorithm to regulate a
controlled variable. The controller input receives information about the status of the process variable
and then provides an appropriate output signal to the final control element.

6.  Control Loop: (See Closed Loop.) Control Range: The range of valve travel over which a control valve
can maintain the installed valve gain between the normalized values of 0.5 and 2.0.
 Control Valve:
 Control Valve Assembly: Includes all components normally mounted on the valve: the valve body
assembly, actuator, positioner, air sets, transducers, limit switches, etc.
 Dead Band: The range through which an input signal can be varied, upon reversal of direction,
without initiating an observable change in the output signal. Dead band is the name given to a
general phenomenon that can apply to any device. For the valve assembly, the controller output (CO)
is the input to the valve assembly and the process variable (PV) is the output as shown in figure 1-1.
When the terming fluid, such as gasses, steam, water, or chemical compounds, to compensate for the
load disturbance and keep the regulated process variable as close as possible to the desired set
point.

7. CONTROL VALVE

9. Basic sliding stem control valves
 The following terminology applies to the physical and operating characteristics of
standard sliding-stem control valves with diaphragm or piston actuators. Some of
the terms, particularly those pertaining to actuators, are also appropriate for rotary-
shaft control valves. Many of the definitions presented are in accordance with ISA
S75.05, Control Valve Terminology, although other popular terms are also included.
Additional explanation is provided for some of the more complex terms. Component
part names are called out on accompanying figures 1-3 through 1-6. Separate
sections follow that define specific rotary-shaft control valve terminology, control
valve functions and characteristics terminology, and other process control
terminology.

10.  Actuator Spring: A spring, or group of springs, enclosed in the yoke or actuator
casing that moves the actuator stem in a direction opposite to that created by
diaphragm pressure.
 Actuator Stem: The part that connects the actuator to the valve stem and transmits
motion (force) from the actuator to the valve.
 Actuator Stem Extension: An extension of the piston actuator stem to provide a
means of transmitting piston motion to the valve positioner

11. valve

12. actuator

13. ACTUATOR DEATAILS

14. ACTUATOR DETAILS

18.  Actuator Stem Force: The net force from an actuator that is available for actual
positioning of the valve plug.
 Angle Valve: A valve design in which one port is co-linear with the valve Bellows Seal
Bonnet W6434/IL stem or actuator, and the other port is at a right angle to the valve
stem.
 Bellows Seal Bonnet: A bonnet that uses a bellows for sealing against leakage
around the closure member stem.
 Bonnet: The portion of the valve that contains the packing box and stem seal and
can guide the stem. It provides the principal opening to the body cavity for assembly
of internal parts or it can be an integral part of the valve body. It can also provide for
the attachment of the actuator to the valve body. Typical bonnets are bolted,
threaded, welded, pressure-seals, or integral with the body. (This term is often used
in referring to the bonnet and its included packing parts. More properly, this group
of component parts should be called the bonnet assembly.)
 Bonnet Assembly: (Commonly Bonnet, more properly Bonnet Assembly): An
assembly including the part through which a valve stem moves and a means for
sealing against leakage along the stem. It usually provides a means for mounting the
actuator and loading the packing assembly

19.  Bottom Flange: A part that closes a valve body opening opposite the bonnet opening. It can
include a guide bushing and/or serve to allow reversal of the valve action.
 Bushing: A device that supports and/ or guides moving parts such as valve stems.
 Cage: A part of a valve trim that surrounds the closure member and can provide flow
characterization and/or a seating surface. It also provides stability, guiding, balance, and alignment,
and facilitates assembly of other parts of the valve trim. The walls of the cage contain openings that
usually determine the flow characteristic of the control valve. Various cage styles are shown in
figure 1-8. Closure Member: The movable part of the valve that is positioned in the flow path to
modify the rate of flow through the valve. Closure Member
 Guide: That portion of a closure member that aligns its movement in either a cage, seat ring,
bonnet, bottom flange, or any two of these.
 Cylinder: The chamber of a piston actuator in which the piston moves (figure 1-7). Cylinder Closure
Seal: The sealing element at the connection of the piston actuator cylinder to the yoke.
 Diaphragm: A flexible, pressure responsive element that transmits force to the diaphragm plate and
actuator stem.
 Diaphragm Actuator: A fluid powered device in which the fluid acts upon a flexible component, the
diaphragm. Diaphragm Case: A housing, consisting of top and bottom section, used for supporting
a diaphragm and establishing one or two pressure chambers
 Diaphragm Plate: A plate concentric with the diaphragm for transmitting force to the actuator stem.
 Direct Actuator: A diaphragm actuator in which the actuator stem extends with increasing
diaphragm pressure.
 Extension Bonnet: A bonnet with greater dimension between the packing box and bonnet flange
for hot or cold service.

20. Globe Valve: A valve with a linear motion closure member, one or more ports, and a body
distinguished by a globular shaped cavity around the port region. Globe valves can be further
classified as: two-way single-ported.
Lower Valve Body: A half housing for internal valve parts having one flow connection. The seat
ring is normally clamped between the upper.
Actuators:
Pneumatically operated control valve actuators are the most popular type in use, but electric,
hydra land manual actuators are also widely used. The spring-and-diaphragm pneumatic
actuator is most commonly specified due to its dependability and simplicity of design.
Pneumatically operated piston actuators provide high stem force output for demanding service
conditions. Adaptations of both spring-and-diaphragm and pneumatic piston actuators are
available for direct installation on rotary-shaft control valves. Electric and electro-hydraulic
actuators are more complex and more expensive than pneumatic actuators. They offer
advantages where no air supply source is available, where low ambient temperatures could
freeze condensed water in pneumatic supply lines, or where unusually large stem forces are
needed. A summary follows, discussing the design and characteristics of popular actuator styles.

21. Diaphragm Actuators
Pneumatically operated diaphragm actuators use air supply from controller, positioner,
or other source. Various styles include: direct acting (increasing air pressure pushes
down diaphragm and extends actuator stem, figure 3-32); reverse-acting (increasing air
pressure pushes up diaphragm and retracts actuator stem, figure 3-32); reversible
(actuators that air pressure pushes down on diaphragm, which may either open or close
the valve, depending on orientation of the actuator lever on the valve shaft, figure 3-
34). Net output thrust is the difference between diaphragm force and opposing spring
force. Molded diaphragms provide linear performance and increased travels. Output
thrust required and supply air pressure available dictate size. Diaphragm actuators are
simple, dependable, and economical.

22. Piston Actuators
 operated using high-pressure plant air to 150 psig, often eliminating the need for
supply pressure regulator. Piston actuators furnish maximum thrust output and fast
stroking speeds. Control Valve with Double-Acting Piston Actuator W0320-1/IL
Piston actuators are double acting to give maximum force in both directions, or
spring return to provide fail-open or fail-closed operation. Various accessories can
be incorporated to position a double-acting piston in the event of supply pressure
failure. These include pneumatic trip valves and lock-up systems. Also available are
hydraulic snubbers, handwheels, and units without yokes, which can be used to
operate butterfly valves, louvers, and similar industrial equipment. Other versions for
service on rotary-shaft control valves include a sliding seal in the lower end of the
cylinder. This permits the actuator stem to move laterally as well as up and down
without leakage of cylinder pressure. This feature permits direct connection of the
actuator stem to the actuator lever mounted on the rotary valve shaft, thereby
eliminating one joint or source of lost motion.

23. Manual Actuators
 Manual actuators are useful where automatic control is not required, but where ease
of operation and good manual control is still necessary (figure 3-37). They are often
used to actuate the bypass valve in a three-valve bypass loop around control valves
for manual control of the process during maintenance or shutdown of the automatic
system. Manual actuators are available in various sizes for both globe-style valves
and rotary-shaft valves. Dial-indicating devices are available for some models to
permit accurate repositioning of the valve plug or disk. Manual actuators are much
less expensive than automatic actuators. R

24. Rack and Pinion Actuators
 Rack and pinion designs provide a compact and economical solution for rotary shaft
valves (figure 3-38). Because of backlash, they are typically used for on-off
applications or where process variability is not a concern.

25. Electric Actuators
 Traditional electric actuator designs use an electric motor and some form of gear
reduction to move the valve. Through adaptation, these mechanisms have been
used for continuous control with varying degrees of success. To date, electric
actuators have been much more expensive than pneumatic for the same
performance levels. This is an area of rapid technological change, and future designs
may cause a shift towards greater use of electric actuators.

26. Electrohydraulic Actuators
 Electrohydraulic Actuators Electrohydraulic actuators require only electrical power to
the motor and an electrical input signal from the controller. Electrohydraulic
actuators are ideal for isolated locations where pneumatic supply pressure is not
available but where precise control of valve plug position is needed. Units are
normally reversible by making minor adjustments and might be self-contained,
including motor, pump, and double-acting hydraulically operated piston within a
weatherproof or explosion-proof casing.