What is instrument ?
Instrument is a devices which is used to measure,
monitor, display etc. of a process variable.
• What are the process Variable ?
The process Variable are :
A process of ( liquid, gas, or electricity) move steadily and
continuously in a current or stream.
What is Flow ?
go from one place to another in a steady stream, typically in
large numbers. .
Flow can be measured in a variety of ways. Positive-
displacement flow meters accumulate a fixed volume of fluid
and then count the number of times the volume is filled to
What is pressure ?
Pressure is force per unit area applied in a direction
perpendicular to the surface of an object .
Pressure is the ratio of force applied per area covered …
P = F/A he unit of pressure is the Pascal
Pa = N = kg m/s2 =kg
The Pascal is also a unit of stress and the topics of pressure and
stress are connected.
Bed of nails (not really pressure but shear strain?)
Finger bones are flat on the gripping side to increase surface area in
contact and thus reduce compress ional stresses .
What is Temperature ?
In a qualitative manner, we can describe the temperature of an object
as that which determines the sensation of warmth or coldness felt
from contact with it.
Temperature is a degree of hotness or coldness the can be measured
using a thermometer. It's also a measure of how fast the atoms and
molecules of a substance are moving. Temperature is measured in
degrees on the Fahrenheit, Celsius, and Kelvin scales.
A temperature is a numerical measure of hot or cold. Its
measurement is by detection of heat radiation or particle velocity or
kinetic energy, or by the bulk behavior of a thermometric material. It
may be calibrated in any of various temperature , Celsius,
Fahrenheit, Kelvin, etc. The fundamental physical definition of
temperature is provided by thermodynamic .
What is Level ?
A device for establishing a horizontal line or plane by means of a
bubble in a liquid that shows adjustment to the horizontal by
movement to the center of a slightly bowed glass tube
A measurement of the difference of altitude of two points by
means of a level .
Horizontal condition; especially : equilibrium of a fluid
marked by a horizontal surface of even altitude <water seeks its
The magnitude of a quantity considered in relation to an
arbitrary reference value; broadly : magnitude, intensity <a
high level of hostility>
What is meaning of Loop in instrumentation ?
In computer programming, a loop is a sequence of instructions that is
continually repeated until a certain condition is reached. Typically, a certain
process is done, such as getting an item of data and changing it, and then
some condition is checked such as whether a counter has reached a
prescribed number. If it hasn't, the next instruction in the sequence is an
instruction to return to the first instruction in the sequence and repeat the
sequence. If the condition has been reached, the next instruction "falls
through" to the next sequential instruction or branches outside the loop. A
loop is a fundamental programming idea that is commonly used in writing
programs. An infinite loop is one that lacks a functioning exit routine . The
result is that the loop repeats continually until the operating system senses
it and terminates the program with an error or until some other event occurs
(such as having the program automatically terminate after a certain duration
Define the types of loop control ?
Open loop control system .
Close loop control system .
Cascade loop control system .
What is open loop system ?
An open-loop controller, also called a non-feedback controller, is a type
of controller that computes its input into a system using only the current
state and its model of the system.
A characteristic of the open-loop controller is that it does not use feedback
to determine if its output has achieved the desired goal of the input. This
means that the system does not observe the output of the processes that it is
controlling. Consequently, a true open-loop system can not engage in
machine learning and also cannot correct any errors that it could make. It
also may not compensate for disturbances in the system.
An open-loop controller is often used in simple processes because of its
simplicity and low cost, especially in systems where feedback is not critical.
A typical example would be a conventional washing machine .
What is close loop system ?
Closed loop control systems are those that provide feedback of the actual state of the
system and compare it to the desired state of the system in order to adjust the system.
The closed loop control system is a system where the actual behavior of the system is
sensed and then fed back to the controller and mixed with the reference or desired
state of the system to adjust the system to its desired state. The objective of the
control system is to calculate solutions for the proper corrective action to the system
so that it can hold the set point (reference) and not oscillate around it.
When a scale out triggering event occurs, the input parameter that triggers the event
is monitored around its set point. The system increases and decreases capacity on
demand to stay as close as possible to the set point for the triggering parameter.
With closed loop systems, you can evaluate the system near the set point using a PID
control algorithm or similar control scheme. A simpler approach, such as hysteretic,
can be very effective and can be implemented with less complexity and tuning.
Close loop system diagram .
Close loop Block diagram
What is Cascade loop control system ?
A cascade control system is a multiple-loop system where the primary variable is
controlled by adjusting the set point of a related secondary variable controller. The
secondary variable then affects the primary variable through the process.
The primary objective in cascade control is to divide an otherwise difficult to control
process into two portions, whereby a secondary control loop is formed around a
major disturbances thus leaving only minor disturbances to be controlled by the
The use of cascade control is described in many texts on process control
Cascade control is most advantageous on applications where the secondary closed
loop can include the major disturbance and second order lag and the major lag is
included in only the primary loop. The secondary loop should be established in an
area where the major disturbance occurs.
Cascade loop control system Diagram.
Pressure measurements .
Pressure is the force exerted per unit area
Pressure is the action of one force against another force. Pressure is force applied to, or
distributed over, a surface. The pressure P of a force F distributed over an area A is
defined as P = F/A
TOTAL VACUUM - 0 PSIA
NOM. 14.7 PSIA
Pressure Measurement Terms.
Measured above total vacuum or zero absolute. Zero absolute represents total lack of
The pressure exerted by the earth’s atmosphere. Atmospheric pressure at sea level is
14.696 PSI. The value of atmospheric pressure decreases with increasing altitude.
Same as atmospheric pressure.
The pressure above atmospheric pressure. Represents positive difference between
measured pressure and existing atmospheric pressure. Can be converted to absolute by
adding actual atmospheric pressure value.
The difference in magnitude between some pressure value and some reference
pressure. In a sense, absolute pressure could be considered as a differential pressure
with total vacuum or zero absolute as the reference. Likewise, gauge pressure (defined
above) could be considered as Differential Pressure with atmospheric pressure as the
mm of Hg 5171
mm of WC 70358
in of WC 2770
A Pressure Gauge is used for measuring the pressure of a gas or liquid.
A Vacuum Gauge is used to measure the pressure in a vacuum.
A Compound Gauge is used for measuring both Vacuum and Pressure.
Pressure Gauges are used for Indication only.
Bourdon tube measuring element is made of a thin-walled C-shape tube or
spirally wound helical or coiled tube. When pressure is applied to the
measuring system through the pressure port (socket), the pressure causes the
Bourdon tube to straighten itself, thus causing the tip to move. The motion of
the tip is transmitted via the link to the movement which converts the linear
motion of the bourdon tube to a rotational motion that in turn causes the
pointer to indicate the measured pressure.
Gauge Construction types .
“C” Type Bourdon
The device contains a micro switch, connected to a mechanical lever and set pressure
spring. The contacts get actuated when process pressure reaches the set pressure of the
It can be used for alarming or interlocking purposes, on actuation.
It can be used for high / high-high or low / low-low actuation of pressure in the process .
The set range can be adjusted within the switch range.
The sensing element may be a Diaphragm or a piston
Pressure/Vacuum Switch - A device that senses a change in
pressure/vacuum and opens or closes an electrical circuit when
the set point is reached.
Pressure switches serve to energize or de-energize electrical
circuits as a function of whether the process pressure is normal
The electric contacts can be configured as single pole double
throw (SPDT), in which case the switch is provided with one
normally closed (NC) and one normally open (NO) contact.
Alternately, the switch can be configured as double pole double
throw (DPDT), in which case two SPDT switches are furnished,
each of which can operate a separate electric circuit.
Pressure Transmitter Advantages
A Pressure Transmitter is used where indication and/or record of
pressure is required at a location not adjacent to the primary
A Pressure Transmitter is used for both indication and control of
A Pressure Transmitter is used where overall high performance
Both Electronic and Pneumatic Transmitters are used.
These can be either Gauge, Absolute or Differential Pressure
Transmitter Measuring Principle
The diagram shows an electronic differential pressure
sensor. This particular type utilizes a two-wire
Another common measuring technique is a strain gauge.
Process pressure is transmitted through isolating
diaphragms and silicone oil fill fluid to a sensing
The sensing diaphragm is a stretched spring element that
deflects in response to the differential pressure across it.
The displacement of the sensing diaphragm is
proportional to the differential pressure.
The position of the sensing diaphragm is detected by
capacitor plates on both sides of the sensing diaphragm.
The differential capacitance between the sensing
diaphragm and the capacitor plates is converted
electronically to a 4–20 mA or 1-5 VDC signal.
For a gauge pressure transmitter, the low pressure side is
referenced to atmospheric pressure.
The Orifice Plate
The orifice plate is the simplest and cheapest. It is simply
a plate with a hole of specified size and position cut in it,
which can then clamped between flanges in a pipeline
The increase that occurs in the velocity of a fluid as it
passes through the hole in the plate results in a pressure
drop being developed across the plate.
After passing through this restriction, the fluid flow jet
continues to contract until a minimum diameter known as
the vena contracta is reached.
Working principle & Advantages
The orifice plate is the simplest and cheapest.
The increase that occurs in the velocity of a fluid as it passes through the hole in
the plate results in a pressure drop being developed across the plate. After passing
through this restriction, the fluid flow jet continues to contract until a minimum
diameter known as the vena contracta is reached.
The equation to calculate the flow must be modified
The Venturi Tube
The classical or Herschel Venturi tube is the oldest type of differential pressure
flow meter (1887).
The restriction is introduced into the flow in a more gradual way
The resulting flow through a Venturi tube is closer to that predicted in theory so the
discharge coefficient C is much nearer unity (0.95).
The pressure loss caused by the Venturi tube is lower, but the differential pressure
is also lower than for an orifice plate of the same diameter ratio.
Advantages of Venturi Tube
The smooth design of the Venturi tube means that it is less
sensitive to erosion than the orifice plate, and thus more
suitable for use with dirty gases or liquids.
The Venturi tube is also less sensitive to upstream disturbances,
and therefore needs shorter lengths of straight pipe work
upstream of the meter than the equivalent orifice plate or
Like the orifice plate and nozzle, the design, installation, and
use of the Venturi tube is covered by a number of international
The disadvantages of the Venturi tube flow meter are its size
The nozzle combines some of the best features of the orifice plate and Venturi
It is compact and yet, because of its curved inlet, has a discharge coefficient close
There are a number of designs of nozzle, but one of the most commonly used in
Europe is the ISA-1932 nozzle, while in the U.S., the ASME long radius nozzle is
more popular. Both of these nozzles are covered by international standards
Rota meter consists of a conical transparent vertical glass tube containing a “bob”.
The flow rate is proportional to the height of the bob.
The Rota meter is characterized by:
Simple and robust construction
Low pressure drop
Axial Turbine Flow meters
The modern axial turbine flow meter is a reliable device capable of
providing the highest accuracies attainable by any currently available flow
sensor for both liquid and gas volumetric flow measurement. It is the
product of decades of intensive innovation and refinements to the original
axial vaned flow meter principle first credited to Wolman in 1790, and at
that time applied to measuring water flow.
The initial impetus for the modern development activity was largely the
increasing needs of the U.S. natural gas industry in the late 1940s and
1950s for a means to accurately measure the flow in large-diameter, high-
pressure, interstate natural gas lines.
Today, due to the tremendous success of this principle, axial turbine flow
meters of different and often proprietary designs are used for a variety of
applications where accuracy, reliability, and range ability are required in
numerous major industries besides water and natural gas, including oil,
petrochemical, chemical process, cryogenics, milk and beverage,
aerospace, biomedical, and others.
Turbine Flow meter & working principle
The meter is a single turbine rotor, concentrically mounted on a shaft
within a cylindrical housing through which the flow passes.
The shaft or shaft bearings are located by end supports inside
suspended upstream and downstream aerodynamic structures called
diffusers, stators, or simply cones.
The flow passes through an annular region occupied by the rotor
blades. The blades, which are usually flat but can be slightly twisted,
are inclined at an angle to the incident flow velocity and hence
experience a torque that drives the rotor.
The rate of rotation, which can be up to several ×104 rpm
A magnetic pick up coil detect the rotation
Glass Tube with Option of Graduations
Not Popular for Process Applications
Typically Used for Calibrating Metering Pumps (Calibration Tubes)
Flat Glass Gauges
Glass Sections on Opposite Sides of the Chamber
View the Liquid Level through the Gauge
Used on Interface Applications and Dirty or Viscous Liquids
Illuminators Can be Used to Diffuse Light Evenly on the Back of the Gauge
Reflex Flat Glass Gauge
Single Glass Section with Prisms Cut in the Glass on the Process Side
Light Striking the Vapor Phase is Refracted to the Viewer which Appears
Light Striking the Liquid Phase is Refracted into the Liquid which Appears
Used on Clean, Clear, No corrosive Liquids
Magnetic Level Gauge
Consists of a Non-Magnetic Chamber, Internal Float with Magnet and Bi-
Colored Indicator Wafers
Float / Displacer
The Visible Length Should Cover the Full Operating Range of Interest
Including any Other Level Instrumentation on the Vessel
If More than One Gauge is Required, the Gauges Must Overlap Each Other
Level Chamber Needs to be Installed Vertically Level to Reduce any Possible
Friction with the Float
Require Jig Set Connections
May Require a Magnetic Trap
Level Float / Displacer
Long Visible Lengths
Corrosive or Toxic Liquid Applications
Adaptable to Variations in Fluid Densities
High Pressure or Temperature Applications
Affected by changes in fluid density
Coating media may seize moving parts
Over Pressuring can Implode Float
Long ranges may require additional support
When Air Pressure Enters a Dip
Pipe with a Pressure Greater Than
the Hydrostatic Head of the Process
Fluid, the Air will Bubble out the
Bottom of the Dip Pipe
As the Liquid Level Changes, the
Air Pressure in the Dip Pipe also
Consists of Pressure Regulator,
Rota meter and Pressure Gauge
Along with a Stilling Well
Types of Temperature Instrument
Resistance Temperature Detector (RTD)
Filled Thermal Systems
Various Units of Temperature Measurement
°C – degrees Celsius (or Centigrade)
°F – degrees Fahrenheit
K – Kelvin
R – Rankin
Relationship between different units
°C = (°F - 32)/1.8
°F = 1.8 x °C + 32
K = °C + 273.15
R = °F + 459.67
Conversion tables or software can be utilized to facilitate
with converting between these units.
In 1821 a German physicist named See back discovered the thermoelectric effect
which forms the basis of modern thermocouple technology. He observed that an
electric current flows in a closed circuit of two dissimilar metals if their two junctions
are at different temperatures.
The thermoelectric voltage produced depends on the metals used and on the
temperature relationship between the junctions.
If the same temperature exists at the two junctions, the voltage produced at each
junction cancel each other out and no current flows in the circuit.
With different temperatures at each junction, different voltages are produced and
current flows in the circuit.
A thermocouple can therefore only measure temperature differences between the two
junctions, a fact which dictates how a practical thermocouple can be utilized.
Thermocouple measuring circuit
Needs to be held constant to give a
fixed reference. ( early methods held
cold junction at 0ºC using ice or
Standard Thermocouple Alloy Conductor Combinations
CODE CONDUCTOR COMBINATION TYPICAL OPERATING RANGE ºF
B Platinum-30% Rhodium / Platinum-6% Rhodium +2500 to +3100
C Tungsten-5% Rhenium / Tungsten-26% Rhenium +3000 to +4200
D Tungsten-3% Rhenium / Tungsten-25% Rhenium +2800 to +3800
E Nickel Chromium / Constantan 0 to +1650
J Iron / Constantan +0 to +1400
K Nickel Chromium / Nickel Aluminium 0 to +2300
N Nickel-Chromium-Silicon / Nickel-Silicon-Magnesium 1200 to +2300
R Platinum-13% Rhodium / Platinum 1600 to +2600
S Platinum-10% Rhodium / Platinum 1800 to +2600
T Copper / Constantan -300 to +650
• Normally element is in a thermowell
• Commonly element is 1/4” outside Diameter
• Sheath material, normally Stainless steel but can be
special material such as Inconel, Incoloy, Hastelloy
• Duplex thermocouples have 2 elements inside one
RTDs (Resistance Temperature Detectors) operate under the principle that the
electrical resistance of certain metals increases and decreases in a repeatable
and predictable manner with a temperature change.
Wire Wound Element
Precise lengths of wire are wrapped
around a ceramic mandrel, then inserted
inside a ceramic shell which acts to
support and protect the wire windings.
Inner Coil Element
Wires are coiled then slid into the holes of a
ceramic insulator. Some manufacturers backfill
the bores with ceramic powder after the coils
are inserted. This keeps the coils from shorting
against each other.
Thin Film Element
Metallic ink is deposited onto a ceramic
substrate. Lasers then etch the ink to provide a
resistance path. The entire assembly is
encapsulated in ceramic to support and protect.
RTD Lead wire Configuration
2-wire: Should only be used with
very short runs of leadwire. No
compensation for leadwire
3-wire: Most commonly used for
industrial applications. Leadwire
4-wire: Laboratory use historically,
moving more into industrial
applications. Full compensation
for leadwire resistance.
The most common method for measuring the resistance of an RTD is to use a
Wheatstone bridge circuit. In a Wheatstone bridge, electrical excitation current
is passed through the bridge, and the bridge output current is an indication of the
Temperature Element Assembly
Head Nipple-Union-Nipple Thermowell
Thermo well Insertion Modification
Low level inputs
mV from thermocouples
High level outputs
Digital (i.e. Fieldbus)
Thermistors are temperature sensing devices that are similar to RTD’s in that
their resistance changes as temperature changes.
The major difference is that for most thermistors the resistance decreases as
Thermistors are an inexpensive alternative to RTD’s when temperature ranges are
below 150°C. Thermistors can be used from temperatures of –80°C to 300°C.
Most thermistors have base resistances, which are much higher than RTD’s.
One of the greatest advantages of using a thermistor sensor is the large change in
resistance to a relatively small change in temperature. This makes them very
sensitive to small changes in temperature.
A Bimetallic Thermometer consists
of an indicating or recording device,
a sensing element and a means for
connecting the two. Basic example:
Two metal strips expand at different rates as
the temperature changes.
A pointer is attached to the rotating
coil which indicates the
temperature on the dial.
Coil rotation is caused by the
difference in thermal expansions
of the two metals.
Gas chromatography & analyzer
Gas chromatography - specifically gas-liquid chromatography - involves a
sample being vaporized and injected onto the head of the chromatographic
column. The sample is transported through the column by the flow of inert,
gaseous mobile phase. The column itself contains a liquid stationary phase
which is adsorbed onto the surface of an inert solid.
The carrier gas must be chemically inert. Commonly used gases include
nitrogen, helium, argon, and carbon dioxide. The choice of carrier gas is
often dependant upon the type of detector which is used. The carrier gas
system also contains a molecular sieve to remove water and other impurities.
Sample injection port
For optimum column efficiency, the sample should not be too
large, and should be introduced onto the column as a "plug" of
vapor - slow injection of large samples causes band broadening
and loss of resolution. The most common injection method is
where a micro syringe is used to inject sample through a rubber
septum into a flash vaporizer port at the head of the column. The
temperature of the sample port is usually about 50°C higher than
the boiling point of the least volatile component of the sample.
For packed columns, sample size ranges from tenths of a micro
liter up to 20 micro liters. Capillary columns, on the other hand,
need much less sample, typically around 10-3 mL. For capillary
GC, split/split less injection is used. Have a look at this diagram
of a split/split less injector;
There are two general types of column, packed and capillary (also known as open
tubular). Packed columns contain a finely divided, inert, solid support material
(commonly based on diatomaceous earth) coated with liquid stationary phase. Most
packed columns are 1.5 - 10m in length and have an internal diameter of 2 - 4mm.
Gas Turbine for Power Generation
The use of gas turbines for generating electricity dates back to 1939 Today, gas turbines are one
of the most widely-used power generating technologies. Gas turbines are a type of internal
combustion (IC) engine in which burning of an air-fuel mixture produces hot gases that spin a
turbine to produce power. It is the production of hot gas during fuel combustion, not the fuel
itself that the gives gas turbines the name. Gas turbines can utilize a variety of fuels, including
natural gas, fuel oils, and synthetic fuels. Combustion occurs continuously in gas turbines, as
opposed to reciprocating IC engines, in which combustion occurs intermittently.
How Do Gas Turbines Work?
Gas turbines are comprised of three primary sections mounted on the same shaft: the compressor,
the combustion chamber (or combustor) and the turbine. The compressor can be either axial flow
or centrifugal flow. Axial flow compressors are more common in power generation because they
have higher flow rates and efficiencies. Axial flow compressors are comprised of multiple stages
of rotating and stationary blades (or stators) through which air is drawn in parallel to the axis of
rotation and incrementally compressed as it passes through each stage. The acceleration of the air
through the rotating blades and diffusion by the stators increases the pressure and reduces the
volume of the air. Although no heat is added, the compression of the air also causes the
temperature to increase.
Principle of turbine
The compressed air is mixed with fuel injected through nozzles. The fuel and
compressed air can be pre-mixed or the compressed air can be introduced directly into
the combustor. The fuel-air mixture ignites under constant pressure conditions and the
hot combustion products (gases) are directed through the turbine where it expands
rapidly and imparts rotation to the shaft. The turbine is also comprised of stages, each
with a row of stationary blades (or nozzles) to direct the expanding gases followed by
a row of moving blades. The rotation of the shaft drives the compressor to draw in
and compress more air to sustain continuous combustion. The remaining shaft power
is used to drive a generator which produces electricity. Approximately 55 to 65
percent of the power produced by the turbine is used to drive the compressor. To
optimize the transfer of kinetic energy from the combustion gases to shaft rotation,
gas turbines can have
Final instrument of process control valve
Control Valve Characteristics and Types
Control Valve Parts
Control Valve Accessories
Control Valve Operation
Valve Hand Jack and Minimum Stop
Introduction of control valve
The control valve is composed of a valve with an externally powered
actuator. The control valve is designed specifically for reliable continuous
throttling with minimum backlash and packing friction. The control valve is
involved with the disposition of energy in a process. It dispenses energy from
the source, dissipated energy that exists within the system, or distributes
energy in the system in one way or another.
The chemical and petroleum industries have many applications requiring
control of gases, liquids, or vapors processes. Many process operation require
regulation of such quantities as density and composition, but by far the most
important control parameter is flow rate. A regulation of flow rate emerges as
the regulatory parameters for reaction rate, temperature, composition, or a
host of other fluid properties. For this purposes the control valve is using as
the process control element.
Control Valve Characteristics and Types
The different types of control valves are classified by a relationship between
the valve stem position and the flow rate through the valve. This control valve
characteristic is assigned with the assumptions that the stem position indicates
the extent of the valve opening and that the pressure difference is determined
by the valve alone. There are three basic types of control valves whose
relationship between stem position (as percentage of full range) and the flow
rate (as a percentage of maximum)
• Quick Opening:-
This types of valve is used predominantly for full ON/full OFF control
applications. The valve characteristic shows that a relatively small motion of
valve stem results in maximum possible flow rate
Through the valve. Such a valve, for Example , may allow 90% of the flow
rate with only a 30% travel of the stem.
This type of valve, as shown in picture, has a flow rate that varies linearly
with the stem position. It depends the ideal situation where the valve alone
determines the pressure drop.
3. Equal Percentage:-
Equal percentage is the characteristic most commonly used in process
control. The change in flow per unit of valve stroke is directly proportional to
the flow occurring just before the change is made. While the flow
characteristic of the valve itself may be equal percentage, most control loops
will produce an installed characteristic approaching linear when the overall
system pressure drop is large relative to that across the valve.
The types of the valves as follows,
Knife edge valves
Control Valve Parts
The Valves has Two Main Parts
Body Assembly Parts
The pressure retaining housing through which the service fluid flows. It has inlet and
outlet connections, and houses the trim components
Trim component the plug makes contact with to close the valve.
Trim component which clamps the seat ring in place. The seat retainer does not guide
the plug, and should not be confused with a cage
are used in control valves to prevent leakage. around the seat ring, bonnet or pressure-
Part that moves in and out of the seat ring to open and close the valve. It can also be
used to characterize the flow.
The valve component which houses the guides and packing. It also seals one opening
to the body.
Flange that attaches the bonnet to the body.
Guides — Bushings contained in the packing box which align the plug with the seat
Guides — Bushings contained in the packing box which align the plug with the seat
Packing — Material used to seal the valve from leaking around the plug stem.
Packing Box — Internal bore of bonnet which contains guiding and packing.
Actuator Assembly Parts
Actuator — Device which develops sufficient thrust to open or close the
valve. Common designs include piston, diaphragm, hydraulic, manual hand
wheel and electro-hydraulic actuators.
Lifting Ring: Used for Lifting The Valve
Adjusting Screw: Part used to compress the actuator spring.
Cylinder: Actuator part used for containing air pressure and enclosing the
Spring Button: The part that prevents movement of the actuator spring and
permits the adjusting screw to compress the spring.
Spring: In piston actuators, the part which provides force for fail-safe
operation; in diaphragm actuators, the part that provides force to counteract air
pressure from the opposing chamber.
Piston: Part used to separate two air chambers of piston actuator.
Actuator Stem: Part used to connect the valve plug with the piston actuator.
Yoke: A component which secures the actuator to the valve body.
A valve Positioner is like a proportional controller. The set point is the control signal
from the primary controller and the controlled variable is the valve position. The
Positioner compensates for disturbances and nonlinearities.
The use of positioner are as follows,
When the signal pressure is not enough to operate the control valve.
To make split range between the valves.
It can be used to reverse the action of the actuator from air to open to air to close
and vice versa.
To minimize the effect of hysterisis effect.
To minimize the response time for the valve.
If the actuator is spring less positioner will be used.
If the valve has high friction.
The operation of the most common positioner as follows. In construction, pneumatic valve
positioners have diaphragms or bellows to sense the incoming signal from the controller and
feed back devices from the valve stem. The unit may be position balanced or force balanced.
Any error in the two signals causes a proportional change in the output of a pilot valve.
In our plant we are using Valtek beta positioner and the main parts are shown in the picture.
Instrument Signal Capsule: It will receive The Signal from
I/P Transducer & move The Pilot Stem.
Range Adjustment Locking Screw
Range Adjustment Gear
Zero Adjustment Locking Knob
Transducers convert a current signal to a pneumatic signal. The most common transducer
converts a 4-20 mA electric signal to a 3-15 psig pneumatic signal.
An increase in the dc signal to the coil increases the magnetic field around the coils. This field
increases the magnetic strength in the armature and the magnetic attraction across the air gap
between the armature and the pole pieces. The magnetic attraction will therefore downward at the
nozzle end and upward at the feed back bellows end, resulting in a torque that rotates the
armature about the torsion rod to cover the armature nozzle. The resulting restriction produces an
increased pressure in the nozzle, in the upper chamber of the relay, and in the feed back bellows.
The relay responds to the increase in nozzle pressure to increase the output pressure to the
actuator and control valve.
Volume Boosters are used on throttling control valves to provide fast stroking
action with large input signal changes. At the same time, the flow boosters allow
normal positioner air flow (and normal actuation) with small changes in the
positioner input signal. Depending on actuator size, packing set and the number
used, boosters can decrease valve stroking times up to 90 percent.
Air to Open
Air to Close
Air fail to Lock in the same position
Fail Safe System for Valves:-
Where service conditions exceed the capabilities of the standard fail-safe
spring to drive the valve to its failure position, and where specially
failure springs may be both mechanically and economically unfeasible, air
spring fail-safe systems on Valtek control valves provide the thrust
necessary to drive the
plug to its failure position. An air spring provides a pressurized volume of air to drive
the actuator piston in the failure direction. The volume of air is sometimes provided
within the actuator itself, or where the cylinder volume is insufficient, a separate
external volume tank is provided.
Air spring systems are used primarily to close valves upon air failure. And sometimes
they must open valves upon air failure. A fail-closed valve is customarily operated
with the flow direction over the plug. Thus, with the plug on the seat, the upstream
pressure acts to hold the valve closed.
Fail-open valves customarily operate with the flow direction under the plug. Thus,
when a general system failure occurs, the upstream pressure will keep the plug off the
seat and the valve open.
Air springs on valves are practical because the locked-up air is used only at the instant
of air failure to drive the valve to the fail position. Line pressure will insure that the
valve stays either closed or open.
• Occasionally, service conditions require that the valve remain in the last operating position upon
loss of air supply. For such applications, valves can be equipped with a fail-in-place lock-up
system. If air failure occurs, the system activates two pilot-operated lock-up valves that trip and
lock the existing cylinder pressures on both sides of the piston, thus maintaining the last throttling
Signal-to-open, Fail-closed Signal-to-close, Fail-open