Tips on Selection of automatic valve in industries

Tips on Selection of automatic valve in industries

1. Some Tips on Selection of Valve Body
for Automatic Control Valve
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2. ValveBody
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First : Some Basics
It usually consists of a BODY or VALVE and an ACTUATOR, which responds to a signal from the controlling
system and changes the position of a FLOW CONTROLLING ELEMENT in the valve.
Three major control valve components:
VALVE Body
The valve body is mounted in the process fluid line and is used to control the flow of fluid in the process. Valve body are
classified into two general types based on the movement of the valve’s closure part:
1- linear (sliding stem)
2- rotary

3. BASICS TYPES OF VALVES
•Globevalvesconsistofamovabledisk-type
elementandastationaryringseatinagenerally
sphericalbody.
•Thevalvestemmovesaglobeplugrelativetothe
valveseat.
•Theglobeplugcanbeatanypositionbetween
fullyopenedandfullyclosedtocontrolflow
throughthevalve.
RecommendedUses:Applicationsrequiring:
••Preciseflowregulation
••Frequentandwidethrottlingoperation
••Suitedtoveryhighpressuredrops
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Globe valve
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4. GateValve
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Gate valves use linear type of stem motion for opening and closing of a valve.
Best Suited Control: Quick Opening
Recommended Uses:
• Fully open/closed, non-throttling
• Infrequent operation
• Minimal fluid trapping in line

5. Butterfly Valve Bodies
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The butterfly valves are used for limited throttling where a tight shut off is not required
Butterfly valves consist of a disc attached to a shaft with bearings used to facilitate rotation.
Butterfly Valve Recommended Uses:
• Fully open/closed or throttling services
• Frequent operation
• Minimal fluid trapping in line
• Applications where small pressure drop is desired

6. BallValve
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Ball valves are a quick opening valves that give a tight shutoff.
Ball valves are considered high recovery valves, having a low pressure drop and relatively
high flow capacity.
Best Suited Control: Quick opening, linear
- ball valves have very high flow coefficients.
- High capacity results from the unobstructed flow path when the ball is in the fully open position.
Recommended Uses:
• Fully open/closed, limited-throttling
• Higher temperature fluids

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8. Direct And Reverse Actuator Selection Criteria
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The selection of a particular actuator action (direct or reverse) is generally determined by the fail mode that is desired.
Reverse & Direct Valves
Valve PlugStem
ValvePlug
ValveSeat
(a) DirectActing (b) ReverseActing

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Valve
Positioner
Control valve positioner:
1- Analog I/P: the positioner performs the same function as the one above, but uses
electrical current 4-20 ma as an input signal and it translate this to a required valve position
and supplies the valve actuator with the required air pressure to move the valve. . Pressure is
typically modulated between (3 to 15 psig) to move the valve from 0 to 100% position
2- Digital: although this positioner functions very
much as the analog I/P, it differs that the electronic
signal conversion is digital rather than analog.
This type of positioner is a microprocessor-based
instrument. The microprocessor enables diagnostics
and two-way communication to simplify setup and
troubleshooting.
In old days positioner and I/P used to be separate,
nowadays positioner and I/P analog coming as
one instrument and became smarter.

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Role
Theroleofthepositioneristoachievevalvestempositioningaccuracy,evenwhenvalvefriction
andfluidforcestendtopreventtheactuatorfromaccuratelypositioningthecontrolvalvestem.

CONTROL VALVE POSITIONER
The fundamental function of a positioner is to deliver pressurized air to the valve actuator, such that the position of the
valve stem or shaft corresponds to the set point from the control system
Positioner Action - While many instruments must be selected as direct-acting or reverse-acting devices, most
control valve positioners can be configured to provide either action.
Gauges - Positioners may include up to three
gauges, Gauges provide a visual indication of the
following:
• The supply pressure
• The input signal pressure
• The positioner output pressure
Gauges provide useful information to personnel
who have responsibility for troubleshooting,
maintenance, and calibration.

11. Self, pilot, spring or Air Supply pressure (on/off by solenoid).
Supply pressure regulator if using Air Pressure:commonly called air sets,
reduce plan air supply to valve positioners and other control equipment. The
regulator mounts integrally to the positioner, or nipple-mounts or bolts to the
actuator.
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2- Supplying pressure:

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13. valve characteristic
-theflowcapacityofacontrolvalve,withaspeciallyshapedplugcannotbesimplydeterminedarithmetically.
-somemeasurementsunderstandardizedconditionsarenecessarytodefinetheflowcapacityofthevalveandmakeit
comparable
-Theprincipleofthemeasuringarrangementisrepresentedbelow
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Control Valve Capacity - Cv:
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. The relationship between pressure drop and flow rate
through a valve is conveniently expressed by a flow coefficient (Cv).
What is Flow Coefficient (Cv)?

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Flow coefficient (Cv) is defined as the number of gallons per minute (gpm) at 60°F that will pass through a full open valve with
a pressure drop of 1 psi. Simply stated, a control valve which has a Cv of 12 has an effective port area in the full open position
such that it passes 12gpm of water with 1 psi pressure drop. The Cv for water is usually determined experimentally by
measuring the flow through a valve with 1 psi applied pressure to the valve inlet and have a 0 psi pressure at the outlet.
For incompressible fluids like water, a close approximation can be found mathematically by the following equation;
Where,
• Cv = Valve flow coefficient
• Q = Fluid flow, US GPM (also given by Area of pipe x mean velocity)
• S = Specific gravity of fluid relative to water @ 60ºF
• ∆P = Pressure drop (P1 – P2) across the control valve at maximum flow, psi
The equation shows that the flow rate varies as the square root of the differential pressure across the control valve.
Greater the pressure drop, higher will be the flow rate. Pressure drop across a valve is highly influenced by the
area, shape, path and roughness of the valve.

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ChokedFlow:
The flow coefficient (Cv) equation illustrates that the flow rate through a valve (Q) increases with the pressure
differential (∆P). Simply stated, as the pressure drop across the valve gets larger, more flow will be forced through
the restriction due to the higher flow velocities. In reality, As the pressure drop across the valve is increased, it
reaches a point where the increase in flow rate is less than expected. This continues until no additional flow can be
passed through the valve regardless of the increase in pressure drop. This condition is known as choked flow.

It is necessary to account for choked flow during the sizing process to ensure against undersizing a valve. In other
words, it is necessary to know the maximum flow rate that a valve can handle under a given set of conditions.
When selecting a valve, it is important to check the pressure recovery characteristics of valves for the
thermodynamic properties of the fluid. High recovery valves, such as ball and butterfly, will become choked at lower
pressure drops than low recovery valves such as globe which offer a more restricted flow path when fully open.
Flashing;
As previously mentioned, at the point where the fluid’s velocity is at its highest, the pressure is at its lowest.
Assuming the fluid is incompressible (liquid), if the pressure falls below the liquid’s vapor pressure, vapor bubbles
form within the valve and collapse into themselves as the pressure increases downstream. This leads to massive
shock waves that are noisy and will certainly ruin the equipment.
Requirements for occurrence of flashing:
• The fluid at the inlet must be in all-liquid condition, but some vapor must be present at the valve outlet;
• The fluid at the inlet may be in either a saturated or a subcooled condition; and
• The valve outlet pressure must be either at or below the vapor pressure of the liquid.

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Cavitation:
Cavitation is similar to flashing in a way that the liquid pressure drops to value below its vapor pressure,
causing a liquid to vaporize into vapor bubbles.
Flashing effects:
• Material damage is associated with the formation of sand-blasted surfaces;
• Decreased efficiency - valve ability to convert pressure drop across the valve into mass flowrate is compromised.

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CONTROL VALVE CHARACTERISTICS
Each valve has a flow characteristic, which describes the relationship between the flow rate and valve travel. As a
valve opens, the flow characteristic, which is inherent to the design of the selected valve, allows a certain amount of
flow through the valve at a particular percentage of the stroke. This enables flow regulation through the valve in a
predictable manner. The three most common types of flow characteristics are:
1. Linear
2. Equal percentage
3. Quick opening
Linear valve characteristics:
This characteristic provides a linear relationship between the valve position and the flowrate. The flow through a
linear valve varies directly with the position of the valve stem. This flowtravel relationship, if plotted on rectilinear
coordinates, approximates a straight line, thereby giving equal volume changes for equal lift changes regardless of
percent of valve opening. These valves are often used for liquid level control and certain flow control operations
requiring constant gain.
Equal percentage valve characteristics:
The equal percentage valve plug produces the same percentage change in flow per fixed increment of valve stroke at
any location on its characteristic curve. For example, if 30% stem lift produces 5 gpm and a lift increase of 10% to
40% produces 8 gpm or a 60% increase over the previous 5 gpm, then a further stroke of 10% now produces a 60%
increase over the previous 8 gpm for a total flow of 12.8 gpm.

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Quick opening valve characteristics:
A quick opening valve plug produces a large
increase in flow for a small initial change in stem
travel. Near maximum flow is reached at a relatively
low percentage of maximum stem lift. Quick
opening plugs are normally utilized in two position
“On-Off” applications but may be used in some
linear valve applications. This is possible because
of its initial linear characteristic at a low percentage
of stem travel. The slope of this linear region is very
steep which produces a higher initial gain than the
linear plug but also increases the potential
instability of the control valve.
Why Use Equal Percentage?
Provides equal percentage increases in rate of flow for equal
increments of plug movement.
Provides the best choice of flow characteristic for most systems.

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At this point, let’s define two additional terms: 1) Rangeability, and 2) Turndown and define their relationship with
respect to valve authority.
Valve Rangeability - Rangeability indicates the extent of flow values that the valve can reliably regulate and is
often reported as a ratio of the largest to the smallest flows that canbe controlled acceptably.
A control valve with higher rangeability will control flow over wider flow rates. For example, a valve with a
rangeability of 50 and having a total flow capacity of 100 GPM, will control flow accurately down to as low as 2 GPM
even fully open. Rangeability is affected by three factors:
1. Valve geometry – inherent rangeability due to the design of the body and the regulating element.
2. Seat leakage – excessive seat leakage can cause instability as the valve lifts off of the seat.
3. Actuator – diaphragm actuators are seldom accurate at less than 5% of the valve opening, whereas piston-
cylinder actuators can provide control within 1% of valve lift due to the presence of air in two chambers.
Valves with high rangeability are sometimes desirable but these will be expensive to manufacture since very close
tolerances are involved between the disc and the seat. A typical commercial valve generally has a rangeability of
about 35 to 50.
Turndown - It is the ratio between maximum usable flow and the minimum controllable flow; usually less than the
rangeability. For instance, as stated above, after the 100 GPM valve has been applied at a job, it might turn out that
the most flow you would ever need through the valve is 68 GPM. Since the minimum controllable flow is 2 GPM, the
turndown for this valve is 34 to 1.

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In comparing rangeability and turndown, we may say that rangeability is a measure of the predicted stability of the
control valve, and turndown is a measure of the actual stability of the valve.
Note:
The term rangeability applies to the valve whereas the term turndown applies to the application. The rangeability of
the selected valve must exceed the turndown requirements of the application.
Mathematically we can define rangeablity as: R = (Qmax / Qmin) x β½
Where:
• R = valve rangeability
• Qmax = design flow rate
• Qmin = controllable flow rate
• β = installed valve authority
The minimum controllable flow rate through a valve is a function of the valve design. It is directly affected by all
sources of friction within the valve assembly. In an ideal valve, any change in the signal applied to the actuator,
even an infinitely small change, will force the valve stem to move, even if that movement is infinitely small. However,
friction represents a force that must be overcome by the actuator. When the actuator exerts sufficient force to
overcome friction, the valve stem will move some finite amount. When this occurs when the valve is fully closed, this
finite movement results in a certain minimum flow rate.

This means the valve must have a manufacturer's rangeability rating of 123:1 or greater. A typical commercial valve
generally has a rangeability of about 50:1. Industrial valves can have a rangeability as high as 200:1. This is part of
the reason why industrial valves are so much more costly than a commercial valve.

22. Linear flow characteristic:
◼Avalvewithanideallinearinherent
flowcharacteristicproducesflowrate
directlyproportionaltotheamountof
valveplugtravel,throughoutthetravel
range.Valvewithalinear
characteristicareoftenspecifiedfor
liquidlevelcontrolandforflowcontrol
applicationsrequiringconstantgain.
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Equal percentage flow characteristic:
◼Ideally for equal increments of valve
plug travel, the change of flow rate
regarding maybe expressed as a
constant percent of the flow rate at the
time of the change. The change in flow
rate observed regarding travel will be
relatively small when the valve plug is
near its seat and relatively high when
the valve plug is nearly wide open.

23. Quick opening flow characteristic:
◼A valve with a quick opening flow characteristic provides a maximum
change in flow rate at low travels.
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Cage-Guided Valves
•In a cage-guided valve, the flow characteristic is determined by the shape of the cage
windows, or openings.
•the cage window shapes for the three flow characteristics that were previously described.

24. Post-Guided Valves
Post-Guided Valves-For post-guided valves, flow
characteristics are determined by the contours of
the valve plug .
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Different types of Plug Valve
Ports, which affect the flow
significantly, and in turn
affected by characteristics of
process fluids

25. Plug valve disk: Plugs are either round or taper cylinder. They may have various types of port openings, each with
a varying degree of the opening area.
Plugs are available with
•Rectangular Port
•Round Port and
•Diamond Port
Below in the photo are the list oftaper plug with Rectangular, Round, and Diamond Port design.
•Rectangular Portis the most common for plug valve. The rectangular port represents at least 70% of the
corresponding pipe’s cross-sectional area.
•Roundport plughas a round opening through the plug. It is available in full bore and reduced bore design.
Valves with reduced ports are used only where restriction of flow is not important.
•Diamond Portplug has a diamond-shaped port through the plug. All diamond port valves are venturi restricted
flow type. This design is for throttling service.
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26. Types of Plug Valves
Plug valves can devide into lubricated tyoe or non-lubricated typeand with different styles of port openings
through the plug.
Lubricated Plug Valve
The plug inside a lubricated plug valve includes a cavity in the centre along its axis. You can observe this within
the image. Lubricant chamber at the end and also the sealant injection fitting at the very top make sure the way
to obtain lubricant.
Small check valve underneath the injection fitting prevents the sealant from flowing within the reverse direction
when the sealant is injected in to the cavity.
Plug surface will get constantly lubricated through the sealant that moves in the center cavity through radial
holes into lubricant grooves around the plug surface. Now why we needed all of this? Many plug valves have
all metal construction.
The narrow gap round the plug may allow leakage, and when you lessen the gap further, it will raise the friction
and plug could get stuck within the valve body.
The lubricant cuts down on the pressure needed to spread out or close the valve and enables smooth
movement from the plug. The lubricant also prevents corrosion from the plug.
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27. The lubricant material should be suitable for the fluid
from the pipeline. It shouldn't dissolve or wash away
through the flow medium because this could
contaminate the fluid, and damage the seal between
your plug and the entire body, leading to leakage.
Also, the sealant used must have the ability to
withstand a temperature from the flow medium.
Lubricated plug valves can be found in the big size
range, and they're fit to operate in high-pressure
temperature services. These valves are susceptible
to less put on and supply better corrosion resistance
in certain service environments.
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28. Non-lubricated Plug Valves
A non-metallic elastomeric sleeve or liner can be used
in this kind of plug valve. This sleeve is installed in
your body cavity from the valve. The polished tapered
plug functions like a wedge and presses the sleeve
from the body.
This nonmetallic sleeve cuts down on the friction
between your plug and also the valve body. Non-
lubricating plug valves needed minimum maintenance.
Because of the non-metallic seat, these valves aren't
utilized in high-temperature services.
Lubricating and non-lubricating plug valves can handle
supplying a bubble-tight shutoff and therefore are of
lightweight.
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29. Multi-Port Plug Valves
Below is the photo of3-way multiport plug valve. The top image is of 3-way 3-port design and bottom is 3-way
2-port design.
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This is 4-way design

30. Multiport valves are utilized in transfer lines as well as for diverting services. Just one multiport valve may satisfy
the requirements of 3 or 4 gate valves or other kinds of the shutoff valve. However, sometime multiport valve
doesn't completely turn off flow. Great care ought to be drawn in indicating the specific port arrangement for correct
operation.
Plug ValveApplication
•Plug valve used as on-off stop valves and capable of providing bubble-tight shutoff.
•It can be used in different types of fluid services such as Air, gaseous, vapor, Hydrocarbon, slurries, mud, and
sewage applications.
•Plug valve can be used in a vacuum to high-pressure & temperature applications
Pro Side of Plug Valve
•Simple design with few parts
•Quick to open or close
•inline maintenance possible
•Offers minimal resistance to flow
•Provides reliable leak-tight service
•Multiple port design helps reduce the number of valves needed and permits a change in flow direction
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Con Side of Plug Valve
•It requires greater force to operate, due to high friction
•Larger valves cannot be operated manually and required an actuator
•Pressure drop due to reducing port
•Cost of Plug valves may be more than ball valves for given size and class

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100 % Port area in plug valve, is excellent for abrasive media as fluid does not accelerate causing wearing, also sludge
or sediments flow without resistance, that help avoid blockage, can be used at pump suction to avoid pressure drop

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Mechanical aspects
An important factor when choosing automation is what is needed to mount the valve mechanically. ISO 5211,
VDI/VDE or Namur are standards for the mechanical coupling (interface) between valve and actuator. They also
specify the standard for the coupling between actuator and positioner (or limit switch). The product type and
manufacturer determine which standard applies. It is important to know who takes responsibility for the
functioning of the valve when different parts are assembled by different parties.
For example, larger valves have much greater forces to handle than smaller ones and should be able to do so for
a long time. By sizing the valves correctly, we ensure that the mechanical interfaces are correct, and can
therefore guarantee long trouble-free operation regardless of external conditions, with special anti-backlash
coupling to avoid any hysteresis that would compromise the controllability and accuracy of our solution.
What is ISO 5211?
ISO 5211 is an international standard that specifies a standardized interface for mounting actuators (such as
electric, pneumatic, or hydraulic actuators) on industrial valves. This standard defines the dimensions and
requirements for the mounting flange and drive connection to ensure compatibility and interchangeability between
valves and actuators from different manufacturers.

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Materials of construction:
General Service valves are specified with commonly found materials to match the pipe
material. The standard materials are:
• Carbon Steel
• Stainless Steel
• Chrome-moly
For special services, the construction is often guided primarily by the type of fluid, service temperature, etc. Here
are few examples:
• Concentration of the material in the fluid is very important. Most chemicals are easier to handle in dilute
concentrations. However, acids—such as sulfuric acid—become more aggressive as they are diluted with water.
Some organic materials that are not corrosive by themselves become so in the presence of water.
• Temperature is an important factor in choice of materials because high temperature increases corrosion. In
addition, at very elevated temperatures, the pressure rating of the valve can be severely lowered due to
deterioration of either metallic or nonmetallic material properties.
• Materials used in valves in hydrocarbon service experience sulfide stress cracking. NACE standard MR-01-75
provides specific guidelines for their selection criteria.
• Cast carbon steel (ASTM A216 – Grade WCB) is the most popular steel for valve bodies in moderate service
such as air, superheat or saturated steam, non-corrosive fluids.
• Cast chrome-moly steel (ASTM A217 – Grade WCB-C9) has addition of chromium/Molybdenum that provide
corrosion resistance and also suitable for temperature up to 1050ºF.
• Cast type 304 SST (ASTM A351 – Grade CF8) is for oxidizing and very corrosive fluids.
• Cast type 316 SST (ASTM A351 – Grade CF8M) is same as 304 SST but since it has addition of Molybdenum it
has better resistance to corrosion.

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• Cast Iron (ASTM A126) is used for steam, water, gas and non-corrosive fluids and is inexpensive.
• Cast Bronze (ASTM B61 and B62) is used for steam, air, water, oil and non-corrosive fluids.
Some Important Questions
What are the common control valve applications?
Control valves are used in following applications:
• Pressure Reducing
• Pressure Relief/Sustaining
• Pump Control
• Rate-of-Flow Control
• Level Control
• Cavitation Control
• Surge Anticipation
• Electronic Control
• Metering
What is the rate of a flow control valve?
Rate of flow control valve maintains a maximum flow rate setting downstream
regardless of pressure changes. It is typically installed within distribution systems and
process control applications.
Can two control valves be used in series in high pressure drop applications?
Dropping the pressure across two valves rather than one is theoretically better. However, in practice, the two valves
will not usually control well together unless the process can operate with a very low proportional band with slow
response times.

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Can two control valves be used in parallel to handle high turndown
applications?
Two valves in parallel working on split range signals can give very high turndown capability. The situation that
should be avoided if possible is that the larger valve operates in the "cracked open" position – one way to avoid this
is to program the PLC or DCS to shut the small valve and use only the larger unit once the capacity of the small
valve is exceeded.
An alternative to the two valves in parallel is to select a valve with a high rangeability such as a V-port ball valve.
Is flow through a Control Valve – Turbulent or Laminar?
Flow through control valves is almost always turbulent Laminar flow takes place with liquids operating at low
Reynolds numbers. This occurs with liquids that are viscous; working at low velocities. Laminar flow in gases and
vapors very seldom will be experienced in process plants.
The control valve size is essentially always equal to the line size. Is this correct?
NO. The control valve does not need to be of the same size as the pipe. Don’t assume a 4” valve will handle the
flow in a 4” line. The flow path is the critical factor and shape of the flow passage in the valve can be a problem.
Look at the pressure through the valve and if the valve creates a pressure drop, then a slightly larger valve may be
needed. Always calculate the flow coefficient. Even for a given size of valve, there is no industry-wide standard
pressure for valve sizes; no two manufacturers design a 4" valve the same way, and different designs have
different pressure considerations. Contrary to conventional wisdom, it is better to make an error in under sizing a
control valve than to oversize it.
A better and usually less expensive approach is to use a valve that is designed with multiple pressure drop
restrictions inside the trim.

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TIPS AND TRICKS
Take the following tips into consideration when choosing a valve.
1. Choose a valve that will pass the maximum flow when about 90% open.
2. Choose a valve that will pass the minimum flow when about 10% open.
3. Choose a valve that will pass the normal flow when about 60-70% open.
4. Size control valves to absorb about 1/3rd of the total system pressure drop.
5. Control valves should not be less than half the pipe size. Normally the valves exclusively envisaged for shut-off
service shall be line size. Alternatively, they could be sized as control valves.
6. In the case of lines with a diameter of up to 1" the valve size shall normally equal that of the line. In the case of
lines with a diameter larger than 1" the valve size shall not be less than 1".
7. Valves shall generally have flanged connections as per rating envisaged in the line specification with the
exception of valves with a nominal diameter smaller than or equal to 1½" which shall have a minimum rating of
300 ANSI.
8. If you are dealing with a corrosive fluid, choose the valve body and trim material to match the pump casing and
impeller.

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9. Velocity is the key to handling abrasive materials. Keep line velocity of about 10 ft/s for clean liquid. If you
have a fluid that is abrasive, keep the velocity as low as possible, without having the particles drop out of
suspension. When dealing with high pressure drop situations try always to keep the velocities below 0.3 mach
on the inlet
pipe, valve body, and outlet pipe.
10. Always sense pressure where you want to control it. Many control valves and pressure regulators do not
function properly simply because they are sensing pressure at one point and being asked to control it
somewhere else.
11. If you use a transducer in a control loop, specify a positioner on the valve. Otherwise the transducer will rob
the actuator of available thrust, and the valve will leak when it is supposed to shut off.
12. In cavitating fluids “be sure to allow a straight run of downstream pipe after the valve even when the control
valve has cavitation trim in it”. If there is a pipe "T" or elbow immediately downstream, the flow will choke out
and back up into the valve.
13. Remember that control valves only do what you tell them to. Many control valve problems turn out to be a
problem somewhere else.