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CONTROL VALVES

CONTROL VALVE

CONTROL VALVE

ISA-S75.05: A Control Valve is a power-operated device used to modify the fluid flow rate in a process system.

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CONTROL VALVE

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Inst Air

Control valve in FLOW CONTROL

controller

Inst Air

dP Flow measurement

Control valve in LEVEL CONTROL

controller

Inst Air

Control valve in PRESSURE CONTROL

controller

Inst AirInst Air

Pressurized vessel

Control valve in Temperature Control

controller

Inst Air

Super heated Steam in Hot water out

Steam outCold water in

Controllercontrollermeter valve

INPUT OUTPUTSET POINT (commonly from the operator)

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Control Valve Classification

Control Valve

Linear Motion

Rotary Motion

Globe

Globe

Single Seated

Double Seated

Angle 3 way

Diaphragm Pinch Ball Butterfly Plug

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1.1. Single Seated

1.2. Double Seated1.2. Double Seated

1.3 Three way

2. Diaphragm

3. Pinch

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Pinch Valves

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The flexible sleeve allows the valve to close drop tight around valve to close drop tight around solids , solids that would typically be trapped by the seat or stuck in crevices in globe, diaphragm, butterfly, gate or ball valves.

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) (Flow Factor/Valve Coefficient/Capacity Coefficient

) (Flow Factor/Valve Coefficient/Capacity Coefficient

CvCv

Cv = Flow coefficient or flow capacity rating of valve. F = Rate of flow (US gallons(3.73liters) per minute). SG = Specific gravity of fluid (Water = 1). P = Pressure drop across valve (psi) @ 16oC.

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Cv - gallons per minute (GPM) of 60oF water with a pressure drop of 1 psi (lb/in2).

Cv Kv

Cv Kv

Cv (Gallon/Min) 1 0.268

Kv (Liters/Min) 3.73 1

Valve undersizedStarving for fluidBuildup of upstream pressureHigher backstream pressure damaging equipmentsCavitation & Flashing

Too small Cv

Large oversized valve is selectedCost, size & weight increasesHigher pressure drops and faster velocity causingCavitation, flashing & corrosionBath tub stopper effect

Too Large Cv

- Flow Characteristic


Quick Opening: Quick open plugs are used for on-off applications designed to produce maximum flow quickly

Linear: produces equal changes in flow per unit of valve stroke regardless of plug position, used regardless of plug position, used where valve pressure drop is a major portion of the total system pressure drop

Equal Percentage: change in flow per unit of valve stroke is directly proportional to the flow occurring just before the change is made, generally used for pressure control

In each angle, pressure differential should be kept constant; then flow measurement is done.

C-Type BALL VALVE

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Eccentric type:

Common Body Designs; 1Common Body Designs; 1

In-Line

In-line style bodies feature smooth, streamlined, constant internal area passages with no pockets, permitting high capacity with minimum turbulence.


Angle Form

The angle-style body form for many applications such as handling erosive fluids. The angle valve incorporates a selfdraining design. The design also allows for smaller space requirements than as globe valve.


Three-Way Form

Three-way bodies are used for either combining or diverting services.A standard globe valve usually, easily converts to three-way service with the addition of a three-way adaptor, upper seat ring, two gaskets, a three-way plug, and bonnet flange bolting.


Offset Form

When inlet and outlet piping can be offset, this design is the simplest, least expensive barstock style. It works best in self cleaning applications. Other than the body, the offset design is completely interchangeable with the standard globe valve.

Common Body Designs; 5Common Body Designs; 5Steam Jacketed

Steam jackets are used to heat the fluid passing through the control valve.

Expanded Outlet

The expanded outlet valve(Reduced Bore), permits the installation of a small valve in a larger line without using line reducers or expanders. The valve is a standard in-line globe valve, except for the body which incorporates expanded outlets. Because line expanders and reducers are not used, field installation expenses are reduced.

Common DesignsCommon Designs

Designed for most gas and liquidapplications. The valves unique construction handles pressures from vacuum to 15,000 psi (1034 Bar) and temperatures from -423 to 1500F (-253 to 816C).High positioning accuracy, repeatability, controlled high speed, and instant response.

Common DesignsCommon Designs

The ShearStream ball valve features a segmented V-notch ball to reduce clogging, to improve shearing action and to exceed 300:1 rangeability. The one-piece body provides high performance.

A high performance rotary control valve using an eccentric plug which provides high rangeability, zero breakout torque and durable trim with a significant increase in valve life.Applications from petrochecmical to low content slurry and pulp service to severe service.The eccentric plug provides rangeability greater than 100:1, compared to 50:1 for typical globe valves and 20:1 for most butterfly valves.The shutoff rating reaches Class IV for metal seats and Class VI for soft seats.

Special DesignsSpecial Designs

The Offset design is used for installation in offsetpiping configurations. All parts, except the body, are identical to the conventional globe type.

high pressure classes but low sizeswhere fast delivery is required

Special DesignsSpecial Designs

The pressure-balanced regulator used primarily by the aerospace industry in high temperaturegas applications. Upstream pressure is delivered to a small piston within the diaphragm actuator. (This piston has the same area as the plug.)

Steam Jackets are used to heat the fluid passing through the control valve. The steam jacketed valve body uses a standard globe-style body with oversized, blind flanges for a full jacket or standard flanges for a partial jacket. The jacket usually is rated for 150 psi and comes equipped with 3/4-inch NPT supply and drain connection.

Severe ServicesSevere ServicesThe trims are designed to eliminate cavitation, reduce high noise levels andhandle flashing applications often associated with high pressure drop service.

This cartridge uses specially designed channels and intersecting holes (plenums) in series to holes (plenums) in series to prevent single point, large pressure recovery which can cause cavitation and hydrodynamic noise.For less serious cavitation applications, use of some retainers to minimizes cavitation damage by controlling the location of imploding vapor bubbles are offered.

Severe ServicesSevere Services


Attenuator reduces gaseous noise levels with staged pressure reduction through a series of drilled-hole cylinders.

Above design effectively reduce gaseous and hydrodynamic noise more and eliminate the damaging effects of cavitation in liquids.

Downstream DevicesDownstream Devices

Installed between raised face flanges immediately downstream from the valve, the plate incorporates a series of stages to control line turbulence and

Silencer Plate; upto 15dB

stages to control line turbulence and absorb the pressure drop. The number of stages varies according to the application.


Diffusers share the high pressure drop with the valve. The length of the diffuser and the number of holes vary to accommodate the flow capacity required.

Diffuser; upto 30dB

accommodate the flow capacity required.


In a blow off or vent system, tremendous energy in the form of noise is released at the open exit. Vent silencers attenuate this noise energy before it is released to

Vent Element; upto 25dB

this noise energy before it is released to the outside environment.

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Valve Sizing Valve Sizing1- Flow Capacity; Cv1- Flow Capacity; Cv

The valve sizing coefficient most commonly used as a measure of the capacity of the body and trim of a control valve is Cvbody and trim of a control valve is CvOne Cv is defined as one U.S. gallon per minute of 60 degree Fahrenheit water that flows through a valve with a one psi pressure drop.

Valve Sizing Valve Sizing2- Pressure Profile2- Pressure Profile

Maximum velocity and minimum pressure

occur immediately downstream from the occur immediately downstream from the

throttling point at the narrowest constriction of

the fluid stream, known as the venavena contractacontracta

().

Downstream from the vena contracta, the fluid

slows and part of the energy (in the form of

velocity) is converted back to pressure.

Valve Sizing Valve Sizing3- Allowable Pressure Drop3- Allowable Pressure Drop

The curve departs from a linear

relationship at the onset of "choking"

described using the FiFi factor.described using the FiFi factor.

The flow rate reaches a maximum,

qmax, at the fully choked condition at the fully choked condition due

to effects of cavitation for liquids or

sonic velocity for compressible fluids.

The transition to choked flow may be

gradual or abrupt, depending on valve

design.

Valve Sizing Valve Sizing3- Allowable Pressure Drop3- Allowable Pressure Drop

For liquid ; ANSI/ISA sizing equations use a pressure recovery factor, FL, toto calculate the calculate the P at which choked flow is assumed for sizing P at which choked flow is assumed for sizing P at which choked flow is assumed for sizing P at which choked flow is assumed for sizing purposes.purposes.For compressible fluids ; a terminal pressure drop ratio, xT, similarly describes the choked pressure drop for a specific valve.Pa : When sizing a control valve, the smaller of the actual pressure drop or the choked pressure drop is always used to determine the correct Cv.This pressure drop is known as the allowable pressure drop, Pa .

Valve Sizing Valve Sizing4- Cavitation4- Cavitation

In liquids;when the pressure pressure anywhere in the liquidliquiddropsdrops belowbelow thethe vaporvapor pressurepressure of the fluid,

5- Flashing5- FlashingIf the downstream pressure is equal to or less than the vapor pressure, the vapor vapor bubbles created at the vena contracta do not bubbles created at the vena contracta do not dropsdrops belowbelow thethe vaporvapor pressurepressure of the fluid,

vapor bubbles begin to form in the fluid stream. As the fluid decelerates there is a resultant increase in pressure. If this pressure is higher than the vapor pressure, thethe bubblesbubblescollapsecollapse (or implode) as the vapor returns to ) as the vapor returns to the liquid phasethe liquid phase.This two-step mechanism called cavitation produces noise, vibration, and causes erosion damage to the valve and downstream piping.

bubbles created at the vena contracta do not bubbles created at the vena contracta do not collapsecollapse, resulting in a liquid-gas mixture downstream of the valve. This is commonly called flashing.When flashing of a liquid occurs, the inlet fluid is 100 percent liquid which experiences pressures in and downstream of the control valve which are at or below vapor pressure. This two phase (vapor and liquid) fluid at the valve outlet and in the downstream piping; the velocity of this two phase flow is usually very high and results in the possibility for erosion of the valve and piping components.

Valve Sizing Valve Sizing6- Liquid Pressure Recovery Factor, FL6- Liquid Pressure Recovery Factor, FL

The liquid pressure recovery factor, FL, predicts the , predicts the amount of pressure recovery that will occur between the amount of pressure recovery that will occur between the vena contracta and the valve outlet.vena contracta and the valve outlet.

7- Liquid Critical Pressure Ratio Factor, FF7- Liquid Critical Pressure Ratio Factor, FF

The liquid critical pressure ratio factor, FF, multiplied by the vapor pressure, predicts predicts the theoretical vena contracta the theoretical vena contracta

vena contracta and the valve outlet.vena contracta and the valve outlet.FL is an experimentally determined coefficient that accounts for the influence of the valves internal geometry on the maximum capacity of the valve. It is determined from capacity test data.High recovery valves High recovery valves such as butterflybutterfly and ball valves ball valves have significantly lower pressures at the vena contracta have significantly lower pressures at the vena contracta and hence recover much farther for the same pressure drop than a globe valve. Thus they tend to choke (or cavitate) at smaller pressure drops than globe valves.* High recovery valves High recovery valves are valves that lose little energy due to little flow turbulence.

the theoretical vena contracta the theoretical vena contracta pressure pressure at the maximum effective (choked) pressure drop across the valve..

Valve Sizing Valve Sizing8- Chocked Flow8- Chocked Flow

in gases and vapors ; Choked flow occurs when the fluid velocity reaches sonic values at any point in the valve body, trim, or pipe. As the pressure in the valve or pipe is lowered, the specific volume increases to the point where sonic velocity is reached.

9- Reynolds Number Factor, FR9- Reynolds Number Factor, FR

The Reynolds Number

Factor, FR, is used to

velocity is reached.In liquids, vapor formed as the result of cavitation or flashing increases the specific volume of the fluid at a faster rate than the increase in flow due to pressure differential. Lowering the downstream pressure beyond this point in either case will not increase the flow rate for a constant upstream pressure. The velocity at any point in the valve or downstream piping is limited to sonic (Mach = 1).As a result, the flow rate will be limited to an amount which yields a sonic velocity in the valve trim or the pipe under the specified pressure conditions.

correct the calculated correct the calculated CvCv

for nonfor non--turbulent flow turbulent flow

conditionsconditions due to high

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valve Cv .

Valve Sizing Valve Sizing10- Piping Geometry Factor, FP10- Piping Geometry Factor, FP

Valve sizing coefficients are determined from tests run with the valve mounted in a straight run of pipe which is the same diameter as the valve body.If the process piping configurations are different from

11- Ratio of Specific Heats Factor, Fk11- Ratio of Specific Heats Factor, Fk

The ratio of specific heats

factor, Fk, adjusts the adjusts the

equation to account for equation to account for If the process piping configurations are different from the standard test manifold, the apparent valve capacity is changed. The effect of reducers and increasers can be approximated by the use of the piping geometry factor, FP.

equation to account for equation to account for

different behavior different behavior of gases gases

other than air.other than air.

12- Terminal Pressure Drop Ratio, xT12- Terminal Pressure Drop Ratio, xT

The terminal pressure drop ratio for gases, xT, is used to predict the choking point where additional pressure drop (by lowering the downstream pressure) will not produce additional flow due to the sonic velocity limitation across the vena contracta.This factor is a function offunction of the valve geometry and varies similarly to FL, depending on the valve type.

Valve Sizing Valve Sizing13- Expansion Factor, Y13- Expansion Factor, Y

The expansion factor, Y, accounts for the

14- Compressibility Factor, Z14- Compressibility Factor, Z

The compressibility factor, Z, is a The expansion factor, Y, accounts for the

variation of specific weight as the gas passes

from the valve inlet to the vena contracta.

It also accounts for the change in cross-

sectional area of the vena contracta as the

pressure drop is varied.

The compressibility factor, Z, is a

functionfunction of the temperature and the

pressure of a gas.

It is used to determine the density

of a gas in relationship to its actual

temperature and pressure conditions.

Valve Sizing Valve Sizing15- Velocity15- Velocity

As a general rule, valve outlet velocities should be limited to the following maximum values:

Gas applications where special noise

attenuation trim are used should be

limited to approximately 0.33 Mach.

In addition, pipe velocities

downstream from the valve are critical

The above are guidelines for typical applications. In general, smaller sized valves handle slightly higher velocities and large valves handle lower velocities.Special applications have particular velocity requirements; a few of which are provided here:

downstream from the valve are critical

to the overall noise level.

ExperimentationExperimentation has shown that

velocities around 0.5 Mach can create

substantial noise even in a straight pipe.

The addition of a control valve to the

line will increase the turbulence

downstream, resulting in even higher

noise levels.

Valve Sizing Valve Sizing15- Velocity15- Velocity

Liquid applications where the fluid where the fluid temperature is close to temperature is close to the the saturation pointsaturation point should be limited to 30 feet per second to avoidavoid reducing the fluid pressure below the vapor pressure. This is also an appropriate limit

In flashing services, velocities become much higher due to the increase in volume resulting from vapor formation.For most applications, it is important For most applications, it is important to keep velocities below to keep velocities below 500 500 feet per feet per secondsecond. Expanded outlet style valves vapor pressure. This is also an appropriate limit

for applications designed to pass the full flow rate with a minimum pressure drop across the valve.Valves in cavitating service should also be limited to 30 feet per second to minimize damage to the downstream piping. This will also localize the pressure recovery which causes cavitation immediately downstream from the vena contracta.

secondsecond. Expanded outlet style valves help to control outlet velocities on such applications.Erosion damage can be limited by using chrome-moly body material and hardened trim.On smaller valvesmaller valve applications which remain closed for most of the time such as heater drain valves higher velocities of 800 to 1500 feet per second may be acceptable with appropriate materials.

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Calculating Cv for LiquidCalculating Cv for Liquid

The Equation for the flow coefficient (Cv) in non-laminar liquid flow is:

Calculating Cv for LiquidCalculating Cv for LiquidThe following steps should be used to compute the correct Cv, body size and trim number:

Step 1: Calculate Actual Pressure DropStep 1: Calculate Actual Pressure DropThe allowable pressure drop, Pa, across the valve for calculating Cv, is the smaller of the actual P from Equation 3.2 and choked Pch from Equation 3.3. (P1 and P2 Should be given.

p1 p2.(

Step 2: Check for Choked FlowStep 2: Check for Choked FlowStep 2: Check for Choked FlowStep 2: Check for Choked FlowUse Equation 3.3 to check for choked flow:

If Pch (Equation 3.3) is less than the actual P(Equation 3.2) , use Pch for Pa in Equation 3.1.

Calculating Cv for LiquidCalculating Cv for LiquidIt may also be useful to determine the point at which substantial cavitation begins. The following Equation defines the pressure drop at which substantial cavitation begins:

In high pressure applications, alternate analysis may be required; verify analysis with factory if P > P (cavitation)> 300 psi (globe valves) or 100 psi (rotary valves).

The required Cv for flashing applications is determined by using the appropriate P allowable [Pch calculated from Equation 3.3].


Step 3: Determine Specific GravityStep 3: Determine Specific GravitySpecific gravity is generally available for the flowing fluid at the operating temperature. The appendix provides fluid property data for 268 chemical compounds, from which the specific gravity, Gf can be calculated.

Step 4: Calculate Approximate CvStep 4: Calculate Approximate Cv

Generally the effects of nonturbulent flow can be ignored, provided the valve is not operating in a laminar or transitional flow region due to high viscosity, very lowvelocity, or small Cv. In the event there is some question, calculate the Cv, from Equation 3.1, assuming Fp=1, and then proceed to steps 5-7. If the Reynolds number calculated in Equation 3.6a is greater than 40,000, FR can be ignored (proceed to step 8 after step 5.

Sizing Valve using Cv for LiquidSizing Valve using Cv for Liquid

Step 5: Select Approximate Body Size Based on CvStep 5: Select Approximate Body Size Based on CvFrom the Cv tables in section 4, select the smallest body size that will handle the calculated Cv.

Step 6: Calculate Valve Reynolds Number Rev and Reynolds Number Factor FRStep 6: Calculate Valve Reynolds Number Rev and Reynolds Number Factor FRReynolds Number Factor FRReynolds Number Factor FR

Use Equation 3.6a to calculate valve Reynolds Number Factor:

Use Equation 3.6b to calculate valve Reynolds Number Factor FR if Rev < 40,000, otherwise FR = 1.0:


Step 7: Calculate the Final CvStep 7: Calculate the Final Cv

If the calculated value of FR is less than 0.48, the flow is considered laminar; and the Cvis equal to Cvs calculated from Equation 3.6c. If FR is greater than 0.98, turbulent flow can be assumed (FR = 1.0); and Cv is calculated from Equation 3.1. Do not use the piping geometry factor Fp if FR is less than 0.98. For values of FR between 0.48 and piping geometry factor Fp if FR is less than 0.98. For values of FR between 0.48 and 0.98, the flow is considered transitional; and the Cv is calculated from Equation 3.6e:

For laminar and transitional flow, note the P is always taken as P1 - P2 .


If the pipe size is not given, use the approximate body size (from step 5) to choose the corresponding pipe size. The pipe diameter is used to calculate the piping geometry factor, Fp, which can be determined by Tables 3-III and 3-IV. If the pipe diameter is the same as the valve size, FP is 1 and does not affect Cv.

Step 8: Calculate Piping Geometry FactorStep 8: Calculate Piping Geometry Factor

the valve size, FP is 1 and does not affect Cv.

Step 9: Calculate the Final CvStep 9: Calculate the Final CvUsing the value of FP, calculate the required Cv from Equation 3.1.

Step 10: Calculate Valve Exit VelocityStep 10: Calculate Valve Exit VelocityThe following Equation is used to calculate entrance or exit velocities for liquids:


Step 11: Recalculate the Cv if Body Size ChangedStep 11: Recalculate the Cv if Body Size Changed

After calculating the exit velocity, compare that number to the acceptable velocity for that application. It may be necessary to go to a larger valve size.

Recalculate Cv if the FP has been changed due to selection of a larger body size.


First identify if the valve will be used for on/off or throttling service. Using the Cv tables in Section 4, select the appropriate trim number for the calculated Cv and body size selected. The trim number and flow characteristic (Section 9) may be affected by how the valve will be throttled. When cavitation is indicated, refer to Section 14 to evaluate

Step 11: Select Trim NumberStep 11: Select Trim Number

the valve will be throttled. When cavitation is indicated, refer to Section 14 to evaluate special trims for cavitation protection.

Sizing Valve using Cv for LiquidSizing Valve using Cv for LiquidExample1; p1


Example1; p2


Example2; p2

Flashing Liquids Velocity CalculationsFlashing Liquids Velocity CalculationsWhen the valve outlet pressure is lower than or equal to the saturation pressure for the fluid temperature, part of the fluid flashes into vapor. When flashing existsWhen flashing exists, the following calculations must be used to determine velocity. Flashing requires special trim designs and/or hardened materials.Flashing velocity greater than 500 ft/sec requires special body designs.

If flow rate is in lb/hr:

if the flow rate is given in gpm, the following Equation can be used:

Flashing Liquids Velocity CalculationsFlashing Liquids Velocity Calculations

The % flash (x) can be calculated as follows:

Calculating Percentage FlashCalculating Percentage Flash

For water, the enthalpies (hf1, hf2 and hfg2) and specific volumes (vf2 and vg2) can be found in the saturation temperature and pressure tables of any set of steam tables.


Example1; p1

Assume the same conditions exist as in Example1, except that the temperature is 350F rather than 250 F. By referring to the saturated steam temperature tables, you find that the saturation pressure of water saturation pressure of water at at 350350F F is is 134134..5 5 psiapsia, which is greater than the outlet pressure of 105 psia (90 psia). Therefore, the fluid is flashing. Since a portion of the liquid is flashing, Equations 3.9 and 3.10 must be used. x (% flashed) can be determined by using Equation3.10:Equations 3.9 and 3.10 must be used. x (% flashed) can be determined by using Equation3.10:


Example1; p2

Therefore, the velocity in a 3-inch valve can be determined by using Equation 3.9:

Flashing velocity is less than 500 ft/sec, which is acceptable for Mark One bodies. Hardened trim and CavControl should also be considered.

Calculating Cv for GasCalculating Cv for Gas

Because of compressibility, gases and vapors expand as the pressure drops at the vena contracta, decreasing their specific weight. To account for the change in specific weight, an expansion factor, Y, is introduced into the valve sizing formula. The form of the equation used is one of the following, depending on the process variables available:

Calculating Cv for GasCalculating Cv for GasThe following steps should be used to compute the correct Cv, body size and trim number:

Step 1: Select the Appropriate EquationStep 1: Select the Appropriate EquationBased on the information available, just select one of the four equations to calculate Cvfrom flow rate: 3.11, 3.12, 3.13 or 3.14. the calculation will be done later(step 5).

Step 2: Check for Choked FlowStep 2: Check for Choked FlowStep 2: Check for Choked FlowStep 2: Check for Choked FlowDetermine the terminal pressure drop ratio, xT, for that particular valve by referring to Table 3-V. Next, determine the ratio of specific heats factor, Fk, by using the Equation below:


Calculate the ratio of actual pressure drop to absolute inlet pressure, xabsolute inlet pressure, x, by using Equation 3.16:

Choked flow occurs when x reaches the value of FkxT. Therefore, if x is less than FkxT, the flow is not choked. If x is greater, the flow is choked. If flow is choked, then If flow is choked, then FFkkxxTT should be should be used in place of x (whenever it applies)used in place of x (whenever it applies)

Calculating Cv for GasCalculating Cv for GasStep 3: Calculate the Expansion FactorStep 3: Calculate the Expansion Factor

The expansion factor, Y, may be expressed as: * (If the flow is choked, use FkxT for x)

Step 4: Determine the Compressibility FactorStep 4: Determine the Compressibility Factor

To obtain the compressibility factor, Z, first calculate the reduced pressure, Pr, and the reduced temperature, Tr, Using the factors Pr and T, find Z in Figures 3-4 or 3-5.

Step 4: Determine the Compressibility FactorStep 4: Determine the Compressibility Factor

Sizing Valve using Cv for GasSizing Valve using Cv for Gas

Step 5: Calculate CvStep 5: Calculate CvUsing the above calculations, use one of the four gas sizing Equations to determine Cv :(assuming Fp is 1)

Step 6: Select Approximate Body Size Based on CvStep 6: Select Approximate Body Size Based on CvStep 6: Select Approximate Body Size Based on CvStep 6: Select Approximate Body Size Based on CvFrom the Cv tables of valves, select the smallest body size that will handle the calculated Cv.

Step 7: Calculate Piping Geometry FactorStep 7: Calculate Piping Geometry FactorIf the pipe size is not given, use the approximate body size (from step 6) to choose the corresponding pipe size. The pipe size is used to calculate the piping geometry factor, Fp, which can be determined by Tables 3-III or 3-IV. If the pipe diameter is the same as the valve size, Fp is 1 and is not a factor.


Step 8: Calculate the Final CvStep 8: Calculate the Final CvWith the calculation of the Fp, figure out the final Cv.

Step 9: Calculate Valve Exit Mach NumberStep 9: Calculate Valve Exit Mach NumberStep 9: Calculate Valve Exit Mach NumberStep 9: Calculate Valve Exit Mach Number

Equations 3.20, 3.21, 3.22 or 3.23 are used to calculate entrance or exit velocities (in terms of the approximate Mach number).Use Equations 3.20 or 3.21 for gases, Equation 3.22 for air and Equation 3.23 for steam.Use downstream temperature if it is known, otherwise use upstream temperature as an approximation.

Sizing Valve using Cv for GasSizing Valve using Cv for GasAfter calculating the exit velocity, compare that number to the acceptable velocity for that application. Select a larger size valve if necessary. Refer to section 13 to predict noise level.

Step 10: Recalculate Cv if Body Size ChangedStep 10: Recalculate Cv if Body Size Changed

CautionCaution: : Noise Noise levels in excess of levels in excess of 110 110 dBAdBA may cause vibration in valves/piping resulting in may cause vibration in valves/piping resulting in equipment damage.equipment damage.

Step 10: Recalculate Cv if Body Size ChangedStep 10: Recalculate Cv if Body Size ChangedRecalculate Cv if Fp has changed due to the selection of a larger body size.

Step 11: Select Trim NumberStep 11: Select Trim NumberIdentify if the valve is for on/off or throttling service. Using the Cv tables in Section 4, select the appropriate trim number for the calculated Cv and body size selected. The trim number and flow characteristic (Section 9) may be affected by how the valve is throttled.

After selecting trim number, calculate the mach number for trim(bore) size for not being larger than 1.0. if mach number is greater then select larger trim; if the trim size is maximum, you should choose larger body size.


* Step 12: Calculate Valve Trim Mach Number* Step 12: Calculate Valve Trim Mach NumberAfter selecting trim number, calculate the mach number for trim(bore) size for not being larger than 1.0. if mach number is greater then select larger trim; if the trim size is maximum, you should choose larger body size. Note: it is possible that if you select small trim size in large body size while mach number is Note: it is possible that if you select small trim size in large body size while mach number is high, the velocity of gas reaches over the sonic velocity which it is not possible! So you should consider the mach number in trim not to be mare than 1.0 .

Sizing Valve using Cv for GasSizing Valve using Cv for GasExample1; p1


Step 12: by considering the mach number during pass of flow from trim it is clear that if the exit velocity is close to 0.5 mach, the square of body size to bore size should not be more than 0.5 (i.e. the trim size should be more than 4.6) . Hence the selected trim no. is not suitable and we have to select trim size 5.0 .not suitable and we have to select trim size 5.0 .By selecting this trim size, Cv will be large no. which results in lower controllibility.

Noise PredictionNoise PredictionControl valve noise is generated by turbulence created in the valve and radiated to the surroundings by the downstream piping system. Major sources of control valve noise are mechanical vibration of the valve components, and hydrodynamic and aerodynamic fluidnoise.Noise produced by mechanical vibration is usually well below 100 dBA and is described as a mechanical rattling.Aerodynamic noise levels can be above 100 dBA and reach as high as 150dBA in certain Aerodynamic noise levels can be above 100 dBA and reach as high as 150dBA in certain services.

Noise Sources :Noise Sources :

Noise PredictionNoise PredictionThe predicted noise value can be considered accurate to + 5 dBA when the outlet velocity is less than sonic and is correct for single throttling point trims only.NOTE: any level below 70 dBA usually does not require low noise trim or other accessories.

For calculating noise level, these data should be available:oo RequiredRequired(not installed) valve sizing coefficient, Cv .o Upstream pressure, psia, P1 .o Downstream pressure, psia, P2 .

At the next Slides, the method of calculating(predicting) noise level will be presented. After Calculating Noise Level, add below quantities if the corresponding condition is ready:1. For a valve installed near a reflective surface (a hard floor or wall), add 3 dBA.2. For a valve installed near two reflective surfaces (a hard floor and wall), add 6 dBA.3. If the valve is near three reflective surfaces (two hard walls and hard floor), add 9 dBA.4. A valve installed in a small room with all reflective walls, floor, and ceiling can easily

produce noise levels 30 or 40 dBA.

o Downstream pressure, psia, P2 .o Flowing temperature of fluid, T .o Flowing fluid, Q .o Pipe size and schedule D, sch,

Hydrodynamic Noise PredictionHydrodynamic Noise Prediction

1- Hydrodynamic Noise Prediction1- Hydrodynamic Noise Prediction

To obtain DPs and Rs, it is first necessary to calculate the pressure drop ratio, DPf, where:

The total sound pressure level is easily found by finding DPs, Cs, Rs, Ks, and Ds from Figures 13-2 through 13-5 and Tables 13-I and 13-II and then substituting them into the noise equation.

Hydrodynamic Noise PredictionHydrodynamic Noise PredictionIf DPf calculated is greater than 1, flashing is occurring in the valve and these formulas do not apply to flashing service.


Example1; p1


Example1; p2

Aerodynamic Noise PredictionAerodynamic Noise Prediction

2- Aerodynamic Noise Prediction2- Aerodynamic Noise Prediction

The total sound pressure level is easily found by finding Vs, Ps, Es, Ts, Gs, As and Ds from Figures 13-6 through 13-9 and Tables 13-I and 13-III and 13-IV then substituting them into the noise equation.


Example1; p1


Example1; p2

Noise AttenuationNoise Attenuation

1. CHANGING THE PROCESS FLOW CONDITIONS

2. CHANGING THE LOCATION OF THE VALVE

3. CHANGING THE STYLE OF THE VALVE BODY

4. CHANGING (Reversing) THE INSTALLED FLOW DIRECTION OF THE VALVE BODY

5. USE MULTIPLE VALVES OR DOWNSTREAM RESTRICTION DEVICES5. USE MULTIPLE VALVES OR DOWNSTREAM RESTRICTION DEVICES

6. USE OF HARDENED TRIM

7. USE OF THICKER PIPING WALL SCHEDULE

8. USE OF DOWNSTREAM NOISE SUPPRESSION(plate, diffuser, )

9. INCREASE FRICTIONAL LOSSES

10. USE OF EAR PROTECTION OR INSULATION

11. Use Of Special Designs or Equipment

Flow CharacteristicsFlow Characteristics

Inherent flow characteristicVs.

Installed flow characteristics

When a constant pressure drop is maintained across the valve, the characteristic of the valve alone controls the flow; this characteristic is referred to as the inherent flow characteristic. Installed characteristics include both the valve and pipeline effects. The difference can best be understood by examining an entire system.


Equal PercentageEqual 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.

Quick ReviewQuick Review

when the overall system pressure drop is large relative to that across the valve.

LinearAn inherently linear characteristic produces equal changes in flow per unit of valve stroke regardless of plug position. Linear plugs are used on those systems where the valve pressure drop is a major portion of the total system pressure drop.

Quick OpenQuick open plugs are used for on-off applications designed to produce maximum flow quickly.


Example1; p1

A centrifugal pump supplies water to a system in which a control valve is used to maintain the downstream pressure at 80 psig. The pump characteristics are shown below:

Effect of other equipmentsEffect of other equipments


The maximum flow required is 200 gpm. at which the pump discharge pressure (P1) is 100 psig. Piping losses are negligible. Using the ISA liquid sizing formula, the flow coefficient, or Cv, can be determined :

Example1; p2

the flow coefficient, or Cv, can be determined :

To determine the plug characteristic which should be specified, let us analyze the installed flow characteristic of equal percentage and linear trim in this valve.


Example1; p3


Example1; p4


Example2; p1

The previous example was idealized in that the downstream pressure was held constant and the pressure drop variation was due to the pump characteristic alone. Now consider a more realistic installation where the delivered pressure must be held

Effect of other equipmentsEffect of other equipments

consider a more realistic installation where the delivered pressure must be held constant after passing Through the valve and with some line restriction, R, in series with the valve.


To find the installed characteristics of equal percent and linear trim in a suitably sized valve, a pressure drop distribution must be chosen. A suitable choice would be 4 psi across the valve at a flow of 200 gpm. The control valve can then be sized for the

Example2; p2

across the valve at a flow of 200 gpm. The control valve can then be sized for the maximum required Cv :

Since the pressure drop across the restriction will vary with flow in accordance with the square root law ( Q = R sqrt(P) ) the available pressure drop across thevalve at various flowing quantities can be determined, keeping in mind the pump characteristic. This is shown in Table 9-II.


Example2; p3

Example2; p4


Actuator SizingActuator Sizing

In throttling services three questions must be answered:

1. Will the actuator handle the throttling differential pressure?2. Will the actuator provide sufficient thrust to overcome application pressures to open or close the valve, and generate enough seat loading for tight shutoff with the given air supply pressure?

Linear Actuator Sizing, Pressure-balanced TrimLinear Actuator Sizing, Pressure-balanced Trim

supply pressure?3. Will the spring fail the valve in the proper direction?

With on/off services, only questions 2 and 3 must be answered.

Actuator SizingActuator SizingLinear Actuator Sizing, Pressure-balanced TrimLinear Actuator Sizing, Pressure-balanced Trim

The equations in this section use the following variables:


When an actuators stroke length exceeds the longest stroke length shown for that size actuator in Table 16-VI, the actuator will not have a spring. For actuators without a spring, SE = SR = 0. AR is not used when sizing actuators for valves with standard trim or when sizing actuators for Class 150 through 600 MegaStream valves.


Determine the maximum allowable throttling pressure drop (Pa) that the selected actuator can handle by using equations (16.7) and (16.8):

Step 1: Determine Actuators Maximum Allowable Throttling Pressure Drop

Step 1: Determine Actuators Maximum Allowable Throttling Pressure Drop


Compare the maximum allowable throttling pressure drop to the actual pressure drop. If the actual throttling drop is less than Pa, the selected actuator is sufficient.However, if the actual throttling drop is greater than Pa, the next larger actuator size should be chosen and the above calculation should be repeated.


Calculate the actuator cylinder areas required by using the applicable group of equations in the following tables. Compare the calculated areas to the corresponding areas for the actuator size selected in Step 1. Actuator areas are listed in Table 16-III. If the calculated

Step 2: Determine Actuators Size For Actuation ThrustStep 2: Determine Actuators Size For Actuation Thrust

actuator size selected in Step 1. Actuator areas are listed in Table 16-III. If the calculated areas are less than or equal to the corresponding areas for the selected actuator, the actuator size is adequate. If the calculated areas are larger, an actuator with cylinderareas larger than the calculated areas must be selected.When determining the required actuator size, various service conditions should be considered. For each sizing equation, the conditions to be considered for that equation are listed with the equation. Each equation should be evaluated for each listed condition that will actually occur. The condition numbers refer to the following list.


Service Conditions to be considered:1. P1 and P2 for flow conditions. If more than one flow condition will occur, each should

be evaluated.2. P1 and P2 at shutoff. If more than one set of pressures will occur during shutoff, each set of pressures should be evaluated. The possibility of P2 dropping to atmospheric set of pressures should be evaluated. The possibility of P2 dropping to atmospheric pressure (0 psig) should be considered. Pressures used to bench test thevalve should also be considered.3. P1 and P2 equal to the maximum value of P. This condition may occur if the pipeline is pressurized and the pipe downstream from the valve is blocked. For this condition, set RSL = 0 in the sizing equations.4. P1 and P2 equal to 0. This condition will occur if the pipeline is depressurized. This condition will also occur during bench testing of the valve. For this condition, set RSL= 0 in the sizing equations.


NOTES:1. On valves larger than 24-inch, the weight of the plug may need to be accounted for; contact factory.

Step 2: Determine Actuators Size For Actuation ThrustStep 2: Determine Actuators Size For Actuation Thrust

contact factory.2. A negative number calculated for AL or AU indicates that the smallest available actuator will work for the condition being evaluated.3. For valves with a trim number smaller than the plug stem diameter, AS-Astem will be a negative number. In this case, the negative sign must be retained during the sizing calculations.


If it will be necessary for the valve to stroke open or closed upon loss of air supply pressure, the fail-safe spring must be sized. The required spring force is calculated by using the applicable equations in the following tables. Each sizing equation should be evaluated for the listed conditions that will actually occur. The condition numbers refer to the service conditions listed in step 2.

Step 3: Determine Spring SizeStep 3: Determine Spring Size

the service conditions listed in step 2.After the required spring force is calculated, it must be compared to the standard spring force for the actuator selected in steps 1 and 2. This spring force is listed in Table 16-VI. If the required spring force is less than the standard spring force of the selected actuator, a standard spring will be sufficient. If the required spring force is greater than that of a standard spring force, compare the required spring with the dual (or heavy-duty) spring force for the same size actuator (see Table 16-VI). If the dual spring force is larger than the required spring force, the dual spring should be used. If the dual spring force is not large enough, a volume tank or larger actuator will be required. Section 19 contains volume tank sizing information. If the spring or actuator size selected to provide sufficient spring force is different from that used during step2, the calculations of step 2 must be verified using the New spring or actuator information.

MaterialMaterial

Emergency Conditions

HandWheel:To bypass Control Valve

By pass Valves By pass Valves

PositionersPositionersPositioning is based on a balance of two forces:one proportional to the instrument signal and the other proportional to the stem position.

PositionersPositioners


I/P


Linearity Independent

Hysteresis - Maximum position error for the same value of input when approached from opposite directions.Dead Band - Maximum change in input required to cause a reversal in valve stem movement.

Performance DataPerformance Data

movement.

Response Level - Maximum change in input required to cause a change in valve stem position in one direction.Resolution - Smallest possible change in valve stem position.

Repeatability - Maximum variation in position for the same value of input when approached from same direction.


Steady State Air Consumption

Supply Pressure Effect - Position change for 10 psi supply pressure change.

Open-loop Gain - Ratio of output pressure unbalance to instrument pressure change with locked stem @60 psi.

Performance DataPerformance Data

change with locked stem @60 psi.

Maximum Flow Capacity

Frequency Response

Stroking Speed : Closed to open ; Open to closed

PositionerPositioners are instruments that help improve control by accuratelypositioning a control valve actuator in response to a control signal

Positioners receive an input signal either pneumatically orelectronically and provide output power to an actuatorelectronically and provide output power to an actuator

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Increase control system resolution i.e)fine resolution.

Allow use of characteristic cams.

Minimize packing friction effects.

Allow Split Ranging.

Overcome seating friction in rotary valves.

Facilitate operation when the higher number in the bench-set range is greater than 1ksc Permit use of piston actuators.

Allow distance between controller and control valve using the advantage of

Reasons To use Positioners

Allow distance between controller and control valve using the advantage of

4-20ma signal.

POSITIONERSA device attached to an actuator that receives An electronic or pneumatic signals fromthe controller And compares this signal to the actuators position.

3-way positioners

Send and exhaust air to only one side of a Single acting actuator that is opposed by a range Spring.

4-way positioners

Send and exhaust air to both sides of an actuator which is required for double acting actuators.

Valve Positioners Pneumatic valve positioners are the most commonly used valve accessories. A valve positioner is a device which will accurately position a control valve in

accordance with the pneumatic control signal.

The control signal is routed to the positioner where comparison of the valve position (actual) to the control signal (desired) is used to develop an output pneumatic signal which operates the valve actuator.

The positioner compares the control signal (the requested valve position)with the actual valve position through the mechanical feedback linkage.with the actual valve position through the mechanical feedback linkage.

If the valve position is incorrect, the positioner will either load or exhaust air from the valve actuator until the correct valve position is obtained.

A positioner requires both a control signal and an instrument supply air for normal operation.

Most positioners come equipped with three gauges to indicate supply air pressure, control signal pressure and actuator diaphragm signal (output) air pressure.

186ERT 212/3 Process Engineering Skills

Valve Positioners

Valve Positioners Advantages of the valve positioner include:1) Minimizing the effect of friction, hysteresis and

deadband on the valve stem. With a high pressure system, tighter valve stem packing is needed to prevent leakage and a high frictional force is generated. With a positioner valve stem movements generated. With a positioner valve stem movements of as little as 25 m are possible.

2) Enables signal range change. A positioner can amplify the incoming control signal when a greater actuating force is needed. A 20-100 kPa control signal can be amplified to 40-200 kPa before being applied to the actuator.


Valve Positioners3) Allows signal reversal. A positioner can operate in

either direct or reverse acting mode. For example, in reverse acting mode, an increase in control signal pressure causes a decrease in positioner output air pressure. For example, in reverse mode, a 100 - 20 kPa actuator signal would correspond to a 20 - 100 kPa control signal from the I/P transducer.kPa control signal from the I/P transducer.

4) Increases the speed of response of the actuator. The speed of response of the valve actuator depends on:(a) the actuator volume, and(b) the flow rate of the control signal air.


Valve PositionersAllows valve flow characteristic to be changed.

Most valve positioners employ a rotating cam in the feedback system. This cam can be changed to simulate different valve flow characteristics. A linear globe valve can be used to respond in an equal globe valve can be used to respond in an equal percentage manner.

Allows split range operation. In a split range control loop, one controller is used to drive two control valves.


Valve Positioners

A positioner ensures that there is a linear relationship between the signal input pressure from the control system and the position of the control valve.

This means that for a given input signal, the valve will


This means that for a given input signal, the valve will always attempt to maintain the same position regardless of changes in valve differential pressure, stem friction, diaphragm hysteresis and so on.

Valve Positioners A positioner may be used as a signal amplifier or booster. It accepts a low pressure air control signal and, by using

its own higher pressure input, multiplies this to provide a higher pressure output air signal to the actuator diaphragm, if required, to ensure that the valve reaches


diaphragm, if required, to ensure that the valve reaches the desired position.

Some positioners incorporate an electropneumatic converter so that an electrical input (typically 4 - 20 mA) can be used to control a pneumatic valve.

Some positioners can also act as basic controllers, accepting input from sensors.

Positioner Working

Some of the mechanisms

i. Force Balance Positionersi. Force Balance Positioners

ii. Motion balance Positioners

iii. Electronic Positioners

194

Force Balance Positioners

195

Motion balance Positioners; 1

196

Motion balance Positioners; 2

197

Electronic Positioners

Sensor to detect valve position

Micro-Processor Error = Position - Signal

198

DeZurik OperationRotary Control Valves

October 2005

Actuators

Actuator

The purpose of the valve actuator is to accurately locate the valve plug in a position dictated by the control signal

202

Types of Actuators

Pneumatic Valve Actuators: Adjust the position of the valve by converting air pressure into rotary motion or linear motion. Piston and Diaphragm Actuators are examples of Pneumatic Actuators

Electric Valve Actuators: An electric actuator is powered by Electric Valve Actuators: An electric actuator is powered by motor that converts electrical energy to mechanical torque

Hydraulic actuator: Consists of a cylinder or fluid motor that uses hydraulic power to facilitate mechanical operation. The mechanical motion gives an output in terms of linear, rotary or oscillatory motion

203

Types of Actuators Operation

Multi-turn actuatorgives torque for at least one full

Part-turn actuatorgives torque for less than one fullgives torque for less than one full

Linear actuatoropens and closes valves that can be operated via linear force

204

Electric Actuators

Electric actuators use an electric motor with voltage requirements in the following range: 230 Vac, 110 Vac, 24 Vac and 24 Vdc.and 24 Vdc.

There are two types of electrical actuator 1. VMD (Valve Motor Drive)

2. Modulating.

VMD ( Valve Motor Drive )

This basic version of theelectric actuator has threestates:

I. Driving the valve open. II. Driving the valve closed. III. No movement

The controller positions thevalve by driving the valveopen or closed for a certaintime, to ensure that itreaches the desiredposition, Valve positionfeedback may be used withsome controllers.

Modulating In order to position the control valve in

response to the system requirements a modulating actuator can be used. These units may have higher rated motors (Step Motor) and may have built-in electronics.

A positioning circuit may be included in the modulating actuator which accepts an the modulating actuator which accepts an analogue control signal (typically 0-10 V or 4-20 mA). The actuator then interprets this control signal, as the valve position between the limit switches.

To achieve this, the actuator has a position sensor (usually a potentiometer), which feeds the actual valve position back to the positioning circuit. In this way the actuator can be positioned along its stroke in proportion to the control signal.

Pneumatic Actuators

They are basically of two types

1. Piston actuator

2. Diaphragm actuator

Piston Actuator

They can withstand higher input pressures. Can offer small cylinder volumes. They are generally used where the stroke of a diaphragm

actuator would be too short or the thrust is too small.

Diaphragm Actuators They have compressed air applied to a flexible membrane called the diaphragm

They are single acting They are single acting i.e. air is supplied from single side of the diaphragm

Classification of control valve on the basis of actuator action

Classification of control valve on the basis of valve action

Failure mode

Actuator action

Valve bodyaction

Control valveaction

Failure mode Valve Color

Direct Direct Air to close FAIL OPEN Green

Direct Reverse Air to open FAIL CLOSE Red

Reverse Direct Air to open FAIL CLOSE Red

Reverse Reverse Air to close FAIL OPEN Green

Diaphragm Actuators

Air To Close Valve

P FLOW =P1-P2 P SHUTOFF =P1-PLOAD1. USUALLY P SHUTOFF > P FLOW 2. Methods to increase P SHUTOFF

1. Increase PLOAD2. Increase ADiaphragm3. Reduce FSpring3. Reduce FSpring4. Reduce Packing

Fup = FdownP1 * APort+ FSpring+ Packing=PLOAD* ADiaphragm+

P2 * APlug

Note: Inlet Pressure tends to push open plug

Air To Open Valve

P FLOW =P1-P2 P SHUTOFF =P1-PLOAD1. USUALLY P SHUTOFF > P FLOW 2. Methods to increase P SHUTOFF

1. Decrease PLOAD2. Increase ADiaphragm3. Increase FSpring4. Reduce Packing 4. Reduce Packing

Fup = FdownP1 * APort+PLOAD* ADiaphragm + Packing= FSpring +P2 * APlug

Note: Inlet Pressure tends to push open plug

Accessories Positioners ; pneumatic input 3-15 psi

Electro-pneumatic 4-20 ma (HART, Fieldbus, Profibus)

Limit switches Position feedback

a standard, pneumatic positioners use 3-15, 3-9, 9-15, 3-7, 7-11 and 11-15 psi input signals; electro- 3-7, 7-11 and 11-15 psi input signals; electro-

pneumatic positioners use 4-20, 4-12, 12-20 and 10-50 mA input signals. Other non-standard signals, such as 6-30 psi, are also available.

Valve Sizing

What is Valve Sizing?

Flasing; Cavitation; Noise

October 2005

Double Single Bal. Split EccentricKey Features Seat Seat Trim Angle Y Pattern 3- Way Body Ball Butterfly PlugCapacity 1 1 1.2 1 to 2 1.5 0.7 1 3 3.2 1.3Shut Off % Rated Cv 0.5

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ACTUATORS

ACTUATORS

A device mounted on a valve that in response to a Signal, automatically moves the valve tothe required position using an outside power source. The addition Of an actuator to athrottling valve is called a

Pneumatic Electronic motor Electro hydraulic

Diaphragm Piston

PNEUMATIC ACTUATORSAir is relatively inexpensive, 90% of the Industries employ these actuators.

HYDRAULIC ACTUATORSExceptional stiffness & high thrust are required, Fast stroking speeds. Hydraulic fluid above and below a piston to position the valve.

ELECTROHYDRAULIC ACTUATOR

Self contained hydraulic system, electrical Signal feeds to an internal pumps, which uses hydraulic fluid from a reservoir to feed hydraulic fluid above or below the piston.

Pros Exceptionally stiff because of the incompressibility of liquids.

Cons Expensive and Bulky

ROTARY ACTUATOR

Valve Actuators

The diameter of the diaphragm plate determines the force that will be applied to the actuator stem.

For example, if the maximum input signal pressure is 100 kPa and the plate diameter is 30 cm, then:kPa and the plate diameter is 30 cm, then:

Force applied to stem = Pressure x Plate Area= 100 kPa x 3.14 x (0. 15)2 m2

= 7.07 KN (1590 lb)


CONTROL VALVES

CONVALCONVAL

EmersonISA

EmersonISAISA

Forms()Selection criteria

ISAForms()

Selection criteria