control valve functional testing - Swetha

Control Valve Function Testing

BySwetha Sundaram, P.Eng

BY: SWETHA SUNDARAM P.Eng, Instrumentation & Controls Engineer

Control Valve Function Testing

Why it is required?What are some of the things that are checked?Types of testing:Common Testing

1)Test procedure to verify valve travel.2) Setting limit switches on valves.2) Hydrostatic Testing.3) Valve Seat Leakage Test

Special Testing1) Capacity Test.2) Fugitive Emissions.


FUNCTIONAL TESTING

Functional Testing is done on finished products to verify a range of performance

parameters.

The control valve is completely assembled and fitted with all accessories

such as positioner, solenoid valve etc.

All accessories are tested to see if they work the way they are supposed to.


CONTROL VALVE – FUNCTIONAL TESTING

• Why is Functional Testing important for control valves?

• The control valve behavior has a major impact on the performance of the Control Loop.

• Functional testing is done to verify the proper performance of the control valve.


Valve AccessoriesSome of the valve accessories are:

• Actuator – a device used to move something. Used to open or close a valve.

• Taps in our homes are examples of valves.Anyvalve can be made into a control valve by

adding an actuator.


Valve Accessories contd

• Positioner – receives signal from controller & sends signal to actuator to move the valve.

• Limit Switches – provides confirmation signal to PLC or DCS.

Actuator

DVC Positioner


• Hand Wheel – used to manually open/close the valve.

• Solenoids – allows air into actuator when energized.

• Volume Boosters – supplies air to actuator when high stroking speed is required.

Valve Accessories contd


What are some of the things that are checked?

• The gauges on the positioner are checked to see if they are working properly.

• The calibration of the valve positioner is verified and adjusted as needed.


Functional test Procedure

• The valve, I/P positioner, current source generator, and instrument air supply are set up .

• Instrument air supply P is checked. It is set according to the I/P positioner requirements.

• 4 mA current signal is applied to the I/P positioner. The valve stem shall be in 0% travel. 20 mA current signal is applied to the I/P positioner; the valve stem shall be in 100% travel.

• If the valve stem travel indication didn’t show correctly 0% or 100% travel, then travel is calibrated using the HART communicator


• The five point stem travel is checked for hysteresis and linearity check on 0%, 25%, 50%, 75%, 100% and then 100%, 75%, 50%, 25% and 0% stem travel.

• The three point stem travel is checked for deadband check on 5%, 50%, and 95% stem travel.

•


Diagram showing typical test set-up. http://www.youtube.com/watch?v=m-b1Llp6ik0&feature=related


http://www.youtube.com/watch?v=m-b1Llp6ik0&feature=related

What are some of the things that are checked? contd

• All required stroking times are checked & valve rated travel is verified.

• http://www.youtube.com/watch?v=jzffBXYrsxo&feature=results_video&playnext=1&list=PL4572CC2C362FEE57

• http://www.youtube.com/watch?v=NDYR53Tdwdo

• It is checked to see if the valve fully opens & closes.


http://www.youtube.com/watch?v=jzffBXYrsxo&feature=results_video&playnext=1&list=PL4572CC2C362FEE57

http://www.youtube.com/watch?v=NDYR53Tdwdo

What are some of the things that are checked? contd

• If the control valve is equipped with a hand wheel, the valve is fully closed and opened using the hand wheel.

• Limit switches on control valves are checked to see if it is set properly for the open & close position.

• http://www.youtube.com/watch?v=SUI1U5I9bbc&feature=related


http://www.youtube.com/watch?v=SUI1U5I9bbc&feature=related

Hydrostatic Testing

• Hydro tests can be done on just the valve body or with the actuator assembled.

• This is a safety test to verify the mechanical integrity of the valve body.


• The valve is filled with a suitable liquid and is pressurized in accordance with the table shown below.


• The minimum duration of test pressure before the start of inspection is as per the following table.

Once the test is complete, the valve is inspected for any visible leaks or deformities.


Seat Leakage Test

Seat Leakage Test is performed to check

If valve will shut-off properly when it is

required to do so throughout the service life

when installed in field.


If proper shut-off is not met

- there is loss of fluid downstream of

the valve.

- Leakage results in trim damage to

both seating surfaces on the plug

and seat ring.


• ANSI specifies 6 classes for seat leakage.

Class I: Not required to shut-off at all.

Class II to IV: the allowable leakage

decreases from Class II to Class IV.

Class V: For metal seated valves to

meet class V sophisticated changes

to seating profile is reqd.

Class VI: Is for soft seats.


• Class V is tested @ actual service pressure. Other classes are tested at a lower pressure.

• Valve may pass the seat leakage test but may notmeet the real requirements in the field. This is true for all classes except Class V.

• The testing medium is either air or water.


If valve failed the seat leakage test it could be because

a) We don’t have good contact between seating

surface & plug. Mating surfaces have not been

machined properly.

b) Actuator & valve stem are not properly connected

and aligned.

c) Sufficient seat load required to bring the

plug & the seat surface together and to

hold them tightly under service condition has not

been met.


Capacity Testing

• Applies to custom designed valves only or for some special design cases.

• Flow Coefficient Cv of a valve is a relative measure of it’s efficiency at allowing fluid to flow.

• The valve flow coefficient takes into account all factors that affect flow including dimensions and flow direction changes.

• the flow coefficient Cv is the volume (in US gallons) of water at 60°F that will flow per minute through a valve with a pressure drop of 1 psi across the valve.


The Test Valve is isolated between two throttling valves in the above set-up so that the DP across the test valve can be carefully controlled.


The upstream & downstream throttling

valves are adjusted so that the DP is 1 PSI.

The flow is measured in USGPM.

The measured flow in USGPM is the Cv of

the valve.

Example: If @ 60 F & a DP of 1 PSI across

the valve, the flow rate is measured as 60

USGPM then the Cv is 60.


Fugitive Emission

• Tests for fugitive emissions are not part of the regular functional testing for valves.

• They are performed by the packing manufacturer so as to certify the packing meets the requirements.


Test to check fugitive emission

• Fugitive emission is the unanticipated emission of process fluid to the environment.

• This can cause significant damage to the environment.

• affect the health & safety of people.

• And lastly, can result in economic losses as well.

• http://www.youtube.com/watch?v=XLaz29jGwGc&feature=results_video&playnext=1&list=PL4572CC2C362FEE57


http://www.youtube.com/watch?v=XLaz29jGwGc&feature=results_video&playnext=1&list=PL4572CC2C362FEE57

Assignment

• Capacity testing is done on a 3” Fisher valve. The valve model is ED and the trim characteristic is equal %.

• The test conditions are listed below. The valve is wide open.

• Inlet T 60 F

• DP 1 PSI across the valve.

• Flow rate Q 136 USGPM

• What is the rated CV of the valve?


control valve functional testing - Swetha

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