Equal Percentage Control Valve Calibration

Control valves cannot be calibrated. They can only be set up. A typical set-up would be:

• 4mA: end of range
• 8mA: 1/4 or 3/4 of range, depending on whether it is set-up direct acting or reverse acting
• 12mA: half range
• 16mA: 3/4 or 1/4 of range, depending on whether it is set-up direct acting or reverse acting
• 20mA: full range

though they could be set up, in principle, to whatever the user requires.

As PWSlack stated,

I like to add, a PID Control loop would be required. Proportional, Integral, Derivative

Indeed.

Selection of the correct valve characteristic allows the best compromise to be found for the P, I and D parameters in the controller, and therefore the optimum performance from the system. Good point.

Dear Mr Rajeshbalki ,

If you have gone through the curves of linear and eq% valves than this curve holds true for linear stem travel, because the trim is design within itself for eq% and lin flow rates

regards

find2am

if you can give me the stem movement percentage against standard 4,8,12,20mA will be helpful

There's no need. Just adjust the positioner as you did with linear valves. Adjust it for full closed at 4mA and full open at 20mA. The equal percentage valve will take care of the flow.

regards,

Vulcan

I thank you all for your immediate reply. So what i have concluded from all your answers is that the positioner will be having Linear Characteristic and the TRIM portion will take care of the characteristic of flow wether Linear / Equal Percentage. If am not wrong shall i ask you further, then what is the use of Equal Percentage CAM on positioner? If the positiner characteristic is set to equal percentage, how can be the stem movement? is it same as Linear or differed? Kindly assist me incase if am wrong on my concept. thanks and regards, bala.

If the positioner has a Linear/Equal Percentage cam, your control valve is, most probably, a linear control valve. The cam allows you to convert the valve into an equal percentage valve without changing the valve plug.

If that is what you have, then, yes, you'll want to know the linear position of your valve at different input signals. Here's a simple way of making the adjustment.

Set your cam for equal percentage and inject 4mA and adjust your positioner for full close (again, assuming air-to-open). Next inject 20mA and adjust your positioner for full open. Then inject 17.33mA and check (just check, no adjustment) if your positioner is at 50%. If it is, you're done.

Good luck.

regards,

Vulcan

I agree with the comments by Vulcan.

Zia-ur-Rehman

Dear Vulcan, Thank you very much for your valuable reply which has cleared me the concept. can you plz give the details for 25% and 75% corresponding mA which will be clear me 100% to cross check the valve operation. Thanks & Regards, Bala

it is important to check specs of the valve where it do mention about the type of trim and cam position if you come to know the specs than go for calibration you can also check the tag plate of the control valve .

regards

find 2 am

14mA = 25%

19mA = 75%

Hi Mr.Vulcan, Thanks for your inputs. I will be more effective with your inputs.

This curves will help you in future for calibration of the valves

Hi,

-What happens if the control valve is %EP, but we install a linear cam positioner or vice versa? what happens to the process?

-during normal process if DCS sends signal of say 60% to an equal percentage valve , what will be the local reading?will it be at 60% opening or different?

>-during normal process if DCS sends signal of say 60% to an equal percentage valve , what will be the local reading?will it be at 60% opening or different? The valve E/P or I/P or positioner will attempt to position the valve stem at 60% of its travel. The flow rate is determined by the internal trim and the process conditions. A positioner uses valve stem position (rotary or linear) as its feedback; some value between 0 & 100%. The positioner has no clue as to what the flow rate is, or whether the trim is linear or equal percentage, or custom.

Hi,

But when we calibrate the equal percentage vlv, we dont get linear response say 12mA-50% opening valve.

Usually operators in my plant will compare the control output (CO) of the controller and compare it with the positioner's feedback. And they complain about the feedback and CO difference. So we have to have to tell them that the equal percentage vlv works differently. Am I right?

I made an assumption of a linear positioner response, and the situation posed by your site operators tells me that they are expecting a linear positioner response as well. However, that might very well not be the case.

When you 'calibrate', you are presumably checking the valve stem position in response to an input signal from the controller.

Any positioner has some form of characterized response to an input signal. That characterized response is either linear, equal percentage (=%), quick opening or custom.

Pneumatic and electromechanical positioners have a mechanical cam installed that determines its stem positioning response to the control signal.

Smart positioners have a stem positioning response 'setting' that determine the stem position in relation to the control signal.

If the positioner is not setup for a linear response to the control signal, then the valve stem goes to some position other than the control signal percentage value, because that's what it is supposed to do. That's what characterized response means. That can happen if the non-smart positioner's cam is not a linear cam or if the smart positioner's response setting is not 'linear'.

If the valve stem position does not match the control signal value (in percentage), then you need to check to see how the positioner is setup.

The pneumatic or electromechanical positioner's response is whatever the installed cam is.

The smart positioner's response is whatever its output response setting is. The name used for 'output response' varies from manufacturer to manufacturer.

>we have to have to tell them that the equal percentage valve works differently.

Probably not.

Checking the valve stem position in relation to the output signal only involves the POSITIONER, not the valve.

The valve is the chunk of metal in the pipeline that drops pressure to regulate flow.

When you say 'equal percentage valve', those terms refer to a valve's internal trim which also has a flow characteristic: linear, =%, quick opening or custom.

This trim flow characterization is typically hidden or disguised in the valve's model number, one of the digits in the long 19 digit part number.

Typically a valve with =% trim has a positioner that produces a linear response to the control signal. I suspect that this is what your control room operators are looking for: a linear response from the POSITIONER that positions the valve stem linearly with the controller's output.

You might well have a positioner set up as equal percentage output response that positions the valve stem at some percentage opening not anywhere close to the control signal value, but if that's how the positioner is setup, then it's doing what it is supposed to do.

The real question is, how is the positioner set up? Linear or =% ?
How should it be set up? Linear or =% ?

You might have to tell the control room guy that an equal percentage POSITIONER works differently (but nothing's been mentioned about what VALVE's trim characteristic is).

Thanks bud for ur answer. It helps me a lot to understand.

FYI, we have =% valve trim and we also installed a =% smart positioner (1:25).

Yes you are right, local means valve stem positions.