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Tubing Effect on Control Valve

02/23/2011 5:28 AM

Hi,

I am wondering is there any effect if I am using different size of tubing say 3/4 inch to 1/4 inch on the performance of the control valve?

The tubing is from the air regulator to the control valve diapghram?

Can it cause the control valve to be "hunting"?

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#1

Re: tubing effect on control valve

02/23/2011 6:28 AM

Is it a motorized valve? What is it controlling (pressure, flow, actuator position etc)? Is it in a feedback loop? None of the above?

More information, please.

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Active Contributor

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#4
In reply to #1

Re: tubing effect on control valve

02/23/2011 9:28 PM

The control valve is used for Fuel Oil Pressure Control.

The positioner used is smart positioner ABB TZIDC.

Originally using copper tubing which is sized 3/8 inch.

we are using now 1/4 inch.

TQ

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#2

Re: Tubing Effect on Control Valve

02/23/2011 8:58 AM

On its own, the line size cannot cause the valve to "hunt".

If the air signal is driven off an air pressure regulator as stated, then the valve will not "hunt", as there is no controller action that can alter the air pressure signal in response to any measured process value other than the human that tweaks the knob on the regulator.

In such circumstances, it doesn't much matter about the size of the air line for short-ish lengths, though increasing the size either in length or in diameter will give the valve a reduction in speed for a change to a new pressure setpoint. The difference between 1/4in and 3/4in is so slight over, say 20m, that it can be disregarded for all practical purposes.

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#3

Re: Tubing Effect on Control Valve

02/23/2011 10:08 AM

I concur with PWSlack with regards to "reduction in speed" if I am correct in my assumption to mean valve reaction time.

If this valve is to be used on a high speed process line, conveyor, et. al., I would stick to what the manufacturer spec'd as far as tube size throughout the tube run (pre and post regulator).

I saw this happen on reject chutes for a high speed Smirnoff Ice bottling line that was being commissioned. As a reject bottle would pass the detector, the rejected bottle would pass the reject chute but the next two bottles would get bumped out the reject chute.

The mechanical contractor pi$$ed away a whole day trying to adjust the regulators "just right" until the client noticed the two different sized tubes and demanded that the regulator to valve tubes be changed to match the pre-regulator line as specified.

Within 5 minutes of the mismatched tubes being changed, low and behold, the reject circuits work as specified.

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#5
In reply to #3

Re: Tubing Effect on Control Valve

02/23/2011 10:05 PM

Indeed! If anything, the downstream tubing should even be larger.

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#6

Re: Tubing Effect on Control Valve

02/23/2011 10:58 PM

Depends on the length of tube, will cause more or less restriction and it will have an effect on your system's speed or time constant.

Try cutting tube length in half, or doubling it to see the effect.

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#7

Re: Tubing Effect on Control Valve

02/24/2011 2:27 AM

If you've found some "hunting" in the valve performance, the air restriction caused by the smaller tubing size can be the problem.

Also, the reduction in tubing size surely would change the stroke time of the valve if it's use sometimes as on-off. As you mention a positioner, it's used (also?) as modulating. The reduction in air flowrate can affect also some other parameters as hysteresis and response time.

All this "possible" effects will be more pronounced as the length of tubing increases.

Kind regards

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#8

Re: Tubing Effect on Control Valve

02/24/2011 10:04 AM

If you change the size of the tubing from the air regulator to the control valve you may have the following problems:

1.- The speed of action of the valve is slower, since the actual air volume to the valve will be slower to fill the volume.

2.- The possibility of having the valve react due to the process upsets, since less air volume is reaching the valve.

In real life the speed of action and maintaining the position required are directly referenced to the air pressure and volume reaching the valve.

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