HI friends;

I have a pressure control valve model is as under mallard company 5500-2j6-g73rs-3ae. and the control is60-030r1-1 Beaumont-Texas.

the problem is that the incoming pressure is 350psi. after regulate i set it on 150 psi. we required the flow 50 MSCFH. but when gas turbine starts it decrease the pressure and fluctuate between 130 to 170. i have no procedure of that valve how to increase its time to regulate the pressure rapidly. the main pressure still 340 no drop in that pressure.

please help me
if any one have its setting procedure plz send me a link.

thanking u i will remain

 

HI;

I suggest before learning how to tune your system, first you must study the characteristics of the variation of the process variable you are controlling. For instance:

1. What is the response time of your valve. Meaning from full close to full open what is the time to travel? Or what is the time response for your actuator to adjust from 130 psi to 170 psi?

2. What is the minimum response time of your controller? (usually if digital controllers this is fast enough)

3. What is the characteristics with respect to time of the fluctuations you are having during the gas turbine running? If you have a chart recorder then it would be easy for you to determine. Or what is average time for the fluctuations to reach from 130 psi to 150 psi and 150 psi to 170 psi ?

The logic behind those questions above, is that maybe your actuator cannot cope up with the frequency of the fluctuations just like a running race between a turtle and a monkey. YOur actuator is the turtle and the fluctuations is the monkey. Even if your controller is fast enough, you must have an actuator that can cope up with the fluctuations so you have a good control system. Even if its to a nearest response time, it can be compensated already by adjusting the PID. But not so slow otherwise the system would be haunting.

randy

 

I would add to this that trying to tune a pressure regulating valve for these kinds of flow-rates with no flow through the valve (which is what you seem to be saying you are doing), that doesn't usually result in very good pressure control, just as you have described.

The first question I would ask is: When did this problem start? After some maintenance outage? After the valve was removed for refurbishment and replaced? Has this been an ongoing problem since commissioning of the system? Is this a new valve that is replacing an older one? Has the gas fuel supply pressure changed that require the installation of a pressure reducing (regulating) valve?

Per the Mallard brochure for 5500 Valves and Actuators available on the World Wide Web at several sites, the actuator (the 73R portion of the valve part number) is a simple diaphragm-and-spring (reverse-acting), so downstream gas pressure is acting on one side of a diaphragm against a spring, driving an equal percentage trim valve. (It would have been nice had you provided us with this information.)

Once the gas fuel flow has started, have you tried to re-adjust the pressure regulator to maintain the desired setpoint?

Do the pressure fluctuations occur only during starting and acceleration, or when the unit reaches operating speed also?

A lot of times these pressure regulators (and their actuators) were not properly sized for low flow-rates experienced during starting and acceleration; they were only sized for flow-rates at rated load.

The second question I would ask is: Exactly how, and when (when there is flow through the valve or when there is no flow through the valve) are you adjusting the pressure regulator?

Because, if you are adjusting the downstream pressure when there's no flow through the regulating valve, and you have no method of reducing the pressure to determine if the valve can maintain the setpoint, then that's part of the problem.

Think about it; you are trying to adjust a pressure setpoint with no flow through the valve, and as you increase the setpoint the pressure increases. But that's only pressurizing the line up to the downstream valve that is closed. That means it doesn't take much opening to get the pressure to build up. And when the pressure exceeds the setpoint, that valve will close!

And, then when there is flow through the valve, it takes an increased opening to maintain the same pressure, and the opening needs to keep increasing as the flow-rate increases in order to maintain the pressure setpoint.

So, for these types of valves and actuators, it's usually best to adjust them when there's flow through the valves, say at FSNL, when the flow is stable (so, not during starting and acceleration).

It might also be that once you've adjusted the pressure when there is flow through the valve, that when the unit stops or trips that the pressure increases downstream of the valve. Remember, there's usually no "vent" downstream of the valve so the pressure has 'nowhere to go' if the valve isn't quick enough or the piping volume between the valve and the valve that closed is small. So, that pressure regulator isn't going to be able to reduce the downstream pressure if it's too high because there's no method of venting the line, and the gas can't flow back into the upstream side of the valve (against higher pressure!).

I've also seen some diaphragm-and-spring regulators that required a needle valve in the sensing line to reduce the pressure fluctuations to help maintain stability. If there is a valve in the sensing line, have you tried closing it slowly to see if it helps to reduce the fluctuations? Remember, if you close it too much you will slow down the valve's response, but if the valve is a needle valve you should be able to adjust it to at least reduce, if not eliminate, the pressure fluctuations.

But, it would be nice to know when this problem started. And to know how you were able to resolve the problem!