ROCLINK 800 PID LOOP Troubles

[nick88]

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I'm using a PID loop instruction in a Floboss 107 RTU for an oil and gas application. There are two loops, a primary and override loop - primary is for flow control, and override loop for pressure control. The PV's of the primary and override loops are from a USM meter and pressure transmitter respectively. The pressure transmitter is downstream of control valve, and USM is upstream of control valve. The output of the loop is a 4-20ma signal to a GE oil and gas DNGP valve. I'm using a low select between each loop, so which ever output is lowest should be in control. If flow or pressure is below set point, we want valve to open, valve will close when either PV is above SP.

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I'm having an issue with the loop select toggling between two loops. Our PID controller was in pressure control for the past several weeks with SP's around 684 psig and 165000MSCFD. The flow was around 40000 MSCFD, pressure stayed at 684 psig since we were in pressure control.

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When operators raised pressure SP to 720 psig (flow sp remained the same,) the pressure raised to around 702 psi and flow jumped up to 100000 MSCFD when the loop select started toggling between primary and override. The issue is the primary flow control loop was trying to close valve and pressure loop was doing just the opposite. Valve position remained at around 59% open due to the toggling action.

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Is there anyway to adjust parameters so that the pressure loop remains in control until the flow PV is 10000 MSCF or so from SP and the flow loop starts to take action?

Here are other configured variables in PID.

Primary Variable EU Span 0 - 200000 MSCF/DAY

Secondary Variable EU Span 0 - 1500 PSI

Output Span 0 - 100 % (0% = close)
Loop Period - 0.5 seconds

Primary Deadband - 300.0 MSCF/DAY

Override Deadband - 0.00 PSIG

Override Gain 27.0
Override Reset .1
Override Derivative 0.00

Primary Gain 2.3
Primary Reset 1.4
Primary Derivative 0.00

Nick

 

[DwSoFt]

For starters your gain looks way too high. And your scale factor for flow looks way too low.
If you want to control on pressure first then flow the way I would do it is have 2 separate primary only.loops and the use the fst (or ds800) logic to bring both values in and make it select what you want for output in the logic and then go out to the analog output. You will want to also adjust your max range to prevent the loop not in co trol from ramping all the way open (clamping of say +5%) and you will need to account for auto/manual actuation.
I can send you an fst example with the pid screenshot later that shows an example of this as I usually do it this way when I what 3mode control but I think you just need to get the settings in order in my opinion.

 

[nick88]

Dan,

I set the flow scale factor = -.0005 which is calculated from (span of output)/(span of input) per the Roclink software manual.

Would you please send me the fst example and screenshot, I would like to see this.

 

[DwSoFt]

I will get to that yet but your gain especially the pressure one is too big