# Clamping PID in Compact Logix

## 5 posts in this topic

Hello all,

There are two pressure gauges in a line with a flow of water. The flow is controlled by the pumps which is controlled by the PID whose control variable is pressure gague -2 . In ideal condition the pressure gauge -2 remains at 55 psi and pressure gauge -1 stays at 82 PSI. But in the line if someone opens a manual valve then pressure gauge - 1 reads 100 psi because the pump is running as per pressure gauge 2 . Is is possible to clamp the PID so that pressure gauge 1 remains at 82 psi.

FYI: Pressure gauge 1 is right after the pumps and pressure gauge 2 is at the end of the line.

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Just thinking "out loud" can you use the Software Manual and Set Output features of the PID to accomplish what you want.

When PG1 first exceeds 82 PSI capture the PID Output value, then place the PID in Manual at the set output captured.

When PG2 exceeds 55 PSI then return the PID to Automatic.

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Think about the physical situation--flow causes a pressure drop between the gauges.  Limiting the pressure at gauge1 will also drop the pressure at gauge2.  If sufficient pressure at the end of the line is more important than excess pressure at the beginning of the line, then leave it alone, as it is working properly.  If keeping 82 psi at gauge1 is more important than sufficient pressure at the end of the line, then change your PID instruction to simply use gauge1 instead of gauge2.

If you need 55psi at gauge2 while keeping 82psi at gauge1 (or something less than the observed 100psi), then you need to install a bigger pipe.  Or a separate pipe.

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hello pratik,  if I don't miss something, you have customary users sustaining a steady state flow plus random users that produce disturbance. as you have been told, firstly you should decide if min 55 psi is mandatory to have or not, if mandatory you need to keep your system reactive in order to cope with disturbance or change the piping design, otherwise you could smooth the feedback signal from PG 2 to provide some hysteresis and avoiding the pump runnning at full bore if not strictly required. Moreover you should know how your PID works and the mass of fluid in the line because this affects the reactivity of the system as well.

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@BobLfoot - I cannot run the PID in manual as the output is different when 1,2 or 3 pumps are running.