PID Question.

Mark Buskell

Member
Join Date
Sep 2003
Location
Florida
Posts
892
I am using PID'd to control chemical flow. The basic setup is fixed speed pump and we vary the flow rate with a output (4-20) to a electric solenoid (Badger Valve) and monitor the flow with a micro motion meter (4-20). This works fine and we have been doing this way for a number of years now. Flow is from 1000 to 40000 grams.

AB-ControlLogix currently.

Say the setpoint is changing from 10000 to 30000 and back.
The question is when going from a lower setpoint to a higher setpoint it takes around 10 seconds max for the loop to stablize.
But when you go from a higher setpoint down to a lower setpoint, it takes up to twice a long.

It was never an issue in the past years but due to a change in production techniques, we are now looking closely at this issue.

We thought it might be a pressure issue with the valves, but the manufacturer swears they put up to 300 psi through them and they reacted the same in both directions. (Around 6 seconds from fully closed to open and back. We operate right at 60 psi.

Right now, the solution I will be trying is when going from high to low, I will lock the valve is manual (PID Manual bit) for 4 or 5 seconds, force the output down low and then release it.

Has anyone else seen then type of problem before when working with PID's?
 
Divide and Conquer

Does the valve react the same in both directions? Surely you have an inner loop for the valve control and can verify this.

Chances are your process in non-linear and the gains change as a function of the PV changing. A really clever person can figure out how the gains as a function of PV or temperatures differences and change the gain accordingly on-the-fly.
 
I think your idea of using manual mode is a good one if you have constant targets you are trying to hit. Each target can have a valve preset position.

As far as nonsymetrical operation, I would start by checking the valve. Does it travel in each direction at the same rate, and see how nonlinear it is. Then plot out the inputs and outputs for the PID and see what's going on for each iteration of the PID.
 
Mark.
When you are controlling flow you will usually see this effect. As you increase the flowrate from Low to High the losses in the pipework work against the flow and provide a damping effect. When you go from high to low the losses decrease with flow and damping is removed.
Andybr
 
The badger valves use a non linear trim kit.
There are 2 types of trims kits that I am famaliar with. One is a linear trim kit. In a linear trim kit the stem is shaped like a "V". In a non linear the stem is shaped like a "U". The "U" stem provides much better stability when you are trying to maintain low flow.
Tomorrow, we are setting up a bunch of tests on the valve.
We are going to try to determine if the problem might lie in the trim kits, but it may also be a combination of the non linear trim kits as well as trying to program a PID that has a non linear flow.
We will also be testing response times with and without pressure flow on the valves.
 
Control Valve Trim

The type of trim that should be used is based on the characteristics of the flow system. If you have a significant piping run, then a non-linear trim such as equal percentage would be appropriate. Since the flow versus pressure drop curve is non-linear, the valve must also be non-linear to achieve the desired overall result; 0-100% output equals 0-100% flow, a linear loop. Then pressure drop across the valve is high at low flows and frictions and low at high flows. (the greater the flow, the greater the friction and pressure drop at a fixed diameter of pipe). If your system has a short piping run like most chemical injection systems; the trim may be non-linear or linear depending on the configuration. Are you pumping into a header than maintains a constant pressure? Then a linear trim may work.
I would also check other issues such as mechanical wear on the valve, the positioner (if there is one) or the calibration of the valve without a positioner (0ma=0%, 12ma=50%, etc.).
 

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