i have two pumps at very different stages in thier life cycles and one PID loop that controls both pumps driven from a tank level. One pump has recently been rebuilt so as the pumps change over the max output now has to be changed each time to allow for huge differences in flow rates between the old and rebuilt pump. Is this a tuning issue or is it better practice for each pump to have its own PID loop as we have numerous situations like this?

 

If the output is that drastically different, sounds to me like the second pump is also due for a rebuild. If that's not feasible, either politically or financially, I'd say have two loops. Another option I've implemented at times when you have two alternative control variables driving the same process variable is to only have one loop (since you only need one loop at any given time), tune each case independently, and store gain sets that you switch in when you change between systems.

 

Hey Steve,

thanks for your reply

 

I designed a single PID controller to regulate the level for a waste treatment plant using six pumps, 3 fixed speed and 3 variable speed.

The PID loop operates using normalized Pumping Units (PU) where 100 PU = 1 Fixed Speed Pump with a range of 100 to 600 PU. That made it easy to load share between the fixed and variable speed pumps and determine stop and start conditions.

The variable speeds operated with a 70 to 100% control output signal that provided 50 to 100% flow so they operate with 50 to 100 PU.

If you normalize the regulator to Pumping Units then you can put a gain on the worn out one and it won't affect the regulator tuning.

 

That's a clever solution for a situation where you have a range of pumps designed to offer different flow rates. Effectively you are adding a "characterizer" or gain setting to the controller output depending on the actuator selected.

However, if your pump has degraded to the point where its noticeably not delivering its design flow, then complete failure is probably not that far away (and murphys law says it will be at 3 am). As ever, you need to solve the real problem and fix the pump, not patch round it with software.