Level Control PID for Duty & Assist Pumps

Join Date
Jul 2007
Location
Kiruna
Posts
600
Hi,

I've recently tuned a PID for a level control in a channel application. Everything was functional fine until a new feed pump was added to the system. The additional feed pump naturally has increased the flow to the chamber that my PID is level controlling.

The clients control philosophy calls for an assist pump in the chamber to start when the Duty pump has reached 95% of MaxSpeed. The assist pump will remain on until the Duty pump has dropped to 45%.

My thinking was I could use the PID output to modulate both pump speeds but as I'm sure you more experienced guys are about to point out this is not possible for this application.

The results observed where as follows:
When the Duty pump hit 95% the assist kicked in at 95% also. This in turn drove the level down drastically (small channel) cuasing both pumps to stop in a Low level setpoint.

The channel level then increased above the Duty start setpoint causing the duty pump to start flat out and again calling the Assist pump.

How do you guys control an application like this? My next thoughts were to perhaps only control the assist on the PID when its needed but how should I decide what speed to control the duty pump?

Does this make sense? Any comments greatly appreciated.
 
Without specifics, I guess you would have to trim the speed ref. of each pump to 47.5% when the assist pump kicks on to match the same flow as the 95% of the one pump. Then restart the PID.

Why was the assist pump added?
It sounds like the equipment was purchased without considering how it was to be controlled.
 
I would use the assist pump as a trim.
At 95% ( I think would start this at about 80%) start the assist pump and ramp it up ( slowly). Leaving the PID on the duty pump only. As the assist pump ramps up the duty pump output will start to come down. At about 50% stop the ramping of the assist pump and leave it at whatever % its at. If the duty pump raises back up to 95% then ramp the assist pump up some more.
On the way down start ramping the assist pump down at about 25% on the duty pump. This will increase the output on the duty pump. At about 50% stop the assist pump ramping.Leave it in place.

At some point you shut off the assist pump lets say at 15%(assist pump output) or what ever your systems requires.
 
Start the assist pump at 10% speed. Give it a little time to react to the additional pump capacity. If the duty pump remains above your high threshold level or when it returns to it, increase the assist pump by another 10%. Lather, rinse, repeat. Adjust the speed increments for the assist pump for best response.

On the way down, when the duty pump reaches the low threshold level, decrease the assist pump speed by 10%. Lather, rinse, repeat.
 
That's great guys, something to work with tomorrow. So all agreed then that the PID should remain controlling only the Duty Pump?

NB: The additional feed pump always existed. It just wasn't required up until now. I commissioned the system with a single feed pump and it worked a treat until the second pump was added. Another thing to note is the second feed pump may not always be running. So my PID should work for a single duty pump also.

Do you think the suggested methods here will allow for this?
 
I usually use PID only for one pump, if another pump is needed, PID controls one pump and second pump runs at constant speed.

Ex if PID output goes over 95% certain time => Pump2 is started at constant speed 95% and at same time manipulate PID output to ex. 30%. Now PID output starts to increase from 30% if needed.

If PID outputs goes below 30% for certain time => Second pump is stopped and same time plc program puts PID output to 95%.

Control cut-in and cut-off values from HMI or scada if you can, don't use constants for these on plc code, because you have to tune also these on commission...
 
You have a classic example of the limits of PID control - when the load or the supply system changes dramatically the tuning needs to change.

Lare's system might work, but it is dependent on the turndown and pump characteristics. If you are using centrifugal pumps two pumps at 47.5% speed you won't necessariy get half the flow of one pump at 95% speed.

One issue with Lare's system is that it still isn't stepless. Even ideally when you start that second pump you go from 95% flow to 125% flow, which means you will get a big jump in flow rate.

One solution is to have two sets of tuning parameters, one for a single pump and one for two pumps. HAve a cascaded control loop, where the output of the level control is a flow rate, and the flow rate is the input to each pump control. When you start the second pump cut the speed to both so you equal flow of one at 95%, load the second set of tuning parameters into the pumps' control loops, and run them at equal flow rates. Note that equal flow may not be equal speed, depending on the piping configuration.
 
I put 30% for example, it depends of pump and were water is pumped (back pressure), some pumps don't make any flow below certain speed.

When second pump is started you need to solve first to were you have to "drop" pid 's output, on some systems it have to be 0%, or flow will jump a lot, on another system it can be something between 0-xx%, it really depends of system and pumps as Tom pointed.

That is why I said, you have to tune these turn-on and turn off parameters at same time when you solve other PID parameters.

Putting PID to zero, when second pump is started is not allways good idea, because if pump makes flow only above 30% you will get little bit slow response when PID is restarted from 0%. Of course if pump make flow right after zero speed, you have to stard PID from 0%, when second pump is needed.

That way there is no big flow difference when second pump is started, because flow remains it state when pumps are increased.
 
Last edited:
The two basic techniques discussed here will both work, they each have good points.

Assist pump on separate control
(Mickey, Steve, Lare)
Stepping or ramping the speed of the assist pump based on the output of the PID can work well. On the good side, the PID loop remains simple and the control for the assist pump is also relatively simple. What is basically happening here is you are using the assist pump to cancel out some of the load on the PID and move it back towards the center of it's operating range. The process gain for the PID loop doesn't change. On the bad side, this may only work well if the load change is relatively slow compared to the step rate or ramp rate (which seems to be the case here). If a load change causes the PID loop output to go to max or min, the reponse will be limited to the step or ramp rate.

Assist pump on PID control with main pump.
(Gerry, Tom)
Controlling both pumps with the same PID can work well. It will require 2 sets of tuning constants because the process gain is 2x when two pumps are running. On the good side, we lose the concern about load changes vs step/ramp rate. On the bad side, the transition from one pump to two pumps and back creates more problems. Choosing the right transition points and matching the flow at those transition points is key. There needs to be enough overlap in the operating flow range of one pump mode and two pump mode so that the number of transitions is minimized. You may also want to add some optimizing code so that you don't stay in two pump mode indefinitely when you are above the switch point but within one pump capacity for a minimum time.
 
Last edited:
Gerry M has got it spot on, Both pumps would start at a reduced Level for a timed period to allow the pumps to settle before the PID took control of both pumps. I would suggest starting at about 55% ( the pumps are not proportional).
 

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