Bumpless pump control sequencing

Dryhops

Member
Join Date
Jan 2018
Location
California
Posts
90
I recently made a simple program which controls four pumps to maintain line pressure of a glycol system. Two pumps are running at any given time, and are sequenced based on the load and running time. Both running pumps get their reference speed from the same PID, no lead lag.

When I transfer control between two pumps, the line pressure drops precipitously and the pump that is still running will ramp up to compensate until the new one as accelerated.

Can anyone suggest an elegant technique for managing this better? It seems like a common design paradigm so I'm hoping there is a good solution to address this.
 
Is there any issue running three pumps? Also, do you have ramp or coast to stop on your VFDs? When I have this situation, I like to run an additional pump (third pump in this case) and have all three pumps sync up, then ramp to stop the pump to be stopped. This should provide minimal pressure drop.
 
One method that I have used before is to do the following:
1. When transfer is initiated, capture the current CV value.
2. Startup new pump and start ramping at a slow rate from 0 to CV.
3. Keep current pump running on the control loop, the PID will start to wind down as new pump is ramping up.
4. When the new pump speed > current PID CV, stop ramping and instead set the pump speed = PID CV. Also now turn off the first pump/send to 0 speed.
 
freewheel stop on VFD? Try ramp stopping. If acc and stop ramps are same, then bump should be small as other accelerate and other ramps down.
Also is your PID ouput going to 0 when pump change. Is there difference on PID output if pump change from 1 to 2 or from 2 to 1?
 
I implemented some of your suggestions, thanks for the advice. The problem is improved, but not totally eliminated. Here is how I programmed it:

1. Two pumps running in tandem.
2. Transfer is initiated, both old pumps remain on while the third new pump begins to ramp.
3. Wait until the new pump is at reference speed, only then stop only of the old pumps.

This regime works a lot better than a simple start/stop, but I'm still getting a pressure spike. I've attached the trending to demonstrate:

I'm going to try perhaps only waiting until the new pump is at 50 or 75% reference speed before stopping the other pump. Any other better ideas to try to mitigate this spike?

Capture.PNG
 
Are these actually setup on a PID control loop? It looks like they just run at 60hz??

If a PID control, can you tune your loop faster and also perhaps slow down the new pump on command ramp?
 
Yes, they are controlled by a PID, but in that image the PID is at 100% due to plant demand. Maybe the spike will be reduced when the demand is lower and the pumps are not operating at their max. I will have to wait until later in the day to test.

I checked, all of my VFDs are set to 'Ramp CF' for stop mode. I can modify accel/decel times, but I still need the VFDs responsive enough to prevent substantial spikes in line pressure. I'd rather have special cases to handle the transfer and have my PID tuned to my process.
 
Let's assume three identical pumps.

You need to know the flow. This can ideally be measured, but if necessary it calculated from pump performance curves based on pressure. If flow can't be measured you should also use the pump curves to determine the speed at which the pump deadheads at the actual pressure.

Disable the PID loop. Ramp the lag pump up to just over the deadhead speed while ramping the lead down until the flow decrease equals the flow at deadhead. Then start to gradually ramp down the lead pump (the one being stopped) while simultaneously ramping up the lag pump so that the decrease in flow on the lead is matched by the increase in the lag. When the lead reaches the deadhead speed shut it off.
 

Similar Topics

Hi I am trying to understand the best method of providing bumpless transfer to the PID function (not PIDE) The existing code shows In Manual...
Replies
10
Views
4,552
I didn't see an exact answer to what I was looking for, so here it goes.* Right now a chilled water control valve is in PID auto the majority of...
Replies
5
Views
1,656
Morning all, Could anyone shed some light on whats happening with my PID Loop? On our plant we have a 'fast ramp' on the PID Instruction done...
Replies
14
Views
7,514
Hello- Looking for suggestions on a PID startup sequence with bumpless transfer. Basically I need to force and hold the output to different values...
Replies
2
Views
1,887
Hello all, I having trouble transferring from a manual control to automatic control. At the moment when the valve is in auto, it uses the PID loop...
Replies
8
Views
2,720
Back
Top Bottom