controlling underground pump using level and flow

Dear,

We have a Variable speed pump located in an underground pit, the purpose is to perform the following
1- When the level is high, the pump will need to run at a maximum allowable frequency
2- When the level is low, the pump will need to run at the minimum frequency
However, we have flow constrain in order frequent start/stopping of the pumps. flow should vary between 2.7 LPS and 4.2 LPS.
I was thinking of controlling the VFD based on level using a PI loop and at the same time measure the flow in order not to allow it to go below 2.7 LPS or above 4.2 LPS.
What kind of control shall use in order to maintain the pump running within the needed flow range

thank you!!!

Don't mess with the PI control. Use P control only.
It doesn't sound like exact level is that critical so keep it simple.

OutPut% = 100%*(level-LowLevel)/(HighLevel-LowLevel)

Simple.

Peter Nachtwey said:

Don't mess with the PI control. Use P control only.
It doesn't sound like exact level is that critical so keep it simple.

OutPut% = 100%*(level-LowLevel)/(HighLevel-LowLevel)

Simple.

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Thank you, but how to ensure that the control doesn't fall outside the stated range ( 2.7 LPS to 4.2LPS)

Use proportional control for the SP in LPS then use PI to control the pump delivery.

This sounds like cascade control might be your solution. Control the level by controlling the flow. You control the level by controlling the flow between 2 limits, in this case 2.7 LPS to 4.2LPS.

Level is your master, so you set the SP of the level at whatever you want. As you want the level in the pit to be as low as possible, you can set it at 10% or something. It does not matter much, but will be determined by how much water you are allowed to have in the sump. Sometimes it is better for the pump to rather run continuously, than start/stop all the time. So maybe a higher setting might be advisable. Anyway, you decide when you commission the system.

Instead of the output of your Master loop(Level) going to the VFD, you take it as the SP of the Slave loop(Flow). If the level gets high, the OP form Master will increase the SP of the Slave loop, which should speed up the VFD. The Slave loop uses the flow as its PV. The OP of the Slave loop needs to be clipped so as to not exceed the 2.7 LPS to 4.2LPS flow value. These values could be determined in Manual mode.

So, at low level, the VFD will run at minimum flow and at high level the VFD will run at max flow.

If my brief was your post, I would just scale the pump output to the tank level using a SCP and use GRT or LES to stop the pump going above or below your limits.

3 or 4 lines of code and it's done.

KISS

So what pump protection do you have when you are limiting the flow to the lower value and the level continues to fall?
Similarly what do you do when the pump flow is at maximum flow limit and the level continues to rise?

Calistodwt said:

So what pump protection do you have when you are limiting the flow to the lower value and the level continues to fall?
Similarly what do you do when the pump flow is at maximum flow limit and the level continues to rise?

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That would be catered for by your protections. My solution is for the control under normal circumstances.

#1 you would have to trip the pump.
#2 could be problematic, as his brief was a max flow rate.

Process design would stipulate if the max LPS may be exceeded in special circumstances. I would guess this not to be a problem though, because if so, the process design would be wrong. You can't have 10LPS flowing into your pit sump, but limit the outflow to 4.2LPS.

Lord Farquaad said:

That would be catered for by your protections. My solution is for the control under normal circumstances.

#1 you would have to trip the pump.
#2 could be problematic, as his brief was a max flow rate.

Process design would stipulate if the max LPS may be exceeded in special circumstances. I would guess this not to be a problem though, because if so, the process design would be wrong. You can't have 10LPS flowing into your pit sump, but limit the outflow to 4.2LPS.

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dear lord Farquaad,

thanks a lot and yes I agree with you, 4.2 LPS was determined by design so this shouldn't be exceeded.

I assume clipping is easy to be done on the flow control to make it between 2.7 and 4.2lps.
please find attached schematic showing your idea.

Charbel said:

dear lord Farquaad,

thanks a lot and yes I agree with you, 4.2 LPS was determined by design so this shouldn't be exceeded.

I assume clipping is easy to be done on the flow control to make it between 2.7 and 4.2lps.
please find attached schematic showing your idea.

Click to expand...

Your schematic looks right. As for the clipping, there are some simple ways to do it, and some elegant, but complex ways.

Simplest way is to increase the speed of the VFD manually and see at what frequency you get the min and max LPS. Then set those as your output limits. Unfortunately that would not cater for wear and tear, process disturbances etc. For that, you will need to constantly look at the Slave PV and limit/clamp the output when your LPS limit values come into play.

Could be a lot of fun to see how many solutions you could come up with for this.

I guess better to use PI control for both flow and pressure.

Charbel said:

I guess better to use PI control for both flow and pressure.

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Yes, these loops will not be slow enough to use D. P for sensitivity, I for offset. Although, with your parameters and application, offset would not be a problem. You are emptying a sump, not controlling a critical vessel level.

My recommendation would be the same as Lord Farquaad's.

It is customary to use float switches as a backup for the level transmitters. A high level float would sound an alarm if the pump couldn't keep up with influent flow rate. A low level switch would stop the pump and sound an alarm if the minimum pump flow exceeded influent flow rate. This also provides protection in case the level transmitter goes bad.

Note that you may need a minimum frequency or minimum flow rate to avoid getting too far to the left on the pump performance curve. At less than the supplier's minimum (dead head) flow internal recirculation may cause vibration and temperature issues that can damage the pump.

Tom Jenkins said:

My recommendation would be the same as Lord Farquaad's.

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Yes, but do ye think that he is compensating for something?

mbartoli said:

Yes, but do ye think that he is compensating for something?

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