3 pump Pressure control.

Hi, I am new to PLC's and have been tasked with building an MCC panel for a pumphouse supplying water from a dam to a fruit packaging and storage facility which includes admin and engineering offices and also staff housing. The disturbances could therefore vary greatly in magnitude and frequency. Hopefully someone has worked on a similar process and has it all figured out. Thanks in advance for your responses.

Equipment:
Modicon M340
Schneider ATV 61 VSD's.(2 x 30kW, 1 x 15kW)
Schneider HMI
Interface between VSD's and PLC will be analogue cards on the PLC.
Pressure Transmitter: 4-20mA

The process is as follows:

3 pumps, Duty, Standby 1 and Standby 2.(Duty sequence is user selected via HMI)
1)Duty pump controls to setpoint.(This needs to be smooth from startup as rapid pressure changes may damage old pipework).
2)If Duty pump not coping Standby 1 starts.
3)If Duty and Standby 1 not coping, Standby 2 Starts.
4)Stop Standy 2 then Standby 1 when they are not required.

This seems straightforward at first glance but how do I control each pump with a PID loop without each creating a disturbance for the others to overcome? Would it be better to have the Duty Pump on a PID loop and the standby pumps stepping up by preset amounts. What happens when all three pumps are running but no longer required. Do we start stepping down if the pv to the Duty pump is less than a set value and stop stepping down when it is greater than another set value(i.e a deadband)? What happens if a large valve closes and the pressure spikes up. How do we drop the standby pumps out quickly but without overcompensating?

Your thoughts greatly appreciated.

I suggest that you add a flow meter.
Use the flow to determine which pumps should be running and pressure feedback to control the pump speeds.

Mendonsy is correct - this is usually done with a cascade loop, with pressure control setting a system flow rate and speed control of the pumps controlling flow. You may experience the following problems, among others:

- The flow/pressure curve of two pumps operating in parallel is different than a single pump. Three pumps in parallel are different than two. Therefore, you may need different tuning for each situation.

At high heads and low flows with two pumps running it may be possible for one pump to make the other dead head - no flow from that pump.

You probably only get around 50% or so turndown on each pump, so running one full out and then bringing on the second will probably result in a big jump in flow and pressure.

The impact of the above depend greatly on the type of system you have. If you have mostly static head (lots of lift) the pumps will behave one way. If you have mostly friction head (low lift, long force main) they will behave differently.

Another point also .......
Many pumps do not respond well to speed control and some can actually be damaged by attempting it. You should contact the pump manufacturer to see what they recommend for the model you are using.
You may have to run the pump at full rated speed and use a proportional valve on the output side or a bypass regulator to control the pressure.

Hi, sorry for lack of response. I have been away for a while. Just wanted to thank you for your responses. From what you have suggested and from discussions with local engineers I think the whole strategy seems questionable. The problem is we are upgrading an existing system which I now understand has never worked satisfactorily. I think a bit more planning is required before proceeding.

Croc there is a simple solution to this. You can get a pump control card for the ATV61. So you biggest motor will get this card and the card will control the switching on and off of the other motors. The card needs a pressure feedback from the line. You give it a pressure sp and the main pump will start running and speed up and down according to pressure. If the pump cannot hold the pressure when running full speed after x time it will start the next pump. If the pressure gets to high again it will stop the 2nd or 3rd pump again. So there is actually only need for 1 vsd. the other two pumps can be normal soft start or DOL depending on the line.If the pressure stays high the main pump will slow down and eventually go into a "sleep" mode where it will monitor the pressure to see if it will be necesary to start up again
Easy to setup and does a very good job.

PM me if yo want the manual

I have programmed 2 similar 3-pump systems, except that the primary pump was changed by a selector switch. That way the pumps get equal run time. The first system used an HMI for all the logic, using VBScript. It is still running the house water for a major hospital. The second system(another hospital, still in operation) used a PLC for logic. It is fairly straightforward. You should be able to program an output on the drive to come on if max speed is maintained for X number of seconds. That means that the primary pump can't keep up and you should start up pump #2. You then need to keep pump #2 on for a minimum amount of time. Use the same logic for the pump 2/3 control. Each drive should have their own PID loop. Don't worry, they won't fight each other as long as you don't have them tuned too hot. You then also program an output to come on when minimum speed is set for X amount of time. You then know you can remove the boosting pump. I know I am leaving out a lot of details, but that is the basic premise.

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