How to control 6 pumps in cascade

Hi, I need to control 6 pumps in a cascaded control. I read some threads about controlling 2 pumps, but how about 6?
The pumps are all the same with a max. flow of 300 m3/h and all have a vfd. The pumps are used to deliver cooling water for 10 evaporators. The water is pumped from a nearby chanel.

This is what I think how it should work so far:
The first pump starts with a PID controller which controls the pressure. If the PID reaches >90% for some time start the second pump. So far so good. Then what? Control both pumps with the PID controller? And start them with e.g. 60%?
And what if the third pump is needed?
Is it a good idea to control all 6 pumps with the same PID contoller? This way all pumps have the same speed.

One technique that is more commonly used is to take your pressure reading start one pump, read again if less than 50% start pump two, repeat adding a pump each time when you get to less than what another pump would add to the pressure full out then use the last pump as your pressure compensator, This one is the PID controlled. In essence your adding fuzzy logic and PID control. Use the fuzzy logic technique to get the pumps close to the desired pressure then maintain with the PID conrolled pump. You can however also use the PID to run all pumps at the same speed but you ned to ask yourself if all the pumps actually needed to maintain pressure? Or are the extras merely standby pumps in case of a faulted pump.
Ron beaufort posted an excellent example of pump control with an out of service detection. as well as mimimum running hours usage. He used the fuzzy logic technique rather than PID for this application. In his case he is using fans. Perhaps this will provide some ideas.

One other question is how exact does the pressure need to be? You may be able to use just fuzzy logic as per the post above rather than add a PID loop

This sort of application is always complicated by the pump curve.

Centrifugal pumps do not produce any pressure until well into their speed range, 40% is typical, and I have seen some as high as 65%. Also the pumping efficiency falls off towards the top end, so that going from say 90% to 100% produces no appreciable change in output.

You will need to determine your low and high cut-off points, and factor these into the algorithm for starting and stopping pumps.

A HTHW (High-Temperature Hot Water - 130°C) plant I worked on had two sets of cascaded pumps, 4 to provide flow-rate in the loop and to supply demand from users, and 3 to maintain system pressure, which was critical to prevent boil-off.

For each of the controls, we used one PID, and split the output into ranges to drive the individual pumps, overlapping the top end of one and the bottom end of the next and so on. The overlap points were determined during commissioning. I don't remember the exact detail, but we got the 2 controls near perfect with some experimentation.

If your pumps are in parallel, as ours were, you will need non-return, or aut-valves, to stop recirculation through unused pumps. It is possible to put your pumps in series, though.

HTH

mordred said:

One other question is how exact does the pressure need to be? You may be able to use just fuzzy logic as per the post above rather than add a PID loop

Click to expand...

The pumps are used to deliver cooling water for about 10 evaporators, if the pressure fluctuates, these processes wil also be influenced. So the pressure should be pretty accurate. I don't have exact numbers.

K if possible I would look at something along the lines of what Daba suggested

daba said:

This sort of application is always complicated by the pump curve.

Centrifugal pumps do not produce any pressure until well into their speed range, 40% is typical, and I have seen some as high as 65%. Also the pumping efficiency falls off towards the top end, so that going from say 90% to 100% produces no appreciable change in output.

You will need to determine your low and high cut-off points, and factor these into the algorithm for starting and stopping pumps.

A HTHW (High-Temperature Hot Water - 130°C) plant I worked on had two sets of cascaded pumps, 4 to provide flow-rate in the loop and to supply demand from users, and 3 to maintain system pressure, which was critical to prevent boil-off.

For each of the controls, we used one PID, and split the output into ranges to drive the individual pumps, overlapping the top end of one and the bottom end of the next and so on. The overlap points were determined during commissioning. I don't remember the exact detail, but we got the 2 controls near perfect with some experimentation.

If your pumps are in parallel, as ours were, you will need non-return, or aut-valves, to stop recirculation through unused pumps. It is possible to put your pumps in series, though.

HTH

Click to expand...

Are the pumps running in different speeds? Or all with the same speed?
Your method sounds very good.

You can't control what you don't understand

Maik Fuchs said:

The pumps are used to deliver cooling water for about 10 evaporators, if the pressure fluctuates, these processes wil also be influenced. So the pressure should be pretty accurate. I don't have exact numbers.

Click to expand...

The ability for the cooling system to remove heat is a function of flow and the cooling water's temperature relative to what is being cooled. I don't think controlling to a pressure is the right thing to do unless you are controlling flow indirectly this way.

Water under pressure will remove little heat if there isn't any flow.

Also, the relationship between flow and pressure is not always linear. You must have a table around there that provides the relationship between pressure and flow. Some times the pressure goes up as a function of the flow squared so to get twice as much flow you need four times as much pressure drop across the system.

Peter Nachtwey said:

The ability for the cooling system to remove heat is a function of flow and the cooling water's temperature relative to what is being cooled. I don't think controlling to a pressure is the right thing to do unless you are controlling flow indirectly this way.

Water under pressure will remove little heat if there isn't any flow.

Also, the relationship between flow and pressure is not always linear. You must have a table around there that provides the relationship between pressure and flow. Some times the pressure goes up as a function of the flow squared so to get twice as much flow you need four times as much pressure drop across the system.

Click to expand...

I understand what you mean but I can't control the flow because I don't know what it should be. The 10 evaporator all have their own control valve for the cooling water. Depending on the inlet temperature the evaporator need more or less water. So in winter we need less water... I will look for some specifications of the evaporator to see how much cooling water they need. But this also depends on the product...

Now we are getting somewhere.

Maik Fuchs said:

I understand what you mean but I can't control the flow because I don't know what it should be. The 10 evaporator all have their own control valve for the cooling water.

Click to expand...

OK, but you should have mentioned this earlier because it is these control valves that are controlling the flow and not the pumps. This makes sense. This isn't much different that the hydraulic systems I deal with.

In a servo hydraulic system it is critical to maintain a constant pressure just as in your system. What will make controlling your system easier is to put an accumulator on the out put of the pumps. This will slow down the pressure changes and make the system much more controllable and now you truly can have the pumps controlling pressure. What pressure must you maintain?

The accumulator will help linearize your system.

Don't be stingy with information. Sizing you accumulator properly is important. The bigger the accumulator the slower the pressure will change as a function of flow demand but also flow supply. After getting this right the pump control will be easy. This accumulator could be a water tank or a bladder type accumulator depend on the volume you are talking about.

Imagine if you had a big water tank above the cooling valves which is high enough that is supplied a constant pressure. Now your control problems become much easier. Especially when the surface are of the tank is large.

BTW, shame on the mechanical designers that don't know any better.

We can get to how to turn on and off the pumps once this matter is resolved.

Peter Nachtwey said:

OK, but you should have mentioned this earlier because it is these control valves that are controlling the flow and not the pumps. This makes sense. This isn't much different that the hydraulic systems I deal with.

In a servo hydraulic system it is critical to maintain a constant pressure just as in your system. What will make controlling your system easier is to put an accumulator on the out put of the pumps. This will slow down the pressure changes and make the system much more controllable and now you truly can have the pumps controlling pressure. What pressure must you maintain?

The accumulator will help linearize your system.

Don't be stingy with information. Sizing you accumulator properly is important. The bigger the accumulator the slower the pressure will change as a function of flow demand but also flow supply. After getting this right the pump control will be easy. This accumulator could be a water tank or a bladder type accumulator depend on the volume you are talking about.

Imagine if you had a big water tank above the cooling valves which is high enough that is supplied a constant pressure. Now your control problems become much easier. Especially when the surface are of the tank is large.

BTW, shame on the mechanical designers that don't know any better.

We can get to how to turn on and off the pumps once this matter is resolved.

Click to expand...

The setpoint for the pressure is 3,5 bar.
I told the engineer also we need an accumulator but they don't agree. "We are replacing the pumps which are running for 20 years without an accumulator. Why would we need one now!?" It's always the same story...

Your problem isn't how to control the 6 pumps it is the engineers.

Without the accumulator the rate of change in pressure in you system is:
dP/dt=β*(Qin-Qout)/Volume
Where:
dP/dt is the rate of change in pressure
β is the bulk modulus of oil
Qin is the inflow from the pumps
Qout is the out flow through the cooling control valves
Volume is the total volume of water between the pumps and the valves.

http://en.wikipedia.org/wiki/Bulk_modulus

My (Qin-Qout) is the same as the ∂V in the wiki page.
You can see that a little difference between the flow in and out cause the pressure to change rapidly and this will be a problem when switching pumps.

a flow mismatch of just 0.001 cubic meters per second will result in a pressure change of ( 2.2*10^9 N/m^2 * 0.001 m^3/sec ) / 100 m^3=22000 Pa/sec or 2.2 bar per second ASSUMING there is 100 cubic meters of water being compressed. That is a big error percentage wise. How much water is being compressed? What are the like errors in flow rates going to be. I suggest you find out what the volume of water is that is being controlled and do this calculation with real numbers.

Perhaps momentary pressure transients are OK since the pumps combinations can switch quickly compared to the time constant of the cooling towers.

When controlling this I would have 1 PID that has an output of 0 to 100% flow and each pump handles 16 2/3% of the flow. The trick is dividing the100% output among the 6 pumps. 16 2/3% is scaled to full RPM for each pump. I would add another pump as the flow demand is increases by 16 2/3% and divide the total flow demand between the running pumps. When the flow demand starts to drop the running pumps will start slowing down until then get to a point like perhaps 20% of the 16 2/3% I would shut down the longest running pump and divide the flow demand among the other running pumps. I would always ramp the RPM signal to the pumps to avoid shocks. Doing this in ladder is not going to be easy but I believe Ron Beaufort has this worked out. I would do this in structured text.

This would be a good example for a motion controller.


If you are just know asking to how control the 6 pumps then was the pressure controlled before? How well was the pressure being controlled before?

i am using this all the time, however a little cheaper:
only one pump needs the VFD the rest can be with simple contactors. better energy too.
as this is a cooling proces probably no need for exact control, stability is more important.
so i would start up pumps every minute until pressure is at setpoint and not rising anymore (fuzzy or just plain control is best). and stepdown with 2 minute interval. pumps perform best at nominal rpm. and can be damaged when running in other rpm.
btw i am close and if needed i can help you. i am specialist on small (wago, beckhoff plc types.) and yes i do know cooling tech very good.