How do you control pump speed from output pressure

We have a community water scheme consisting of a three phase submersible pump which tranfers water into members storage tanks over
night.The tanks have ball type float valves which control the inflow to the storage tank. When all tanks are full the pressure in the supply line /pipe builds up [approx 620 kpa 90lb} and a pressure switch turns the pump off which is locked off until resarted the next night. As the pump has only two speeds i.e flat out or stopped the last tank/s to fill experience very heavy inflows and pressure.The problem with this is that the associated water meters are being driven at far in excess of the specified flow rate and consequently being worn out well before they should. What I want to do is have a set point of say 400 kpa and control the speed of the three phase {4Kw } pump, so that as tanks fill and less water is being delivered the pressure set piont is maintained.The turn off function would have to be some way other than a pressure switch.Any suggestions please?
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For starters, check out that (a) the motor will sustain VVVF operation and (b) that the flow can be controlled stably by reducing the motor speed. High static head applications can be very hard to stabilise with variable speed. If you can advise the static head between the pump and the discharge it should be possible to give an indication on this, but there's no substitute for trying it. The pump manufacturer may be able to assist, but don't count on it. Assuming that both these aspects are resolvable, then pressure control should be pretty straightforward using available devices. The simplest way to handle the turnoff would be to do it when the 3 tanks are all full, presumably the pressure solution is nice because you don't need signals from the tanks back to the pump. With pressure control, what would happen would be that the pressure would rise somewhat when the last flow was cut off, and the VSD would then back the pump speed off further. This might be a usable approach, but that sort of thing can be a bit chancey. It would need to be trialled. Another possibility would be a flow switch in the common line, you wouldn't need a continuous measurement which would keep costs down.
One other variable that may help or may hinder is entrapped air in the lines, if the system geometry allows that to occur. That can influence your dynamics significantly, making for variable loop characteristics. Been caught by it more than once.
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Controlling the pump motor frequency sounds like a complex and expensive solution. If you were to introduce a suitably chosen fluid resistance element at a far end location, the flow rate would decrease here. The negative factor to this suggestion is that a higher pressure would be reflected to the pump and could cause it to turn off prematurely. However, with a well chosen value of resistance, you might achieve the required result. ... The emphasis here is on 'well chosen'. You might have to experiment until you achieve an acceptable resistance value. The fluid resistance component might be implemented in the form of a manually adjustable valve, or even a few meters of a reduced diameter pipe, placed at a strategically located point. A partially closed gate valve might work. ... The member storage tanks would require a longer period of time to fill, and the pump would have to run for an extended period. However, you would not have the excessive flow rates to contend with.

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neilr wrote:

Controlling the motor speed might be hard. Depending on the pump type, flow vs. rpm might be very non-linear, so control might be unstable. There are three last tanks, and the controller must be stable for each.
How important is efficiency? A bypass valve that shunts excess flow back to the well when two tanks are full is an inefficient solution that is simple to install and inexpensive. The cost would include either sensors in the tanks to determine when they have shut and two-out-of-three logic, or velocity sensors at the meters to open the bypass when any one goes out of range.
Jerry
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Jerry,After further consideration of your approach, and that of John Popelish I could try this method without to much effort. We already have a pressure relief valve [set at approx 850kpa] at the pump.This was installed to protect the pipeline[class 9 pipe= 900kpa] from excess pressure and possibility of bursting.If I was to set this relief valve at say 400kpa and by-pass some of the excess flow back into the river while the last one or two tanks are filling Switching off the pump would be by sensing the increased pressure or flow rate back into the river once the last tank has filled.
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neilr wrote:

It sounds like a plan! You might consider two relief valves, one of which opens when only one tank is filling, the other, when all tanks are full. It's time to shut the pump when the second valve opens. A pressure switch might replace the second relief valve.
You might need to throttle the outlet(s) of the relief valve(s) to ensure stable operation. In that case, an unthrottled valve to protect the pipe might be prudent.
Good luck.
Jerry
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One can use the following approach to estimate the reduction in pump pressure needed to keep your water meters from exceeding their maximum rated flow rate. Using the familiar (Q, flow rate) = K x (square root(P1 - P2))equation one can estimate K for your main supply line. K is a proportionality constant for your main supply line, P1 is your pump's output pressure and P2 will be assumed to be zero for this estimate. This is a reasonable assumption as when your pump is first turned on and all of your member tanks are filling simultaneously the equivalent flow resistance of multiple branch lines is very low. You didn't specify what the flow rating of your pump was so for this estimate I will use a 25 gpm, 94.6 liters per minute, rating. For P1 I will use 400 kpa, 58 psi, as it is the only number given in your original post. Then: K = 94.6 / (sgrt(400)) = 4.73 Then say the maximum rated flow rate of you water meters is 5 gpm,18.93 liters per minute. For a worse case analysis assume P2 is zero. Then if you wanted to limit the flow rate in your main supply line to 18.93 liters per minute when the last tank is filling your pump's output pressure must be less than ((18.93 /4.73) x (18.93 / 4.73)) = 16 kpa, 2.32 psi. This might appear to be very low but don't forget the pressure drop is proportional to the square of the flow rate. Flow resistance in your branch lines will help. However, you said your water meters were seeing flow rates far in excess of their rating so there can't be very much flow resistance in the branch lines in your present system. My suggestion is the best way to keep your water meters from exceeding their maximum rated flow rate is to add a flow restrictor in every branch line. The restrictor should be sized so that a water meter's maximum rated flow rate is not exceeded when full system pressure is applied to a branch line. Howard
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neilr wrote:

Have you considered a bypass control valve at the pump that recirculates the water back to the storage at the pump inlet, to control pressure? You could detect the last tank full condition by watching the valve position. When it has been open enough to be carrying the full flow for some period of time, turn the pump off. The pressure setpoint could be high enough that, when all 3 tanks are filling, the control valve would be fully closed, so it would not much affect the total fill time.
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My own thoughts exactly!
Kelvin B. Hales Kelvin Hales Associates Limited Consulting Process Control Engineers Web: www.khace.com
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Kelvin Hales wrote:

Even though it may not be as elegant as a variable speed drive, it would probably be easier to maintain without a maintenance engineer.
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John Popelish wrote:

Isn't that basically what I wrote way back? How could I have put it more clearly?
Jerry
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Jerry Avins wrote:

Not much. You got there first, but I didn't see your reply before posting mine.
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Because controlling the speed of a three phase AC motor is difficult and expensive alternate schemes like installing an accumulator at the pump should be investigated. You could then use one set of pressure switches and on off control to keep the pressure in the accumulator within a suitable band. Then once all member storage tanks are full and the pump stops running for say 30 minutes or more use a timer to lock out your pump until it is reset the following night.
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I don't know what you consider expensive, but you can purchase a 3 phase AC drive (inverter) for a 4kW motor for around $500. I'm sure there is currently a starter and circuit protection (fuses, overload) which could be easily replaced physically by a Variable Frequency Drive with proper fusing. Use an analog signal from a P/I transducer to control the speed of the motor. All of these other bypasses and whatnot seem extremely labor intensive. Using an inverter would be a simple "out of the box" electrical installation.
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gnoge wrote:

Do you have any idea how motor speed translates to flow/head at a particular head/flow? Does anybody? How will severe nonlinearities here affect loop stability?
Jerry
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gnoge wrote:

This is more in line with my approach.Grundfos have an off the shelf solution but involves completely replacing the existing control panel containing relays,timers,c/ct breakers etc with a new control panel containing 3 phase AC drive[inverter] as well as relays timers,c/cct breakers,[ certainly more than approx.$2000.]My preferred choice would be to retrofit existing control panel with the necessary sensors /controllers.
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Hello All. what i would like to say with confidence that it is definitely not expensive to control the speed ( via frequency control ) of motor by using a drive. The product in which i am expert can do this very well. It needs only PID function which will control the speed as per the set point and error value. Also the auto off function is there available. For more information please feel free to e mail anytime at snipped-for-privacy@ra.rockwell.com Where ever your application is in the world we are ready to help you ! Thanks. Vijay
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snipped-for-privacy@gmail.com wrote:

That would be a loop within a very non-linear loop. The final controlled variable is not motor speed, but hydraulic pressure. There is not only the pump nonlinearity to consider, but the dynamics of the piping system.
Jerry
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Try study this system http://www.lowara.com/product.php/4724
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Thank you. Any controllers for submersible pumps.
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