Pump different modes ?

[mech212]

We have a system with a centrifugal pump with only one speed which means one H-Q curve . Now the system flow rate needs changes in 3 modes . I=mode 1 it needs Q=10 m3/h and H= 3 bars , mode 2 it needs Q=15 m3/h and H= 3.2 bars , mode 3 it needs Q = 20m3/h and Q=3.4 bars .. WE will install a control valve with three different openings to achieve the 3 different modes by changing pressure drop so moves the system curve,, but does a pump will be capable of producing this different heads and flows at every valve position ? How things goes in such a situation and how to deal with CV and pump suppliers

 

[Artisi]

do you have the pump operating curve?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[mech212]

No , we don't install the pump yet

 

[LittleInch]

You size your pump duty base don worst case allowing for some level of pressure drop across the control valve.

SO in your case the worst case is 20m3 @3.4 bar. Allow 0.5 barg for a CV at that flow rate and then go get your pump.

Once you have your pump curve you can generate a CV data sheet with your three flow rates and your differential pressure. highest flow is 0.5 barg pressure drop (say), the others you need to work out what the pump output is at your flows of 10 and 15 m3/hr and then subtract your required pressure (3.0 and 3.2).

The CV vendor will then size your valve for you.

Simples

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[mech212]

But as you can see if this is a pump curve , the flow rate of 15 will never match with head of 3.2 even if we use a control valve , so we need to work on point 3 so we need a pump with variable speeds , so is it better to determine the 3 points i need of the 3 modes so that the supplier can find a pump curves that fit to what i need with variable speeds ?

 

[LittleInch]

Yes it will. If you want 15 m3 at 3.2 bar just add a control valve and close it a bit. I don't see why this is so difficult to understand. It's no different to turning on a tap in your house and modulating flow accordingly.

You can do it with variable speed as well but your OP said you only had a pump with one speed. Make your mind up please.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ashtree]

Perhaps to aid understanding , the control valve not only regulates flow but also has a pressure drop across the valve proportional to flow but which is also adjustable, so in effect controls pressure as well.
What LittleInch says is exactly true. There will be some position of the control valve where a flow of 15m3/hr and a pressure of 3.2 bar is achieved. The trick will be sizing a control valve that will be easily adjustable to give you the range required without exceeding valve limitations. This should not be a problem for a valve supplier who know what they are doing.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"