Pump selection

[IMNO]

Hello,
I am designing a water transmission main from a source to a community. The community contain different Blocks, each one has underground storage tank. The idea is to install the pump the source and fill each tank one by one using the same set of pumps. As the blocks have different length from the source and different elevation difference as compare to each other and source, therefore I calculated the pump head considering the worst head loss required from source to a blocks.
Now using that pump head, I am getting very high terminal head/residual pressure for the remaining blocks. To utilize the available head I reduce the diameters of pipes and limit the velocity in pipe to 3.4m/sec. (we are using HDPE pipe)
Now I have the following questions
1- Is it possible to use centrifugal pump with vfd so that it can supply constant flow rate but vary pump head as per requirement?
2- Is it better to loss the head using valves?
3- Is it safe to take velocity up to 3.4m/sec . May result in water hammer?

 

[bimr]

3.4 m/sec is not an economical pumping velocity. You will be wasting energy if you are pumping any distance.

Suggest you use a velocity of 0.9 to 1.5 m/sec.

The extra pump head will be lost when the flow enters the unpressurized storage tank, there is no point trying to absorb the energy with piping or valves.

Yes, a high velocity may result in water hammer.

A centrifugal pump with VFD may be used to obtain constant flow rate with lowered pump head but don't expect to save significant energy doing so.

 

[LittleInch]

you don't really give us much to go on here about with flow, pressures, ;lengths etc, but here goes.

1- Is it possible to use centrifugal pump with vfd so that it can supply constant flow rate but vary pump head as per requirement?
Yes.

2- Is it better to loss the head using valves?
Depends on how often you're doing it. Reduction of flow using valves can use less power than max flow, but you are wasting power to some extent

3- Is it safe to take velocity up to 3.4m/sec . May result in water hammer?
3.4 is a bit fast, but depends on whether you have the ability to create surge in the first place.

PE is quite forgiving about surge as it's quite flexible and attenuates surge waves much better than steel, but the devil is in the detail.

Some options to maybe consider
If you want the same or similar flowrate to all the tanks you could look at
1) Use a PD pump (screw or similar). size the head / motor for your worst case, but then the nearer tanks will only use whatever you need. You might htough need to use slightly bigger pipe to avoid the high velocities
2) consider using different sized pumps for the different tanks or manifolding two together in series for the far tank and only one for the closer tanks
3) Pump at higher flow rate for the closer tanks for less time with a bigger pipe.

Without knowing the complexity of your network and range of heads / flow curves etc it's very difficult to advise much more.


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