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large volume pump station, variable demand

large volume pump station, variable demand

large volume pump station, variable demand

Forum Participants:

I'm working on a project where the customer has one large diversion point for his water right from a river.  Water will be drawn from the river through a large wet well (like a reservoir can pump) structure using a vertical turbine pump.  Decreed flowrate is 4,500 gpm.  This system will supply center pivots and wheel lines with an instantaneous demand ranging from 400-4,500 gpm.  Under maximum flowrate condtions, required head will need to be ~ 150 ft.  The wet well turbine pump will need to be electrically driven.  
What thoughts do you all have about the best way to meet the variable demands with only one supply point?  The law will only allow this one diversion point for the full decreed rate.  I'm trying to get some fresh approaches to this problem so I have not included descriptions of how the customer's system is currently operated.  If needed I can supply this information at a later date.  What have other irrigators, irrigation districts, municipalities, etc. that you all have worked with done to supply their systems with such large maximum volumes of water that can fluctuate over a large demand range?

RE: large volume pump station, variable demand

Hi jfriddell

with the system you outline, I would look at a variable speed drive pump with a pressure control loop, and perhaps a minimum flow bypass to cope with short term (almost)zero demand conditions. The most important thing is to establish a daily demand profile and then use this to prepare a pressure/flow vs time chart. This will let you know is simple variable speed and bypass is sufficient, or if you need a second smaller pump or perhaps a reservoir. The choice between smaller pump/reservoir or bypass valve may be made on economic (capex vs opex) grounds.
Security of supply is also an issue, especially if the water could be used for firefighting or other emergency use. Care needs to be taken in selecting the location of the pressure tapping; normally at the spur which will see the lowest operating pressure. Also care needs to be taken to ensure piping is not subject to excessive pressure under unusual demand conditions; sometimes a second pressure sensor is used for peak pressure limiting.
Personally, I prefer a pump fed reservoir and gravity supply to users, but this is often not possible.
When specifying your pump, be sure you state it is to be VSD driven, and if you have poor electrical engineering support, consider purchasing the pump and VSD as a single package.



RE: large volume pump station, variable demand

Have you considered a multi-pump installation within the one wet well.
400 - 4500 variation seem too extreme to be done easily.

Naresuan University

RE: large volume pump station, variable demand

Valued Respondents:

Thank you for the advice so far.  As this is strictly an irrigation water supply, there is little need for sophisticated controls.  The greatest level of sophistication that I've considered thus far is a manually controlled VFD, but my turbine pump supplier has leaned me away from using a VFD on the required 300 hp motor driver.

As Artisi points out, the flowrate range is extreme and I realize that one large pump is no the best method, but I'm limited to this configuration as the wet well infrastructure already exists, has weathered major flood events well, and the customer is happy with the structure.  I believe the wet well is made of 24" steel casing, so using multiple smaller pumps in the same well is not feasible.  Additionally, to enter the existing piping network, a below grade pump discharge is required.  The top of the wet well is ~20-25 ft. above ground level as a means of flood protections and to keep the motor and controls out of floodwaters.

Keep in mind that this is a privately owned investment so overall cost of this upgrade project is a major concern and not to be likened to the same critical design factors that say a municipality water supply problem would demand.

Any other comments by others will be greatly appreciated.  Thanks again for the time contributed by Artisi and smckennz!

Thank you,


RE: large volume pump station, variable demand

I would be extremely interested in seeing what your "turbine supplier" is proposing if they are steering you away from VFD control.   


Naresuan University

RE: large volume pump station, variable demand


My turbine supplier has only suggested that a constant pressure valve, such as a Cycle Stop valve (CSV), may be a less expensive alternative and would provide similar pump throttling capabilities without the extremely high initial cost of the VFD for such a large motor.

Do you all see any way that the wide range of flowrates at the required pressures can realistically be handled by this one large wet well pump, or is there some other method that stands out as far superior to the "one large pump to do it all" idea?

The customer is currently using the turbine simply to lift the water from the river to the reservoir.  The pipeline network is then pressurized by two engine driven centrifugals.  The reservoir is old and has very little storage capacity left after ~60 years of sediment build-up.  Customer has had bad experiences cleaning out reservoirs in the area as they tend to be very "leaky" afterwards.

Any futher thoughts?  Thanks to all who have commented.


RE: large volume pump station, variable demand

Since we are just brainstorming I'll throw in my couple of opinions.

VFD could do it, downside is the possibility of repair/ replacement liability exposure.

Artisi is correct about multiple pumps.  The advantages of multiple smaller pumps are many, but consider the following advantages especially.
With smaller pumps you can get lower Specific Speeds (Ns), and probably lower Suction Specific Speeds (Nss).
With Ns in the 2000-3000 range you obtain maximum efficiency, but best of all, you get very smooth operation at all flow rates.  In other words, a low specific speed 1,000 gallon pump may operate smoothly from 200 gpm to 1,100 gpm with no vibration, long reliable life.

Now, couple that same idea with a multistage turbine, which has a diffuser in each bowl and balanced flow out of the impeller through that diffuser, that puppy sings at all flow rates with no vibration, lasts forever.  That is why multistage turbines have an incredible reputation for long reliable service. They can operate very smoothly through huge operating window, and often way off BEP.

Now, with that low specific speed, the power curve is directly proportional to the flow rate.  In other words, as flow increases, current consumption increases, and as flow rate decreases, current consumption decreases, AND ALL WITHOUT A VFD.

Sorry 'bout the casual nature of the writing, but I am tired.  I really, really like discussing these topics.


RE: large volume pump station, variable demand

Pumpdesigner, et. al.:

I get the impression that you are leaning toward a multi-stage turbine with a diffuser in each bowl.  From your discussion, it sounds as though that set-up would accomodate my needs as best as possible.  All I would have to worry about is controlling the discharge pressure when the system has very little demand and the pressure increases.

Any thoughts on how to most effectively do this?  Would a pressure reducing valve be the answer here, or is there some other control technology out there?

I really appreciate all of your discussion/brainstorming.  I would like to be afforded the opportunity to redesign the entire system for the customer, but I know that replacing the wet well with multiple smaller wells, etc. will be far out of the cost of the budget and may not be allowed by the water law.

Keep the thoughts coming as they occur to you all.


RE: large volume pump station, variable demand

You are correct, you need pressure control for sure.
VFD can be used to control pressure.  Flexible and field configurable.  But always, the down side is when they fail, big dollar replacement costs.  On big deal projects the money is there and no one is watching.  But when someone is watching the store, you could get into trouble.  We have many VFD projects working reliable with no trouble ever.  But I do read where big failure problems cost the customer repetitively.  I read about one oil field where the VFD replacement costs every year are hundreds of thousands.  They have brought in multiple consultants trying to reduce failures, no one has been able to help yet.

My point about the power curve being proportional to the flow rate for low Specific Speed pumps was simply to point out that a control valve restricting flow also causes a drop in current, so the energy is conserved when flow rates are low.

DOL (Direct Across the Line) starters, with Low Specific Speed pumps, and a jockey pump.  5000 gpm could be done with 4 main pumps doing 1,000 gpm each, and a 400 gpm jockey pump.

When done correctly that system works reliably for 50 years with little maintenance, and low cost maintenance.

Also, the Feds are pushing efficient systems without VFDs to save electricity.  Pick the correct pump and run full speed, achieve maximum efficiency.

Have a great day.


RE: large volume pump station, variable demand

Quote - The customer is currently using the turbine simply to lift the water from the river to the reservoir.  The pipeline network is then pressurized by two engine driven centrifugals.- Unquote

The method in use now is the best by far, run your big pump at its best efficiency to fill the reservoir on demand, then use a combination of smaller units for the actual irrigation duty.

Seems the system has been pretty good for the past 60 years - why try and re-invent the wheel with complex equipment - just because VFD's and like equipment is available there is no compelling reason that it has to be used.
Look to carefully cleaning the reservoir.

Keep it simple.

Naresuan University

RE: large volume pump station, variable demand

I like what Artisis said.
There were an awful lot of smart guys out there, sometimes I think we are going downhill as those old guys retire.
I'm not speak about BobPE though.


RE: large volume pump station, variable demand


When chosing between adjusting the speed of a big pump or operating several smaller pumps One should look at the system curve,  if the system curve is steep (such as a long pipe) a VFD or other variable speed device provide the best efficiency. when the system curve is flat (pumping from one tank to another through a short lengths or large diameter piping) then many smaller pumps provide the best efficiency under varying flows.

An alternative to a VFD is a magna-drive, it is a magnetic coupling that uses a normally driven motor and couples it to the pump through an induction magnet system.  The speed is controlled by a lever that can be automated or manually adjusted, the lever controls the distance between heavy duty magnets and copper plates.

Anytime you use a valve to reduce pressure there is lost energy, but also anytime you run a pump off of BEP there is lost energy.


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