Pump Pit Design for Vertical Turbine Pumps 1

[ProjectEng]

What are some important considerations when designing a pump pit for vertical turbine pumps? That it should be deep enough to provide the necessary NPSH is a given.

This is for a cooling tower application where there will be 9 pumps lined up over a pit that is 64' wide. Each pump will be capable of 25,000gpm.

Thanks in advance for all the input.

 

[BobPE]

Project Eng:

That is a serious application, I don't mean to bsound rude, but if you are asking questions here, you need professional help. Get an engineer that has done this before for this simple reason that sump design is not easy. Designing one for small flows and simple hydraulics, one can fudge the design without knowing much and it will work most of the time. Something like this sounds like a career breaker.

Bob

 

[ProjectEng]

Ok let me make the question more specific. I've already made the sump deep enough to satisfy NPSH requirements.

I'm specifically interested in guidelines that dictate pump pit volume. Hydraulic Institute recommends having 2mins of pumping capacity (at max flow rate) in the sump pit.

Why is this? Is this intended as surge capacity in case water supply drops off momentarily? (I wouldn't think so because every system is different) Or is it because having this amount of extra volume will improve conditions in the pump pit leading to maximum pumping efficiency, such as eliminating vortexing tendency?

 

[stressriser]

"I've already made the sump deep enough to satisfy NPSH requirements."

You have no idea how strange that sounds. The depth of the sump will only meet it's own requirements. You need to meet the requirements of the system. What goes down must go up.

Ask the PE on the job what he/she thinks.

 

[Kawartha]

I assume that you are considering a rectangular intake structure. In this case the volume of the structure is not the crucial design parameter. It is much more important to design the structure to eliminate free surface and submerged vortices, and to ensure uniform velocity (without swirl) of liquid entering the pump intake bell. Each pump will require its own bay, and liquid velocities in each pump bay should be less than 1.5 ft/sec. Minimum submergence (S) should be calculated based on the Froude number. Dimensions and angles of the structure intake and the pump bays is critical to satisfactory operation.

If the design is not correct, the fix costs could be major. As a minimum, I would suggest that you check your design with someone that has experience designing successful intake structures.

 

[pumper]

Designing a sump is more complicated than you think, and nothing will give you more headaches than a bad sump design. Whom ever you are buying your pumps from will be more than happy to supply you with there recommeded parameters. This will be a large sale for them and they will want your pumps to run trouble free as long as possible. They will engineer this for you I'm sure with no extra cost. Then if there is a problem the ball is in there court. If they have'nt a sump specialist find a pump mfr that does...

 

[BobPE]

pumper:

I would have to agree that the design is complicated, but would idsagree that a pump manufacturer would have the slightest idea how to design a sump....

I say this from experience since most of the sumps I have had to retrofit were the result of a pump manufacturer and an industry that did not know how to get the proper engineering for the project. The pump manufacturer knows flange to flange and everything in between, but beyond the pump and into the system is the domain of engineers.

I have seen all to many nightmares in industry from good intentioned manufatcurers that dont have the expertise, but as you said, want to make the sale. Invest in an engineer that does sumps, its a small cost for a system that someone will have to hang their hat on for years to come. Plus, during sump design, different pump choices may come to light that the manufacturer may not bring to your attention if they are trying their hand at the sump design. I look at it as its all how the information for the induaty's project manager and how he makes desisions. Unbiased information being the best there is.

BobPE

 

[Kawartha]

My experience agrees with BopPE. Pump manufacturers will not provide an intake design. On a similiar application for medium to large pumps (35,000 gpm) that I have been involved with, manufacturers would not even comment on the planned intake design. Secondly, if you find a pump supplier who will assist you in designing the intake structure, they will take no responsibility for the design. If it does not work - it's your problem.

The best you can hope for from the pump manufacturers are several important dimensions: bell diameter, range of distance between suction bell and invert elevation of the intake bay, minimum submergence. All information to help design, but not enough.

 

[stressriser]

I agree. I am a pump designer, we manufacture pumps.

We do not spec systems.

 

[pumper]

I suppose the knowledge base of pump companies dwindles as much as all other fields. I have had the good fortune to work with one pump company who wrote the book on sump design. My 2 decade friend and mentor Herman who spent twenty years on the board of the National Hydraulic Institue has successfully designed 100's of sumps, Worked for this company, as the senior engineer. I concede, as the world knowledge base dwindles responsiblity for any project is left to the PE.
I have lead a sheltered life, with compedent professional comrades.
Pumper