Pump Design for Lift Station

[allie075]

I'm a new engineer who is just getting into designs.
I have to design a lift station to handle 0.127cu m/s of flow. I've calculated the horsepower requirement for the pump(s) but folks at the office think i shouldn't have. They say all i need to give the manufacturer is the flow and TDH.
Is this standard engineering practice? The HP rating is 100. I'm thinking about 2-50HP pumps and 2 more for standby. I was thinking that during low flow, one pump could be used...possibly. Also, more pumps would give the system more robustness.... minimising the risk of the station being out of operation. (Note: This LS serves a large area). The office folks say 1-100HP and another standby. Please tell me what you think, i don't want to fall into lax engineering practices.

One more thing...the equation: Water HP = Q x TDH/3960, would it work for sewage pumps?

Looking forward to your responses,
Allie

 

[CrazyHorse81]

If you feel that you need more technical practice in designing, then size the pumps yourself in addition to providing the mfg with the information. If you come up with something different, then have a discussion with the mfg to see why. They know their pumps like the back of their hand - so it is a good idea to have their recommendation and input.

As long as the pump(s) can perform as designed, the only issue would be cost difference in my opinion - if any between 1-100 and 2-50 (which with back up would be 2 vs 4 pumps) Typically I have not seen a wet well with low/high flow pumps only high and low level alarms to turn a pump on and off.

I know that for clean water the eqn you stated should work as it assumes that there is 8.3454 lb(mass)/gal(water). I wouldn't think this is entirely true with wastewater.

 

[bimr]

One would think that you would be looking at a 3 pump lift station. With one of the pumps being a spare.

With the large capacity that you describe, one would also think that this is a variable speed motor application. The variable speed pumps run constantly, so the use of VFD would allow the pump station wet well size to be substantially reduced, reducing the size of the project.

Sewage is also water, so the same HP calculation can be used.

While you are primarily calculating HP, there are many other factors that go into the selection of the pump. You have issues such as pump efficiency, starting frequency of motors, standard pump sizes, potential for motor overloading, etc. You generally do not have access to this information.

You would be better off working to ensure that the lift station operating characteristics are correct, than trying to size the pump. You can also call more than one lift station pump supplier for additional opinions on pump capacity.

 

[chief]

your equation needs to allow for pump efficiency centrifugal. Typically I use a figure of 0.6 under the line. So your equation becomes HP = Q X TDH / 2376.

Offshore Engineering&Design

 

[jartgo]

The efficiancy parameter that chief mentioned is critical. I wouldn't use an efficiency any greater than 0.75 for theoretical HP calculations, certainly 0.6 is a smart number.

In design, I wouldn't worry with calculating the HP to any real degree of accuracy. I would figure out what my flow is, what my TDH is, and find a pump using the manufacturer's pump curves. The initial HP calculation you are referring to would just get you in the right section of the book of pump curves. The pump curves will more accurately take into account the efficiency, impeller size, power, etc.

 

[TBCEng07]

VFDs have become so common and much cheaper that we engineers love to use them.