Centrifugal or Vertical Turbine?

[justaname]

We are working on an environmental remediation process design for collection of spring water into a wet well/sump (5' diameter x 10' deep) and then pumping the water through carbon vessels for treatment. Typical flow rates are 10-25 gpm, but during heavy rains can go up to 500 gpm. We are considering 2 parallel lines with one small (5 hp) and one larger (25 hp) pump on each line, using a pressure transducer to control which pumps go online. Originally, we were considering vertical turbine pumps with vfd for increased efficiency. After looking at the sump design, we may only need 3'-4' of suction lift, so centrifugal pumps (also with vfd) may be a more economical option. Any advice on which type of pump to select based primarily on reliability and maintenance issues? The assumed max discharge pressure is 75 psi.

 

[BigInch]

Let me get this straight. Your flowrate varies from 10 gpm to 500 gpm, that's 50 X, but your power only varies by 50/5 = 10 X. That's a bit unusual.

BTW, VFDs don't increase efficiency. In fact they reduce efficiency by somewhere from 3 to 5%

Why have VFDs? One would normally run such an installation by turning on a pump at normal operating speed whenever the level got too high and turn the pump off when it gets low. Otherwise don't run NOTHING you don't need to be running. That operatng philosophy will help both your power consumption and maintenance issues FAR more than worrying about choosing between one or the other type of pump.

**********************
"Being GREEN isn't easy"
.... Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com/

 

[Artisi]

If this is a long term project I would go for vertical turbine pumps rather than above ground pumps which introduce a problem of keeping them always primed, if a short term project you could probably live with above ground pumps- also, why separate discharge lines?

 

[justaname]

Thanks for the help - this is all pretty new to me. VFD's were recommended by the pump rep - not really sure if we need them either, but was worried about excessive cycling (which I'm assuming can be mitigated by proper sizing). The pump sizing is preliminary and yes, needs to be adjusted. This is expected to be a long-term project. We are separating the discharge lines to simplify the piping and valving into two parallel treatment trains which have a maximum flow rating of 200 gpm each rather than combine and split again. This will allow for maintenance of one train during nominal operation.

 

[Artisi]

I would query why they have called up VFD's and their usefullness in the system.

It might be the same old story, just because the technology is there doesn't mean it has to be used.

 

[whammett]

Wait, am I missing something? You said during heavy rains you need the pumps to be able to handle a flow of up to 500 gpm. Yet, you said yourself that the discharge lines can handle a maximum flow rating of 200 gpm each. So your discharge lines can only handle 400 gpm cumuatively.

Either you overlooked something, or you have some way of pumping off the excess water which you failed to mention.

"Scientists dream about doing great things. Engineers do them." -James Michener

 

[justaname]

Sorry, I meant 300 gpm rating per treatment line.

 

[BigInch]

Yes that's right. If you can size your sump and treatment tank so that you can have pumps running at BEP, then you'll be running at maximum possible efficiency. There is a cost trade-off between larger tank and smaller pump, but over the life of the design, it is usually better to size your tanks so that you can have the pumps running at a constant speed and somewhere near or a little above your average flowrate, then run them on or off. The closer you can get to your average flow, the fewer on/off cycles you will have. The higher efficiency and lower energy cost when running for 25 years or more of design life, pays back almost any tank cost.

**********************
"Being GREEN isn't easy"
.... Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com/

 

[JJPellin]

I agree with Artisi. I would prefer to not have an above ground pump with a suction lift because of the possibility of loosing prime. We have a similar application where we collect ground water from a half-dozen sumps and pump that to a larger sump that pumps to a treatment plant. All of our pumps in the smaller sumps are submersible pumps. These are specially made to resist clogging. The large sump has a vertical volute pump (sometimes called a wet-pit pump). This arrangement has been working very well for us. A VFD would be a good idea if this was a transfer with very little pressure needed. The pump could be slowed down to hold constant level rather than started and stopped. But, if a fairly constant 75 psi discharge is needed, the VFD is less useful. If you slow it down to try and reduce the pump minimum flow, you may not have enough head to get into the treatment system. I would select vertical volute pumps or vertical turbine pumps and place them on level control with start / stop limits to prevent the pump from being operated below minimum stable continuous flow or above 120% of BEP.

Johnny Pellin

 

[relentlessratt]

Vertical turbine pumps (open lineshaft) are good for clean services and from what you say, the water should be clean. However, after some time in operation, the sump may have some build up of silt. Be careful and keep this in mind for your selection.

 

[Artisi]

A build-up of silt in any sump is a function of the sump design irrespective of pump style, that is unless the pump is equipped with spray holes in the casing or part of the pumped liquor is re-directed from the discharge pipe into dead area's in the sump to keep it agitated.

 

[impeller1]

you can try to find a vertical pump with an enclosed line shaft if you're fluid is dirty

 

[TenPenny]

If your sump is going to be dirty enough to want an enclosed lineshaft, then I'd suggest that an above-ground self primer would be a lot less headaches.

 

[justaname]

For the original application, our spring water is fairly clean and we don't anticipate much silt accumulation.

 

[QualityTime]

VFDs are good when the allowable number of pump stops and starts at full load are an issue. You can have unlimited number of stops and starts. Also if inrush is an issue