Centrifugal Pump into Empty Main 1

[sow]

Is it acceptable to use centrifugal pumps for discharging into relatively short mains that are initially empty? The kind of thing used for large drainage pumps that simply lift and dump the flow over a relatively low static head without non return valves.

I know that normally one would think of using an axial or mixed flow for this duty, however the head is around 12m and in the range for a centrifugal pump.

Two areas of concern with using centrifugals are:

1) Pumping into an empty main. We would be using soft starts therefore the max flow would not be instantaneous, but ramped up. Would this allow for no head starting in particular problems with being off the curve? If not how would an axial or mixed flow pump cope better with this?

2) Backflow – I cant see any special problem particular to centrifugal pumps with this, as long as the impellor is secured properly.

Any comments much appreciated.

Thanks

Sow

 

[jet1749]

I use self priming Durco pumps on inverter drives on an effluent duty of 75m3hr @11m head which is virtually all system loss. This essentially lifts from one pit and dumps it into an adjacent one. No problems at all. No NRV's and no particular attention paid to the impeller security (reverse flow cannot undo a screwed on impeller)

 

[stanier]

Be careful of the surge in the system when pumps stop and start as it may damage mechanical seals and bearings.

Soft starters only work when you have power. Loss of power can result in surge pressures that can damage pump and pipework.

That said many cntrifugal submersible sewage pumps discharge into gravity carriers without damage. They tend to be robustly designed for this arduous servive.

Check out Grundfos, KSB, Sarlin, ITT FLygt, ABS websites for information.

 

[PUMPDESIGNER]

By centrifugal I assume that you have a scroll type volute with a cutwater?
If so, then that kind of pump can be very efficient for your use, but there is one difficulty and concern.

Scroll volute pumps with only 1-cutwater have extremely high radial thrust reactions to flow rates far from BEP. If you operate close to the BEP you should be ok. Do not trust the pump manufacturers on this easily until they earn your trust. Many know little about radial thrust, don't care, sell the same pump to the same people every year or two because of torn up shafts, bearings, motors, etc.

I personally would not use centrifugal for that application because it is often too hard to determine and control site conditions well enough to be sure the pump operates at or near BEP.

My recommendation would be a 1-stage turbine or submersible. Two advantages to start with, 1-circumferential flow through the impeller (no cutwater), and diffused impeller, so the flow rate is irrelevant, can operate anywhere on the curve smoothly with no vibration.

As Stanier pointed out, surge is a potential problem. Filling an empty line is potential problem always. You may be thinking soft start, but you may need a VFD for the soft starting, depends on the required ramp time for the system. If the pump must operate slowly for minutes to safely fill the line, then a VFD will be needed unless you find a soft start that allows ramp times as long as you will need. I have little knowledge of what is available. We are building a system now with 4 Siemens soft starts, but the max allowable run time for those is only 20 seconds because that is all we need.

PUMPDESIGNER

 

[PUMPDESIGNER]

I forgot to ask, is your centrifugal overhund or between bearings?

PUMPDESIGNER

 

[sow]

Thank you all for your valuable posts.

Pumpdesigner - the pumps I was thinking about were horizontal submersible sewage centrifugal with 4No vanes and a rleativley large throughlet. The impellor is overhung.

How would the cutwater cause problems at low heads during start up/ main filling that it wouldnt have during normal operation?

Also why would the turbine type pump (axial?) not have problems operating away from its BEP? Wouldnt it have vibration problems due to pressure variations and dynamic imbalnces?

 

[PUMPDESIGNER]

If there is one cutwater and the pump is a scroll volute, maximum hydraulic efficiency and hydraulic balance surrounding the impeller occurs at BEP. Radial thrust increases as flow moves away from BEP (above or below BEP).

Explanation & drawings showing radial thrust on scroll volutes at:

http://www.irrigationcraft.com/radial_thrust.htm

See the following sources:
Pump Handbook 3rd Edition(Karassik et al) - Section 2.1
Centrifugal and Axial Flow Pumps (Steppanoff) - 7.3

Concentric flow impellers feeding into diffusers have NO radial thrust at any flow rate.

I agree that an axial flow pump (Ns greather than approx. 10,000), will not operate smoothly at low flows, and indeed cannot be operated at low flows. So we use a turbine in the Francis Vane area (Ns= Approx. 3500 or less)



PUMPDESIGNER

 

[PUMPDESIGNER]

sow,
Are you pumping solids or relatively clear water?
Sewage pumps, solids handling pumps, non-clog pumps, all come with a price for the abilities they have to handle solids, rags, etc. If you are pumping relatively clear fluid then you are better off getting away from those pumps.

The price paid for those abilities is: lower efficiency, more vibration, higher initial cost, and higher maintenance cost.

PUMPDESIGNER