Pump outlet at same level as inlet, piping at elevated level
Pump outlet at same level as inlet, piping at elevated level
(OP)
We have to select a pump for transferring (lean) ash slurry from one pit to another, ~200m apart. The piping configuration requires that the pump lifts the fluid from the underground pit to over 10m height, flows through the pipeline at slight slope and then drops to the other pit. The final discharge in pit 2 is at about the same elevation as the pump CL.
From the pressure drop calculation for the overall system, the total pressure required to be developed by the pump practically is the line losses, as the difference in static head is 0. However, if I select a pump to meet the system resitance at nominal flow, it will clearly be incorrect as the pump may not develope the initial start-up head required to lift the fluid to the highest part of the piping and set the flow. What is the correct technique to specify/ select the pump in such situation? Should we consider a notional initial flow rate and ensure the pump can develope the head required to lift the fluid to the 10m level?
Must be a very elementary problem for experts in pump selection - would appreciate explaining in simple terms the confusion I am having?
thanks in advance,
From the pressure drop calculation for the overall system, the total pressure required to be developed by the pump practically is the line losses, as the difference in static head is 0. However, if I select a pump to meet the system resitance at nominal flow, it will clearly be incorrect as the pump may not develope the initial start-up head required to lift the fluid to the highest part of the piping and set the flow. What is the correct technique to specify/ select the pump in such situation? Should we consider a notional initial flow rate and ensure the pump can develope the head required to lift the fluid to the 10m level?
Must be a very elementary problem for experts in pump selection - would appreciate explaining in simple terms the confusion I am having?
thanks in advance,





RE: Pump outlet at same level as inlet, piping at elevated level
That's not your biggest problem- your biggest problem is that you'll never get a pump to lift 10m. You can only lift by creating a vacuum. 10m of water = approx 1 atmosphere. i.e. if you create a perfect vacuum in the suction line you might- just - be able to lift 10m. In practical terms- you'll never manage it (even if you prime the pump first).
I'm assuming that it's >10m from the pump to the surface of the fluid in the ash pit.
Basically- for that sort of application you need to either get the pump closer to the liquid surface (by moving either) or use a submersible pump.
RE: Pump outlet at same level as inlet, piping at elevated level
My problem is selecting the pump which will have to operate at a much higher start-up head to lift upto this 7.8m, but once flow commences, the head drops as the final delivery point is same as pump CL.
I guess the pump is to be selected for a maximum head (to lift upto 7.8m) for a notional flow and then operate at the nominated duty point which is for the full flow and reduced head (for line friction loss only.
The problem is like pumping over a mountain and then bringing back to ground level again - what is the design philosophy?
regards,
RE: Pump outlet at same level as inlet, piping at elevated level
the suction lift is 7.8m, ie product level below the pump centreline - yes!
what is the highest point in the discharge line in relation to the pump c/line?
What is the height difference between the product level and the discharge point?
RE: Pump outlet at same level as inlet, piping at elevated level
If my interpretation is correct then you must include the 7,8m head in the pump spec as the pump will have to overcome that head at startup. Theoretically you could recover some of that 7,8m head as the slurry flows down the last vertical leg, but my advice would be to disregard that recovery unless you have a massive flowrate and the pumping power is significant.
In order to recover that head the pipe size for the final leg has to be carefully selected to ensure that the pipe runs full, but that the friction losses are not too high. It is difficult to get this exactly right for even one specific flowrate, and if your flowrate can vary you have an impossible situation.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Pump outlet at same level as inlet, piping at elevated level
Suction end level = RL (-)1.5m
Pump Cl = RL 0.0 m
1st elbow in discharge pipe = RL +6.3m
2nd elbow in discharge pipe = RL +6.3m
Discharge pipe end level = RL (-)1.5m
Thanks anyway and do not bother if it the configuration is still not clear.
RE: Pump outlet at same level as inlet, piping at elevated level
Your response is also clear. I realize that unless we get the syphon effect, the head may not be recovered in the discharge leg.
And we have all real life variations, the flow rate will not be constant etc etc.
We were developing the system resistance curve for various flow rates, using a software which analyses the pressure drop from the model geometry. When we model the complete geometry, it invariably considers the gain in static head and the total pressure drop merely becomes the line loss (velocity head constant throughout the constant pipe dia).
From what you said, it appears the logical modelling will be to leave the last discharge leg and calculate the total head required to deliver upto the elbow upstream of the discharge leg. (It means, however, the friction losses in the discharge leg will be ignored too.
I was originally going by the concept that once the flow commences, the 7.8m static head gain will indeed be recovered and the pump will actually operate at a lower duty point corresponding to full flow rate and low head (only to overcome the friction). However, the pump characteristic curve should cover the initial 7.8m start-up static head (plus the line losses), albeit for a lower flow rate. That is so long as the head developed by the pump for 0 flow is say, 10-12m and for the duty flow it is the head dictated by the system resistance, the system will work. You seem to disagree with that, and I can see your reasoning.
But tell me something, could it also be danegrous to select the pump for the maximum static head which means that if we do recover full or some of the static head in the discharge leg, the duty point will be thrown off the pump curve totally?
thanks heaps for the response.
RE: Pump outlet at same level as inlet, piping at elevated level
A way to ensure that this does not happen is to oversize the final leg (one pipe size should do it) so that it does not run full. To be even more sure that there is no recovery, use the larger size and instead of having an elbow at the end of the horizontal section at the top of this leg put a tee with a short open standpipe (say 2m) to guarantee that the pressure at the end of the horizontal section is atmospheric.
Regarding your other concern of ignoring the friction losses in the final down leg - this is usually fine because if the pipe runs full and there is some friction loss it will be less than the head recovery from the syphon effect.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Pump outlet at same level as inlet, piping at elevated level
I agree with everything else you mentioned. Thanks so much for having such a fruitful discussion - much appreciated indeed.
RE: Pump outlet at same level as inlet, piping at elevated level
RE: Pump outlet at same level as inlet, piping at elevated level
The pump will, nevertheless, have the usual features for smooth operation and maintenance.
Thanks cb92.
RE: Pump outlet at same level as inlet, piping at elevated level
RE: Pump outlet at same level as inlet, piping at elevated level
The benefit of the open box is that it can be observed and cleaned if necessary. It is does block it will overflow for all to see.
The gravity leg will be subjected to free surface flows and the velocity will be very high. The abrasion occurring will be extreme.
Can you not direction drill from the pump discharge to the receiving sump and overcome the challenge?
Another alternative is to add a discharge valve at the end of the pipe to create a friction head and make a selection of slurry pump in its range? I know this will be a source of constant maintenance as the wear will be high.