5 replies to this topic

[Robert Larson]

I have an issue that looks to me like a pump starvation issue in a new piping and system application. Pump was designed for 300 gpm @ 165 ft tdh. When installed, pump seems to run off the curve at about 250 gpm @ 70 ft. With no flow, pump dead heads at 180 ft but once you start to allow flow through system, pressure drops off but amp draw doesn't go up past 3/4 load. Pump should overload motor if allowed to run fully open at 70 ft of head. When flow is fully open, amp draw on 20 hp at 460v motor is down below 70% of full load. Concern is that if it is an npsh issue, why does unit run smoooth with no cavitation or surge sounds. I would expect to hear or feel something out of place.

[Zauberberg]

Does the pump follow its Q-H curve?

[Qalander (Chem)]

I have an issue that looks to me like a pump starvation issue in a new piping and system application. Pump was designed for 300 gpm @ 165 ft tdh. When installed, pump seems to run off the curve at about 250 gpm @ 70 ft. With no flow, pump dead heads at 180 ft but once you start to allow flow through system, pressure drops off but amp draw doesn't go up past 3/4 load. Pump should overload motor if allowed to run fully open at 70 ft of head. When flow is fully open, amp draw on 20 hp at 460v motor is down below 70% of full load. Concern is that if it is an npsh issue, why does unit run smoooth with no cavitation or surge sounds. I would expect to hear or feel something out of place.

Dear Larson Hello/Good Night,

Although this may seem somewhat funney to you but somewhat similar situation'starvation like' was faced on one of my previous employors facility while pumping crude oil out using seven stage submersible pump connected via a 24" diameter pipeline all valves verified full open and storage tank was topped up at suction providing ample NPSHa but whenever pump discharge valve slightly opened to increase the flow pump motor amps decreased.

On critical observation of discharge pressure gauge,seal(cooling crude oil) pressure gauge variations and Elecric motor drive amperes dropping pattern.

I suspected as if there comes physical obtruction in the pump suction path (pipeline segment closed to the pump itself)

The pump in question had a bucket type suction strainer and I got the pump isolated with block valves by the staff(operations) and requested mechanical guys to open the bucket strainer which was very difficult to persuade since they very recently cleaned that out.

Finally we succeeded in opening this up and everyone was astonished,as there we found a wooden piece aound 24 inchesX8 inchesX10 inches finding its way from somewhere and used to block partially the suction cross-sectional flow availability area whenever flow was enhanced.

After its removal everything became fine and we all succeeded in troubleshoot.
Hope this real story proves helpful.

[Padmakar Katre]

Dear Larson Hello/Good Night,

Although this may seem somewhat funney to you but somewhat similar situation'starvation like' was faced on one of my previous employors facility while pumping crude oil out using seven stage submersible pump connected via a 24" diameter pipeline all valves verified full open and storage tank was topped up at suction providing ample NPSHa but whenever pump discharge valve slightly opened to increase the flow pump motor amps decreased

Dear,
What is significance of NPSH(Available or required) in case of submersible pumps.

[Padmakar Katre]

I have an issue that looks to me like a pump starvation issue in a new piping and system application. Pump was designed for 300 gpm @ 165 ft tdh. When installed, pump seems to run off the curve at about 250 gpm @ 70 ft. With no flow, pump dead heads at 180 ft but once you start to allow flow through system, pressure drops off but amp draw doesn't go up past 3/4 load. Pump should overload motor if allowed to run fully open at 70 ft of head. When flow is fully open, amp draw on 20 hp at 460v motor is down below 70% of full load. Concern is that if it is an npsh issue, why does unit run smoooth with no cavitation or surge sounds. I would expect to hear or feel something out of place.

Dear,
If you would have been put a simple sketch of the system along with the fluid handled and its conditions at inlet and outlet(at design stage or from the PDS/MDS) it would have better for us to comment on this issue.From your information like 300gpm@165ft of TDH and dead head around 180 ft seems that the pump has flat curve (less than 10% of slope) But I don't understand why it deviates from its Q Vs H curve as you told the head of 70 ft @ 250 gpm. Seconly you say that Unit runs smoothly with no signs of cavitation or surge sounds then where the problem falls just check the ammeter from electrical section(2 ways - 1. ammeter near from pumps swith and 2. from electrical sub-station). Secondly you check for the net outflow from your pump if there is any Flow indication in pump discharge as power should go up with flow. Seems to be weired doubt.

[S.AHMAD]

I have an issue that looks to me like a pump starvation issue in a new piping and system application. Pump was designed for 300 gpm @ 165 ft tdh. When installed, pump seems to run off the curve at about 250 gpm @ 70 ft. With no flow, pump dead heads at 180 ft but once you start to allow flow through system, pressure drops off but amp draw doesn't go up past 3/4 load. Pump should overload motor if allowed to run fully open at 70 ft of head. When flow is fully open, amp draw on 20 hp at 460v motor is down below 70% of full load. Concern is that if it is an npsh issue, why does unit run smoooth with no cavitation or surge sounds. I would expect to hear or feel something out of place.

Dear friend

Two possibilities that I can think off at this moment.

Firstly, there is possible partial blockage at the suction of the pump. However, under this situation, you should hear cracking sound of cavitation unless the temperature is far below the saturation point that cavitation is not possible even at very low suction pressure. Since you did not provide detail information on type of fluid, temperature and NPSHA, then we cannot add any further comment on this one.

Secondly, there is possibility that the pump supplied to you is not the one from which the pump curve was derived. Did you witness the shop test? Probably, the one supplied to you is of different impeller design.