MInimum flow requirement of Submerged Pump

[Trond]

Hi all,

We are designing a plant where one piece of equipment is a sump tank submerged pump. The liquid pumped is hydrocarbon liquids, potentially as low as -20 to -30°C. I have just been informed that there is a minimum flow requirement for this pump, and the client has requested minimum flow protection. I am not used to small submerged pumps mormally being fitted with this, and unfortunately the pump curve is rather flat, so a simple PCV with return flow to the tank will not do the job.

What would be the cause of the minimum flow requirement for a submerged pump? I was thinking that temperature would not be the issue, as any heat generated woudl be removed by the liquid. Is it vibration that is the problem here? What would be the consequence of running a submerged pump for, say 10-15 minutes against blocked outlet?

Any help, hits or suggestions would be greatly appreciated.

Trond

 

[smckennz]

Hi Trond
the first selection consideration of a deadheaded pump is heat generation. You need to work out if there is any danger of vaporisation due to the heat generated from the fluid turbulence within the pump. If there is vaporization, you need to decide if this creates a hazard or a problem with the pump. A typical hazard is explosive potential, and typical pump problems are shaft seal or wearing ring overheating.
The second selection consideration is whether the pump will damage itself through vibration (as you identified) or cavitation. With small pumps of low specific speed (highish pressure) this is seldom a problem, but since you have selected the pump already, you should put it to the manufacturer.

Cheers

Steve

 

[Trond]

Thanks for that input, Steve,

The sump tank is connecteed to a flare system which is permanenlty purged, so explosion hazard due to vapourisation is not really a concern. I am still awaiting a reply from the vendor (via out client), but suspect that operating the pump against a blocked outlet for a limited time (say a few minutes) would not cause any problems. The discharge pressure is only around 5 barg (discharging to an atmospheric vessel), so I would presume that the minimum flow referreed to here has more to do with operability than pump protection. Naturally running the pump permanently with a deadhead would eventually damage it, but as I said above, a few minutes should probably be OK. Will get this confirmed from the supplier, though.

Thanks again for your input!

Trond

 

[25362]

Question: is there a possibility of a surge on power failure that may rotate the pump in reverse acting as a turbine, and what would be the consequences of that effect ?

 

[rmw]

Careful about the reversing of the rotation. If the impeller is threaded onto the pump shaft, and they most often are, changing the rotation for any reason (as mentioned, or bump testing for rotation check) will cause the impeller to unthread, and crash into the casing, and can cause damage to the pump.

Regarding the recirculation requirement, submerged pumps sometimes have internal clearances that can meet the minimum flow requirements to prevent overheating of the pumpage. Check your pump for this feature.

Otherwise, it is a heat transfer problem to determine how long a dead headed pump would take to heat the whole sump up to a point of danger vs just having a recirc flow.

rmw

 

[profengmen]

What would be the cause of the minimum flow requirement for a submerged pump? I was thinking that temperature would not be the issue, as any heat generated woudl be removed by the liquid.

But Temperature IS the issue. Submerged pumps need a minimum flow to keep them cool. They will not cool by just "sitting" in the liquid, they need convection.

 

[buzzp]

Profengman has it right. The liquid is required to haul the heat away from the pump. If the liquid is not moving then there is no heat removal. Since your liquid is pretty cold to begin with it could probably run dead head for a long time before any damage is done.
As far as turning the pump backwards, check valves are generally used in larger installations to prevent this from occurring. Not all pump/motor mfg's have problems with reverse liquid flow unscrewing their pump motors from each other or the casing, depends on how the fastening bolts are tightened, right or left and which way the pump turns. Generally, reverse flow will not cause any problems regardless of how the fasteners are turned to tighten. Problems usually only occur when the pump is shut off and liquid flows in the reverse direction AND you try to start the pump again. Timers are also used in the place of check valves in some applications to allow the fluid to run backwards until it stops.
For smaller HP pumps monitoring the power to the motor can provide fluid flow information since the relationship is almost linear in most pump/motor designs at a fixed head.
I am pretty confident that this method would provide adequate protection for loss of liquid. Larger HP pumps can generally get away with just monitoring the current, although the relationship is not linear.

 

[sailoday28]

See thread407-96694
with
c= spec ht
dt= temp rise
H= pump head

c dt =H(1/n -1)
If one can assume that at low flows
n = m H that is n, pump efficiency is directly proportional to flow, Q.
Then n approaches 0 and temp rise becomes a problem.

 

[BobPE]

Trond:

The pump should only operate on its curve in a calculated pump operation envelop. That is that there is a max and min flow for the pump you are using, flows outside this range could damage the pump. Specifically, low flows will cause recirculation problems, which would cause vibrations and damage the pump or appurtenant facilities....

Low flow control is easy, monitor flow and shut it off when below your range or, bypass adequate flow back to suction.

Depending on your pump, 10 to 15 minutes may not be a problem...the problem will be if it is 10 to 15 minutes 10 to 15 times a day, 365 days a year...then your pump would most likely fail.


Let us know how you make out....

BobPE