Fluid Free Flowing Through Centrifugal Pump
Fluid Free Flowing Through Centrifugal Pump
(OP)
Hello. I have a situation where my system will sometimes need a pump but other times the fluid wont require the pump to move the fluid. This is caused by varying densities of fluid being moved. If I always allow for fluid to be on both the suction and discharge ends of the pump can any cavitation occur? To put it simply when the pump is not needed the fluid will free flow through the pump.
Is this ok? (it is a centrifugal pump)
Is this ok? (it is a centrifugal pump)





RE: Fluid Free Flowing Through Centrifugal Pump
Generally, reverse flow may damage centrifugal pumps.
RE: Fluid Free Flowing Through Centrifugal Pump
RE: Fluid Free Flowing Through Centrifugal Pump
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(2B)+(2B)' ?
RE: Fluid Free Flowing Through Centrifugal Pump
In so far as cavitation, it will happen when suction pressure is less than liquid vapor pressure.
About a sketch of the entire system so that we can understand your circumstances.
RE: Fluid Free Flowing Through Centrifugal Pump
From "BigInch's Extremely simple theory of everything."
RE: Fluid Free Flowing Through Centrifugal Pump
Cavitation will not be an issue as there is no suction being formed in the impeller eye. Vapor bubbles could form as the pressure drops, but there is nothing that could cause them to collapse violently.
If the flow is too great, the pump could overspeed, damaging the pump or the driver. A paper at the last Pump User's Symposium included a study showing that a pump can overspeed by 20% or more even at rated flow if the flow is passing through in reverse. With the flow passing forward, it might produce different results.
What is the driver? What types of bearings are used? How are they lubricated? Does it use a mechanical seal? What is the flush plan? There could be a number of potential problems with bearings and seals.
Johnny Pellin
RE: Fluid Free Flowing Through Centrifugal Pump
So far you have a number of answers, most based on the posters assumptions of the question.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Fluid Free Flowing Through Centrifugal Pump
From "BigInch's Extremely simple theory of everything."
RE: Fluid Free Flowing Through Centrifugal Pump
Of course if this is the case, I see no problem other than the pump adding some head loss in the overal system.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Fluid Free Flowing Through Centrifugal Pump
Thank you.
RE: Fluid Free Flowing Through Centrifugal Pump
1. The impeller could unscrew.
2. Hydrodynamic bearings could fail if the speed is too slow.
3. API Plan 11 or 13 seal flush may not function.
4. Oil rings, slinger or flingers may not function at low speed.
I agree with BigInch. A bypass with a check valve makes more sense and it is probably cheaper than adding a clutch.
Johnny Pellin
RE: Fluid Free Flowing Through Centrifugal Pump
A potential problem with the bypass check suggested by BigInch is that the check valve may chatter in conditions when the pump is powered with a low forward speed, but some forward flow already exists - this can happen for my particular pumping arrangement.
All of the above may or may not be a problem, for your particular pump. But you should be aware of the problems, and have a plan for fixing them if they should occur.
RE: Fluid Free Flowing Through Centrifugal Pump
With respect to the forward-facing check valve bypass, you could use a weighted or spring loaded check to cut down on chatter, or simply mount the swing check in the vertical position.
Regards,
SNORGY.
RE: Fluid Free Flowing Through Centrifugal Pump
I am beginning to like the check valve set-up. It is a lot simpler. My question now is: Would you have to apply a motor brake to stop the imprellor from spinning when system is in pump bypass mode?
RE: Fluid Free Flowing Through Centrifugal Pump
My particular check was spring mounted. It would only chatter at specific speed/frequency range - probably related to pipe volume, flow velocity and spring rate. We swapped for a swing check, and it changed the frequency and pump speed a little, but still chattered.
A similar test stand had pumps in series also, but just motor starters (no VFDs), and yes, the downstream pump could, if not started first, have the impeller start to unwind (different pump style, and much larger pump). Upon starting, you could hear a godawful crash as the impeller tightened back up and hit its stop. This pump had a set of manual bypass valves, which (to create the problem) were sometimes not used properly by technicians, because it involved a lot of walking back and forth. (If you design something that is hard to use properly, and easy to use improperly even if that causes damage to the equipment, expect the latter to be the normal way people will use it)(even if they paid money for the equipment).
Those large series pumps were replaced some time ago by an even larger pump and VFD.
RE: Fluid Free Flowing Through Centrifugal Pump
Thanks
RE: Fluid Free Flowing Through Centrifugal Pump
Try it with a nut and a stud. Hold the nut and turn the stud until the nut reaches the end of the thread. Continue turning. You are applying torque through the stud to the nut. Now continue twisting in the same direction, but by applying torque on the assembly via the nut.
From "BigInch's Extremely simple theory of everything."
RE: Fluid Free Flowing Through Centrifugal Pump
Think of looking down the axis of a bolt with a nut on it. If you spin the bolt CW, the nut will tighten. If you spin the nut in the same direction, CW, the nut will loosen.
Same thing going on if you allow the impeller to spin the shaft.
RE: Fluid Free Flowing Through Centrifugal Pump
Also worth noting...
On a splitcase (between bearing) pump, most designs have shaft sleeves that slide over the shaft and shoulder up against the impeller. These sleeves are then held on by nuts threaded to the shaft, and they in effect center the impeller in the case. These nuts are what can unthread if pump is spun by the impeller.
One will be right-handed threads, the other left-handed, and can only sustain rotation in one direction. These sleeves will also usually have the packing or seals on them.
Many failures have occured when well meaning mechanics try to flip the shaft over in the case to have it come out of the opposite side of the case if they need to get the driver on the other side for some reason.
They will usually remember to flip the impeller on the shaft but forget that the nuts holding down the shaft sleeves are still threaded for previous rotation. Then the sleeves can unthread, allowing lots of bad things can happen. In this case, the entire shaft needs to be changed out.
RE: Fluid Free Flowing Through Centrifugal Pump
From "BigInch's Extremely simple theory of everything."
RE: Fluid Free Flowing Through Centrifugal Pump