Fluid Free Flowing Through Centrifugal Pump 1

[mmur]

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)

 

[bimr]

What are the fluid properties?

Generally, reverse flow may damage centrifugal pumps.

 

[mmur]

densities range from 1010kg/m3 to 1300kg/m3. The pump would not be running in reverse. The fluid would simply be running through pump. (no work required on the system)

 

[electricpete]

Sliding bearings under load (if present) usually don't like rotating slowly. For example kingsbury thrust bearing on vertical machine or plain sleeve bearings on horizontal machine.

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(2B)+(2B)' ?

 

[chicopee]

Your problem is ambiguous at best as you are not well defining your problem. For example "....If I always allow for fluid to be on both the suction and discharge ends of the pump ...." what do you mean by that? Are you talking about the pump being at rest when the system is shutdown?
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.

 

[BigInch]

Much more efficient to install a tee in suction and a tee in discharge, then connect the two together with a check valve. You will have a bypass that won't lose much head, or cause unnecessary maintenance expense to your pump. When the pump is running, the check will automatically go closed.

From "BigInch's Extremely simple theory of everything."

 

[JJPellin]

There are a number of potential problems. But you will have to provide more details so we can define them. If the pump has a threaded impeller it might unscrew. Even though the rotation will be forward, the torque will be backwards. The pump becomes a turbine.

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

 

[Artisi]

Think you need to expand your problem a little further, I assume there will always be flow from inlet to discharge even if the pump is not in operation.

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.)

 

[BigInch]

He doesn't say anything at all about reverse flow, but many have assumed it to be so.

From "BigInch's Extremely simple theory of everything."

 

[Artisi]

Correct, the OP clearly stated that the pump would not be running in reverse.

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.)

 

[mmur]

Thank you everyone. I am going to simply approach this as the pump being a head loss to the overall system. When the pump is not need I will have a "clutch" to remove it from the motor so it can spin freely. I will be mindful of the pumps RPM when free flowing though.

Thank you.

 

[JJPellin]

With forward rotation at an unknown speed, the following problems could still exist:

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

 

[btrueblood]

I've run forward flow through a pump on a test stand, and while unscrewing the impeller might be a problem, I've never had it happen - with my particular pump. Similarly, the forward flow does not spin the coupled motor/impeller, the pump is just not a very efficient turbine in forward flow direction - with my particular 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.

 

[SNORGY]

It was loosening of the impeller (if the pump is so designed) that made me a bit leery about this as well. Upon subsequent start-up, it can create some issues.

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.

 

[mmur]

Thanks again. I appreciate the help. I know some people are looking for some details on the pump but unfortunately I have spec'd out a pump yet so I cant provide these details. I have simply worked out the hydraulics and realised that with the varying densities the requirement of a pump may not be necessary.

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?

 

[btrueblood]

Snorgy,

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.

 

[mmur]

This may be a stupid question but I have to ask for my own sanity. If the fluid is "free flowing" through the pump, the impeller would spin in the same direction as it would if it were in operation (from suction to discharge), correct? And if it were to spin in the same direction why would it unwind?
Thanks

 

[BigInch]

The applied torque is in the opposite direction.
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."

 

[DubMac]

There is a difference between the shaft spinning the impeller, and the impeller spinning the shaft.

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.

 

[DubMac]

Didn't see your reply BigInch, sorry to duplicate.

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.