Fluid pass through centrifugal pump

[MohdYaseen2010]

can liquid pass through the centrifugal pump whilst it's already stopped (not working) considering the pump suction source is higher than the pump level.
In my thought, there is minor fluid can slip over the pump impeller but not all the fluid. I thought also its dependent on the static head of the vessel & density of the liquid.
In my case the fluid is condensate oil, but I need the view also in all other cases such as heavy and light crude, produced water, natural gas, etc.

Mohd Yaseen

 

[Artisi]

Yes.

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.)

 

[LittleInch]

Yes.

The fluid resistance will vary from pump to pump so head, density and viscosity will make a difference to the flow rate. But I wouldn't call it minor flow.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MohdYaseen2010]

Dears,

Thanks for your feedbacks.

What is the case if the pump is PD type, do you think the fluid will pass through also?

Mohd Yaseen

 

[Strong]

Yes, a PD pump has clearances between gear elements and between housing and rotating elements. Sufficient head could rotate shaft. Does the system have check valves?

Walt

 

[MohdYaseen2010]

yes of course, the system has check valve in the discharge
the case that the pump is unloading the condensate oil in the Flare Knock out Drum to our Drain system. we are worrying about passing of the flared gas contained in the vessel through the pump when its stopped (condensate oil at Low Low) which make us losing part of the gas that suppose to be discharged to the flare header to the benefit of the drain sump.

Mohd Yaseen

 

[MJCronin]

Yes, reverse flow is possible and has happened in centrifugal pumps.

That is why we have check valves in the system ... !!!

Beware of loosening and damage to threaded impellers and pump casings as impellers have been found to completely disengage, bounce around within the casing and cause much damage. Seals are destroyed

Damage can occur in a split second !! ..... Before the motor can trip !!

https://blog.craneengineering.net/can-a-centrifugal-pump-run-backward

https://www.eng-tips.com/viewthread.cfm?qid=133249

https://www.pumpsandsystems.com/pumps/february-2015-how-avoid-costly-damage-one-simple-step

MJCronin
Sr. Process Engineer

 

[MohdYaseen2010]

Dear MJCronin;

Thanks for your feedback & cooperation but the case is opposite than you thought, I am not worrying about the backward motion. My pump system is well protected by placing Check Valve at discharge, PI & PIT @ discharge & suction, suction strainer at suction, etc.

My case is regard to fledding of source fluid through the pump when its stopped! as the pump is synchronized with the source vessel to be stopped when the level of the condensate oil be at low low and in that case we afraid that the gas contained in the vessel fled away through the pump.

Mohd Yaseen

 

[Artisi]

The running of a centrifugal pump and its driver in reverse from return flow will not unscrew an impeller. This is entirely different action from starting a centrifugal pump in the reverse direction that will / can / maybe unscrew the impeller from the shaft, usually only for a few revolutions before the inevitable disaster takes place.


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.)

 

[CheemaJ]

Is it possible to pump through a centrifugal pump without unintended consequences such as damage to pump (when the pump is idle)?

 

[SandCounter]

ChemaJ, pumping forward through an idle centrifugal pump can unscrew the impeller. It can be done if proper precautions are taken (remove the impeller, secure the impeller from spinning). Pumping backward through the pump without locking the shaft can cause the impeller to free-wheel at higher than design speeds and come apart.

I used to count sand. Now I don't count at all.

 

[georgeverghese]

If flare drum pressure is sometimes higher than drain drum pressure and condensate is free flowing through the FKOD pump to the drain drum, you may have to install a shutdown valve on pump suction (or discharge) to stop flare gas and condensate from going to the drain drum. You can auto close this SDV when condensate level is low low, or whenever the pump is stopped. SDV will have to be reopened ( say 5seconds before pump start) on auto as part of pump start sequence. If not, you may even overload the LP flare system and cause high backpressure on all sources connected to LP flare. ( Am assuming the drain drum is connected to LP flare).

 

[nondimensional]

CheemaJ,
To understand the answer to your question well, please read about FOUR QUADRANT CURVES in publications by R.T.Knapp (1930s). This knowledge gives you an understanding of what the pump (overall as a unit) is doing when it is spinning - no matter which direction, no matter whether there is torque generated or applied, etc...

Now, to answer your question directly, it depends on the pump construction. If your impeller is keyed, there may be no damage to your impeller/pump as long as the free spinning speed is less than 125% of full speed of the pump (this limit is typically to protect the bearings). These kinds of free spinning forward speeds are typically not possible unless you are in a downward pumping situation under very high suction pressures (think mountainous terrain, RO processes, etc). So, pump can be protected by adjusting the design if this is a known issue before pump construction.

If the pump design cannot be changed, then simply applying some type of control logic to force a discharge valve close just before the pump is shut down. This ensures a positive shutoff. Other avenues to make this happen is a siphon-break valve, etc and that depends on your piping arrangement too. Anti-siphon or siphon-break valves are fairly common while transferring fuel, hazardous chemicals, etc.