Centrifugal pump stops working....

[ksjm019]

This is a very basic question, just to confirm my rationale - what happens when a centrifugal pump stops working?

 

[pennpiper]

ksjm019,
You have NOT given us any data witch to approach an answer.
- What is the fluid?
- What is the Operating Temperature?
- What is the operating Pressure?
- What is the suction source elevation relative to the pump?
- What distance above or below the pump?
- What is the suction line size?
- What is the linial distance of the suction line?
- What kind of centrifugual pump is it (Single stage-End suction/Top discharge, Multiple Stage-Side Suction/Side Discharge, etc.)
- What is the discharge line size?
- What is the linial distance of the discharge line?
- What is the elevation of the discharge line destination equipment?
-

 

[ksjm019]

This is a very general question and not for any specific scenario. From an unit operation stand point, what would happen if the pump just stops working? Would the liquid backflow and oiverflow from the vessel, would the pump work till it reaches it's shut-off pressure? Assume it's pumping water...

 

[SNORGY]

Typically what happens in the plants I design is that the standby pump starts working.

Regards,

SNORGY.

 

[SNORGY]

OK...the above post was somewhat glib...

Confining the scope of my response to the pump and its fluid, irrespective of the impact on controls and affected equipment:

When the pump stops, the impeller (or impellers) will go from full spped to full stop at some ramp rate. (The ramp may be steep.) The fluid on both sides of the pump will also go from full speed to full stop in the piping, but will reach full stop somewhat sooner than the pump impeller does due to slip effects. On the discharge side of the pump, if the fluid line velocity is high enough and there is enough fluid mass and inertia, a phenomenon known as column separation may occur. This is a condition in which the decelerating fluid mass tends to pull itself apart locally, creating "vacuum boiling" (localized voids) for a short period of time until the piping pressure recovers. The pressure recovery in such instance typically produces a fluid hammer due to the void collapse. The instantaneous pressure rise can be several times higher than the pumping pressure and can, indeed, break the pump and connected equipment and piping. In my experience, you probably want especially bigger pumps to ramp down slowly when they "quit"; that's a controls issue and doesn't really help in situations when a shaft or an impeller key shears and the impeller stops turning immediately irrespective of what's happening on the other side of the failure.

I guess I am not sure about the scope of your question.

Regards,

SNORGY.

 

[rmw]

I think that he/she is asking if what is pumped uphill will flow back downhill. In the absence of a properly functioning check valve it will.

The wording also indicates that pumping against shut off pressure might be the issue.

Other than that, the OP is vaguely worded.

rmw

 

[Compositepro]

A stopped centrifugal pump will merely behave as a narrow point in the pipe. It will not cause flow to stop, but will have more resistance that a section of pipe.

 

[ChEMatt]

The liquid will equilibrate between the source and destination. So if you're pumping from one tank to another the levels will eventually even out given the elevation of the system (i.e. change in Z).

This will probably happen even with check valves. Check valves always worked in chemical engineering class, but in the real world they're not as reliable. In some situations people will drill a hole in 'em, actually.

I like what Compositepro said, the pump will just act like a narrow point in the pipe. Essentially it will act as a reducer.

A centrifugal pump is not a check valve.

Onwards,

Matt