Can BUMP starting a Centrifugal pump cause wear ring melting together?

[Ayden]

I have one question about starting or BUMP starting a pump.
We are discussing WHY THE IMPELLER AND CASING WEAR RING MELTED TOGETHER. OfFcourse we couldn't find out from operators. Before starting this VTB pump we checked if pump was rotating FREELY. Yes it was turning but a little bit stiff. Then after 10 min. operator is calling on radio telling that pump is stuck no rotation.

After this happening we took pump to shop and saw that wear rings at IB were melted together.....
We also notice that the discharge piping was plugged....

My question here is: CAN THE PUMP GET STUCK BY BUMP STARTING THE PUMP?????

Many thanks in advance for any answer on my question...

 

[vpl]

Do you think the plugged discharge pipe might have something to do with it? Especially if the pump was running for 10 minutes with no flow?

Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[Ayden]

NO NO NO, The pump never was started as per operator.
Offcourse if pump was started with discharge piping plugged they would have melted together without any doubt. BUT PUMP WAS BUMP STARTED AS PER OPERATOR.... Bump start is as I could understand pushing starting button for couple os seconds and turning back off. This they do to check amp. om motor and example for rotation.

So the pump never ran according to operator....

 

[vanstoja]

I have a few questions.
1. What was the operator doing during the 10 minute interval from turning the rotor manually till telling you the pump wouldn't start? Was he repeatedly jogging the pump and getting multiple no-starts?
2. What was the pump rotational speed set for when the jogging attempt(s) were made? How long does it take for the pump to come up to that speed during a normal startup?
3. Does the pump have labyrinth teeth wear ring seals? Plain annular seals? Other kind?
4. Were the seals rubbing lightly during manual turning of the rotor shaft? Was the impeller hotter than the casing wear rings due to operation just prior to jogging attempts?
It seems possible to melt labyrinth seals tooth tips during jogging events that bring the pump up to an appreciable fraction of the normal running speed, particularly if the seal clearance gap is near-zero from previous running or jogging. With the discharge pipe plugged the pump was operating at shutoff head where radial hydraulic thrust is at a maximum level. Possibly, motor unbalanced magnetic pull loading would also contribute to high radial loading on the impeller at the seal gaps. Flash temperatures at the surface of rubbing metals can readily produce melting in typical wear ring seal materials particularly if labyrinth teeth with small contact areas are involved.

 

[PUMPDESIGNER]

Just a few little comments that may help.
There should be no contact at the wear ring, just close fit.
So, theoretically bump starting will not damage the wear ring, and that is our usual experience. I have never seen a wear ring sieze from bumping. Greatest danger from bumping and reason not to do it is actually the seal, not the wear ring.

But in reality, if the wear ring was toucing slightly by poor fit at the factory, then you will have galling, and if the rubbing was serious and if you ran for more than a bump, then you could sieze, but that would take a bit longer than a single bump I am afraid.

My guess, and I feel pretty strongly about this, the wear ring was rubbing heavily and was not correct.

PUMPDESIGNER

 

[Ayden]

Pumpdesigner thanks for your commend. It is very strange though because the pump was on "Hot standby" when a warmup line cracked and spilled. The service is Vacuum tower Bottom. So we fixed the warmup line but had concerns if product had crystalized(freeze-up). So then we checked manualy if pump was turning.

The other thing is these pumps are brand NEW pumps and were put in service mid september....

Thanks

 

[tstead]

Ayden,

Bump starting is common place to ensure that the pump is rotating in the proper direction.

Also, if motor amp data is taken during the bump starts then do yourself a favor. Throw that data in the trash because it is essentially worthless.

Tim

 

[abeltio]

first some considerations

C1. bump check of a MOTOR is a typical action to check rotation during initial commissioning or after DISCONNECTING/RECONNECTING the motor leads for whatever reason...AND ALWAYS WITH THE MOTOR AND DRIVEN EQUIPMENT UNCOUPLED!!! SERIOUS DAMAGE CAN OCCUR TO THE DRIVEN EQUIPMENT IF ROTATED IN THE WRONG DIRECTION.
if the pump was already commissioned AND motor leads were never disconnected there is no reason whatsoever to do a bump.

C2. when required the "bump" is performed by keeping the motor energized for 1 second or less, and on motors up to 100~150 HP...
for bigger motors, or main leads (medium or high voltage gear, generally at sub-station or plant level) the recommended procedure is to first check phase rotation with the specific instrument for that purpose.

C3. hopefully there is a control system that records pressures, flows, and: breaker closure

C4. the "10 min" time to do the bump check is inconsistent with a bump check

C5. like tstead pointed out... amps can only be checked after the motor completed the start which could last a couple of seconds at least (even more if it is a delta/wye start)

C6. as patricia pointed out... the damage seems consistent with a no-flow operation.


C7. regardless of what the operator says, because the poor thing could be scared s***less of retribution, or does not know how a proper bump check is performed... running the pump for 10 min is not a bump... even running it for 5 sec is not a bump... see C1

C8. if what the operator says is correct the amp readings should have been consistent with a "locked rotor" condition (i.e. much higher than normal, may be you should have also seen an OVERLOAD alarm).


Now the questions:
The timeline is critical to perform a root cause analysis...
Have you checked the control system automatic log to determine:
Q1. how long was the breaker of the pump closed for?

Q2. any pressure/flow readings during that period that may support the no-flow (i.e. shutoff pressure) condition?

Q3. did the op record any amp readings before the failure?

Q4. did the control system log any overload alarm?

Q5. if so, how long after the breaker was closed? (a locked rotor would generate an overload condition in less than 20 sec usually)

Didn't answer your question... in fact, may be i just added to the general confusion...
Happy 2004

saludos.
a.

 

[macmil]

As pointed out above, the reason for a "bump" is to check rotation, typically after motor leads have been re-connected.
Well; was the direction of rotation correct?
If not, then all kinds of damage cpould take place, including melting of wear rings, if the wrong rotation unscrewed the impeller.

 

[scalleke]

If the rings were melted the pump was running.

It is possible to damage a pump during a bump even. During a bump the hydraulic load on the impeller and thus on the shaft is not what it is supposed to be during normal operation close to BEP.

If the discharge is plugged the impeller sees a liquid on which he tries to pass energy, the discharge is blocked so discharge pressure pieks. The pressure pushes the shaft to the side and touches and te impeller runs into the wear ring. If the pump keeps on turning, the liquid heats up and the liquid turns into gas. Load on the shaft shifts again.

in case of a bump you have local damage, in case of running the result is meltdown.

Best regards.

Scalleke

 

[Parado]

Bump starting, checking of correct rotational direction should be done un-couple as mention by Abeltion. In any case there is a possibility for the impeller locknut to work loosen even reverse turning for a couple of second.
Secondly for some high speed machine equipped with integrated oil pump require to ensure healthy oil system by a flip of the pump.
Your pump must have initial touch between the casing & impeller wear ring (able to turn but stiff, a sign of radial rub). Here I assume your pump although is under hot standby (from your note) but stagnent and had no flow (repair of warming up line). During the down time, the temperature of this "Hot" stand-by is reducing. By a couple of flip in later stage, the fresh liquid will come into mixing with the initial liquid which have a delta T with the fresh liquid. This mixing will not be even due to the quick start and stop. Distortion can happen to pump casing or shaft. If this combine to the initial rub, there is a possibility of wear ring melting due to excessive force induced to the rub.
Pump run with a plugged discharge in such a short duration should give better liquid hydraulic balance radially due to high internal recycle and pressure distribution. Should not be considered contributor to the seizure unless pump was run too long with high recycle & temperature, i.e vaporisation happen internally.

 

[smckennz]

Hi Ayden

You question was;
CAN THE PUMP GET STUCK BY BUMP STARTING THE PUMP?????
The answer is yes if it is a screwed on impeller and the pump starts in reverse.

Can the pump get stuck by wear rings being welded together on a 1 sec bump? Assuming a normal metal pump, of course not.
Wear rings often do rub when dry however. The liquid cushion between the faces prevents seizing during normal operation.
The operators probably misunderstood the instruction and maybe started the pump with the suction valve closed and ran it for a few minutes. This could be enough to weld the rings together. In polite circles we call this a "dry rub".
It happens from time to time, almost always the result of misunderstood/unclear instructions. We call a bump a bump and not a bump start, which is something I do with a motorcycle that has a flat battery.

Cheers

Steve McKenzie

 

[piffer]

Consider self lubricating stationary wear ring (Graphalloy) when you rebuild the pump...What is the wear ring material combination used in your pump?