5 replies to this topic

[Mahesh@A&M]

I appreciate if someone can give me some inputs on our pump impeller failure. I have attached those picture here. This pump is used for 100 deg F water service with a strainer on the upstream and mixer in the downstream to heat the water to 180 deg F with steam. I predict it is due to hammering in the pipe. I noticed pressure fluctuations from 15psig to 22 psig while the pump is in operation. If this is what you think, what would be the best solution?

Thank You
Roopa

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•  000_0012.jpg   1.19MB   38 downloads
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[Art Montemayor]

Roopa:

I had to study the photos well to find what I believe is the impeller failure you indicate. I believe the impeller failed by having its “nose” (the cylindrical extension of its inlet eye) sheared or cut off. The portion that was cut off is the thin cylinder or ring that we can see being propped up to where it was originally joined to the impeller eye and we also can see it laying on the table as the impeller is being extracted. Am I correct? If not, please describe the nature of the failure.

If I am correct, then it is a mechanical failure that can be caused by one of several factors:

The impeller could have been vibrating or “wobbling” on the shaft due to a mechanical failure such as a worn or damaged bearing or it could have suffered repeated process shocks such as those from continued cavitation. The inlet to the impeller wasn’t cut off by the cavitation (if it was cavitation); the cavitation could have caused traumatic and repeated vibration of the impeller, causing it to rub against a harder casing surface that “machined” or cut off the separated piece. We really are just guessing at this point because although your pictures are great shots and have excellent focusing and detail, we still lack a lot of other details. We don’t know the condition or results of the front casing. We also don’t know about the process conditions (temperatures, pressures, vibrations, noises, etc.) prior to the failure – and for how long. You say you are injecting steam to heat the 100 ^oF water being pumped but you don’t give use a drawing or sketch showing where, exactly, this injection takes place and how long this pump has work successfully – if at all.

The impeller seems to be made of cast iron. Depending on the grade of the cast iron, it could be mild and soft material instead of meehanite or close-grained, which would make it susceptible to being machined off with a different, tougher metal.

Yes, steam hammering in the discharge pipe could be causing impeller vibration in this type of cantilevered impeller design. However, the hammering doesn’t cause the impeller to be cut off directly. It may be causing impeller deflection due to the constant hammering in the line. If so, how close is the hammering source? Can it be placed further away from the impeller? Is one or two check valves placed between the hammering source and the pump’s discharge port? What make and model of pump is it?

Can you furnish a sketch of the set up and the piping on an Excel spreadsheet?

[Mahesh@A&M]

Montemayor,

Thanks for your inputs. Yes the cylindrical inlet section of the eye has cut off from the impeller. I have attached an Iso of this installation with details on pump, tank and pipeline.

During steam injection, I have seen pipe vibrating and it is noisy as you predicted. This pump lasted 3.5 years. Steam pressure is 140psig and discharge pressure is 60psig( not 16psig).

There is no check valve in the discharge line. Most surprising part is P&ID shows a check valve in the discharge line but there is none in the field. My maintenance says that it is original installation and P&ID might be wrong. I assume P&ID is right but installation persons have installed a ball valve instead of check valve.

Thank you
Roopa

[Mahesh@A&M]

oops. I forgot to attach drawing in my previous post. Here it is


Roopa

Attached Files

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[Art Montemayor]

Roopa:

Without at least one check valve – I would use two in series – I’m afraid you will continue to experience back pressure surges on the pump’s impeller. I wouldn’t chance it. The P&ID is correct and it should be As-Built. Obviously, somebody removed the original check valve - for whatever reason.

I have used this type of hot water maker before on more than one occasion and it works well just as you show the piping - except that I include a downstream accumulator tank to act a surge accumulator or bladder. You don’t show your steam control valve and the downstream temperature controller, but I assume you are using one and the resultant hot water temperature is automatically controlled by regulating the steam injection. There is an 80 psi pressure difference between the pump’s discharge pressure and the steam pressure. I would protect the pump from pressure fluctuations. If your pump is being dead-headed temporarily by any excess steam pressure or hammering, it will be forced to compensate for these deviations from normal design conditions. One way it can “compensate” is that the shaft deflects. You don’t want this to happen.

You show a butterfly throttling valve on the suction of the pump and no discharge throttling. I hope that you have a control valve doing the throttling on the discharge of the pump and that you are not throttling the suction. I would not throttle the suction of any centrifugal pump.

Carver makes a good, reliable pump. Have you consulted with them? I am willing to bet they would agree with me on the check valves and on the discharge flow control.

I hope this experience helps you out.

[Mahesh@A&M]

Art,

You mean pump might experience back pressure surges with two check valves in series? There is a control valve on pump discharge side and TIC( temp indicator control valve) on steam line. I will install two check valves in the line. This will protect the pump from pressure fluctuations.

Once again thank you for your invaluable suggestions. I have decent academic stats(GPA 4.0) now I will gain more knowledge from experience in the field and experienced gentlemen like you. I spoke to them and mailed few pictures of the impeller. I am still waiting for their feedback.

Thank You
Roopa