Diagnosing Wet End Pump Damage

[KACC27]

I have three end-suction pumps in an application that results in severe pitting damage to the casing and impeller. The casing is carbon steel and the impeller is a 400series stainless. Both parts have been extensively damaged from the suction side of the impeller all the way into the dicharge volute. The suction piping and discharge nozzle of the pump have no signs of damage. I suspect some form of cavitation, but typically have seen cavitation damage more localized instead of widespread. 410SS wear rings used to get eaten as badly until we discovered that nickel and/or 316SS rings hold up in service without a scratch. We pump a dilute caustic liquor (with trace suspended alumina solids during upsets). I would like to attach pictures if I can figure out how.

 

[Artisi]

Photo's would be a great help.
Hopefully somebody can help with advise in getting your pics into the system somehow.

International College
Naresuan University
Phitsanulok
Thailand

 

[PUMPDESIGNER]

If you can post them somewhere on a website, just give us the link and we can go to the site, we have done that many times on this forum.

It would be very helpful to give us the following information even if you cannot get photos up:

PUMP
Specific Speed
Suction Specific Speed
RPM
BEP head and flow rate

SYSTEM
Range of flow rates
Controls
Intake line size

PUMPAGE
Temperature
Vapor Pressure
PH

Anything else you can think of.

PUMPDESIGNER

 

[KACC27]

It took me a couple of days to gather this info and link images.

Here goes: RPM = 1765; BEP = 4600gpm@435'; Range of flow = 1750gpm to 5200gpm; typical flow = 4600gpm; controls = none, runs full speed; suction line dimater = 12"; Temp of fluid = 170°F; vapor pressure = 4.8; P.H. = 10

I am still working on the specific speeds.

The links to the photos are below. Sorry, but I have a different URL for every image.

http://www.IMGSpot.com/u/04/139/13/MVC-222S.JPG

Case overview

http://www.IMGSpot.com/u/04/139/14/MVC-223S.JPG

Damage behind case ring

http://www.IMGSpot.com/u/04/139/14/MVC-305S72042.JPG

Close up of casing damage

http://www.IMGSpot.com/u/04/139/14/MVC-224S.JPG

1st Cutwater

http://www.IMGSpot.com/u/04/139/14/MVC-225S.JPG

2nd Cutwater

http://www.IMGSpot.com/u/04/139/14/MVC-226S.JPG

Pump Discharge

http://www.IMGSpot.com/u/04/139/14/MVC-229S.JPG

Impeller overview

http://www.IMGSpot.com/u/04/139/14/MVC-304S.JPG

Side by side comparison with new impeller

http://www.IMGSpot.com/u/04/139/14/MVC-231S.JPG

Inlet vanes

http://www.IMGSpot.com/u/04/139/14/MVC-232S.JPG

Impeller vane tip

http://www.IMGSpot.com/u/04/139/14/MVC-228S.JPG

Used 316 stainless impeller wear ring

http://www.IMGSpot.com/u/04/139/14/MVC-305S.JPG

Close up of wear on impeller shroud

 

[d23]

KACC27,

If the head is 435 feet and the flow is 4600 GPM the Pump Ns will be 1257.

You said there are no flow controls, but the Q is between 1750 and 5200 GPM. What is the TDH required at 1750 and 5200? Something within the system is causing the flow to change.

The other info that would help is the NPSHr and NPSHa at several flows. Having the NPSH numbers and TDH required at several different flows would help validate what could be recirculation.

Great Pics!

D23

 

[Artisi]

Looking at your pics, I would suggest that it is a typical case of severe corrosion erosion and not a hydraulic problem although running back to 1750 gpm could be giving you a hydraulic / mechanical problem - you have not addressed any problems in this area so we can assume that the pump is happy operating at this reduced flow.

On re-reading your original message, you seemed to have already answered your own question when you said that the 410ss wear rings suffered the same degradation which was cured by changing to nickel and or 316 ss rings. It is therefore reasonable to say that the pumps materials are incompatable for this application.


International College
Naresuan University
Phitsanulok
Thailand

 

[PUMPDESIGNER]

d23 - Good to see you out and about.
I have been having a problem here, so I have been delayed getting back on this.

I agree with Artisi, chemical attack is the culprit in my opinion for the following reasons.

Cavitation damage is easy to detect because of specific, localized, and well defined patterns of damage. In your case, the damage covers large areas evenly, not a trait of cavitation which is always localized.

The deterioration in the photographs is most evident in high velocity areas as follows: impeller front shroud, vane tips, volute casing close to the impeller.

Note that the discharge nozzle area is not damaged as the other areas are. I feet that is diagnostic and important. That discharge area is not high velocity, fluid movement in that dicharge area is slow and moving more by diffusion until you get well beyond BEP.

It is common for pumps to be made of materials that are resistant to chemical attacks but within limtits such as: chemicals up to certain concentrations, PH up or down to certain limits, within certain pressure and temperature limits, and also within certain velocity limits.

In your case you have exceeded a velocity limit in certain areas of the pump, accelerating the aging rate of the material when exposed to the pumpage.

PUMPDESIGNER