Shaft rupture in certrifugal pump

[srfgp]

In our refinery we have three towers to strip acid water. The pumps at the bottom of these towers pump the stripped water back to the plants. The suction pressure is 4,3 kg/cm2 and the discharge pressure is 27,5 kg/cm2. One of these pumps has failed frequently by shaft rupture, the other ones never had this kind of failure. The pump that has failed is a different model from the other ones. The rupture is caused by fatigue and the frequency is approximetely one by month. This pump has a very narrow impeller (three times more narrow) then the other pumps to the same flow. Does someone has any experience with this kind of failure?

 

[hbw]

Pump manufacturers publish shaft horsepower rating. Contact your local manufacturer's representative and ask for the HP rating of your shaft. It is usually designed to handle the maximum impeller size for the pump.
Rarely do shafts shear if properly selected and installed.

I suspect that your problems arise from stress corrosion cracking, improper shaft alignment or machine imbalance.
Avoid austenitic stainless steel (316, 304, Alloy 20)shafts where halides (chloride, bromides, florides, iodides) are in the pumpage. See pump vendor for available alternative shaft materials.

Check pump/motor alignment and vibration.
Verify that fluid viscocity is as expected.

 

[Artisi]

assuming you have a pump from a reputable manufacturer and you are breaking shafts something is radically wrong with the system, installation, alignment etc etc. Pump shafts are usually over engineered by a large margin.

Where are the shafts breaking, is it always the same starting point for the failure?
are you using OEM shafts or pirated shafts?
Have you had the shaft failures carefully analysed or just assumed it is a fatigue failure?
Have you analysed the pump operating frequency.
Have you been in contact wqith the pump manufacturer?


Analyse the mode of failure if possible and then look for what can cause this type of failure by looking carefully at the pump installation / operation.

Naresuan University
Phitsanulok
Thailand

 

[stewbaby]

We had a similiar situation and it was determined to be stress corrosion cracking. Chloride levels were to high and metallurgy had to be changed accordingly. Along with re-evaluating the hydraulic design, see if metallurgy is different in question pump from other pumps and also recommend sending a sample of a failed shaft to a metallurgist experienced in corrosion analysis.

 

[JJPellin]

Most of the more likely causes are already mentioned in the above replies. But I would classify the options as these:
Internal misalignment between bushings or bearings; torsional resonance; excessive starts per day; up-thrust resulting in shaft buckling (for long shaft vertical configuration); material selection; shaft design (sharp steps with stress concentration). Or it could be a combination of more than one. The presence of cavitation or imbalance could be making the situation that much worse. I suggest that you get as much information as possible from the manufacturer and carefully analyze your vibration to check for obvious problems.

 

[Bjegovic]

There was a very useful thread here, with almost identical question, about six months ago.

Check your operating point at the pump q-h curve. If you are far away from BEP this may cause radial and axial forces on the shaft. In addition to everything stated above, this may be the cause.

 

[checman]

Srfgp

Most likely someone here has already touched on the reason for your pump shaft failures. But what I find most interesting is the size difference of the impellers. The thickness generally reflects capacity and the diameter reflects head or pressure. The flow of this pump would be much lower than the other two if suction and discharge pressures are the same and the impeller diameters are similar. You might want to double check that this pump is running on a good part of its curve.

Regards checman