Ross Mackay Forum Moderator 94 Posts
Re: Pump shaft failures14 December 2006 at 1:29pmIf we assume that this is a well designed pump from a reputable manufacturer, I would have a tendency to discount a pump design problem, particularly because all three pumps are failing at the same intervals and in the same location. It is highly unlikely that 3 or more pumps would have a weakness in the same spot.
Many years ago I stopped worrying about radial loads on a double suction pump. This is frequently a problem with an end suction design, but I have never seen such a problem on a D/S pump. This tends to point me towards a shock loading of some sort that would be a result of a system imbalance or improper or inappropriate operation. i.e. slamming a valve shut on the discharge line when operating at full speed, etc.
We have three double suction pumps operating in parallel. These pumps are high suction specific speed pumps handling cooling water, having a differential head of 43 m and a flow rate per pump of 7,000 h3/hr. The pumps are motor driven and run at a speed of 753 rpm at 50Hz. The pumps have two radial journal bearings and one thrust bearing at the non-drive end. Shortly after commissioning, there are shaft failures at the location near suction eye towards the motor. No other damages/indications are evident. Can you give me the probable reasons for the failure of shafts? How do I calculate the radial load acting on the shaft? We have tried to solve the problem by increasing the shaft material strength, but in vain. The failure is happening on all the three pumps. The interval between failures varies from 11 days to 2 months. The failure is brittle in nature. The breakage is not at the change in diameter. In fact, the failure is at location of maximum diameter, where the impeller is mounted on the shaft. No seal or bearing failure has been noticed in all of these failures.
Have an insight or suggestion?
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