Breaking seals, bearings on centrifugal pump
Breaking seals, bearings on centrifugal pump
(OP)
I have problem with two pumps i serial in a steam turbine system. The pumps are pumping
hot water. Sometimes when the turbine trips, the seal and
occasionally the bearings of the HPP fail and break. Observe that it
is the high pressure pump. When the turbine trips the pumps are running on minflow approx. 32 kg/s
BEP: Flow 137 kg/s, LPP 7-17 bar, HPP 17-37 bar. Inlet temp ca. 160 C.
When the turbin trips then pumps are cooled by a cooled (30 C) water inlet before the LPP.
The pumps looks exactly the same: overhung, end suction impeller, with a circular case. (No volute)
The impeller is recessed. After the impeller the water is passing outlet vanes and turns 90 degree
out in the "big" case. Specific suction speed 12875, specific speed 1252 (gpm,ft)
HPP-high pressure pump
LPP-low pressure pump
Ideas:
Cavitation (in the HPP ?)
Internal circulation (=> cavitation at 17 bar ?)
Radial thrust (in a circular case ?)
Axial thrust (how then, through an impulse force when the water is turning 90 degree.)
hot water. Sometimes when the turbine trips, the seal and
occasionally the bearings of the HPP fail and break. Observe that it
is the high pressure pump. When the turbine trips the pumps are running on minflow approx. 32 kg/s
BEP: Flow 137 kg/s, LPP 7-17 bar, HPP 17-37 bar. Inlet temp ca. 160 C.
When the turbin trips then pumps are cooled by a cooled (30 C) water inlet before the LPP.
The pumps looks exactly the same: overhung, end suction impeller, with a circular case. (No volute)
The impeller is recessed. After the impeller the water is passing outlet vanes and turns 90 degree
out in the "big" case. Specific suction speed 12875, specific speed 1252 (gpm,ft)
HPP-high pressure pump
LPP-low pressure pump
Ideas:
Cavitation (in the HPP ?)
Internal circulation (=> cavitation at 17 bar ?)
Radial thrust (in a circular case ?)
Axial thrust (how then, through an impulse force when the water is turning 90 degree.)





RE: Breaking seals, bearings on centrifugal pump
RE: Breaking seals, bearings on centrifugal pump
1. Seal and bearing failures are frequently due to vibrations or loads applied by axial/radial thrust (and I agree these aren't likely in your configuration) or by misalignment.
2. Typically I don't permit pumps in water service to have a suction specific speed in excess of 10,000 rpm, and impose a minimum flow of about 30% BEP. At 23% even with a 10,000 rpm pump, I would expect to see some suction side recirculation. At a 12,875 rpm pump, where I'd expect at least a 40-50% of BEP minimum flow, I'd be surprised if you weren't getting some recirculation.
3. Suction side recirculation does cause cavitation sometimes, however it can also set up some powerful eddy currents that induce a lot of vibration into the pump. Overhung construction pumps are particularly vulnerable to this kind of excitation cause shaft deflections strong enough to damage seals and eventually bearings.
Suggestions would either be to get new impellers with lower suction specific speeds and/or raise your minimum flow, if at all possible. Have you checked with the original equipment manufacturer to make see if agree with the current minimum flow?
RE: Breaking seals, bearings on centrifugal pump
if you are suddenly feeding 30 deg C water into a 160 deg C pump, then some thermal shock problems are to be expected unless the pump design allows for this.
If the pump problem occurs after trip and when the cool water is fed into the pump, then you might look further at thermal shock. Many mechanical seals are intolerant of thermal shock, and face cracking can occur. Not so sure about the bearings; possibly the casing is cooling before the shaft and reducing the bearing clearance but it depends on the bearing arrangement. I note the HPP does more work than the LPP as its pressure rise is twice as much.
Cheers
Steve
RE: Breaking seals, bearings on centrifugal pump
Some clearance.
The pump heads are equal, i.e. LPP 7-22 HPP 22-37
During the turbine trip the system pressure is decreasing fast. The pumps are protected against cavitation by the cool water inlet (in the beginning mixed with hot water).
The explaination so far from my collegues is cavitation. But I think it is something else. The damages occur on the HPP, the axial bearing has been damaged and more often the sealing. I think it has to do with som axial thrusts. But how is it created ? Does internal recirculation creates axial forces ? Notice that the impeller is recessed axial from the pump outlet.
RE: Breaking seals, bearings on centrifugal pump
RE: Breaking seals, bearings on centrifugal pump
Are you asking / saying that the bearings and seals only fail following an unsheduled shut down.
If this is the case I'm with smckennz and would suspect thermal shock as the likely cause of the seal failure- the bearing failure appears to be something else - have you had the bearing failure analysed by a bearing company. Is the bearing failure co-incidental and only apparent when the pump is dismantled for the bearing failure.
Naresuan University
Phitsanulok
Thailand
RE: Breaking seals, bearings on centrifugal pump
Thinking a bit more about it, a bearing failure analysis is important as this will help point you in the right direction in what to look for from the pump operating point of view.
Naresuan University
Phitsanulok
Thailand