Magnetic drive pump failure in 40secs from start-up

[mech225]

Good day all,

The situation: A mag drive pump has been installed in the plant and is the bottoms pump for a vessel. The pump liquid is Ethane and condensate. On start-up of the pump the pump fails due to journal bearing failure. The bearing is contained within a silicon carbide sleeve which runs against a carbon graphite housing which is attached to the impeller back-plate.

Hence liquid (Ethane and condensate) is directed to the volute casing(suction), then to the discharge of the mag drive pump, whereby a certain quantity is routed to the filter to remove solids then to a cooler. After cooling the liquid, the liquid is routed through channels to the journal bearing (cooling and lubrication) and between the magnets for cooling. Once this stage is completed the liquid exits the internals of the pump via a hole drilled through the centre of the shaft of the pump.

The problem: The journal bearing has failed on start-up of the pump. The bearing is able to handle radial and axial thrust developed. Approximate time of running before failure: 40 to 90secs.Also note that the temperature of the liquid being pumped is -52deg. Celsius.

Pump specifications:
Suction pressure: 460Kpa(g)
Discharge pressure: 1437Kpa(g)
NPSHa: 4.3m
Differential head: 203m
Vapour pressure:519Kpa(A)
Specific gravity: 0.492
Viscosity: 0.11Cp @ -52deg Celsius
Rated power: 48.55KW
Efficiency: 46%

Question: I require your help(just brainstorming) to determine what could be wrong with this set-up???

Also does anyone know of a Anthony E. Stavale, he has published some very interesting papers on magnetic drive pumps, bearing materials, extreme operating conditions of mag drive pumps and dry running of silicon carbide bearings.

Below is a list of his papers that i require.

-Managing extreme requirements with plastic magnetic coupling pumps
-Dry running test utilizing silicon carbide bearings and polymer lubricated strips with conductive and nonconductive containment shells in an ANSI magnetic drive pump.
-Analysis and testing of alternative bearing materials for application in sealless pumps.

Thanking you all for your help.

Mech225

 

[ClicketyClack]

Can you give the manufacturer and model of the pump? I am having a difficult time visualizing the internal design.

Also, describe the failure (crack, shatter, shear, torque, etc.)

The conditions you list, are these actual at the time of failure or rated? The temperature really sticks out to me.

 

[dickon17]

It is the 40 seconds which seems significant. The high differential pressure would infer some fairly high bearing loads. Should the bearing flush curcuit have been primed before start up?

 

[Rotator]

40 to 90 seconds suggests that the bearing is starting dry and may stay dry due to the heat generated by contact friction.

You are dealing with a high vapour pressure fluid, and unless the bearing is designed to run dry you need to ensure that it remains a liquid all the way through the bearing circulation circuit. As the liquid circulates past the magnets and through the bearing it picks up heat and loses pressure. Is your cooler providing enough cooling? Look at the expected temperature rise/heat input from the magnets and bearing, (or can you measure temperature athe bearings) and see if the vapour pressure at that temperature exceeds suction pressure (absolute). Vapour pressure may be very different to that at -52degC.

Good luck!

 

[TenPenny]

Was the pump flooded before startup?

I think in a system like this, it will be crucial to make certain that the pump, the cooling and lubricating loop, and all parts of that system are filled and vented prior to startup - a good mag drive can only run for a minute or two dry without catastrophic failure. I think Ethane would be even less forgiving.

 

[pumpking]

Number of issues to look at here, temperature already mentioned, priming also mentioned. What also concerns me is the low density of the product, what you can find is that low density products ALSO carry the characteristic of poor lubrication characteristics in bearing faces, which would require possibly looking at different bearing materials (we had problems pumping hydrogen peroxide which is also poor lubricant, and found that we had to go to Zirconium Oxide bearings to overcome the specific characteristics.

What also concerns me is the high pressure of the pump, it has already been mentioned of the high loads this would have internally and we are running against a graphite material (not good in my experience).

Can you add a pressure gauge to inlet of bearing flush material to make sure you are getting adequate circulation around the back end ?

Incidentally, you can easily add 10'C intot he product through eddy currents through Isolation shell, as well as friction generated by bearings.

Ash Fenn

http://www.cdrpumps.co.uk