Sundyne LMV-311 high speed shaft damage 2

[SS108]

Hi all,

A Sundyne LMV-311 (9500RPM) pump was recently removed for a leak on its degassing line. The pump itself did not fail however we used this opportunity to inspect the pump. During disassembly, the high speed shaft was inspected and wear and breakage on the splines were noticed. See picture attachment.

We are in the process of trying to determine the root cause of the shaft damage. Wear can be seen on all the splines and 1 spline is broken.

The pump has been operating at a constant (100% BEP) flow since its installation. Suction pressures and other parameters have been monitored and have been maintained within limits for the entire operation of the pump.

Please can anyone help with determining the cause/s of the damage?
Thank you.

 

[JJPellin]

I did not think that spline drives were normally used on LMV-311 impellers. Are you sure that is the correct model?

The damage noted in the picture appears to be fretting from movement. This impeller is clamped in place by either the inducer (if it is equipped with an inducer) or the acorn nut. In either case, the inducer or nut should be torqued to 36 to 40 ft-lbs. The procedure from Sundyne then calls for locking a tab on the star washer. The problem is that the star washer only has about 5 tabs and the inducer only has two slots. In order to get a tab to line up with a slot, you often have to over-tighten or under-tighten the inducer. We no longer use the star washer.

I have seen the inducer come loose on a Sundyne pump twice in the past 20 years. In one of them, the threaded hole in the inducer was not machined deep enough and the inducer stud bottomed out in the thread rather than clamping the stack up of parts properly. On the other one, we believe that the mechanic backed off on the inducer to get a tab on the washer to line up.

I would suspect that you did not have the correct torque on your inducer or nut. The impeller came loose and started to move. The surfaces of all of the teeth fretted and one of them cracked from fatigue.


Johnny Pellin

 

[JJPellin]

Another possibility occurred to me. There are many surfaces in the stack-up with the impeller; the inducer to the impeller, the impeller to the lower seal face, the lower seal face to the sleeve, the sleeve to the gearbox seal face, the gearbox seal face to the shaft step. If you have a dual mechanical seal, there are even more. All of this is clamped together when you tighten the inducer. We have had problems if any of these surfaces are not flat. We hand lap and blue check these surfaces (except for impeller and inducer) before assembly.

I suggest you inspect all of these other surfaces for signs of wear, poor contact, fretting from movement, etc. For reassembly, I would suggest lapping each surface. I would eliminate the lock washer and torque the inducer to the higher end of the recommended range. In order to torque the inducer, you will need an open-ended adapter fitting for your torque wrench.


Johnny Pellin

 

[GPRnD]

Agreed it looks like fretting damage. Loosness in the assembly seems like the most likely cause. It is possible that the spline fit was manufactured too loose, but without access to the original drawings and measurements, that is just speculation.

Sundynes zoom along so fast that everything becomes more critical. In addition things will wear quicker.

What was the total hours on the pump and waht is the shaft material ? Did you see similar wear on the impeller splines ?