A typical bearing housing with a few elusive vulnerabilities
December 2, 2009
Letter to the Editor,
I have great respect for pump specialists who correctly and with the voice of experience give sound guidance on pump selection. The latest write-up following this highly commendable pattern was provided by Larry Bachus and published in the December 2009 issue of Energy-Tech. We might assume that space limitations prevented the "Pump Guy" from including in his article issues of mechanical vulnerabilities with cheap pumps. Cheap pumps cause costly repeat failures, and repeat failures are the precursors of far more serious industrial incidents. The pitfalls of selecting pumps on the basis of price and hydraulic fit alone are thus glaringly evident, and shedding more light on mechanical deficiencies takes on added importance.
A purchasing entity in the Plains States recently specified better pumps, but was turned away by every one of the companies that had responded to its invitation to bid. The disappointed owner-user company then asked me to summarize the attributes of better drive ends (power ends) for pumps.
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/>There are a number of reasons why well-versed reliability engineers will refuse to accept pumps that incorporate the drive-end as shown in Fig. 1. The short summary of reasons is that he or she takes seriously the obligation to look for ultimate, not just short-term, cost of ownership. Serious reliability professionals learned long ago that price is what one pays, and value is what one gets. Anyway, while at first glance the reader might see nothing wrong, the pump drive-end of Fig. 1 does not measure up to the expectations of reliability-focused owner-operators. Again, the issue is highly relevant and is at the core of unexplained repeat failures of pumps that siphon off profits at many U.S. user facilities.
Here are the main deficiencies:
1. Oil rings are risky components; they are applied where high shaft velocities argue against partially submerging bearing balls because this submergence would generate excessive heat. In transit, they have been known to get wedged between shaft surface and the long limiter screw shown in our illustration. At certain operating speeds they tend to skip around and even abrade. To avoid this eventuality, the shaft system would have to be truly horizontal, depth of ring immersion in the lubricant must be as required, and ring eccentricity, surface finish and oil viscosity must be within close limits. Taken together, these parameters are rarely found within acceptable boundaries in real world installations. Therefore, a stainless steel flinger disc fastened to the shaft should be supplied. Such a disc then contacts the oil and flings it into the bearing housing. The disc O.D. must exceed the outside diameter of the thrust bearing. 2. The back-to-back oriented thrust bearing of the Fig. 1 set must be placed in a cartridge. The cartridge outside diameter (equal to the housing bore) must be dimensioned to allow the steel flinger disc to pass through the housing bore upon assembly. 3. Fig. 1 does not show bearing housing protector seals. An advanced version of such a protector seal must be supplied for both the inboard and outboard bearings. Lip seals are not good enough, and neither are old-style rotating labyrinth seals with one "dynamic O-ring" directly opposite a sharp edged groove (risk of degraded O-ring material getting into lubricant) and with only a single O-ring clamping a rotating part to the pump shaft (risk of rotor instability). The right mindset is: "Know what you're buying and see beyond advertising claims." Look for and understand better options! 4. As a matter of routine, the housing or cartridge bore must have a passage at the 6 o'clock position to allow pressure/temperature equalization and oil movement from one side to the other side of the bearing. Note that such a passage is shown in Fig. 1 for the radial bearing, but not for the thrust bearing set. 5. Once items (1) through (4) have been properly implemented, the breathers (or vents) are no longer needed. They should be removed and one of them should be plugged. 6. A pressure-balanced constant level lubricator should be supplied and its balance line should be connected to the second of the two breather ports. As of today, most constant level lubricators are of the considerably more vulnerable pressure-non balanced variety.
