Know the pitfalls of excessively high oil viscosity

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Author: Heinz P. Bloch
Date: June 2019
Publisher: Gulf Publishing Co.
Document Type: Editorial
Length: 1,773 words
Lexile Measure: 1330L

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Since its inception in June 1990, more than half of the 347 "HP Reliability" columns published in Hydrocarbon Processing have incorporated consulting experiences and communications that span the globe. The event and analysis in this month's issue are again based on actual consulting experience. They relate to an incident involving a pump-related fire; the pump-and-driver layout resembled FIG. 1.

Initially, the sequence of events was investigated by refinery staff; a 6-hr meeting was convened later. This close-out meeting was attended by a consulting engineer whose site notes and report later supplemented the observations offered by refinery staff.

Pump lubrication. In this instance, visualize a hot service pump that was lubricated with pure oil mist. Oil mist, or "oil fog," consists of atomized oil suspended in clean instrument air. The oil mist is carried to each bearing housing in a large header leading to small-bore tubing and "reclassifier" orifices that terminate at the bearings. The facility used ISO VG Grade 100 in its oil mist system, which is generally quite acceptable for the rolling element bearings in this 3,560-rpm, API-style pump. Therefore, some deviation from normal dimensions or best-available application practice, or an external trigger event, must have existed to cause bearing failure. A process of elimination was utilized to narrow the possibilities of what had probably happened.

The oil mist was properly applied between the bearing housing end caps and bearings; nevertheless, the angular-contact, back-to-back-oriented thrust bearing set had failed. Calculations verified that the axial thrust load during normal operation was well within the bearings' allowable load range. At the time of failure, the various operating parameters had been normal.

How motor lubrication differed. The 150-hp or 200-hp/3,560 rpm induction motor was equipped with babbitted sleeve bearings that carried the radially acting rotor load. On its drive end (DE), the sleeve bearing had an integral thrust land that limited the rotor's axial travel. Lubricating oil was fed to each babbitted sleeve bearing by way of oil rings and constant-level lubricators.

Little doubt exists that ISO VG 32 was the required lubricant viscosity for these sleeve bearings. However, the DE bearing had failed and a considerable amount of the low-melting-point babbitt had overheated and melted. It is known that serious complications would result if one were to use, perhaps inadvertently, ISO VG 100 where ISO VG 32 is needed. Oil rings will slow down in thick oil or, if the rings are not concentric within 0.002 in. (0.05 mm), they would probably stop moving altogether. Moreover, a thick oil may not carry away enough heat to prevent babbitt from softening. With excessive heat buildup, babbitt will quickly weaken to the point of melting.

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Gale Document Number: GALE|A602105723