- 20 December 2004 -

Dual magnetic hermetic sealing devices for oil mist lubricated pumps and electric motors

Oil mist technology has become the predominant method of lubricating pumps at modern US refineries. In this article, Chris Rehmann of Aesseal Inc, and consulting engineer Heinz P. Bloch, discuss the concept of oil mist lubrication and briefly review past and current best practices before focusing on opportunities for cost-effectively improving the reliability of this lubrication method.

When a survey was recently made of eight well-known US Gulf Coast oil refineries, their pump bearing protection and lubrication practices became the subject of closer scrutiny. As anticipated, these refineries had adopted the important industry-wide practice of using pure oil mist lubrication on thousands of pumps. However, although it had been assumed that the implementation details for oil mist technology had followed state-of-the-art practices, this was not always found to be the case. The survey pointed out opportunities for cost-effective reliability enhancements – the subject of this article. But first, an overview of the concept of oil mist lubrication, along with a brief review of past and current best practices. This overview then leads to a discussion of how to apply novel, cartridge-type

API-610 9th edition-compliant (2003), dual hermetic bearing housing seals in a conscientious improvement programme. The end result will be the optimization of pump and electric motor bearing life in reliability-focused process plants.

Oil mist technology is prevalent

Targeted surveys confirmed that oil mist lubrication is the predominant method of lubricating pumps throughout the refining industry in the USA. Estimates of oil mist usage at 30 refineries in the Beaumont-Port Arthur area ranged from 80% to 90%. In this part of Texas, about 80% of the pumps at each refinery were serviced by the oil mist system. A US

West Coast consulting engineer estimated that over 50% of all USA refineries are using oil mist. Indeed, oil mist lubrication is recognized as a key part of an API-based maintenance programme at many refineries, and pump MTBFs (mean times between failures) of four to six years are being achieved rather routinely. MTBFs as high as 8.6 years are on record at some of the more exceptional refineries.

Process basics

Oil mist lubrication has been in use in petrochemical plants since the 1960s. The basic process description is as follows: Firstly, no liquid oil is placed in the bearing housing. Instead, an entirely static ‘oil mist generator’ creates the oil mist in a central module that services all pumps within about a 600–1000 ft (c.180–300 m) radius. The mist is a mixture of atomized (<3 micron) oil droplets mixed with air in a ratio of about 1:200 000. The mist, which has the appearance of light smoke, is moved out to the pumps via 2” or 4” piping in a low-pressure (<1 psi, approximately 5–6 kPa), low-velocity (<7 ft/s, about 2 m/s) process.

At the pump, the oil mist is inserted into each pump bearing housing through a small-diameter application fitting or ‘reclassifier’, which greatly increases the mist velocity, causing the droplets to collide and coalesce into larger, liquid drops of oil.

This air/oil mixture passes into the housing, where some of it contacts and lubricates the bearings, while much of it falls straight through to the bottom of the housing. The coalesced oil finally passes through a hole in the bottom of the housing and is collected for disposal.

The advantages of oil mist lubrication include:

1.Almost completely maintenance-free and self-checking. It is no longer necessary to rely on operators to check and fill housings with oil.

2. Better lubricant properties, as the oil on the bearings is always new.

3. Lower operating temperatures, up to 10–20 °F (approximately 6–12 °C).

4.Protection of non-running (‘standby’) pumps.

5.Reduced power requirements, since the bearings operate in a thin film of oil instead of ploughing through a pool of oil.

The MTBF benefits of oil mist over oil sump lubrication have been well-documented, and the oil mist lubrication option was included in the API-610 7th  edition in 1989.

Plant-wide oil mist lubrication evolved at about the same time as rotating labyrinth isolators (1960s and 1970s). As the oil mist enters the larger bearing housing, its velocity will slow down. Also as it enters, the droplets will have become larger, so they tend to fall out of suspension to the bottom. It should also be noted that certain bearing cages create ‘windage’ – a fan effect that tends to push the mist away from the rolling elements.

However, to be effective the oil droplets must contact the bearings and are thus compelled to travel horizontally. This horizontal movement is accomplished by using either straight labyrinths or a rotating labyrinth isolator. Either labyrinth style allows continuous mist leakage, thus creating a flow path for the air/oil mixture through the bearings and out into the atmosphere. With much of the oil mist having taken the path of least resistance and collecting at the bottom of the housing, more oil mist must be supplied. To compensate for this bypass loss and to properly lubricate the bearings, refineries over-mist by a factor of at least four times, in order to ensure that at least some oil passes through the bearings.

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