December 2019

Columns

Reliability: When slow-rolling machines can cause failures, and how to avoid them

All the book learning in the world will fail if we do not use common sense. We could also say that we need to think things through and should recognize that intuitive logic can lead us astray. If that sounds like semantic banter, let us zero in on three practical field examples that illustrate the issues at hand.

Bloch, Heinz P., Hydrocarbon Processing Staff

All the book learning in the world will fail if we do not use common sense. We could also say that we need to think things through and should recognize that intuitive logic can lead us astray. If that sounds like semantic banter, let us zero in on three practical field examples that illustrate the issues at hand.

Offshore pump testing

Offshore fire water pump testing is a first example. This particular scenario involved a diesel engine with an idle speed of approximately 600 rpm and a normal operating speed of 1,800 rpm. The engine was coupled to the horizontal input shaft of a right-angle gear unit with a 1:1 speed ratio; the output shaft was connected to a multistage, vertical firewater pump.

Fig. 1. Spider bearing sandwiched between two flanged pipe sections. Two threaded shaft sections and a surrounding coupling are shown above the bearing.
Fig. 1. Spider bearing sandwiched between two flanged pipe sections. Two threaded shaft sections and a surrounding coupling are shown above the bearing.

The pump was designed and installed with a series of flanged sections of pipe forming a long, vertical column. Its pipe sections and pump shaft lengths had been manufactured in 10-ft increments. At each of the flanged joints, carbon-filled PTFE (Teflon) “spider” bearings were installed to keep the shaft aligned and concentric with the 102-ft total vertical column length of the pump (Fig. 1). These carbon-filled PTFE spider bearings were lubricated by the seawater being pumped.

Things were going well, until an unnamed employee was handling the periodically scheduled fire pump test. The employee decided to treat the equipment gently and started the engine, allowing it to idle for an extended warmup run at 600 rpm. Unfortunately, at that low speed the height of the liquid column produced by the vertical pump was insufficient to wet all the spider bearings. As a result, whichever spider bearings remained above the water level inside the pump column were damaged and had to be replaced.

Similar experiences exist elsewhere

A second example is a steam-turbine-driven, 14-stage centrifugal pump in light hydrocarbon service; it was being started up at a refinery in the central U.S. In this example, the operators wanted to “take it easy” on the machines. They slow-rolled the pump-and-turbine set as part of the initial commissioning.

However, operation at slow speed caused the stainless-steel wear rings to make rubbing contact, and the pump seized before its normal operating speed was reached. Galling was severe enough to require rebuilding with new wear rings. Of course, incorporating stationary wear rings made of oriented carbon fiber-filled perfluoro-alkyl (sold as Vespel CR-6100) would probably have eliminated the problem, and/or other preventive measures could have been engineered with a bit of forethought.

Be careful when someone argues for “taking it easy” on equipment; take time to think it through. One significant advantage of using electric motors is that the driven fluid machine comes up to speed quickly. The wear rings then act like bearings and help center the shaft.

On the hunt for Btus and kilojoules

In a third example, a team of graduate energy conservation engineers was engaged in a search for reduced steam consumption. The team issued guidelines to reduce steam turbine slow-roll speeds to the bare minimum needed to keep the casings hot for imminent starts.

After the team issued an edict that small steam turbines should be slow-rolled at no more than 60 rpm, numerous sleeve bearing failures resulted. When slow-roll speeds are too low, the required oil wedge cannot establish itself. An oil wedge is needed to keep shafts from making direct contact with the low melting point bearing alloy typically used in sleeve bearings. Also, a certain oil flowrate will be needed to remove heat from the bearing. By the time a reliability professional was consulted, considerable damage had been done.

Author’s appeal to managers

The message here is really an appeal to management. An organization is made up of individuals having process/operations know-how and people with significant mechanical/machinery experience. The different job functions must communicate and cooperate.

Beware of traditionalists who have no time for perfluoro-alkoxy polymers and other strange-sounding materials. Involve younger engineers who paid close attention in college classes where basic physics were taught. You will not regret it. HP

The Author

From the Archive

Comments

Comments

{{ error }}
{{ comment.comment.Name }} • {{ comment.timeAgo }}
{{ comment.comment.Text }}