Wednesday, August 05, 2009

Prelube Pumps Extend Equipment Life

According to ESE, pre-lubing before every start is an inexpensive way to extend the life of engine-driven equipment. When an engine is running, the engine's oil pump supplies pressurised oil into the centre of the bearings to create a cushion that separates the bearing surface from the crankshaft. Once the engine is running, the surfaces never touch because they ride on this film of oil. As soon as the engine shuts down, however, the oil that comprises the cushion begins to drain away and the film thins.

After only a few seconds, the film thins to the point that 'metal-to-metal' contact can begin to occur. In other words, the cushion is gone and the crankshaft is resting directly on the bearing. Pre-lubing the engine before every start can extend engine life by eliminating metal-to-metal contact during the first several seconds of running. Those few seconds after cranking the engine can be suspenseful for the operator as the oil pressure rises. If the engine has been sitting for extended periods, it can take an uncomfortable amount of time for oil pressure to come up.

Without oil, the engine will not last long. The vast majority of total engine wear occurs during these few brief seconds. A pre-lube system can eliminate this engine start-up wear and extend mean time between critical failures (MTBCF) and the overhaul interval. By filling oil galleries with oil before the service oil pump starts, the 'dry time' is also reduced or eliminated for all lubricated engine parts. This includes the camshaft bearings, valve guides, lifters and rocker assemblies as well as the piston walls and rings. Operators have historically avoided shutting down large engines to reduce start-up engine wear (in addition to other operational advantages).

In colder climates, for instance, letting an engine completely cool could cause significant problems when trying to restart it. Extremely cold engines can still be nearly impossible to start. Until recently, the simplest and most widely used solution to these problems was to leave the engine running. As few as five years ago, for example, long haul trucks would only rarely be shut down. Aside from the few minutes it takes to fuel, they would only shut down for periodic maintenance or when the driver was 'home'.

Fuel was inexpensive and nobody worried about the environment. Truckers believed that the wear associated with start-up was not worth the cost of the tiny amount of fuel a diesel engine uses while idling. As the price of fuel becomes less predictable and truck stop idling is banned, drivers are shutting down engines more. The same type of thing is happening in other industries. Increasing awareness of environmental impact and volatile fuel costs are changing the way in which all engines are operated.

In almost every case, the result is many more lifetime starts. In the trucking industry, the calculated number of lifetime cold starts has jumped from as low as 300 up to 6,300, which is more than 20 times the start-up wear. Pre-lube can help mitigate the wear associated with more lifetime starts. Fully implementing a pre-lube system can cost between USD600 (GBP356) and USD3,000 per engine. The cost justification is the expectation of extending the useful life of an expensive engine. The financial break-even point with pre-lube is generally less than two per cent reduction in wear.

With wear reduction rates commonly as high as 40 per cent, pre-lube can be profitable when the life of the engine determines the useful life of the product. Pre-lube is common in power generation, oil and gas compressors, locomotive primary power and marine power. Industries such as over the road trucking and off highway are also seeing value in reducing start-up wear. A typical pre-lube system for a large engine consists of a pre-lube pump, associated plumbing and electric controls. The suction line is usually at least 0.75in diameter and can be as large as 1.5in.

It runs from the engine's oil sump to the suction port of the pre-lube pump. This line is ideally kept short; the pre-lube pump is usually located within 30in of the oil sump port to prevent pump cavitation when the sump oil is cold and thick. The pressure line runs from the pressure port on the pre-lube pump to a port on the engine where it 'tees' into the oil passages between the engine's own oil pump and the oil filters. The oil re-entering the engine flows toward the oil filter since the engine oil pump will not accept backflow. From there, oil follows the same path as oil from the engine oil pump during normal running.

A check valve is integrated into the pre-lube pump to prevent oil from back flowing through it when the engine is running. As long as the pre-lube pump provides sufficient flow for the engine, all parts that are lubed during normal running will be wetted with oil during the pre-lube cycle. In addition, when the engine oil pump does begin to turn during engine cranking, the oil passages are already full, so the time for the oil pressure to come up to operating level can be reduced to near instantaneous depending on the time gap between the end of pre-lube and the beginning of engine cranking.

The most common way to control the pre-lube cycle is with a button and an oil pressure indicator light at the operator's control panel. The button is held until the pressure light illuminates, at which point the operator can safely crank the engine. Newer engines have sophisticated engine management controls. An increasingly common way of implementing pre-lube is to let the control automate the pre-lube cycle. When the engine management control receives the start command from the operator, it sends the signal to start the pre-lube pump rather than crank the engine immediately. Once a pressure signal is received the control cranks the engine. The pre-lube pump can be left running during engine cranking to prevent oil pressure from falling.

No comments: