by Mark Ashurst, Varna Products
It is no revelation that maximizing equipment life directly impacts the bottom line. When it comes to profitability, every dollar saved on equipment maintenance is a dollar earned. Prelube pumps move oil from the oil sump to the moving parts of an engine before engine start-up. Pre-lubing before every start is an inexpensive way to extend the life of engine-driven equipment.
Inside the Engine
When an engine is running, the engine's oil pump supplies pressurized oil into the center 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.
Prelubing the engine before every engine start offers a way to 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. It is common knowledge that the vast majority of total engine wear occurs during these few brief seconds. A prelube 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.
Prelube Growing in Popularity
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. Some may remember the bad old days of "gelled" fuel. 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 in state after state, drivers are shutting down engines more.
The same type of thing is happening in other industries as well. Increasing awareness of environmental impact and volatile fuel cost are changing the way we operate all engines. 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. Prelube can help mitigate the wear associated with more lifetime starts.
Fully implementing a prelube system can cost between $600 and $3,000 per engine. The cost justification is the expectation of extending the useful life of an expensive engine. The financial breakeven point with prelube is generally less than 2 percent reduction in wear. With wear reduction rates commonly as high as 40 percent, prelube can be profitable when the life of the engine determines the useful life of the product. Prelube 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.
Example of a Typical Installation
A typical prelube system for a large engine consists of a prelube pump, associated plumbing and electric controls.
The suction line is usually at least 0.75 indiameter and possibly as large as 1.5 in. It runs from the engine's oil sump to the suction port of the prelube pump. This line is ideally kept short; the prelube pump is usually located within 30 in 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 prelube 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 reentering 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 prelube pump to prevent oil from back-flowing through it when the engine is running.
As long as the prelube pump provides sufficient flow for the engine, all parts that are lubed during normal running will be wetted with oil during the prelube 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 prelube and the beginning of engine cranking.
The most common way to control the prelube 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 prelube is to let the control automate the prelube cycle. When the engine management control receives the start command from the operator, it sends the signal to start the prelube pump rather than crank the engine immediately. Once a pressure signal is received the control cranks the engine. The prelube pump can be left running during engine cranking to prevent oil pressure from falling.
Mark Ashurst is operations manager for VARNA Products, 530-676-7770 or 888-676-7770, www.VARNAProducts.com