LCIN # 4

October 2002

BENCHMARKING PREVENTIVE MAINTENANCE AND SOME PRACTICAL SOLUTIONS FOR MEDIUM SPEED ENGINES

Below are some benchmarks obtained from industry.

Percentage of total maintenance labor hours spent performing PM. Generally 27-35%--the rest is dedicated to corrective maintenance found in the PM inspections.

Ratio of PM work orders to CM work orders obtained by PM inspections.   6pm to 1 cm.

Ratio of PM Labor Hours to Emergency Labor hours; Greater than or equal to 15:1. Objective of PM is to maintain reliability and therefore there should be fewer emergencies.

The following is a reprint from an article in the Marine Engineers Review by Paul Santos, Chief Engineer, FIMarE providing some practical solutions for medium speed diesel engines.

Checking burnt exhaust valves

For four-stroke medium speed diesel engines running on HFO, a quick and practical check for burnt exhaust valves is to roll the engine with turning gear whilst feeling with the back of your hand expelled com­pression gases at each cylinder indicator cock. Through practice, you can quite quickly find the weak cylinders with valves that may be start­ing to burn.

This test should be the first test carried out when the engine is shut down at the end of each trip.

Testing cylinder leak down

This check is carried out to determine the combustion health of each cylinder on a four-stroke medium speed engine running on HFO. Collect electronic data from your engines, then download and analyze the information to determine overhaul and individual cylinder com­bustion performance. However, due to the difficulty in locating piston ring blow-by with some data collecting equipment, it might be neces­sary to revert back to the tried and tested method of putting air into each cylinder via the indicator cock, with the piston on the compres­sion stroke just after the inlet and exhaust valves have closed. Check that they are closed at the rocker gear (in this case, 140 BTDC.)

Connect a hose complete with ball valve and pressure gauge (after the valve) to the cylinder indicator cock and, using ship’s service air pressure, pressurize each cylinder in turn. Once the pressure stabilizes, at between 5.5-6.0 bar, close the valve and time the leak down. Once the gauge drops to zero, log the time, which, depending upon cylinder condition could be anywhere between 180 and 0 seconds. If the time is near zero, suspect first a burned exhaust valve, or partly opened inlet or exhaust valve. And check that the rocker valve caps are free to turn.

Recorded times of less than 15 seconds may indicate ring blow-by. But by listening for air passing the rings with crankcase doors off can confirm this.

Easy crankcase pressure monitoring

Using a simple homemade U-Tube manometer, you can track any sud­den rise in crankcase pressures of medium speed diesel engines caused by piston ring blow-by. Log the daily-recorded pressures (in mm H₂O) by disconnecting the line to the crankcase mist detector, plug in one end of your U-~Tube and record your reading. 12-34mm H₂0 is good, whereas 34-50 is unfavorable, depending on your engine. Also, if you suspect high pressures, look for oil leaking out from the explosion door seals, or oil mist blowing out the backs of the fuel injection pump fol­lowers and take appropriate action immediately.

Examining cylinder starting air valves

For four-strokes, blank off main airline distributor pilot valves. But for two-stroke slow speed engines close the air to distributor valve. Make:

sure that turning gear is out and the indicator cocks are open.

The engine controls should be in the start position before starting air pressure is applied. By listening for high pressure starting air leaks, first from the indicator cocks and then the starting air lines, a number of leaks can be detected.

You may contact the LCM POC e-mail with comments or suggestions.