BUILD-UP OF INSOLUBLE IN FOUR STROKE ENGINE SYSTEM OIL

Transcription

1 CASE STUDY 2 BUILD-UP OF INSOLUBLE IN FOUR STROKE ENGINE SYSTEM OIL Figure 1: Lube oil treatment system A ship has 4 stroke engine diesel generators of 1500 kw, using 600 cst heavy fuel oil. The generator has a by pass purification system. The objective of the by pass purification system is to keep is to ensure that the pentane insoluble content in the lube oil system tank remains below 1%. For this reason the Q optimum flow rate for engine system oil is about 30% of the maximum centrifuge capacity. This de-rating is done by the purifier supplier by fitting under size oil feed pump to the purifier. Lubricating oil preheating temperature should be 90 O C. The gravity disc is selected using a nomogram on the basis of oil density, flow rate and separation temperature. Page 1 of 5

2 The Challenge Heavy lube oil sludge was continuously coming out the heavy liquid discharge 3, as shown in the picture above. This is not a case of loss of water seal, but excessive impurities in lubricating oil. Extent of damage The lab analysis report showed pentane insoluble was at 3%. The maximum limit is 1%. The crankcase was coated with black residue. The amount of oil in circulation was 1000 ltrs. There was extensive wear and tear of the engine components. The automatic lube oil filter was unable to cope with the impurities accumulated in the oil. The oil needs to be replaced. Oil may need to be replaced very often if this situation continues. 3% Max limit1% Page 2 of 5

3 Investigation Build up of Insolubles from 600cSt Fuel Investigation revealed that because of using 600 cst heavy fuel, the addition of combustion by-products in to the crank case lube oil was quite extensive. The purifier was unable to remove impurities at the rate these were being added by the running engine. Thereby it was unable to maintain the Pentane insolubles in the lube oil sump to below maximum limit 1%. Separation temperature The set point of temperature controller was set at 75 O C but the heater was able to achieve separation temperature of 50 O C only. For good separation the temperature should be 90 O C if 600 cst fuel is being used for combustion. On investigation it was found that oil heater had insufficient heating capacity and its temperature control system was sluggish in operation and also required calibration. 3% Max limit1% Flow rate The purifier flow rate shown above was found to be 1/9 of the maximum rating of purifier, because ship staff had further reduced the flow rate inadvertently by 30% and this added to build up of insolubles to 3%. Purifier should be operated at 1/3 of the maximum capacity, so that it can remove impurities at a rate these are being added by the engine operation. This de-rating has already been done by manufacturer during installation. This is explained below. Page 3 of 5

4 Above example shows a purifier of L/h rated capacity, if it is used for Trunk piston engine lube oil purification, its flow rate should be 4100 L/h. Therefore manufacturer has put in feed pump of 4000 L/h. This flow rate is equivalent to 1/3 of the rated capacity. No further reduction of through put by ship staff is required. Gravity disc Because of excessive sludge coming out of the heavy liquid outlet, the ship staff thought that water seal is breaking. They put a small size gravity disc, and shifted the interface inside the disc stack. This resulted in poor separation and was another factor for build up of pentane insolubles to 3%. Page 4 of 5

5 Quantity of oil in circulation The quantity of system oil in circulation should be at least 1.35kg/kW (CIMAC). For an engine of 1500 kw there should be at least 2250 ltrs of oil in circulation. In this case the amount of oil in circulation was 1000 litres. This overloaded detergent and dispersant additives in the 1000 litre of system oil with insoluble resulting in additive dropout. Lessons learnt Prevent build-up of insoluble: 1. Lube oil purifier should be operated at correct flow rate, separation temperature and gravity disc to correctly position the interface inside the bowl. 2. Ensure lube oil heater and control system is capable of achieving the required separation temperature. 3. The quantity of system oil in circulation should be as 1.35 kg/kw, as recommended by CIMAC so that lube oil additives can handle the impurities and prevent additive drop out. 4. Topping up of make up oil should be done not more than 10% at a time to prevent additive drop out. 5. If 600cSt fuel is being used, the amount of impurities added into the lube oil will increase drastically. 6. Ship staff should carry out Batch Purification at regular interval to maintain Pentane insoluble with in acceptable limits. Page 5 of 5