Compared to many items of equipment a ships turbochargers are relatively simple in design which is fortunate considering that the efficiency of the engine is so dependent on them. That said, a ships turbochargers are subjected to quite harsh operating conditions.
For a start they rotate at high speed; whereas a two-stroke engine will be operating at around 100rpm, its turbochargers will be running at between 10,000 and 15,000 rpm.
This will put stress on the bearings if nothing else. The turbine side of the ships turbocharger is the most affected by normal operation being powered by the exhaust gas stream. Depending upon the condition and cleanliness of the engine, the exhaust gas will contain quantities of oil and soot, corrosive elements and will be at quite high temperature.
How often should ship turbochargers be cleaned?
Cleaning regimes for a ships turbochargers will vary between models and makers and the procedures and intervals will be found in the appropriate manuals or operating instructions.
Both sides of the compressor and the filters require regular cleaning and if a good regime is in place, the need for unscheduled repairs will be much reduced.
The extent of cleaning required on the silencer filters and compressors will depend upon the quality of the inlet air. If taken from a dirty environment, then cleaning will be needed more regularly as salt, oil mist, exhaust gas or dust can all be sucked in with the air.
Failure to keep all parts clean will inevitably affect the efficiency of the ships turbocharger because either the speed will be affected or else the volume of air entering will be reduced, both will impact on the engine by way of reduced power and increased fuel consumption.
Cleaning the filter is a matter of removing the cover, removing the filter ring and washing it with fresh water and mild detergent. The filter should be dried before reassembly. It will also be necessary to check and clean the absorption segments in the silencer body separating them from their metal coverings and cleaning with a mild jet of compressed air, soft brush or a damp cloth.
The compressor is normally washed with fresh water fed into the compressor under pressure when the engine is operating at 85% load or more. The frequency depends upon manufacturers recommendations and the cleanliness of the intake air. ABB for example recommends cleaning every 25 – 100 running hours. Cleaning with fresh water under pressure dislodges rather than dissolves any dirt.
The amount of water and the time over which it is injected will be laid down in the operating manual and this procedure should be followed closely to avoid damage to the turbocharger or the engine. A ships turbochargers normally have a pressure vessel which holds the appropriate amount of water.
The pressure is supplied by the turbocharger itself using an airline from the outlet side of the compressor to the pressure vessel.
Wet and dry cleaning
Cleaning the turbine side can be done in either of two ways – wet or dry cleaning. Wet cleaning is done with the same regularity and in the same manner as for the compressor side with the exception that the engine should be at low load and allowed to cool for at least ten minutes before the water is introduced.
This is done to avoid thermal shock to components and as each the ships turbocharger model may have different requirements, reference should be made to the operating manual to ascertain the exact procedure.
Dry cleaning of the turbine is achieved by introducing an abrasive material into the turbine under pressure in a similar manner to wet cleaning. The abrasive can be carbon granules designed specifically for the task or an alternative; one such alternative that is commonly used is crushed nutshells. Unlike with wet cleaning, there is no need to reduce the engine speed when dry cleaning.
Which method of cleaning the turbine is used will depend upon personal choice of the engineer or superintendent taking due account of maker’s recommendations. It is important that regular cleaning is done because if left for too long, the soot build up can be extensive and dislodgement of large lumps of soot can mean that the risk of damage is greater.
For four-stroke installations ABB Turbocharging recommends only wet cleaning. For two-strokes installations, the operator has the choice of wet or dry cleaning. This differentiation comes about due to the temperature of combustion, variations in the temperature can affect the texture of the residues that accumulate on the turbine components.
It is this texture which ultimately determines whether water or granulate may be used. While granulate or crushed nutshell both achieve good results, it should be noted that dry cleaning has to be carried out more frequently than wet cleaning as it is removes a thinner layer of deposits.
Safety precautions are advisable on a ships turbocharger
Precautions should be taken and the arrangement for introducing the abrasive inspected regularly for tightness of joints and general condition. In 2009, the UK MAIB reported on an incident where safety precautions were overlooked.
In that incident, crushed nutshells were injected into the turbocharger under 7bar air pressure. The injection system used contained a long section of unsupported pipework from the reservoir to the ships turbocharger which unfortunately separated when the engineer was preparing to open the valve to the turbine. This happened because one of the connections in the pipework had vibrated loose.
The engineer raised both his arms to protect himself from the crushed shells which were being sprayed from the failed connection. He suffered severe abrasion wounds to his arms before being able to raise the alarm. Fortunately, the engineer made a full recovery after spending several days in hospital.
However, as the MAIB report points out, had the crushed nutshells been sprayed on to his face then it is likely that he would have suffered far more severe injuries, and possibly blindness.
The accident to the engineer was made worse because his rolled sleeves left his arms bare. Overalls are designed to provide personal safety protection as well as cosmetic protection for dirty tasks.
Sleeves are often cut off or rolled up for comfort reasons, but this only increases the risk of burns or abrasion injuries, as this accident clearly illustrates. The use of PPE, including goggles, gloves and hard hats, should be identified by task risk assessments.
On the ship in question, PPE storage has been retrofitted in the vicinity of the turbocharger. Goggles and gloves are now routinely worn when carrying out the cleaning task.
Maintenance and overhaul of a ships turbocharger
Assuming that there is no abnormal heating of bearings and vibration is absent or within acceptable limits, then it should be possible to suppose that the turbocharger is in good condition. Both heat and vibration are easily measured by sensor so as rotating machines where such conditions must be expected to be found, turbochargers might be thought to be ideal candidates for condition-based maintenance.
CBM is not widely practiced as yet in shipping and while there are some operators who do monitor turbochargers these are very much in the minority. Experienced engineers will usually be able to tell from the sound and feel of turbochargers if there are problems developing.
Recording of engine and turbocharger running parameters on a daily basis will give an early indication of any developing problems. Important measurements include temperatures of intake air before and after the compressor and of the exhaust before and after the turbine, the ships turbocharger speed and lubricating oil pressure and temperature.
In general, most turbochargers on ships will be subject to a periodic maintenance regime established either in accordance with the manufacturers recommendation or based upon the experience of the technical superintendents and chief engineers.
In some instances, this maintenance may be managed by the OEM or by a third party specialist under a maintenance agreement. At the very bottom end of shipping there will always be operators who skimp on maintenance and repair and who will run their equipment to failure but these are in the minority.
Regular cleaning and observations should mean that overhauls are limited to those set out in the manufacturer’s instructions. A routine overhaul of the turbocharger requires it to be dismantled, inspected and parts replaced as necessary. Some of the smaller components are single use only and need to be replaced each time the turbocharger is dismantled. Ensuring that spare parts are available is a necessary prerequisite for ensuring the turbocharger will be available immediately.
The first task in the overhaul is to measure clearances to ensure that they are within limitations. The measurements should be recorded so that when reassembling, the same clearances are preserved. Most overhauls will involve checking the turbine and compressor wheels for damage and the casings for cracks, erosion and corrosion.
The wheels should be cleaned by washing in water or steam cleaned. If the Turbine wheel and nozzle rings is heavily contaminated it may need soaking in hot water or brake cleaner before being brushed to remove the contamination.
Because of being precision parts, needle guns or other means of power cleaning should not be used. All bearings and seals should be inspected for wear or damage and replaced if necessary.
Bearings will need to be replaced at certain times usually at scheduled drydockings.
If you enjoyed this article we would recommend reading the basics and origins of a ships turbocharger.