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| Turbocharger |
Shown in the sketch is a section of turbocharger fitted on a large 2 stroke engine. It consists of a single stage, axial flow exhaust gas driven turbine mounted on a common shaft with centrifugal air compressor.
Turbine : The exhaust gas enters through the nozzle ring and is then directed to the turbine rotor blades at high velocity. The nozzle ring converts the energy in the exhaust gas to kinetic energy. The turbine blades are firmly fitted on the wheel by fir tree shaped roots which give positive fixing and minimum stress concentration.
The blades are held together at the free end by lace wire to dampen vibration. The nozzle ring, turbine wheel, blades and rotor shaft are manufactured from heat resisting nickel chrome alloy steel to withstand high working temperatures. The turbine casing is of cast iron with adequate water cooling spaces. In modern slow speed 2 stroke engines with relatively low exhaust gas temperatures the casings are un-cooled.
Blower : The air blower casing is fitted with filters and silencers at the air inlet of the casing. A inducer is fitted just before the impeller to direct the flow of air to the centre of the impeller without any shock. The impeller is made of light aluminum alloy. The impeller takes in air axially and delivers it radially through a diffuser to the volute casing. The kinetic energy is converted into pressure energy and air is delivered to the air cooler for cooling and then to scavenge manifold. Compressor casing is of cast aluminum and un-cooled.
Labyrinth Seals : Two labyrinth seals are fitted to the shaft, one between thrust bearing and air compressor and the other between turbine and bearing. They are sealed with air under pressure from the compressor discharge through internal passages. The seals prevent possible oil leakage into the turbine and compressor and also prevent exhaust gas leakages into bearing oil.
Bearings : Two shaft bearings are fitted, one at each end. End thrust is taken at the compressor bearing, allowing the turbine bearing free thermal expansion of the shaft. Bearings may be of either plain sleeve types with copper lead bushes on hardened steel sleeves or ball and roller type.
Lubrication : Ball and roller bearings may be lubricated by self contained gear type pumps operated from the shaft and drawing oil directly from the independent bearing sump.
For sleeve type bearings either the lubricating oil from engine lubrication system is used or a complete different system can be used exclusively for the turbochargers. In which case additional pumps, motors, filters and cooler will be required.
Maintenance on Turbochargers :
- Regular checking of oil level in bearing sump and changing oil after 1000 hrs
- Cleaning of air filter after 1000 hrs
- Renewal of bearings after 16000 hrs and gear pumps to be renewed or reconditioned after 16000 hrs
- The cooling water chamber to be cleaned at every 8000 hrs.
- Regular water washing of compressor and turbine.
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| Water washing of Turbocharger compressor |
Turbocharger Cleaning : Under operating conditions turbocharger systems may become fouled, causing reduced efficiency, loss in power and surging.
Compressor :
Oil mist and dust drawn from engine room may get deposited on the compressor surface. Dirt deposits should be dislodged by injection of water during operation.
A small container is provided which is filled with water to clean the compressor. Water is injected using air from the compressor. Cleaning is carried at full load and performed once every day.
1. Open filler of the tank and fill with fresh water. Close vent.
2. Open air supply valve A
3. Open injection valve B and wait for 30 seconds.
4. Close valve A and B and open vent.
5. Check to ensure tank is empty.
Turbine :
Fouling of the turbine can occur due to products from combustion of fuel, ash and any other non-combustibles present in the fuel.
Water Washing of Turbine side: The dirt deposits on turbine side can be reduced by periodic cleaning (water washing) during operation. Dirty turbines lead to higher temperatures of exhaust gas and higher stresses on bearings due to imbalance.
- The engine speed must be reduced to reduce the exhaust temperature and prevent thermal shock of the turbine.
- Once the exhaust temperature is at or below the manufacturer's limit, the turbocharger drain can be opened and freshwater admitted to the turbine casing.
- Water should be admitted slowly until water appears at the drain, then the water flow can be increased.
- Water supply and drain to be closed once fairly clean water starts flowing from the drain.
- After the cleaning is completed the engine must be run on same speed for about 5 mins until all parts are dry.
- This operation is usually carried out on a weekly basis.
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| Arrangement for turbine water-washing |
Dry Cleaning : The turbocharger speed does not have to be reduced when dry cleaning. A container is filled with correct amount of cleaning material, either ground nutshells or small grains of rice. The valve from the container is then opened to blow the material into the turbine casing. This is carried out normally every two days.
New series of two-stage turbochargers :
Two-stage turbocharging is important for the development of new generation of large-bore diesel engines with reduced exhaust emissions. Reducing engine emissions through internal measures is achieved by increasing the mean effective pressure. This requires high charge-air pressures but cannot be achieved through single-stage turbocharging. Two-stage turbocharging enables the charge-air pressure to be increased substantially while simultaneously reducing exhaust emissions, despite the increased specific engine output. MAN Diesel & Turbo has introduced two-stage turbocharging to the market with its TCX series.
Two-stage turbocharging systems consist of two turbochargers of different size connected in series. The exhaust gas coming from the engine drives the turbine of the smaller, high-pressure turbocharger (the first stage) which in turn drives the turbine of the larger, low-pressure turbocharger (the second stage).
The low-pressure turbocharger's compressor draws in ambient air and sends it via an intermediate cooler to the high-pressure turbocharger's compressor. Here, the air is compressed once again and, via a further charge-air cooler, sent to the engine. The system adapts to varying operating conditions either through controlled turbine bypass or by variable nozzle rings (VTA).
VTA – Variable Turbine Area, allows charge air delivery to be optimized by using adjustable vanes. By altering the pitch of the adjustable vanes, the exhaust gas pressure is regulated and thus the air amount can be precisely matched to the quantity of injected fuel at all points in an engine’s load and speed range. The result is reduced specific fuel consumption, reduced emissions HC and CO2 and improved engine response.
Although two stage turbocharging is more efficient than single stage, the additional cost and complexity of the system makes it generally unpopular.
Surging
Surging is a phenomenon that affects centrifugal compressor when the mass flow rate of air falls below a sustainable level for a given pressure ratio.
Surging is a condition whereby an imbalance in demand and supply of air from the turbocharger causes a rapid deceleration. When this occurs the pressure downstream of the compressor is relieved to atmosphere backwards through compressor. This is known as surging and it is accompanied by a loud barking noise and vibration. It was not uncommon on pulse systems in heavy weather, it is less prevalent in modern constant pressure designs.
The turbocharger must produce the required scavenge pressure. When the turbocharger cannot maintain pressure and the air flow decreases, the delivered pressure falls below the scavenge pressure which results in reversal flow and surging occurs.
If surging occurs engine speed must be reduced, the compressor should be water washed and air filters cleaned. If this does not solve problems the engine balance should be checked by taking set of indicator cards.
Conditions leading to Surging:
Surging may occur in heavy weather when propeller comes out of water and governor shuts the fuel almost instantaneously.
Some possible reasons of surging are :
- For multi blower installations surging can occur due to a difference in maintenance of cleaning causing one or more to operate at pressure ratio's above its capability
- When governor shuts fuel instantaneously.
- change in engine speed/ load relationship- say due to hull fouling
- cylinder power imbalance
- faulty injectors or timing
- dirty air filter
- dirty air cooler (air side)
- dirty turbine nozzle ring
- deposits on blades or impeller
- damage to blades
It is also possible that components downstream from the blower exhaust such as a fouled exhaust gas boiler can also lead to surging
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marineGuru
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