TURBOCHARGERS & ITS SURGING

Turbocharger sketch
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.

Water washing of Turbocharger compresso
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.......................................................................................................
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