Air Compressor maintaince and Troubleshooting

In order for ships air compressors to work effectively, at any time, you should know what the most common problems can occur and their causes. You need to know how to take care of vents; How to hold and replace the air valves; Maintenance of pneumatic cylinders and pistons; And like the bearings adjust, crank pins and links. You should be able to replace and repair the lubrication , cooling, control and air systems.

AIR INTAKES

A clean, dry air is essential to the proper operation of the compressors. To do this, the air intake filters should be regularly inspected and cleaned; If the filter is clogged there is a loss of capacity. A clogged air screen or inlet filter can also lead to extraction of oil from a compressor crankcase, around the rings or through oil seals to cause an explosion. Remove the filter element and clean with a jet of hot water or steam or by immersing in a strong solution of sodium hydroxide. The filter housing must be drained and replaced. When the filters are soaked in oil, dipping in clean oil, intermediate and complete emptying must be done before the filter at the inlet changes. Do not clean the filter with petrol or kerosene! Vapors can accumulate and explode in the compressor or the receiver. Make sure that no rain or dew on the inlet side be present and a means to drain the water from suction tube be provided. The lines should be as short and direct as possible. To supply air compressor air to the divers, avoiding compressor absorbs every internal combustion engine exhaust. You should also avoid possible inlets fumes from the fuel tank, spilled oil or gasoline.

AIR VALVES

The inlet and exhaust must be clean and kept in good condition. The valves if have leakage are generally contaminated and cause a loss in capacity. Valves are removed by loosening the fastening screws or clamps and then removing the cover plates. Each relief valve , if provided, can be lifted. Each valve should be checked to ensure that it returns to the same port from which it was removed. The valves removed for inspection should not be removed for cleaning if the conditions so require. Usually, dirt or carbon in the valve hole can be removed without removing the valve. This is done by soaking the valves in kerosene, then giving a stiff, light brushing or light scraping. The action of the valve is to be tested by inserting a screwdriver through the seat opening; The valve must lift and close freely. If necessary, remove the valve, check the arrangement of the various parts so that the corresponding relationship is maintained with the valve assembly. (The periodic reports on board show damage to the piston and valve parts if connected poorly mounted valves in the way of the piston lugs protrude.) Before replacing the cylinder in a cylinder, replace the air valves, check the seals and replace them. Plates coated with copper or asbestos thin and thin copper. Alternatively, they can be used together of compressed asbestos temporary 1/16 inch. Each valve assembly is inserted into the same hole that has been removed. Since it may be difficult in many cases to distinguish between the suction valve and the pressure valve, care must be taken when the valves are inserted into the cylinder. Make sure the suction valves are open to the center of the cylinder and exhaust valves are clear. Failure to do so will result in serious injury or loss of capacity. Then, the valve cap in the cylinder ensures that the gasket is in place; Lower even coverage of nuts and again do not tilt the lid. Tighten the set screw of the valve or clamping screw, pulling the valve in its seat. If no special lock in the threads of the screw of the valve to prevent leakage leakage must be placed through a locking nut in a recess around the screw and placing welding or fuse wire.

CYLINDERS AND PISTONS

The cylinders on pistons should be inspected only AFTER the manufacturer’s technical manual has been consulted.   Be   careful   when   removing   heads,   particularly where metal-to-metal joints are involved, to prevent damage to the joint.

If replacement of piston rings is required because they are worn or broken, take accurate measurements of the cylinder liners. Standard size rings may be used in oversize cylinders if the oversize does not exceed 0.003inch per inch of cylinder diameter. The liner may also need to be replaced if it is badly worn or out of round. When replacing piston rings, first fit them to the cylinder to check for proper end clearance. You can file the ends, if necessary, to make them fit. The side clearance of the rings should be such that the rings will fall easily into the piston grooves, which should be deep enough for the ring thickness.  Ring splits should be staggered.  After you assemble the piston, wire the rings tight with a soft copper wire so that they will enter the bore easily. This wire can be removed through the valve ports after the ring has started into the cylinder bore.

When reassembling the air cylinders and heads, be sure  they  are  all  drawn  down  evenly,  especially on multistage   compressors   where   the   heads   contain cylinders  for  third  and  fourth  stages. Otherwise, the result will be excess wear on the cylinders and pistons.

When a compressor piston has been replaced, the piston end clearance must be checked. This is done by inserting a lead wire through a valve port or indicator connection. Jack the compressor over. When the piston has  moved  to  the  end  of  its  stroke,  the  lead  will  be flattened  to  the  exact  amount of  clearance.  The  wire should  be  long  enough  to  permit  a  reading  near  the center of the piston. These readings should be taken after any adjustment or replacement of the main, crank pin, wrist pin, or crosshead bearings. Methods of adjusting the clearances vary according to the compressor design. You should consult the manufacturer’s instructions for suggested   adjustment.

MISCELLANEOUS  ADJUSTMENTS 

From time to time other miscellaneous adjustments are  required  on  compressors,  including  those pertaining to  wrist  pins,  crosshead  shoes,  reduction  gears, couplings,   and   V-belt   drives.   The  manufacturer’s technical manual will give you specific information forth   care,   adjustment,   and  replacement   of   all   fitted bearings.  Refer  to  the  manufacturer’s  instructions  for detailed information  on  when  and  how  to  make  these adjustments.

Wrist pin bushings are replaced when necessary. This  is  done  when  they  are  worn  to  the  point of becoming noisy. In making a replacement, be sure the oil hole in the bushing is properly lined up with the oil hole in the connecting rod. After being pressed into the rod, the new bushing must be reamed.

Crosshead shoes are provided with shim or wedge adjustment.   Wear   should   be   slight,   but  adjustment should be made when the travel of the piston rod causes movement in the stuffing boxes.

Alignment of reduction gears and pinions should be checked periodically, especially on a new compressor. Misalignment  may  be  caused  later  by  settling,  straining, or  springing  of foundations; pipe  strains  on  turbine-driven compressors; bearing wear; or springing due to heat from a turbine.

Flexible couplings require very little maintenance when they are properly lined up.  Some types require occasional lubrication to prevent excessive wear of springs and bushings. A noisy coupling is an indication that the bushing is worn and requires replacement.

V-belt drives require adjustment for belt tension. Belts generally stretch slightly during the first few months of use. A loose belt will slip on the motor pulley and cause undue heating and wear on the belt. A tight belt will overload the bearings. Belts should be protected against oil and high temperatures.  To prevent rapid deterioration, belts should not be used at temperatures above 130°F. V-belts are usually installed in sets of two or three.  If a single belt is worn or deteriorated, the complete set should be replaced to ensure that each belt will carry its share of the load.

LUBRICATION SYSTEM

Proper care of a compressor lubrication system includes the following:-

1. Keep the oil at a normal level in the reservoir at all times to maintain proper oil temperature.
2. Change crankcase oil periodically, and at the same time clean and flush the crankcase and clean the oil filter.
3. Maintain proper lube-oil pressure by keeping the oil pump in good working order and adjusting the bypass relief valve.
4. Keep  the  oil  cooler  free  from  leaks  (since pressure  on  the  water  side  exceeds  that  of  the  oil)  to prevent oil contamination and emulsification.
5. Properly adjust the lubricator for the specified quantity of oil feed.

COOLING SYSTEM

Proper  care  of  a  compressor  cooling  system includes  the  following  inspections  and  maintenance procedures:

1. Periodically inspect the intercoolers and aftercoolers
2. Remove  collections  of  gummy  oils  or  tarry substances from the cooler tubes by washing tube nests with  a  suitable  solvent  and  drying  them  thoroughly before reassembling.
3. Correct any leakage in tube nests to prevent leaks of water into the compressor while secured or leaks of air into the water side during operation.
4. Inspect and clean the cylinder water jackets periodically with a cleaning nozzle.

When  filling  the  cooling  water  system  after  the compressor  has  been  drained,  open  the  water inlet slightly to allow the water to rise slowly in the cooler shells and water jackets. Vent valves fitted to the water spaces should be opened to permit entrapped air to escape and to remove any air pockets.

CONTROL DEVICES

Because of the great variety of regulating and unloading devices used on compressors, you will have to  consult   the   manufacturer’s   technical   manual   for information regarding the adjustment of these device son particular compressors.

If a control valve fails to work properly, it should be taken apart and cleaned Some valves are fitted with filter  filled  with  a  sponge  or  woolen  yarn  to  prevent particles of dust or grit from being carried into the valve chamber. These filters remove gummy deposits from the oil used in the compressor cylinders. When repacking, use only genuine wool. Cotton will pack and stop the airflow.  Relief   valves   are   very   important   for   safe compressor operation. They should be set as specified by the manufacturer and lift-tested by hand each time the compressor is placed in operation.  To check the setting periodically, test by raising the pressure in the spaces to which they are attached.

SUMMARY

Since an Engine man may encounter so many types of   compressed   air   systems,   air   dryers,   and  air compressors both ashore and aboard Navy vessels, this chapter presented only general procedures and facts.  To maintain, repair, and overhaul specific compressed air systems,  air  dryers, or reciprocating  air  compressors, you  must  refer  to  the  manufacturer’s  technical  manuals. A  definite  preventive   maintenance   schedule   with frequency and assignment of responsibility is required. You should have the manufacturer’s manual handy to establish   minimum   requirements  and   to   follow   its recommendations for maintenance.

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Author Amit                                                                            Article Requested By: Rauhjek

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TURBOCHARGERS & ITS SURGING

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 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.

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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Author marineGuru

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SUPERCHARGING IN MARINE DIESEL ENGINE

Supercharging (or Pressure charging) is a process where a greater mass of air is admitted in the cylinder for combustion and consequently a greater amount of fuel is burnt efficiently.
Power output of the engine is increased with higher thermal efficiency without increasing the size of the engine.

Naturally aspirated engines : It is a term applied to engines where air charge is brought into cylinder only by the downward movement of the piston, without any external aids.

Supercharged engines : is the term used to indicate that weight of air supplied to the engine is considerably increased.
Supercharger can be any device (normally exhaust gas driven turbocharger) which increases the pressure of combustion air supply above that which is normally required.

Air Charge Ratio : This is ratio of volume of air contained in the cylinder at the the start of compression to the swept volume of the piston. For naturally aspirated engines it is about 0.85 where as for supercharged engines it is about 2.5 – 4.0.

Advantages of pressure charging :
1. Substantial increase in power for given speed and size of the engine.
2. Better power to weight ratio i.e. reduced engine weight for given output
3. Improved mechanical efficiency with reduction in specific fuel consumption
4. Reduction in cost per unit of power developed.
5. Increase in air supply has a considerable cooling effect leading to reducing the  severe working conditions and improved reliability.

There are 2 types of Supercharging methods :
Pulse type  and
Constant pressure type.

Supercharging Techniques

PULSE SYSTEM : IN PULSE PRESSURE SYSTEM THE ENERGY OF THE EXHAUST FROM THE CYLINDER IS TRANSFERRED TO TURBOCHARGER BY PRESSURE WAVES OR PULSES.

Pulse Turbo-charging System with Twin entry Blower

• THESE WAVES TRAVEL THROUGH THE MANIFOLD TO THE TURBINE NOZZLES WHERE THEY ARE CONVERTED TO KINETIC ENERGY AT HIGH VELOCITY TO ROTATE THE TURBINE BLADES.
• THIS GIVES A RAPID BUILDUP OF TURBINE SPEED WHEN AN ENGINE IS STARTED OR DURING MANOEURING.
• TO MAINTAIN THE PULSES, RAPID OPENING OF EXHAUST  VALVE, EXHAUST CONNECTIONS OF LIMITED DIAMETER, NO SHARP BENDS & THE TURBOCHARGER IS FITTED CLOSE TO THE ENGINE.
• TO PREVENT BACK FLOW INTO  THESE, THE EXHAUST IS SUBDIVIDED BETWEEN A NUMBER OF MANIFOLDS, EACH CONNECTED TO A SEPARATE NOZZLE BOX AT THE TURBINE.
• UPTO THREE CYLINDERS USE EACH MANIFOLD WITHOUT INTERFERENCE DEPENDING ON THE FIRING ORDER.
• DUE TO SMAL VOLUME OF EXHAUST DUCTING AND DIRECT LEADING OF EXHAUST TO TURBINE INLET THE SYSTEM DOES NOT REQUIRE ANY FORM OF SCAVENEGE ASSISTANCE AT LOW SPEEDS OR WHEN STARTING.

ADVANTAGES :
UTILIZES HIGH KINETIC ENERGY OF EXHAUST GASES
DOES NOT REQUIRE AUX BLOWERS AT LOW SPEEDS OR DURING STARTING OF THE ENGINE.

DISADVANTAGES :
MORE NO OF TURBOCHARGERS NEEDED
EXHAUST PIPING IS KEPT STRAIGHT AND SMALL SO LOCATION OF TURBOCHARGER IS VERY CRITICAL
ACCURATE MATCHING OF EXHAUST REQUIRED TO PREVENT BLOWBACK.
CONSTANT PRESSURE : IN THIS SYSTEM THE EXHAUST GASES FROM EACH INDIVIDUAL CYLINDER IS LED TO A COMMON MANIFOLD OR RECEIVER WHERE THE PULSE ENERGY IN LARGELY DISSIPATED.

THE PRESSURE PULSE IS DAMPED OUT BY EXPANDING THE GAS IN THIS CHAMBER WHICH IS MAINTAINED AT CONSTANT PRESSURE.THE EXHAUST GAS FROM MANIFOLD IS LED TO TURBOCHARGER AT CONSTANT PRESSURE.

THE VOLUME OF THE MANIFOLD MUST BE LARGE ENOUGH TO ACCOMMODATE GAS FLOW AND PREVENT LOCALISED PRESSURE RISE.

ADVANTAGES :
1. STEADY PRESSURE BEFORE TURBINE,EFFICIENT OPERATION
2. BETTER AND MORE RATIONAL UTILISATION OF EXHAUST HEAT
3. COMPRESSOR CAPACITY CAN BE INCREASED AS MORE ENERGY IS AVAILABLE
4. EXHAUST PIPING IS SIMPLER
5. EXPANSION IN CYLINDER CAN BE CARRIED OUT LONGER AS NO PRESSURE PULSE IS REQURED.THIS RESULTS IN EFFECTIVE INCREASE IN STROKE SO MORE OUTPUT.
6. DUE TO EFFECTIVE SCAVENGING SFC CAN BE REDUCED

DISADVANTAGES :
THIS SYSTEM IS SLOWER IN ITS BUILDUP OF PRESSURE WHEN STARTING, & INSUFFICIENT AIR IS AVAILABLE FOR ENGINES DURING MANOEUVRING OR OPERATING AT LOW SPEED.
TO OVERCOME THIS DIFFICULTY ENGINES HAVE ELECTRIC DRIVEN AUXILLARY BLOWER.

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Author marineGuru

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