CoC Oral Exam Preparation (Part – 8): Air Compressor
Effects of leaking valves in Air Compressor:
First Stage Suction:
- Reduce air delivery
- Reduce 2nd stage suction pressure
- Unload the compressor
- Increase running time.
First Stage Delivery:
- Reduce air delivery
- Increase discharge temperature
- Less air drawn in, due to high-pressure air leaking back into cylinder.
Second Stage Suction:
- Reduce air delivery
- High temperature & pressure in 2nd stage suction line
- Increase running time
Second Stage Delivery:
- Reduce air delivery
- Increase suction pressure in 2nd stage
- Increase delivery pressure from 1st stage.
- Back pressure from air bottle.
How to check Air Compressor Efficiency?
- Regular overhauling of valves done or not.
- Check Air Bottle filling time.
- Compare test results and records.
How to check Air Compressor capacity is sufficient?
- Total no. of Air Compressors must be sufficient to fill the empty Air Bottle to maximum pressure within 1 hour.
- Must be sufficient to start at least 12 times for Reversible Engine, and at least 6 times for Non-Reversible Engine.
- Fitted on the shell of Intercooler at waterside.
- Relieves pressure if the tubes burst.
- Rolled Copper Alloy and relief pressure is set while the disc is at softest condition.
- Material tends to harden due to time and surrounding temperature, and set pressure also increased.
- Bursting Disc needed to be annealed, to regain correct relief pressure.
Compound Valves, why used in Air Compressor?
- Give large area of opening and small amount of valve lift.
- Improve Volumetric Efficiency, as valve open and close in minimum time.
- Reduce bumping clearance.
- Reduce wear and tear.
LP Relief Valve opening:
- HP suction valve
- Intercooler choked.
- Relief valve, jammed by foreign particles, in open position.
HP Relief Valve opening:
- HP discharge valve, in closed position.
- After cooler choked.
- Relief valve, jammed by foreign particles, or spring sticking in open position.
(Relief Valves opening pressure are set at ≯ 10% above stage pressure.)
Why Intercooler is fitted?
- Reduce air temperature and volume, and increase air density for next stage.
- Increase Compressor Capacity and Volumetric Efficiency.
- Better lubrication for cylinder and rings.
- Water and excess oil can be drained out, preventing fouling of Intercooler and pipes, Air Bottle corrosion, and starting airline explosion.
- Work done is saved.
- Metal stresses reduced, due to control of temperature.
VE = (Volume of Air drawn into Cylinder) / (Stroke Volume of LP Piston)
VE = (Volume of Air discharged as ‘free air’) / (Stroke Volume of LP Piston)
Why Multistage Air Compressor is built?
- To obtain near to ideal isothermal compression, compressor is to be constructed of multistage with inter-stage cooling.
- Inter-stage cooling reduces air temperature and volume after 1st stage compression, thus increase mass of air for 2nd
- Workdone is saved and air compressor efficiency increased.
Other advantages are:
- Easy to get high final air pressure.
- Easy to control air temperature.
- Easy to maintain correct lubrication.
- Better compressor balancing.
- Reduction in size.
- reduction in clearance volume loss.
Capacity of air compressor:
- Capacity is checked upon how much filling time is lowered.
- Compressor should have enough filling capacity so that sufficient stopping time should be provided between fillings.
- 12 consecutive starts in reversible engine and
- 6 consecutive starts in non-reversible engine.
What is Free Air Delivery, FAD?
- Capacity of Air Compressor is stated in terms of [ m³/ hr ].
- Volume of air actually discharged in 1 hour, that would occupy if expanded down to atmospheric pressure and cooled to atmospheric temperature.
Safety devices on Main Air Compressor:
- Bursting Disc on Intercooler: (At water side)
- Bursting Disc and Fusible Plug (121°C) on Aftercooler
- Relief valves on LP and HP stages. (Set to lift at 10% rise above normal stage pressure.)
- Automatic Moisture Drain Valve.
- Cooling water supply failure alarm.
- Low LO pressure alarm.
- Relief valve on crankcase LO pump.
- Delivery air HT cut out and alarm on Aftercooler outlet. (Max. 93°C)
- LP discharge pressure 4 bars and HP discharge pressure 30 bar
- Intercooler inlet air 130°C and intercooler outlet air 35°C
- Aftercooler inlet air 130°C and Aftercooler outlet air 35°C:
Intercooler is single pass type: Aftercooler, double pass U-tube type:}
Uses of Compressed Air:
- Engine Starting 20 to 25 bar
- Boiler Soot Blowing 20 to 25 bar
- General Service (Whistle, Pneumatic Tools, Lifeboat , service air for cleaning and Pilot ladder) 7 to 10 bar
- Instrumentation and Control 5 bar
- Material: Felt, Metal gauze or Nylon strands
- Removes contaminants from air. Dirt and dust act as abrasives and increase wear.
- Contaminant deposits on valves can become hot and source of ignition.
Hazard of Dirty Filter:
- Very dirty filter or muffling a filter results in large pressure drop.
- Air has to be compressed over higher range.
- In extreme case, discharge air temperature may exceed flash point, or auto-ignition temperature resulting in an
- As a safety against explosion, air temperature is limited to 93° Fusible Plug (121°C) or a High Temperature cut out (93°C) is provided on Compressor.
Pressure Test on Air Compressor:
- Cylinders, cylinder cover, Inter & After coolers are hydraulically tested to:
Air Side: 1.5 x max. Working Pressure.
Water Side: 4 bar or 1.5 x max. WP (whichever is greater)
- Total capacity of air receivers must be sufficient to give at least 12 starts for reversible engine, and at least 6 starts for non-reversible engine, without refilling the receivers.
- There must be two identical main air receivers and one emergency bottle for every vessel.
Mountings of Air receiver:
- Fusible plug:
- composition – Bismuth 50%, Tin 30%, Lead 20%,
- Melting point: 220΄F (104.4΄C). Fitted at the reservoir’s bottom or on reservoir at ship side, when relief valve (safety valve) is not directly fitted on the reservoir.
- Atmospheric relief valve: provided for back-up of fusible plug. In case of ER fire when CO₂ flooding is required, this valve is opened before evacuating ER.
- Spring loaded safety valve: setting pressure: 32 bar (for 30 bar working pressure), with ≯ 10% rise in accumulation of pressure. May be fitted directly or with extension.
- Compensation ring: when a hole is cut or machined in pressure vessel, higher stresses will subject to the material around the hole, and to reduce this, compensation rings are fitted. It is a flange on which a valve or fitting is usually mounted.
- Manual Drain valve or Automatic Drain valve.
- Pressure gauges.
- Access doors.
- Main starting air valve, auxiliary starting air valve, filling valve, service air or whistle air valve.
Internal surface coating:
Graphite suspension in water, Linseed oil, Copal vanish or Epoxy coating having basic properties of anti-corrosive, anti-toxic or anti-oxidation.
Safety devices on Main Air Bottle:
- Fusible plug.
- Pressure Relief Valve
- Atmospheric Relief Valve.
- Low Air Pressure alarm.
- Automatic or remote control Moisture Drain Valve.