Air Compressor & Air Starting System

Question 1

what is meant by Oil Carry Over for Air Compressors on Ships

Answer 1

Starting air lines on board ships may contain large amount of air along with fine droplets of oil mixed with it. Any source of heat when supplied to this rich mixture could result in catastrophic starting air explosion. So it is important to maintain starting air compressors in good condition. Compressor performance, oil separator operation, inter cooler and after cooler efficiency, etc. to be monitored periodically. Also drain all air bottles regularly to get rid of most of the water and oil.

Question 2

list Reasons for Oil Carry Over in Air Compressors

Answer 2

Choking oil drain holes in the scraper rings

Worn out oil scraper ring

Wrong fitting of oil scraper rings

Worn out liner

Choking of crankcase breather (it results in crankcase pressurization and eventually oil carry over

Excessive cylinder lubrication in some types of compressors

Question 3

list ways in how how to minimize oil carry over?

Answer 3

Replace worn out scraper rings

Clear holes of scraper rings

Fit scraper rings properly

Clean and clear crankcase breather

Adjust lubrication to the top of the cylinder

Question 4

briefly outline the materials used for construction for a screw and reciprocating air compressor

Answer 4

Reciprocating Compressor

Crankcase and body – Cast Iron

Crankshaft – Spheroidal graphite cast iron or stainless steel

Connecting rod – Forged Steel

Piston – Aluminium Alloy or Cast / Ductile Iron

Piston Rings – Cast Iron

Screw Compressor

Casing – Cast or Ductile Iron

Screw – Steel or Stainless Steel or Nickel Alloy

Question 5

where are reciprocating air compressor used?

Answer 5

Reciprocating Compressor

Reciprocating compressors are characterized with higher pressures and reduced mass flow rate. They are mainly used in high pressure applications since it can deliver air at about 30 – 40 bar.

1)For diesel engine starting, where electric motor starting becomes costly and impractical.

2)Refrigeration compressors are normally reciprocating type (Single Stage) with a discharge pressure around 10 bar.

3) Air conditioning systems also uses reciprocating compressors (Nowadays trend is changed to screw compressors).

Question 6

where are screw air compressor used?

Answer 6

Screw type compressors provide air at increased mass flow rate but with reduced discharge pressure around 8 bar. Hence applications are also in low pressure systems, such as,

1) Service air compressors used in industry (For cleaning air, etc.)

2) Air conditioning systems nowadays employ screw compressors. (which have low power consumption and increased mass flow rate as advantages)

3) For low pressure air required for running pneumatic tools, pneumatic-hydraulic equipment, etc.

Question 7

what are compressors used for?

Answer 7

Compressors are used to increase the pressure of a gas. Like pumps, compressors can be classified as either kinetic machines, which includes centrifugal and axial compressors, or positive-displacement machines, which include reciprocating and rotary compressors. The compressing medium or ‘gas’ depends on the application, such as, if air is used it is termed as an air compressor. Similarly if refrigerant is used, it is known as a refrigerant compressor. The type of compressor, its discharge pressure and discharge rate is dictated by its use. Marine starting air compressors are part of theVessel Air Systemand play a crucial role in ensuring the smooth and reliable operation of large marine diesel engines. These compressors provide the necessary air pressure to start the engines, allowing vessels to set sail and manoeuvre efficiently.

Question 8

describe the operation of a reciprocating air compressor

Answer 8

Reciprocating Compressor

In a reciprocating compressor, gas is compressed by mechanical variation of the volume of space inside the cylinder, by reciprocating motion of the piston.

For a cycle of operation, there are two strokes such as,

1.) Suction stroke, and
2.) Compression stroke

As the piston moves down, air is sucked from atmosphere to the cylinder through suction valve (a non-return valve). As piston moves up, air is compressed and at the end of compression stroke, air is delivered through delivery valve (which is also a non-return valve). Topmost portion the piston can travel inside the cylinder is is called Top Dead Centre (TDC), and bottom most portion the piston can reach inside the cylinder is called as Bottom Dead Centre (BDC).

Reciprocating air compressors are the most commonly used in marine applications. They utilize a piston and cylinder arrangement to compress air. These compressors can be further classified into single-stage and multi-stage compressors, depending on the number of compression stages involved. Multi-stage compressors provide higher pressure outputs and are often used in larger vessels.

Question 9

describe the operation of screw compressor

Answer 9

Screw Compressor

Screw compressors (also called as helical lobe compressors) are positive displacement machines in which gas is being compressed is forced through the casing by two screws. Unlike the reciprocating compressors which are also positive displacement machines, screw compressors does not typically require internal suction or discharge valves. In addition the flow from screw compressor is generally more uniform and has fewer pulsations than the flow from a reciprocating compressor.
A twin screw compressor consist of two meshing helical rotors mounted on counter rotating parallel shafts that are enclosed within close-clearance casing. One screw is called driving screw which is coupled with a drive, say an electric motor, while the other screw is called as the driven screw, since it is driven by the driving screw. Gears used for driving the screws are called timing gears, since they are properly timed to maintain the close-clearance between the screws.

Screw air compressors are another type used in marine settings. They employ two interlocking rotors to compress air. These compressors are known for their high efficiency, low maintenance requirements, and compact design. They are particularly suitable for applications where a constant and continuous supply of compressed air is necessary.

Question 10

what is the Purpose of Marine Compressed Air Dryer?

Answer 10

The main purpose of a marine compressed air dryer is to eliminate moisture from the compressed air. In a marine environment, humidity is ever-present, and when compressed air is exposed to it, it tends to become saturated with water vapour. When moisture- air passes through pneumatic systems and equipment, it can lead to several detrimental consequences such as:

Corrosion of air tools and equipment –moisture in the compressed air can cause rust and corrosion in pneumatic components, pipes, and machinery, leading to premature failure and potential safety hazards.

Damage to electrical components –moisture can damage sensitive instruments and controls, causing malfunctions and compromising the overall safety and reliability of the vessel.

Reduced efficiency of air tools and equipment –water in the compressed air can hamper the performance of pneumatic tools and systems, leading to decreased productivity and higher operational costs.

Increased maintenance costs

Health and safety hazards

Question 11

what are some important checks/action to make before operating a marine compressed air dryer

Answer 11

To operate a marine compressed air dryer correctly, it is important to follow the manufacturer’s instructions. To ensure the marine compressed air dryer operates efficiently and effectively, follow these checks and actions:

Proper Installation –install the dryer in a clean, well-ventilated area away from potential sources of contamination, such as chemicals or exhaust fumes.

Adequate ventilation prevents overheating and prolongs the lifespan of the dryer.

Filtration –prioritize the installation of filtration systems upstream of the dryer to remove larger particles, oil, and other contaminants that could clog the dryer and reduce its performance.

Adjust Air Pressure –normally dryer is connected to the compressed air supply line. Set the air pressure within the recommended range as per the manufacturer’s guidelines.

High pressures can stress the dryer unnecessarily, while low pressures may result in insufficient drying.

Drain Moisture Regularly –most marine compressed air dryers are equipped with automatic drains.

Ensure these drains are functional and regularly inspect and clean them to prevent blockages and ensure proper moisture removal.

Monitor Performance –regularly check the dryer’s output dew point and pressure levels to verify its efficiency. An increase in the dew point may indicate potential issues that need to be addressed promptly.

Question 12

describe the operating principle of a control air dryer or in other words a marine compressed air dryer

Answer 12

The humid air flows into the air inlet connection and is pre-cooled in the heat exchanger before it enters the evaporator. As the air passes through the evaporator, which is cooled by the liquid refrigerant, the air temperature drops to 10°C, which is the dew point at which the moisture in the air is condensed. The condensed water is now separated from the air and is purged out of the system through the automatic drain trap. The high pressure liquid refrigerant now passes through the expansion valve and is evaporated in the evaporator, before returning to the compressor to continue the refrigeration cycle.

Question 13

what are some maintenance task carried out on a marine compressed air dryer

Answer 13

To ensure the long life and reliable operation of a marine compressed air dryer, it is important to maintain it on a regular basis.

Here are some maintenance practices to follow:

Cleaning –clean the dryer’s exterior regularly and ensure that the surrounding area is free from dust and debris that could obstruct air intake vents.

Filter Replacement –Follow the manufacturer’s guidelines for filter replacement intervals. Clogged or dirty filters can restrict airflow, leading to decreased performance and increased energy consumption.

Heat Exchanger Inspection –Regularly inspect and clean the heat exchanger to prevent a build-up of scale or debris, which can reduce the dryer’s efficiency.

Check Drains –Routinely inspect and test automatic drains to ensure they are functioning correctly and effectively removing moisture from the system.

Lubrication –If the dryer has any moving parts, ensure they are well-lubricated according to the manufacturer’s recommendations.

Question 14

what are some common problems associated with marine compressed air dryers

Answer 14

Insufficient Drying –if the dew point remains high despite correct settings, check for clogged filters, heat exchanger fouling, or malfunctioning drains. Clean or replace components as needed.

Excessive Pressure Drop –a significant pressure drop across the dryer can indicate clogged filters or obstructions in the air passages. Inspect and clean the filters and air pathways to restore normal pressure.

Unusual Noises or Vibrations –noises or vibrations may indicate loose components or worn-out bearings. Inspect the dryer and address any issues promptly to prevent further damage.

Leakage –check for air leaks around fittings, valves, and connections. Repair or replace damaged components to maintain the dryer’s efficiency.

If the dryer is not operating at all –it may have a problem with its electrical connections. In this case, you will need to check the dryer’s electrical connections and make sure that they are properly secured.

If you are still having trouble with your marine compressed air dryer, you should contact the manufacturer for assistance.

Question 15

Describe the following Components of Marine Starting Air Compressors

Air Intake Filter
Cylinder and Piston Assembly
Intercoolers
Lubrication System

Answer 15

Air Intake Filter: The air intake filter prevents dust, debris, and other contaminants from entering the compressor. It ensures that the compressed air is clean and free from impurities that could potentially damage the engine.

Cylinder and Piston Assembly: In reciprocating compressors, the cylinder and piston assembly are the primary components responsible for compressing the air. The piston moves back and forth within the cylinder, creating a pressure differential and compressing the air.

Intercoolers: Intercoolers are heat exchangers used in multi-stage compressors. They cool down the compressed air between stages, reducing its temperature and increasing its density. This process enhances compressor efficiency and prevents overheating.

Lubrication System:Marine compressors have a lubrication system that ensures smooth operation and reduces wear and tear. The system provides lubricating oil to critical components, such as the piston rings and bearings, to minimize friction and extend the compressor’s lifespan.

Question 16

what are some Rules and regulations related to marine starting air compressors?

Answer 16

Rules and regulations related to marine starting air compressors can vary depending on the jurisdiction and classification society governing the vessel. However, there are several international standards and guidelines that provide a framework for the design, installation, operation, and maintenance of marine starting air compressors.

regulations and standards commonly applicable to marine starting air compressors:

The following regulations are relevant to marine starting air compressors:

SOLAS (Safety of Life at Sea) Convention:SOLAS provides comprehensive regulations for the safety of ships and seafarers. It includes requirements for the installation and maintenance of machinery and equipment, including starting air compressors.

MARPOL (International Convention for the Prevention of Pollution from Ships):MARPOL sets standards to prevent pollution from ships, including regulations on air emissions. Compliance with MARPOL Annex VI is essential for controlling air pollution from ships, which may involve ensuring proper operation and maintenance of starting air compressors.

Classification Society Rules:Classification societies, such as the American Bureau of Shipping (ABS), Lloyd’s Register (LR), and Det Norske Veritas Germanischer Lloyd (DNV GL), establish rules and standards for the design, construction, and operation of vessels. These rules often include specific requirements for starting air compressors, covering aspects such as design, materials, installation, testing, and maintenance.

Flag State Requirements:The flag state, under whose jurisdiction a vessel is registered, imposes additional regulations and requirements. These requirements may include periodic inspections, certifications, and surveys related to the operation and maintenance of machinery and equipment, including starting air compressors.

Industry Standards:There are various industry standards and guidelines that provide recommendations and best practices for marine starting air compressors. Examples include:

ISO 10440-1:This standard specifies requirements for the design, manufacture, and testing of reciprocating compressors, including starting air compressors.

ISO 10440-2:This standard covers the design, manufacture, and testing of screw compressors, which are also used in marine applications.

Question 17

what are some essential maintenance tasks for air compressors

Answer 17

Essential maintenance tasks:

Regular Inspection:Inspect the compressor for any signs of leaks, loose connections, or worn-out parts. Check the intake filter for clogging and clean or replace it as necessary.

Lubrication:Monitor and maintain the lubricating oil levels as per the manufacturer’s guidelines. Regularly change the oil and oil filters to prevent contamination and ensure optimal lubrication.

Cooling System:Check the inter-coolers and cooling system for any signs of fouling or blockage. Clean or repair them if required to maintain efficient cooling.

Valve Maintenance:Inspect the valves, such as the intake and discharge valves, for proper functioning. Clean or replace them if they show signs of wear or damage.

Safety Measures:Ensure that all safety devices, such as pressure relief valves and pressure gauges, are in good working condition. Regularly test and calibrate these devices to guarantee accurate readings.

Compressor Manufacturer Guidelines:Compressor manufacturers often provide specific guidelines and recommendations for the operation, maintenance, and troubleshooting of their products. Following these guidelines ensures compliance with the manufacturer’s specifications and enhances the reliability and performance of the compressors.

Question 18

With reference to main starting air compressors:
a) State three maintenance procedures that maintain their efficiency; (6)
b) Describe the procedure for the manual starting of a compressor. (10)

Answer 18

a.) 1. Air filter will prevent any foreign material entering the compressor, which could damage the piston, cylinder or
valves. Filters collect foreign matter or contaminants over time and need to be regularly inspected and cleaned. As clogged filters will result in a reduction in efficiency.

2. starting air compressors use plate valves which need to be regularly inspected to identify any damage or contaminants. Damaged plates need to be replaced to maintain the operating efficiency. Also regularly check piston rings for damage as this would result in a loss in compression and therefore a loss in efficiency
3. An important measurement that is conducted by engineers on start air compressors is the “Bump
   Clearance”
   * This measures the clearance between the piston and the cylinder head at TDC. its the volume of compressed air that wont be delivered.
   * Too Large of a “bump clearance” and the efficiency will be low
   * Too small a bump clearance and there is a potential for damage of piston crown

b.) 1. Ensure all delivery valves are lined up correctly and check the discharge side and suction side.

2. Drain moisture from the air compressor using the drain valve.
3. Open up cooling water valves and monitor cooling water pressure.
4. Check sump oil level with dipstick.
5. open unloader valve to minimise torque load and expel any condensate from the cylinders and relieve any pressure.
6. Start compressor by pressing start button at the control panel.
7. Listen for any abnormal noise or vibrations whilst air compressor is running.
8. Verify cooling water circulation, lub oil pressure.
9. Close unloader valve and verify correct operating pressure.
10. Monitor all parameters at the gauges (air pressure should be the same as the air bottles)

Question 19

a) State how deterioration in the efficiency would be detected; (4)

b) State four causes for deterioration in efficiency; (8)

c) Describe a test for establishing performance. (4)

Answer 19

a.) The compressor runs longer:
this can be seen during routine safety ER rounds where u will see the compressor will be frequently cutting in and out. Though it may be also caused by sudden demand for air or leaky air pipes, the reason to be investigated. Check the unloader for correct operation as diaphragm of the unloaders is susceptible to hardening, deterioration and damage.

Any discolouration of the paint is also an indicator. High air temperature trip is provided to trip the compressor on sensing temperature at the air outlet in excess 80–85deg Celsius. On tripping due to high air temperature, never reset and start the compressor before investigation.

b.) clogged air filter will result in a reduction in efficiency of compressor (replace filter or clean if possible)

damaged air compressor plates will reduce the operating efficiency

damaged piston rings will result in loss of compression and therefore efficiency.

too large a bumping clearance and efficiency will be low, too small and crown of piston can be damaged. high bumping clearance means there’s less compression efficiency because piston sucks in less air and air temperature becomes hotter.

c.) Performance check of compressor can be carried out by draining start air bottles and timing the duration to fill
bottles back to 30 bar.
* records should be logged in the ships planned maintenance
system (PMS). SOLAS provide regulations regarding the time taken to fill up the air bottles but most of the modern reciprocating air compressors take less time then the regulations. Comparing the time taken, with manufacturers and shipbuilder specs (test reports) provide a baseline. If the time taken is more than the prescribed standard, it indicates a fall of performance.

Question 20

With respect to 2-stage reciprocating compressors used for air start purposes:
a) State four reasons for a reciprocating compressor taking too long to fill the
main air starting bottles; (8)
b) State four safety devices fitted to the air starting system of a large marine
diesel engine; (8)

Answer 20

a.) high bumping clearance means there’s less compression efficiency because piston sucks in less air and air temperature becomes hotter.

incorrectly seated air compressor valve and or damaged plate and therefore reducing operating efficiency

worn out / damaged piston rings this would result in a loss in compression and therefore a loss in efficiency.

blocked air suction filter causing damage to pistons and cylinders and valves as well causing a reduction in efficiency because not as much air can get through due to the filter being blocked

b.) flame trap ( Sulzer engine) at air branch pipe

busting disc (man B/W engine) present at branch pipe

safety relief valve at end of air start manifold and air reservoir

non return valve at engine inlet starting air manifold

fusable plug at air reservoir bottom (melt and release air)

turning gear interlock

Question 21

With reference to 2-stage water-cooled reciprocating main engine starting air compressors:

a) Describe, with the aid of a sketch, a protective device fitted to the waterside
of the Intercooler or after-cooler. (8)

b) Explain why the device described in (a) is fitted. (4)

c) Briefly describe the possible consequences should the device described in (a) fail to operate. (4)

Answer 21

a.) see notes for drawing

if pressure exceeds the normal limit a pressure safety relief valve would open. or another device would be a busting disc. both prevent over pressurisation should a tube in intercooler fail. too much pressure would lead to housing failure and high temp cut out on outlet side. 6 bar before there is a burst.

b.) a pressure safety relief valve is fitted to protect against over pressurisation should a tube in intercooler fail. it does this by opening when pressure exceeds 6bar to release the pressure. too much pressure would lead to housing failure and high temp cut out on outlet side.

c.) it can lead to an explosion. Damaged equipment and housing failure , disrupted work flow, and even injury to personnel. Housing failure could be things like ruptured pipes which would lead major leaks and flooding due to air compressors using jacket water for cooling.

Question 22

Sketch a cross section through a two stage main starting air compressor,
labelling the main components. (16)

Answer 22

SEE EOOW ORAL/IAMI Sketch Pack for drawing

Question 23

With reference to a main engine air starting system:

a) List five important safety features fitted to the system; (5)

b) State how a leaking air start valve may be detected; (1)

c) Briefly describe the dangers of allowing the condition mentioned in (b) to persist. (2)

Answer 23

a.) * Turning Gear Interlock
* Safety Relief Valve
* Flame Traps
* Bursting/rupture Disc
* Fusible plug on starting air receiver
* Non Return Valve

b.) the air starting manifold branch pipe would have discolouration or bursting disc has burst/ ruptured and exhaust gas would be leaking into the E/R from air manifold drain pipe

c.) if you allowed this condition to persist hot exhaust gas would be leaking into E/R and therefore would be harmful to crew . if the bursting disc or flame trap has failed this would provide a source of heat for a potential fire to develop.

Question 24

Sketch a simple cross section through an air start valve for a marine diesel
engine.(8)

Answer 24

SEE EOOW ORAL/IAMI Sketch Pack for drawing

Question 25

what is compressed air used for?

Answer 25

Compressed air is used for starting main and auxiliary diesels, operating whistles or typhons, testing pipe lines (e.g. CO2 fire extinguishing system) and for workshop services. The latter could include pneumatic tools and cleaning lances as well as other hand tools and specialist tools such as engine exhaust valve or seat grinding wheels. The high pressure compressed air for the starting of diesel engines will usually be stored in two large air receivers at around 30 kg/cm2 (bar). The compressors will be low-volume, high-pressure machines usually water cooled. Classification societies require that the outlet temperature of the compressed air be kept 98°C due to the risk of air start line fires.

Question 26

what control air used for?

Answer 26

Function of control air in marine engineering is essential for operation of pneumatic valves in Main Engine control (manoeuvring/reversing/start-stop) system. Similar for auxiliary machinery. Control air pressure must be steady. Air mast be dry for which purpose we have several types of humidity/moisture extraction.

Question 27

how many compressors are required on ship?

Answer 27

Class also require that at least two compressors are fitted to a vessel and that one must be propelled by an alternative power source (such as a diesel engine). Each compressor must have sufficient capacity to charge the starting air receiver from atmospheric pressure to full pressure in 1 h. The air receivers must have sufficient capacity to provide a minimum of 12 starts for a reversible engine and 6 starts for a non-reversible engine. It is important that air compressors do not carry over oil into the compressed air lines. If a fault develops in any of the air start valves then hot gasses could start a serious fire or explosion in the air start line if oil is present. Due to the air being compressed there would be more oxygen than usual and the result could be violent. It is considered good watchkeeping practice to shut the isolating valves from the main air receivers while the engine is running to reduce the compressed air in the lines from the receivers to the engine.

Question 28

what is air composed of?

Answer 28

Air is composed of mainly 23% oxygen, 77% nitrogen by mass and since these are near perfect gases a mixture of them will behave as a near perfect gas, following Boyle’s and Charles’s laws. When air is compressed its temperature and pressure will increase as its volume is reduced.

Question 29

what are some important safety aspects with regards to compressed air system?

Answer 29

The compressed air used for powering hand tools such as rotary wire brushes or needle guns is at a much lower pressure than the starting air, but the tools will require a considerable volume of air for them to operate properly and if there are two or three tools in operation at the same time then the air compressors will be working hard to keep up. The working air compressors operate at about 7 kg/cm2 (bar) and are able to produce a high volume of air. The compressed air required for control engineering needs careful consideration as the air needs to be dry and carefully controlled if delicate controls and instruments are to work correctly. Instrument air compressors can be of the screw type which may also be described as oil-injected or oil-free compressors. The compressors up to about 30 kW are generally air cooled and above 30 kW, fresh water cooling is available. The instrument air system must be free from both oil and water contamination for the instruments and controls to work properly which is why the special ‘oil-free’ compressors are available. Instrument air can also be provided via a reducer/dryer combination fitted to the main air or working air system.

Question 30

what is isothermal compression?

Answer 30

Isothermal compression of a gas is compression at constant temperature. This would mean in practice that as the gas is compressed heat would have to be taken from the gas at the same rate as it is being received. This would necessitate a very slow moving piston in a well-cooled small bore cylinder which is not practical for an actual design.

Question 31

what is adiabatic compression?

Answer 31

Adiabatic compression of a gas is compression under constant enthalpy conditions, that is, no heat is given to or taken from the gas through the cylinder walls and all the work done in compressing the gas is stored within it. Again this is not easy to build as a practical solution

Question 32

what is volumetric efficiency?

Answer 32

This is a measure of compressor capacity. It is the ratio of the actual volume of air drawn in each suction stroke to the stroke volume.

Question 33

what stage of compression is used for relatively high pressures?

Answer 33

If we had an infinite number of stages of compression with coolers in between each stage returning the air to ambient temperature, then we would be able to compress over the desired range close to isothermal conditions. However, this is still impracticable and therefore a two- or three-stage compression with inter-stage and cylinder cooling is generally used when relatively high pressures have to be reached. otherwise known as multistage compression.

Question 34

explain what is bumping clearance?

Answer 34

To prevent overheating and consequential damage, cylinders have to be water or air cooled and clearance must be provided between piston and cylinder head. This clearance must be as small as practicable. High-pressure air remaining in the cylinder after compression and delivery will expand on the return stroke of the piston. This expanding air must fall to a pressure below that in the suction manifold before a fresh air charge can be drawn in. Hence, part of the return or suction stroke of the piston is non-effective. This non-effective part of the suction stroke must be kept as small as possible in order to keep capacity to a maximum. This clearance is sometimes referred to as the ‘bump clearance.

Question 35

explain how to measure the air compressor clearance?

Answer 35

Measuring the air compressor clearance

Correct clearance must be maintained and this is usually done by checking the mechanical clearance, between the top of the piston and the cylinder head, (called the bump clearance) and adjusting it as required by altering the height of the piston relative to the cylinder and cylinder head. This is usually done by using inserts under the palm of the connecting rod. Bearing clearances should also be kept at recommended values because any wear in these bearings will also alter the bump clearance by moving the piston relative to the cylinder head. Two possible methods of ascertaining the mechanical clearance in an air compressor are:

1. Remove suction or discharge valve assembly from the unit and place a small loose ball of lead wire on the piston edge, then rotate the flywheel by hand to take the piston over TDC. Remove and measure the thickness of the lead wire ball.

2.Put crank on TDC, slacken or remove bottom half of the bottom end bearing. Rig a clock gauge with one contact touching some underpart of the piston or piston assembly and the other on the crank web. Take a gauge reading. Then by using a suitable lever bump the piston, that is, raise it until it touches the cylinder cover. Take another gauge reading, the difference between the two readings gives the mechanical clearance.

EXTRA SPECIAL CARE MUST BE TAKEN TO ISOLATE THE COMPRESSOR BEFORE UNDERTAKING THIS WORK.

This is a very important point and the flag state examiner will be checking to ensure that candidates appreciate the importance of this procedure. In practice the effective volume of air drawn in per stroke is further reduced by the pressure in the cylinder which on the suction stroke must fall sufficiently below the atmospheric pressure so that the inertia and spring force of the suction valve can be overcome and air under the force of atmospheric pressure will flow into the cylinder.

Question 36

what air compressors are used at sea?

Answer 36

Air compressors are either reciprocating or rotary types, the former are most commonly used at sea for the production of air for starting diesel engines or for driving power tools. Reciprocating air compressors at sea are generally two or three stage types with inter-stage cooling.

Question 37

draw a diagram of a three stage compressor?

Answer 37

see motor sketch pack

Question 38

explain the importance of air filters?

Answer 38

Air contains suspended foreign matter, much of which is abrasive. If this is allowed to enter the compressor it will combine with the lubricating oil to form an abrasive-like paste which increases wear on piston rings, liners and valves. It can adhere to the valves and prevent them from closing properly, which in turn can lead to higher discharge temperatures and the formation of what appears to be a carbon deposit on the valves, etc. Strictly, the apparent carbon deposit on valves contains very little carbon from the oil, it is mainly solid matter from the atmosphere. These carbon-like deposits can become extremely hot on valves which are not closing correctly and could act as ignition points for air–oil vapour mixtures, leading to possible fires and explosions in the compressor. Hence air filters are extremely important. They must be regularly cleaned and where necessary renewed and the compressor must never be run with the air intake filter removed.

Question 39

what’s the purpose of drain valves for a compressor?

Answer 39

Fitted after each cooler is a drain valve, these are essential. To emphasise, if we consider 30 m3 of free air, relative humidity 75%, temperature 20°C being compressed every minute to about 10 bar, about ½ litre of water would be obtained each minute. Drains and valves to air storage unit must be open upon starting up the compressor in order to get rid of accumulated moisture. When the compressor is running drains have to be opened and closed at regular intervals.

Question 40

what reliving device are fitted to compressor?

Answer 40

After each stage of compression a relief valve will normally be fitted. Regulations only require the fitting of a relieving device on the h.p. stage. Bursting discs or some other relieving device are fitted to the water side of coolers so that in the event of a compressed air carrying tube bursting, the sudden rise in pressure of the surrounding water will not fracture the cooler casing. In the event of a failure of a bursting disc a thicker one must not be used as a replacement.

Question 41

Certain factors govern the choice of lubricant for the cylinders of an air compressor what are these?

Answer 41

These are operating temperature, cylinder pressures and air condition. The correct grade of oil must be used when topping up the air compressor. This will be detailed on the vessel’s oil schedule which is kept in the MCR and will be produced by the oil manufacturers. To comply with the Control of Substances Hazardous to Health (COSHH) regulations the specification of the oil should also be available to the ship’s crew.

Question 42

why is operating temperature of compressor important?

Answer 42

This affects oil viscosity and deposit formation. If the temperature is high this results in low oil viscosity, very easy oil distribution, low film strength, poor sealing and increased wear. If the temperature is low, oil viscosity would be high. This causes poor distribution, increased fluid friction and power loss.

Question 43

Air contains moisture that can condense out. Straight mineral oils would be washed off surfaces by the moisture and this could lead to excessive wear and possible rusting. how is this prevented

Answer 43

To prevent this a compounded oil with a rust inhibitor additive would be used. Compounding agents may be from 5% to 25% of non-mineral oil, which is added to a mineral oil blend. Fatty oils are commonly added to lubricating oil that must lubricate in the presence of water; they form an emulsion which adheres to the surface to be lubricated.

Question 44

what compressor are used for starting air system?

Answer 44

Most modern diesel engines use starting air at a pressure of about 26–30 bar and to achieve this, a two-stage type of compressor would be adequate. These compressors are generally of the reciprocating type, with various possible arrangements of the cylinders, or they could be a combination of a rotary first stage followed by a reciprocating high pressure stage.

Question 45

what are the effect of leaky suction and delivery valves for a compressor?

Answer 45

Effects of leaking valves

1.First-stage suction: Reduced air delivery, increased running time and reduced pressure in the suction to the second stage. If the suction valve leaks badly it may completely unload the compressor.

2.First-stage delivery: With high pressure air leaking back into the cylinder less air can be drawn in, this means reduced delivery and increased discharge temperature.

3.Second-stage suction: High pressure and temperature in the second-stage suction line, reduced delivery and increased running time.

4. Second-stage delivery: Increased suction pressure in second stage, reduced air suction and delivery in second stage. Delivery pressure from first stage increased.

Question 46

what’s the purpose of an air compressor?

Answer 46

the role of the air compressor is to raise the pressure of air from a suction to a discharge by inputting energy into the medium.

Question 47

explain the methods of regulating air compressors?

Answer 47

The compressed air has several applications and there are different types of compressor that have been developed to handle the different applications. These different types of compressor also have different methods of control depending upon the type and application of compressor. Therefore, to allow compressors to respond to fluctuations in system demand they are linked to an automatic pressure regulation controller and the controller will start a process to alter the output of the compressor. The most popular methods in use to date are explained below.

Start stop control

A general observation would be that the torque required to drive a compressor increases with the speed of the machine. Also the starting torque can be very high as is the case with reciprocating compressors. Some are fitted with star–delta starters but others are still direct online and for this reason the start–stop technology will only be suitable for electrically driven units. A pressure transducer attached to the air receiver set for desired max–min pressures would switch the current to the electric motor’s starter either on or off. Drainage would have to be automatic and air receiver relatively large compared to the compressor unit requirements so that the number of starts per unit of time is not too great. It must be remembered that the starting current for an electric motor is about double the normal running current. During its operation the compressor does operate at its optimum efficiency and if the machine is stopped for long periods of time then the overall performance is acceptable.

Constant running control

This method of control is the one used most often for the higher volume machine running at a relatively low pressure. The compressor runs continuously at a constant speed and when the desired air pressure is reached the air compressor is unloaded in some way so that the air is NOT delivered and practically no work is done in the compressor cylinders. The methods used for compressor unloading vary, but that most commonly used is to shut off the air to the suction side of the compressor. If the compressor receives no air then it cannot deliver any. Or if the air taken in at the suction is returned to the suction again no air will be delivered. In either case virtually no work would be done in the compressor cylinder or cylinders and this would provide an economy compared to discharging high pressure air to the atmosphere through a relief valve. When the discharge air pressure reaches a desired value it will act on the piston causing the spring-loaded valve to close shutting off the supply of air to the compressor. An alternative method of unloading the compressor, while continuing to run it, is to hold the suction valve open. When the compressor is unloaded the suction valve plates are held open by pins which are operated by a relay valve and piston. When the pressure in the air reservoir falls to a pre-set level, the piston’s chamber is vented and return springs push out the holding pins allowing the suction valve to operate normally. An alternative method of optimising the use of compressors is to have several smaller machines running in parallel. The number of machines running can be adjusted depending upon the demand.

Question 48

what are the advantages of variable speed control for compressors over other methods of regulating air compressors?

Answer 48

Variable-speed control

The ability to vary the speed of the compressor presents a number of major advantages over other methods of control.

These are:

• A gradual start-up and increase in speed meaning that there are no sudden peaks in the current supply to the motor. The stress of sudden acceleration on the mechanical components is reduced.
• The pressure can be controlled to a much finer tolerance because the speed and therefore the flow rate can be adjusted to match the demand. This reduces the range of the pressure fluctuations and also the stress on the pressure parts of the system. Initially systems can be designed using smaller receivers.
• Due to the efficiency being optimised so is the use of energy and therefore there will be a fuel saving for the vessel.
• Variable speed control is also suited to compressors operating in parallel. Here one of the machines can be optimised by speed control while the others operate on an on–off basis.

Question 49

with the aid of a sketch explain the operating principle of an automatic drain?

Answer 49

Figure 7.11 shows an automatic air drain trap which functions in a near similar way to a steam trap. With water under pressure at the inlet the disc will lift, allowing the water to flow radially across the disc from A to the outlet B. When the water is discharged and air now flows radially outwards from A across the disc, the air expands increasing in velocity ramming air into C and the space above the disc, causing the disc to close on the inlet. Because of the build-up of static pressure in the space above the disc in this way, and the differential area on which the pressures are acting, the disc is held firmly closed. It will remain so unless the pressure in the space above the disc falls. In order that this pressure can fall, and the trap reopen, a small groove is cut across the face of the disc communicating B and C through which the air slowly leaks to the outlet. Obviously this gives an operational frequency to the opening and closing of the disc which is a function of various factors, for example, size of groove, disc thickness and volume of space above the disc. Therefore, it is essential that the correct trap be fitted to the drainage system to ensure efficient and effective operation. These traps should be checked by the watchkeeper by listening for their operation. After a while in operation debris in the water can cause grooves to form across the disc and they stop working.

Question 50

what’s the purpose of an automatic drain?

Answer 50

basic purpose is to automatically remove and drain the water collected at a certain point in the compressed air system automatically using compressed air itself as the driving force.

Question 51

explain the construction of an air
reservoir / receiver with the aid of a sketch?

Answer 51

Material used in the construction must be of good quality low carbon steel similar to that used for boilers, for example, 0.2% carbon (max.), 0.35% silicon (max.), 0.4% manganese, 0.05% sulphur (max.), 0.05% phosphorus (max.), u.t.s. 460 MN/m2. Welded construction has superseded the rivetted types and welding must be done to class 1 or class 2 depending upon operating pressure. If above 35 bar approximately, then class 1 welding regulations apply. Some of the main points relating to class 1 welding are that the welding must be radiographed, annealing must be carried out at a temperature of about 600°C and a test piece must be provided for bend, impact and tensile tests together with micrographic and macrographic examination. Mountings generally provided are shown in figure 7.12. If it is possible for the receiver to be isolated from the safety valve then it must have a fusible plug fitted, melting point approximately 150°C, and if carbon dioxide is used for fire fighting it is recommended that the discharge from the fusible plug be led to the deck. Stop valves on the receiver generally permit slow opening to avoid rapid pressure increases in the piping system, and piping for starting air has to be protected against the possible effects of explosion. Drains for the removal of accumulated oil and water are fitted to the compressor, filters, separators, receivers and lower parts of pipelines.

Question 52

what must be done before you start to fill up the air reservoir or air bottle?

Answer 52

Before commencing to fill the air vessel after overhaul or examination, ensure that:

1.Nothing has been left inside the air vessel, for example, cotton waste that could foul up drains or other outlets.

2.Check pressure gauge against a master gauge.

3.All doors are correctly centred on their joints.

Run the compressor with all drains open to clear the lines of any oil or water, and when filling open drains at regular intervals, observe pressure. After filling close the air inlet to the bottle, check for leaks and follow up on the door joints. When emptying the receiver prior to overhaul, etc., ensure that it is isolated from any other interconnected receiver which must, of course, be in a fully charged state. Cleaning the air receiver internally must be done with caution. Any cleaner which gives off toxic, inflammable or noxious fumes should be avoided. A brush down and a coating on the internal surfaces of some protective, harmless to personnel, such as a graphite suspension in water could be used.

Question 53

explain the operation of an air compressor?

Answer 53

Compressed air has many uses on board ship, ranging from diesel engine starting to the cleaning of machinery during maintenance. The air pressures of 25 bar or more are usually provided in multi-stage machines. Here the air is compressed in the first stage, cooled and compressed to a higher pressure in the next stage, and so on. The two-stage crank machine is probably the most common. Air is drawn in on the suction stroke through the first-stage suction valve via the silencer/filter. The suction valve closes on the piston upstroke and the air is compressed. The compressed air, having reached its first-stage pressure, passes through the delivery valve to the first-stage cooler. The second-stage suction and compression now take place in a similar manner, achieving a much higher pressure in the smaller, second-stage cylinder. After passing through the second-stage delivery valve, the air is again cooled and delivered to the storage system. The machine has a rigid crankcase which provides support for the three crankshaft bearings. The cylinder block is located above and replaceable liners are fitted in the cylinder block. The running gear consists of pistons, connecting rods and the one-piece, two-throw crankshaft. The first-stage cylinder head is located on the cylinder block and the second-stage cylinder head is mounted on the first: each of the heads carries its suction and delivery valves. A chain-driven rotary-gear pump provides lubricating oil to the main bearings and through internally drilled passages in the crankshaft to both connecting rod bearings. Cooling water is supplied either from an integral pump or the machinery space system. The water passes into the cylinder block which contains both stage coolers and then into the first and second stage cylinder heads. A water jacket safety valve prevents a build-up of pressure should a cooler tube burst and compressed air escape. Relief valves are fitted to the first and second-stage air outlets and are designed to lift at 10% excess pressure. A fusible plug is fitted after the second-stage cooler to limit delivered air temperature and thus protect the compressed-air reservoirs and pipework. Cooler drain valves are fitted to compressors. When these are open the machine is ‘unloaded’ and does not produce compressed air. A compressor when started must always be in the unloaded condition. This reduces the starting torque for the machine and clears out any accumulated moisture in the system. This moisture can affect lubrication and may produce oil/water emulsions which line the air pipelines and could lead to fires or explosions. The compressor motor is started and the machine run up to speed. The lubricating oil pressure should be observed to build up to the correct value. The first-stage drains and then the second-stage drains are closed and the machine will begin to operate. The pressure gauge cocks should be adjusted to give a steady reading. Where manual drains are fitted they should be slightly opened to discharge any moisture which may collect in the coolers. The cooling water supply should be checked, and also operating temperatures, after a period of running loaded. To stop the compressor, the first and second-stage cooler drain valves should be opened and the machine run unloaded for two to three minutes. This unloaded running will clear the coolers of condensate. The compressor can now be stopped and the drains should be left open. The cooling water should be isolated if the machine is to be stopped for a long period. Automatic compressor operation is quite usual and involves certain additional equipment. An unloader must be fitted to ensure the machine starts unloaded, and once running at speed will load’ and begin to produce compressed air. Various methods of unloading can be used but marine designs favour either depressors which hold the suction valve plates on their seats or a bypass which discharges to suction. Automatic drains must also be fitted to ensure the removal of moisture from the stage coolers. A non-return valve is usually fitted as close as possible to the discharge valve on a compressor to prevent return air flow: it is an essential fitting where unloaders are used. The compressed air system for the supply of starting air to a diesel engine is described in Chapter 2. Control or instrument air supplies have particular requirements with regard to being moisture and oil free and without impurities. A special type of oil-free compressor may be used to supply control air or it may be treated after delivery from an ordinary air compressor. This treatment results in the air being filtered and dried in order to remove virtually all traces of oil, moisture and any atmospheric impurities. Maintenance involves the usual checks and overhauls common to reciprocating machinery, e.g. crankcase oil level, cooling water system, operating temperatures and pressures, etc. The suction and delivery air valves for each stage will present the most work in any maintenance schedule. These valves are automatic, requiring a small pressure differential to operate. The constant rapid opening and closing action of the valves may require the seats to be refaced. Overheating, use of incorrect lubricating oil, or the presence of dirt may result in sticking or pitting of the surfaces. The various buffer plates, spring plates, valve plate and seat which make up a suction or delivery valve can be seen in Figure 7.3. The valves should be stripped and all parts carefully cleaned and examined, any worn parts replaced and the valve seat and plate lightly lapped separately on a flat surface before reassembly to ensure a good seal.

Question 54

What are the causes of starting air line explosion on ships ?

Answer 54

1.The main cause of starting airline explosion is the leaking starting air valve or jamming atopen position of the valve.
2.Initially, the oil which is discharged from the air compressor to starting air line system, it willdeposit as a thin moist film on the internal surface of the pipes but not ready to combustion.
3.If starting air valve leaked or jammed at open position, hot gas or flame may enter thestarting air manifold, vaporize the oil and set the fire to oil mist and greasy matters, which generallydeposit on the surface.
4.At that condition,mostly arrival first startfor manoeuvring time, high pressure compressedair coming into contact with the fire and may cause explosion.

Question 55

How to prevent starting the air line explosion ?

Answer 55

1.Regular overhaul and maintenance of starting air valve.
2.Before departure, test the air starting valve leakage.
3.Regular drain off the air bottle drain valve.
4.Regular drain off air starting system.
5.Regular cleaning of the compressor suction air fitter and use oil wetted suction filter.
6.Feed minimum absolute cylinder lubrication to compressor.

Question 56

What are the safety devices on starting air line ?

Answer 56

1.Spring loaded safety valve or bursting disc. (if bursting disc fitted, no need of relief valve)
2.Flame trap. (At Joint where manifold to each cylinder starting line)
3.Starting air line drain valve. (Inlet of automatic air starting valve)
4.Turning gear interlock
5.Reversing interlock

Question 57

How to test starting air valve leakage for main engine on ships ?

When Engine Running
When Engine Stopped

Answer 57

When Engine Running

By hand touch feeling to adjacent pipe to the valve while in engine is operating.
If leak, we can feel the pipe hotter than others.

When Engine Stopped

Shut the isolating valve of starting air distributor
All indicator cocks must be opened
Disengage the turning gear
Open the main air bottle stop valve
Set the control lever to starting position
Check all the indicator cocks
Escape of air through indicator cock will show a leaking starting valve
It should be replaced immediately.

Question 58

with the aid of a system sketch describe the operation a starting air system?

Answer 58

Diesel engines are started by supplying compressed air into the cylinders in the appropriate sequence for the required direction. A supply of compressed air is stored in air reservoirs or ‘bottles’ ready for immediate use. Up to 12 starts are possible with the stored quantity of compressed air. The starting air system usually has interlocks to prevent starting if everything is not in order. A starting air system is shown in Figure 2.21. Compressed air is supplied by air compressors to the air receivers. The compressed air is then supplied by a large bore pipe to a remote operating non-return or automatic valve and then to the cylinder air start valve. Opening of the cylinder air start valve will admit compressed air into the cylinder. The opening of the cylinder valve and the remote operating valve is controlled by a pilot air system. The pilot air is drawn from the large pipe and passes to a pilot air control valve which is operated by the engine air start lever. When the air start lever is operated, a supply of pilot air enables the remote valve to open. Pilot air for the appropriate direction of operation is also supplied to an air distributor. This device is usually driven by the engine camshaft and supplies pilot air to the control cylinders of the cylinder air start valves. The pilot air is then supplied in the appropriate sequence for the direction of operation required. The cylinder air start valves are held closed by springs when not in use and opened by the pilot air enabling the compressed air direct from the receivers to enter the engine cylinder. An interlock is shown in the remote operating valve line which stops the valve opening when the engine turning gear is engaged. The remote operating valve prevents the return of air which has been further compressed by the engine into the system. Lubricating oil from the compressor will under normal operation pass along the air lines and deposit on them. In the event of a cylinder air starting valve leaking, hot gases would pass into the air pipes and ignite the lubricating oil. If starting air is supplied to the engine this would further feed the fire and could lead to an explosion in the pipelines. In order to prevent such an occurrence, cylinder starting valves should be properly maintained and the pipelines regularly drained. Also oil discharged from compressors should be kept to a minimum, by careful maintenance. In an attempt to reduce the effects of an explosion, flame traps, relief valves and bursting caps or discs are fitted to the pipelines. In addition an isolating non-return valve (the automatic valve) is fitted to the system. The loss of cooling water from an air compressor could lead to an overheated air discharge and possibly an explosion in the pipelines leading to the air reservoir. A high-temperature alarm or a fusible plug which will melt is used to guard against this possibility.