The starting air system is supplied by two main starting air compressors that supply two 5.0m3 starting air receivers. The compressors are of the two stage reciprocating type and are cooled by fresh water from the fresh water cooling system. A three-way temperature controlled valve is used to regulate the flow of water to the compressors to maintain the correct operating temperature.

Each starting air compressor has its own cooling water circulation pump which operates whenever the compressor  is running. The starting air compressors operate on a ‘lead’ and ‘lag’ principle and are set to automatically start when the pressure in the starting air receiver falls to a preset value. The lead/lag control is located on the main switchboard No.1 main air compressor starter panel on GSP No.1. The lead compressor cuts in at 2.8MPa and the lag compressor at 2MPa on falling air pressure. Both compressors stop when the pressure reaches 2.94MPa and are controlled by ‘auto start’ and ‘auto stop’ pressure switches fitted on the pipeline between the compressors and the air receivers. The local starter panel is for the main air conpressors amnd the general service air compressor and comprises start pushbuttons and stop/lock off buttons. Selection of automatic operation is carried out at the main switchboard

Both of the compressors have an automatic unloading arrangement on the first and second stage valves which operate when the compressors start and stop. This allows the compressors to start and stop off load so reducing the loading on the electric motor and the wear and tear on the drive train.

The main air receivers are used to supply the starting air for the main engine and for the three diesel generator engines with the air being supplied to the three generator engines using a separate pipeline from that used for supplying the main engine. Illustration above shows the layout of the system and how the pipework is configured. The control air and general service air systems can also be supplied from the starting air system through a reducing station should the control and general service air compressor become inoperative.

Both compressors are equipped with a high air temperature trip which shuts down the machines when the set point is exceeded. A low level LO trip is also provided which will shut down the machine when the oil level in the sump drops below a preset level. A time delay has been provided to compensate for the movement of the vessel in heavy weather.

The  auxiliary  air  receiver  is normally  supplied  from  the  main  starting  air compressors but can be directly supplied from one of two hand air compressors in the event of ‘dead ship’ conditions. The compressed air in this receiver is used to start at least one of the three generator engines when the main starting air receivers are isolated. This will allow a generator to be connected to the main switchboard and normal electrical supplies to the engine room resumed. The starting air is supplied to the air motors on each of the engines which then engage with the toothed rim of the engine flywheel. The air is supplied to the starting air motor via a reducing valve fitted on each of the engine starting air lines.

Note:   The principal method of starting the vessel from dead ship remains via the emergency generator, which is supplied with 2 battery sets, and in the event of starter motor failure, a spare starter motor.

The generator engine starting air supply valves on the main air receivers and on the air supply lines are normally left open to ensure that any standby generator can be started automatically or locally at any time.

Switches  at  the  compressor  starter  panels  enable  the  compressors  to  be manually started and stopped if required. When in remote operation mode, the starting air compressors are normally selected for automatic operation. Before a compressor is started the compressor outlet and the air receiver filling valves must be open. The instrumentation  and pressure switch valves must also be open to allow the compressor to operate automatically. It is also necessary to ensure that the automatic drain traps fitted to the bottom of both air receivers are open at all times.

Procedure for Operating the Starting Air System

a)     Ensure that all of the pressure gauge and instrumentation valves are open and that the instrumentation is operating correctly.

b)     Check the oil level in the compressors and check also for the presence of water in the crankcase.

c)     Ensure  that  the  auxiliary   fresh  water  cooling   system  is operational  and  that  the  compressor  cooling  water  inlet  and outlet valves are open. Check that the temperature controlled three-way valve is operational .

d)     Only one receiver should be in use during normal operations at sea as this will maintain a reserve should a pressure loss occur in the system. One main starting air receiver will, therefore, be maintained at full pressure with the outlet valves closed. The other main starting air compressor  will have its filling valve open and the outlet valve to the generator engines open.

Note: Condensed water that accumulates must be drained from the starting air receivers and it is essential that the drain traps are operational on both of the main starting air receivers.

e)     Set up the valves as shown in the table below.

The valve settings  listed below  have assumed  that both of the starting  air compressors  are  available  for  operation  and  that  both  of  the  starting  air receivers are open for filling and air discharge:

Position	Description
Open	No.1 starting air compressor discharge valve
Open	No.1 starting air compressor outlet valve
Operational	No.1 starting air compressor drain valve
Open	No.2 starting air compressor discharge valve
Open	No.2 starting air compressor outlet valve
Operational	No.2 starting air compressor drain valve
Closed	Line drain valve
Open	Automatic start pressure switch valve
Open	Automatic stop pressure switch valve
Open	Automatic stop pressure switch valve
Open	No.1 main air receiver filling valve
Open	No.1 main air receiver drain trap valves
Open	No.1 main air receiver pressure gauge valve
Open	No.1 main air receiver main engine air supply valve

Position	Description
Open	No.1  main  air  receiver  generator  engine  air supply valve
Closed	No.1 main air receiver manual drain valve
Open	No.2 main air receiver filling valve
Open	No.2 main air receiver drain trap valves
Open	No.2 main air receiver pressure gauge valve
Open	No.2 main air receiver main engine air supply valve
Open	No.2  main  air  receiver  generator  engine  air supply valve
Closed	No.2 main air receiver manual drain valve
Closed	Main engine starting air line drain valve
Open	Generator engine air supply line valve
Closed	Generator engine air supply line drain valve
Open	No.1 generator engine air inlet valve
Open	No.2 generator engine air inlet valve
Open	No.3 generator engine air inlet valve
Closed	Auxiliary air receiver filling valve from main air compressors
Closed	Auxiliary air receiver filling valve from No.1 hand air compressors
Closed	Auxiliary air receiver filling valve from No.2 hand air compressors
Closed	Auxiliary air receiver filling valve
Closed	Auxiliary air receiver outlet valve
Closed	Auxiliary air receiver outlet line drain valve
Open	Line supply valve to control air/general service air reducing valve
Closed	Line drain valve
Open	Line valve from reducing valve to control air/ general service air systems

Automatic Operation

For automatic operation of the starting air compressors:

f)     Select the lead/lag compressor at the main switchboard.

g)     Ensure that power is available to the main air compressor starter panel. Set the mode switch to AUTO at the main switchboard

g)     Press  the START  pushbutton.  The  lead  compressor  will  run off-load for approximately 15-20 seconds after which time the drain/unloading solenoid valves will be energized to close and allow the discharge of compressed air to the system.

The compressors will now be controlled by the air system pressure switches according  to the setting  of the selector  switch  for the lead  and  follow-on compressors. At 5 minute intervals the drain solenoid valves are de-energised for 2 seconds to drain water from the compressor air cooler water separator bowls.

When  the pressure  switch  indicates  a full pressure  in the air receiver,  the solenoid valves are de-energised. The drain opens and the compressor unloads and stops. When the pressure in the air receiver falls to the recharging pressure a recharge signal is sent to the compressor. The compressor starts again with the drain valve open so that the compressor actually starts off load. When the compressor is running at normal full speed the drain valve is closed and the compressor loads.

If the air receiver pressure continues to fall despite the lead (duty) compressor operating,  the standby  compressor  will start when the pressure  falls to the preset  low  low  pressure.  This  compressor  will  start  and  load  in the same manner as the duty compressor and will unload when the cut out pressure for this machine has been reached.

Manual Operation

For manual operation of the starting air compressors:

h)     Ensure that power is available to the main air compressor starter panel and press the start pushbutton

The  compressor(s)  are  now  started  and  run  in  the  manual  mode.  The compressors will continue to be protected by their alarms and shutdowns as in the automatic mode of operation.

The compressors may be operated individually in the manual mode either from the local starter panel or from the main switchboard room.