5.6 Safety functions of gas supply systems
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Statutory Documents - IMO Publications and Documents - Resolutions - Maritime Safety Committee - Resolution MSC.285(86) – Interim Guidelines on Safety for Natural Gas-Fuelled Engine Installations in Ships – (Adopted on 1 June 2009) - Annex – Interim Guidelines on Safety for Natural Gas-fuelled Engine Installations in Ships - Chapter 5 – Control, Monitoring and Safety Systems - 5.6 Safety functions of gas supply systems

5.6 Safety functions of gas supply systems

  5.6.1 Each gas storage tank should be provided with a tank valve capable of being remote operated and should be located as close to the tank outlet as possible.

  5.6.2 The main gas supply line to each engine or set of engines should be equipped with a manually operated stop valve and an automatically operated “master gas fuel valve” coupled in series or a combined manually and automatically operated valve. The valves should be situated in the part of the piping that is outside machinery space containing gas-fuelled engines, and placed as near as possible to the installation for heating the gas, if fitted. The master gas-fuel valve should automatically cut off the gas supply as given in table 1.

  • 5.6.2.1 The automatic master gas fuel valve should be operable from a reasonable number of places in the machinery space containing gas-fuelled engines, from a suitable location outside the space and from the bridge.

  5.6.3 Each gas consuming equipment should be provided with a set of “double block and bleed” valves. These valves should be arranged as outlined in .1 or .2 (respectively shown as alternatives 1 and 2 in figure 1) so that when automatic shutdown is initiated as given in table 1, this will cause the two gas fuel valves that are in series to close automatically and the ventilation valve to open automatically and:

  • .1 two of these valves should be in series in the gas fuel pipe to the gas consuming equipment. The third valve should be in a pipe that vents to a safe location in the open air that portion of the gas fuel piping that is between the two valves in series; or

  • .2 the function of one of the valves in series and the ventilation valve can be incorporated into one valve body, so arranged that the flow to the gas utilization unit will be blocked and the ventilation opened.

  5.6.3.1 The two block valves should be of the fail-to-close type, while the ventilation valve should be fail-to-open.

  5.6.3.2 The double block and bleed valves should also be used for normal stop of the engine.

  5.6.4 In cases where the master gas fuel valve is automatically shutdown, the complete gas supply branch downstream of the double block and bleed valve should be ventilated, if reverse flow from the engine to the pipe must be assumed.

  5.6.5 There should be one manually operated shutdown valve in the gas supply line to each engine upstream of the double block and bleed valves to assure safe isolation during maintenance on the engine.

  5.6.6 For one-engine installations and multi-engine installations, where a separate master valve is provided for each engine, the master gas fuel valve and the double block and bleed valve functions can be combined. Examples for the high-pressure system are shown in figures 1 and 2.

Figure 1 Alternative supply valve arrangements for high-pressure installations (single engine or separate master valve arrangement)

Figure 2 Alternative supply valve arrangements for high-pressure installations (multi-engine installation)

  5.6.7 The total loss of ventilation in a machinery space for a single fuelled gas system should, additionally to what is given in table 1, lead to one of the following actions:

  • .1 For a gas electric propulsion system with more than one machinery space: Another engine should start. When the second engine is connected to bus-bar, the first engine should be shutdown automatically.

  • .2 For a direct propulsion system with more than one machinery space: The engine in the room with defect ventilation should be manually shutdown, if at least 40% propulsion power is still available after such a shutdown.

If only one machinery space for gas-fuelled engines is fitted and ventilation in one of the enclosed ducts around the gas pipes is lost, the master gas fuel and double block and bleed valves in that supply line should close automatically provided the other gas supply unit is ready to deliver.

  5.6.8 If the gas supply is shut off due to activation of an automatic valve, the gas supply should not be opened until the reason for the disconnection is ascertained and the necessary precautions taken. A readily visible notice giving instruction to this effect should be placed at the operating station for the shut-off valves in the gas supply lines.

  5.6.9 If a gas leak leading to a gas supply shutdown occurs, the gas fuel supply should not be operated until the leak has been found and dealt with. Instructions to this effect should be placed in a prominent position in the machinery space.

  5.6.10 A signboard should be permanently fitted in the machinery space containing gas-fuelled engines stating that heavy lifting, implying danger of damage to the gas pipes, should not be done when the engine(s) is running on gas.

Table 1 Monitoring of gas supply system to engines

Parameter Alarm Automatic
shutdown
of main
tank valve 		Automatic shutdown
of gas supply to
machinery space
containing
gas-fuelled engines 		Comment
Gas detection in tank room above 20% LEL X      
Gas detection on two detectors 1) in tank room
above 40% LEL 		X X    
Fire detection in tank room X X    
Bilge well high level tank room X      
Bilge well low temperature in tank room X X    
Gas detection in duct between tank and
machinery space containing gas-fuelled
engines above 20% LEL 		X      
Gas detection on two detectors 1) in duct
between tank and machinery space containing
gas-fuelled engines above 40% LEL 		X X 2)    
Gas detection in compressor room
above 20% LEL 		X      
Gas detection on two detectors1) in compressor room above 40% LEL X X 2)    
Gas detection in duct inside machinery
space containing gas-fuelled engines
above 30% LEL 		X     If double pipe fitted in machinery space containing gas-fuelled engines
Gas detection on two detectors1) in duct inside
machinery space containing gas-fuelled
engines above 40% LEL 		X   X 3) If double pipe fitted in machinery space containing gas-fuelled engines
Gas detection in machinery space containing
gas-fuelled engines above 20% LEL 		X     Gas detection only required for ESD protected machinery space
Gas detection on two detectors1) in
machinery space containing gas-fuelled
engines above 40% LEL 		X   X Gas detection only required for ESD protected machinery space containing gas-fuelled engines. It should also disconnect non certified safe electrical equipment in machinery space containing gas-fuelled engines
Loss of ventilation in duct between tank and
machinery space containing gas-fuelled engines 6) 		X   X 2) 4)  
Loss of ventilation in duct inside machinery
space containing gas-fuelled engines6) 		X   X 3) 4) If double pipe fitted in machinery space containing gas-fuelled engines
Loss of ventilation in machinery space
containing gas-fuelled engines 		X   X ESD protected machinery space containing gas-fuelled engines only
Fire detection in machinery space containing
gas-fuelled engines 		X   X  
Abnormal gas pressure in gas supply pipe X   X4)  
Failure of valve control actuating medium X   X5) Time delayed as found necessary
Automatic shutdown of engine (engine failure) X   X5)  
Emergency shutdown of engine manually released X   X  
1) Two independent gas detectors located close to each other are required for redundancy reasons. If the gas detector is of self monitoring type the installation of a single gas detector can be permitted.
2) If the tank is supplying gas to more than one engine and the different supply pipes are completely separated and fitted in separate ducts and with the master valves fitted outside of the duct, only the master valve on the supply pipe leading into the duct where gas or loss of ventilation is detected is to close.
3) If the gas is supplied to more than one engine and the different supply pipes are completely separated and fitted in separate ducts and with the master valves fitted outside of the duct and outside of the machinery space containing gas-fuelled engines, only the master valve on the supply pipe leading into the duct where gas or loss of ventilation is detected is to close.
4) This parameter is not to lead to shutdown of gas supply for single fuel gas engines, only for dual fuel engines.
5) Only double block and bleed valves to close.
6) If the duct is protected by inert gas (see 2.7.1) then loss of inert gas overpressure is to lead to the same actions as given in this table.
Parent topic: Chapter 5 – Control, Monitoring and Safety Systems
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