2.7.1.1 Gas supply lines passing through enclosed
spaces should be completely enclosed by a double pipe or duct. This
double pipe or duct should fulfil one of the following:
-
.1 the gas piping should be a double wall piping
system with the gas fuel contained in the inner pipe. The space between
the concentric pipes should be pressurized with inert gas at a pressure
greater than the gas fuel pressure. Suitable alarms should be provided
to indicate a loss of inert gas pressure between the pipes. When the
inner pipe contains high pressure gas, the system should be so arranged
that the pipe between the master gas valve and the engine is automatically
purged with inert gas when the master gas valve is closed; or
-
.2 the gas fuel piping should be installed within
a ventilated pipe or duct. The air space between the gas fuel piping
and the wall of the outer pipe or duct should be equipped with mechanical
under pressure ventilation having a capacity of at least 30 air changes
per hour. This ventilation capacity may be reduced to 10 air changes
per hour provided automatic filling of the duct with nitrogen upon
detection of gas is arranged for. The fan motors should comply with
the required explosion protection in the installation area. The ventilation
outlet should be covered by a protection screen and placed in a position
where no flammable gas-air mixture may be ignited.
2.7.1.2 The connecting of gas piping and ducting
to the gas injection valves should be so as to provide complete coverage
by the ducting. The arrangement should facilitate replacement and/or
overhaul of injection valves and cylinder covers. The double ducting
should be required also for gas pipes on the engine itself, and all
the way until gas is injected into the chamber.footnote
2.7.1.3 For high-pressure piping the design pressure
of the ducting should be taken as the higher of the following:
-
.1 the maximum built-up pressure: static pressure
in way of the rupture resulting from the gas flowing in the annular
space;
-
.2 local instantaneous peak pressure in way of
the rupture: this pressure is to be taken as the critical pressure
and is given by the following expression:
where:
|
p0
|
= |
maximum
working pressure of the inner pipe |
|
k |
= |
Cp/Cv constant pressure specific heat divided by the constant volume
specific heat
|
|
k |
= |
1.31 for CH4
|
The tangential membrane stress of a straight pipe should not
exceed the tensile strength divided by 1.5 (Rm/1.5) when
subjected to the above pressures. The pressure ratings of all other
piping components should reflect the same level of strength as straight
pipes.
As an alternative to using the peak pressure from
the above formula, the peak pressure found from representative tests
can be used. Test reports should then be submitted.
2.7.1.4 For low pressure piping the duct should
be dimensioned for a design pressure not less than the maximum working
pressure of the gas pipes. The duct should also be pressure tested
to show that it can withstand the expected maximum pressure at gas
pipe rupture.
2.7.1.5 The arrangement and installation of the
high-pressure gas piping should provide the necessary flexibility
for the gas supply piping to accommodate the oscillating movements
of the main engine, without running the risk of fatigue problems.
The length and configuration of the branch lines are important factors
in this regard.