Page 71 - Offshore Electrical Engineering Manual

58 CHAPTER 4 Prime Mover Selection Criteria

c. ensuring that the enclosure is always positively pressurised (with air free

from gas), so that gas cannot be drawn in from outside the enclosure,
d. keeping potential gas leak sources in the enclosure (such as fuel gas pipe

flanges) to an absolute minimum.
2. A ‘block and bleed system’ should be included in the fuel gas supply system
so that when the engine is stopped, the entire length of the fuel gas supply pipe
within the enclosure is blocked by isolating valves at both ends and vented
safely to the atmosphere.
3. Electrical instrumentation and controls associated with the fuel gas pipe-
work, such as pressure and temperature transmitters, solenoid valves and
throttle actuators, should all be suitable for safe use in areas where explosive
gas mixtures may be present. With spark-ignition engines, the shielded type
ignition system, as discussed earlier, should be used. This is because the
high ignition voltages are only present within the engine with this type of
system. The subject of hazardous areas is discussed in greater detail in
Part 5 Chapter 4.
4. In reciprocating engines, various extra precautions need to be taken as follows:
a. Drive belts must be of the antistatic, fire-resistant type.

b. Cooling fan blades must be of a type which cannot cause friction sparks if
they come into contact with adjacent parts.

c. Exhaust systems must be fitted with flame traps and spark arresters.
d. A flame trap may be required in the combustion air intake to protect against

a flashback through the induction system (i.e., backfiring). This will be
required even if the intake is in a safe area, if the engine may be run in emer-
gency conditions.

e. All diesel engines should be fitted with a ‘Chalwyn’ or similar air induc-
tion valve to prevent overspeeding if flammable gases are drawn in with the
combustion air. This is particularly important if the engine runs on a fixed
fuel-rack setting, i.e., is not fully governed.
3
f. If the crankcase volume is greater than 0.5 m , relief valves must be fitted to

the crankcase to prevent damage or external ignition due to crankcase explo-
sions. The relief device must be provided with its own spark arrester/flame
trap system.

g. Care must be taken to ensure that any special design features that could
cause external ignition are adequately catered for. For example, turbocharg-
ers must be water-cooled; decompression ports, if absolutely necessary,
should be treated in the same way as exhaust systems.

h. Engine governors, and the fuel injection pumps of diesel engines, should be
designed so as to make reverse running of the engine impossible.
For further information, please refer to Publication MEC-1 of the Engineering
Equipment and Materials Users Association (formerly OCMA) entitled
Recommendations for the Protection of Diesel Engines Operating in Hazardous
Areas.