Page 26 - Uninterruptible Power Supplies

Standby Power Generating Sets
24 Chapter One

engine room there should be a reliable fire detection system not subject
to spurious operation by the oil or exhaust fumes that are always pre-
sent, or by the high ambient temperatures that are sometimes experi-
enced. Suitable fire detection systems are available, one that has been
in use for many years uses fusible metal links mounted above the
engine and flexible cables running around pulleys and extending to a
fire alarm switch, and to any local free-fall fire valves. Whatever fire
alarm system is installed in the engine room, it should be intercon-
nected with any main building fire alarm system.
In the United Kingdom there is a growing requirement for service
tanks to be double skinned or bunded, and for fuel pipework to be dou-
ble skinned or clad in such a manner that any leakage will fall under
gravity to a bunded area. There must be indication that a double-
skinned tank has failed, and indication of any liquid in a bunded area.
This implies that there must be a responsible person who will receive
the information and has the authority to take appropriate action.
Provided that the engine room is kept clean and is not used as a stor-
age area for miscellaneous combustible material, the usual cause of fire
within a diesel engine room is the fracture of a high pressure (injector)
pipe suffering from vibration and fatigue. The fracture results in fuel
being finely sprayed in an indeterminate direction. Diesel fuel is not
easily ignited but if the spray is directed towards a hot turbocharger or
exhaust manifold fire is likely to result. This is a hazard more likely to
be experienced with continuously running engines, standby power gen-
erating sets do not usually build up sufficient running hours to suffer
from fatigue failures.

Waxing of Fuel at Low Temperatures
At low temperatures the wax content of diesel fuel separates out as fine
floating particles and in order to avoid problems with road vehicles the
oil companies distribute different grades of fuel for use during summer
and winter months. For standby installations it is not possible to predict
when the fuel will be used or to purchase winter grade fuel in summer
months. It therefore has to be assumed that, in the United Kingdom, the
fuel stored is summer grade and liable to waxing during winter. The
waxing temperatures are 0°C for summer grade and 9°C for winter
grade. The waxing does not prevent the flow of fuel through pipework
but causes filters and injectors to be clogged.
If the air temperature within the engine room is maintained at a few
degrees above 0°C, the fuel in the daily service tank and in the engine
fuel pipework will not be subject to waxing. During running, cold fuel
supplied to the service tank will mix with the warm fuel already there
and any wax particles will be dissipated.

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