5.1.2 Gas detectors are to be provided in or close to all air intakes serving
safe areas. They are to be capable of initiating early warning of the presence of
flammable and toxic gases likely to be present on the unit, as appropriate to its
purpose or service. The detectors are also to be capable of initiating relevant
shut-down actions, should the concentration of gas increase above the early warning
level. To minimise nuisance shut-downs, consideration should be given to the
provision of duplicated or triple redundant detector heads in each inlet operating
in a voting configuration.
5.1.3 In addition to the detectors required by Pt 7, Ch 1, 5.1 General requirements 5.1.2, for exhaust outlets of accommodation modules
adjacent to gas hazardous areas, consideration should be given to provide gas
detectors to give warning of ingress of gas when the ventilation system is shut
down.
5.1.4 Automatically closed dampers are to be provided in all intakes and
exhausts. When the gas detectors required by Pt 7, Ch 1, 5.1 General requirements 5.1.2 and, if fitted, those to which Pt 7, Ch 1, 5.1 General requirements 5.1.3 refers, have detected gas demanding shut-down
action, all HVAC inlet and exhaust fans and dampers associated with the space/point
where ingress of gas has been detected are to be shut down and closed in addition to
the damper of the duct in which gas has been detected. No reliance is to be placed
on solely shutting dampers without also shutting down the associated fan motors.
Dampers utilised to mitigate against the ingress of gas are to be suitably rated for
this service.
5.1.5 Enclosed spaces with no source of release, but which contain sources of
ignition such as machinery rooms, accommodations blocks or electrical equipment
rooms are to be provided with fast-response gas detection at the air inlets, which
are set with a suitably low gas detection level to initiate appropriate alarm and
trip alarm functionality. The inlet gas detection should be located upstream of
suitably rated fire and gas dampers. This is to ensure that an ignitable
concentration of gas is prevented from entering an area containing an ignition
source. The passage of an ignitable concentration of gas in the ducts should be
stopped before it crosses the boundary of the protected space. Total response time
of detectors, control systems and dampers should mitigate against an ignitable
concentration of gas entering the ventilated space. As such, inlet ventilation
dampers are to be designed to be fully closed within a minimum period of time but in
no case more than five seconds from gas detection at the associated ventilation air
inlet gas detector. Other mitigating measures that can be considered are:
- all machinery and electrical equipment, if any, fitted in the
ducting being of a type suitable for Hazardous area location as applicable ,
see
Pt 7, Ch 2, 5 Machinery in hazardous areas and Pt 7, Ch 2, 8 Electrical equipment for use in explosive gas atmospheres;
- early closure of dampers being initiated upon activation of gas
detection system(s) in the process area, where applicable;
- hydrocarbon concentration within the ducting being the subject
of a dispersion analysis, to allow assessment of hazards created, if any, to
the machinery space being ventilated.
5.1.6 Where a machinery space is not served by redundant air intake ducts,
consideration should be given to the provision of gas detection within the space.
Consideration should also be given to the isolation of electrical equipment, other
than that suitable for installation within a Zone 1 location, see
Pt 7, Ch 2, 8.1 General 8.1.4, when flammable gas is detected within
the space.
5.1.8 Consideration is to be given to the provision of gas detection within
emergency generator spaces and their switchboard spaces as well as in the
ventilation system intakes. In the event of gas being detected in the air intakes,
the ventilation system intake and exhaust fan dampers are to be shut down and
associated fan motors are to be stopped. The emergency generator may continue to
run, provided that aspiration air is drawn separately from outside the space and the
engine induction and exhaust arrangements comply with the relevant requirements of
Pt 7, Ch 2, 7 Engines in hazardous areas. However, if gas is detected within the
emergency generator enclosure, emergency switchroom, or at the engine air intake,
the emergency generator is to be shut down.
5.1.9 Diesel-driven fire pumps will not require to be shut down if gas is
detected in the area or space in which they are sited, provided that no electrical
equipment, other than that suitable for installation in a Zone 1 location,
see
Pt 7, Ch 2, 8.1 General 8.1.4, is required to remain in operation.
Should any equipment not be suitable for such installation (firewater pump drives,
i.e. diesel drive units, etc. are often not certified and are therefore not rated to
operate in a hazardous atmosphere), they are to be suitably protected by other means
(i.e. housed in a safe area, within a suitably rated enclosure with fire rated and
gastight barriers, designed to run with the firewater pump drive enclosure shut down
(i.e. enclosure fire and gas dampers closed, etc.), diesel drives provided with
engine overspeed protection, etc.) to mitigate against gas ingress and enable the
drive to continue to operate. Additionally, such pumps should not be started up with
gas present, and any electrical starting circuits and control and alarm circuits not
suitable for operation in a Zone 1 location are also to be isolated automatically by
the fire and gas panel.
5.1.10 Ventilation systems serving Hazardous areas are to be fully segregated
from ventilation systems serving Non- Hazardous areas.
5.1.11 Drain systems serving Hazardous areas are to be fully segregated from
drain systems serving Non-Hazardous areas.