Page 327 - International safety guide for oil tankers and terminals
P. 327
SAFETY AND FIRE PROTECTION
non-hazardous locations, such as pressurised control rooms, or in
explosion-proof enclosures for location in hazardous areas.
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The remote detection type, which uses remote diffusion detectors,
provides rapid response and good reliability, making this the preferred
Gas analysers should usually be provided with the following features and์นาวี
design.
Alternatively, continuous analysers may also utilise a central detection unit
in which samples are drawn from hazardous areas through tubing to the
central location by means of a suction pump. Central diffusion detection
units, utilising sample lines, are characterised by a relatively slow
response time. Additionally, particulates must be taken into account and
งานห้องสมุด ศูนย์ฝกพาณิชย
the lines must be heated to prevent condensation. Consequently, central
detection units are not generally recommended.
readout and alarm functions, in addition to continuous recording of data:
a) Channels for connection to individual diffusion detection sensors so
that each sampling circuit can analyse samples continuously. Thus,
when an alarm condition occurs, the analyser will home on the sensor
registering the alarm and the alarm will remain actuated until
manually reset.
b) The combustible gas analyser is calibrated in percentage of Lower
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Explosive Limit (LEL) and should be provided with a channel selector,
indicator lamps to show the samples being analysed, and a meter.
Visual and audible alarms should be provided for two levels of
detection. The minimum level most frequently used is 20% LEL. The
second or upper level of detection is usually 60% LEL. Silencing of
the audible alarm should not extinguish the visual alarm until gas
detection falls below the alarm level. Contacts are provided at the two
levels of detection to permit automatic operation of a purging or fire
prevention system.
c) Alarm levels should be adjustable and alarms may be actuated by
contact meters, recorder limit switches, solid-state signal level
detectors, or optical meter relays. Multi-level alarms can be provided
with means to actuate ventilation equipment, to effect transfer pump
shutdown, or to actuate fire extinguishing systems.
d) A means to disconnect the detectors safely from the actuating circuit.
The disconnection capability is necessary for proper routine
calibration and maintenance activities. A key-operated switch with
supervisory alarm is recommended.
e) On complicated or extensive systems, the indication of alarms on a
graphic display, such as an outline plan of a facility, is recommended.
f) Toxic gas analysers should be set to sound alarms at the monitored
location and in the control room when the gas reaches the
predetermined level, for example when an H 2 S concentration reaches
5 ppm. Alarms should generally be both audible and visual.
g) The gas detector head assembly should be suitable for the electrical
classification of the hazardous area and, if installed outdoors, should
be weatherproof and corrosion resistant.
h) The detecting unit included in the head should provide adequate
sensitivity and the necessary stability, under all conditions, to repeat
any reading within ± 2% of the full scale range.
© ICS/OCIMF/IAPH 2006 293