Page 327 - International safety guide for oil tankers and terminals
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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
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