Gas-detection system

Combustible gas detection systems are frequently used in areas of poor ventilation. By the early detection of combustible gas releases before ignitible concentration levels occur, corrective procedures such as shutting down equipment, deactivating electrical circuits and activating ventilation fans can be implemented prior to fire or explosion. Combustible gas detectors are also used to substantiate adequate ventilation. Most combustible gas detection systems, although responsive to a wide range of combustible gases and vapors, are normally calibrated specifically to indicate concentrations of methane since most natural gas is comprised primarily of methane. 

Consensus performance standards and guidance for installation are provided for combustible gas detectors by ISA SI2.13 and RP 12.13 and for hydrogen sulfide gas detectors by ISA SI2.15 and RP 12.15. 

Required locations of gas detectors (sensors) are often specified by the authority having jurisdiction. For example, API RP 14C recommends certain locations for combustible detectors. These recommendations have been legislated into requirements in U.S. Federal waters by the Minerals Management Service. RP 14C should be referred to for specific details, but, basically, combustible gas detectors are required offshore in all inadequately ventilated, classified, enclosed areas. The installation of sensors in nonenclosed areas is seldom either required or necessary. Ignitible or high toxic levels of gas seldom accumulate and remain for significant periods of time in such locations. 

Most combustible gas detector sensors are installed in the upper portions of buildings for the detection of natural gas. However, in many cases the vapor which flashes off oil in storage tanks can be heavier than air. Below grade areas should be considered for sensor installations where heavier-than-air vapors might collect. 

Sensing heads should be located in draft-free areas where possible, as air flowing past the sensors normally increases drift of calibration, shortens head life, and decreases sensitivity. Air deflectors are available from sensor manufacturers and should be utilized in any areas where significant air flow is anticipated (such as air conditioner plenum applicaiion.s). Additionally, sensors should be located, whenever possible, in loca[ion.s which are relatively free from vibration and easily accessed for calibration and maintenance. Obviously, this carmot always be accomplished. It usually is difficult, for example, to locate sensors in the tops of compressor buildings at locations which are accessible and which do not vibrate. 

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Install on-line flammable gas detection system that activates an inerting system... 

Provide damper mechanical position stop to prevent complete closure of damper Eliminate flammables or combustibles Provide inert atmosphere Design ventilation system to keep flammable concentration below lower flammable limit Install on-line flammable gas detection system that activates an inerting system Provide automatic sprinkler protection Use deflagration vents... 

Use CO gas detection system if used indoors or in confined spaces. 

Risk Reduction Factors Coutrol/ rator responses. Alarms, Control system response. Manual anti automatic ESD, Fire/gas detection system Sa/ety System Responses Relief valves. Depressurization system. Isolation systems, High reliability trips. Back-up systems... 

Gas storage cabinets are designed to contain one to four gas cylinders. The cylinders are connected to a gas distribution system which is also contained in the cabinet. Very sophisticated systems are available from cabinet manufacturers. These may include automatic or semiautomatic change-over capabilities, fire sprinklers, purging systems, and gas detection systems which may include alarms and automatic shutoff. Point-of-use scrubbers may also be incorporated into the design, depending on the gas being used (see Fig. 10.43). 

The most important hardware items appeared to be the detectors themselves. The gas detection system gave frequent spurious alarms, and on both platforms the ultraviolet (UV) fire detectors were also prone to spurious activation from distant hot work for example, and had a limited ability to detect real fires. The tmreliability of these systems had a general effect on response time and would, overall, lengthen the time to respond. The second aspect which was related to hardware was fimction and performance testing of the emergency blowdown systems. It is critical that the workers believe the systems will work when required, and this can only be achieved by occasional use or at least fimction testing. 

Stueflotten S., Christensen T., Iversen S., Hellvik J. O., Almas K., Wien T., An infrared fibre optic gas detection system, Proc. Ofs-94 int conf, 1986, p. 87. 

Chambers P., Austin E.A.D., Dakin J.P., Theoretical analysis of a methane gas detection system using the complementary source modulation of correlation spectroscopy, Measurement Science and Technology 2004 15 (8) 1629-1636. 

Austin E.A.D., P. Chambers and J. P. Dakin, Theoretical analysis of a C02 gas detection system using correlation spectroscopy, Europtrode VII, 2004. 

The unmanned compressor building was equipped with a combustible gas detection system. However, it failed to sound an alarm because of a faulty relay in the control room. Automatic fail-safe valves functioned properly, blocking-in the flow of ethylene, but not before 450-11,000 lb of gas had already escaped. 

Piping failure at compressor due to vibration induced stress. Combustible gas detection system inoperability contributed to loss. 

Automatic activation from fire or gas detection systems. 

Various simple and sophisticated fire and gas detection systems are available to provide early detection and warnings of a hydrocarbon release which supplement process instrumentation and alarms. The overall objective of fire and gas detection systems are to warn of possible impending events that may be threatening to life, property of continued business operations, that are external to the process operation. 

Process controls and instrumentation only provide feedback for conditions within the process system. They do not report or control conditions outside the assumed process integrity limits. Fire and gas detection systems supplement process information systems with instrumentation that is located external to the process to warn of conditions that could be considered harmful if found outside the normal process environment. Fire and gas detection systems may be used to confirm the readings of major process releases or to report conditions that process instrumentation may not adequately report or be unable to report (i.e., minor process releases). 

A gas detection system monitors the most likely sources of releases and activates alarms or protective devices to prevent the ignition of a gas release and possibly mitigate the effects of a flash fire or explosion. 

Alarms should be displayed on a conventional dedicated window annuciator panel or if control room based on a dedicated CRT display for fire and gas detection systems. Each detector location should be highlighted with indications for trouble, alarm low, and alarm high. Where annuciator panel window alarms the alarm indication lights should be provided with specific labels indicating the exact alarm locations. 

Commercially available combustible gas detection systems generally use 24 VDC as the power supply for field devices. 24 VDC is inherently safer and corresponds the voltages increasing used by most instrument systems in process areas. A main supply voltage converter can be used to step down or convert from AC to DC power supplies. 

The power for combustible gas detection system should be supplied from the facility s UPS or if this is unavailable normal power with a reliable battery backup source of a minimum of 30 minute duration. 

The detection and alarm circuits of fire and gas detection systems should be continuously supervised to determine if the system is operable. Normal mechanisms provide for a limited current flow through the circuits for normal operation. During alarm conditions current flow is increased while during failure modes the current level is nonexistence. By measuring levels at a control point the health of the circuit or monitoring devices can be continuously determined. End-of-line-resistors (EOLR) are commonly provided in each circuit to provide supervisory signal levels to the control location. 

BHR Group Ltd., Fire Safety Engineering. "Modem Methods of Designing Fire and Gas Detection Systems", BHRA, Cranfield, U.K., 1989. 

Alarms should be initialed by the local or main control facility for the location. Manual activation means should be provided for all emergency, fire, and toxic vapor alarm signals. Activation of fire suppression systems by automatic means should also indicate a facility alarm. Most fire and gas detection systems are also set to automatically activate alarms after confirmation and set points have been reached. Manual activation of field or plant alarm stations should activate the process or facility alarms. 

When the decision is made to install an automatic detection system, the fire/gas detection technology should be carefully selected to match the expected hazard and the environment in which it will be installed. The design and installation of a reliable fire/gas detection system in an industrial facility should only be done by experienced personnel. Otherwise, the facility owner may have an inappropriate system and may be plagued by nuisance alarms and high maintenance costs. 

When evaluating a facility, a review of the existing fire/gas detection system is recommended. Due to recent technology developments, the performance of fire/gas detection systems has significantly improved over the last few years. 

Flammable gas detection systems provide early warning of a hazardous condition resulting from a process release. These detection systems may provide... 

Flammable gas detection systems are typically used to initiate an alarm at a concentration level below the lower flammable limit (LFL). Two gas alarm levels (low and high) are often utilized to allow early warning prior to taking automatic actions. Detection systems may also be used to stop electrical power and initiate process shutdown. The low alarm setpoint should be —20% LFL and the high alarm level set point should be between 40%-60% LFL. Where these devices are used to initiate process shutdown or activate fire protection systems, it is common practice to use some form of voting, typically 2 out of 2, such that the frequency of spurious shutdowns or system activation is minimized. 

Where multiple compressors are involved in the same service or duty, separation should be provided between compressors to reduce mutual exposure. Compressor buildings housing flammable service compressors should be provided with a combustible-gas detection system. The system should alarm at a concentration of 20-25% of the LFL and shutdown the compressor at 40-50% LFL (see Section 8.1.3). The shutdown should include closing all inlet and discharge process lines. For reciprocating compressors in flammable service, explosion vents on the crankcases should be provided. 

Bonn, J.C. 1991. Goal-Setting Design for Fire and Gas Detection Systems. SPE 23305. First International Conference on Health, Safety and the Environment. Society of Petroleum Engineers, Inc. 

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