Flammable gas detection systems

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. 

Once it is determined that flammable gas detection is needed for a processing area, an evaluation of the gas/liquid composition of each credible release can be done. The formation and behavior of gas clouds is primarily governed by whether the material is released as a gas or as a liquid. In order to properly locate detectors, the expected behavior of a gas cloud must be understood. Similar considerations apply to the release of toxic materials and the appropriate location of detectors for these materials. 

The release of pressurized gas to the atmosphere and its dispersion can be described in three stages jet mixing, momentum effects of wind or air currents, and natural diffusion. While the initial properties of the gas at the time of release (e.g. temperature, pressure, density) define the first stage, they have little influence on the second and third stage. The energy associated with the release of a pressurized gas creates a jet mixing effect that causes the gas to be diluted in air. Gas releases can rapidly form explosive clouds depending on the rate of release. 

Placement of detectors should consider the need for protection from physical damage, weather effects (snow, icing, wind-driven rain, or dust), or direct water hose spray during an area wash down. Detector placement should also allow safe access to perform required periodic testing and inspection. Typical approaches to flammable gas detection layout are  

Spatial coverage is a gas detection layout methodology that utilizes a goal setting or at-risk volume approach (Bond, 1993). The objective is to detect gas accumulations that can cause catastrophic escalation through a pragmatic methodology. For flammable gas hazards, this can be based on detection of 

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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... 

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

To develop an appropriate strategy to combat the hazards of combustible/ flammable vapor or gas it is essential to have a better understanding of the basic properties of combustible materials. So, discussions on combustible/flammable gas detection systems start with the general properties necessary for determining a... 

Now let us explore combustible/flammable gas detection systems and their associated technology. 

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. 

Any fuel leakage from the CNG or LPG system detected visibly (evidence such as Ice buildup at fuel system connections and fittings) and verified by either a bubble test using non-ammonia, non-corrosive soap solution or a flammable gas detection meter. (396.3(a)(1))... 

Note 1. In the case of a random leak occurring whilst the operator is present, the gas detection system provides limited protection. This is because the alarm may be concurrent with the hazardous event, ie exposure of the operator to a flammable atmosphere... 

Provide safe separation distances Install fixed fire protection and alarms, water sprays (deluge), and/or foam systems activated by flammable gas, flame, and/or smoke detection devices... 

Consideration shonld also be given to the possibility that the flame arrester may ping, which conld prodnce a vacnnm condition in a low-pressnre tank when the tank is primped out, and implode (collapse) the tank. This may reqnire the installation of a vacnnm breaker or a pressnre-vacnnm conservation valve. If the tank contents are flammable and admission of air may resnlt in an ignitable mixture, it may be necessary to install an inert gas blanketing system on the tank, actuated by a pressnre controller, which would admit a sufficient flow of inerting gas when a vacnnm condition is detected. 

Dry powders are effective on flammable liquid or electrical fires. Special powders are available for use on metals. Dry powder extinguishers may be used on Class C fires, including gases and liquefied gases in the form of a liquid spillage or a liquid or gas leak. This must be accompanied by other actions, e.g. stopping the leak this is necessary to avoid accumulation of an unbumed flammable gas-air mixture which could subsequently result in an explosion. Activation may be automatic by a detection system, or manual. 

Vessel deluge systems are usually designed as open-head systems, which are activated when a fire is detected and/or a flammable gas mixture is detected. The deluge system is usually opened when the flammable gas concentration is a fraction of the LFL (approximately 25 %) or when a fire is detected through heat. Table 7-7 provides descriptions and design specifications for these systems. 

Air intakes to heating and ventilation systems, air compressors for process, instrument and breathing air, and to prime movers for gas compressors, power generation and pumps should be located as far as practical from contamination by dust, toxic and flammable materials release sources. They should not be located in electrically classified areas. If close to possible vapor releases (as confirmed by dispersion analyses( they should be fitted with toxic or combustible gas detection devices to warn of possible air intakes hazards and snutdown and isolate the incoming air ductwork and fans. 

Where it is possible for flammable or toxic gas or vapor released within a hazardous area to migrate to the inlets for HVAC systems serving nonhazardous enclosed areas such as control rooms, detection systems should be installed in those HVAC inlets or connecting ductwork. Detection should be provided in HVAC system intakes if the building, room, or enclosure served is not electrically classified and a flammable (or toxic) gas or vapor could feasibly be drawn into the area, either by mechanical ventilation systems or by differential pressures. The detection system should alarm and automatically shutdown the HVAC to prevent gas or vapor concentration in the protected space from reaching the flammable or toxic range. 

Fire prevention may include providing degassing boots/vent stacks to the top of risers where flammable combustible vapors can be entrained in the water system, i.e. water pressure lower than process pressure. Combustible gas detection at the top ofthe vent stacks can be used to detect flammable material presence in cooling tower. However, the hostile environment and difficult to access location makes the detection challenging to maintain. 

Portable fire extinguishers (carbon dioxide or chemical types), or other fire protection or suppression systems or devices must be available for fire emergencies at storage installations. Only trained personnel should be allowed to operate fire extinguishers. A flame-producing device must never be used for detection of flammable gas leaks. Either a flammable gas detector or compatible leak detection solution should be used. 

Another means of protecting against the explosion hazard in flammable atmospheres is to employ gas detectors. The principle is that the gas detectors detect flammable gas/air mixtures at a low percentage, typically 10%, of the LEL and cause an alarm to sound. If the concentration reaches 25% or so of the LEL, the system automatically initiates a shutdown of the process. This type of technique is commonly used in process plants such as oil and gas refineries, LEG storage tank farms, chlorine doping rooms in water treatment plants, and on vehicles such as electric fork lift trucks operating in Zone 1 and 2 areas. 

If highly toxic or flammable gases are stored inside without gas cabinets, they are usually required to be stored in a separate, gas storage room, where no other occupancies are permitted. This room should have its own ventilation, also directed to a gas treatment system that will abate any hazardous release. If pyrophoric gases are stored then smoke detection should be provided. The ventilation system must cause the pressure in the room to be less than atmospheric, to prevent any escape of gas from the room. The room should be constructed so that an explosion will be allowed to relieve pressure, usually in one direction, while the rest of the room structure remains intact. It is often required to have at least one cabinet on site, so that any leaking eylinders can be safely isolated. 

The general requirements for provision of fire fighting equipment must also be considered together with the provision of gas monitoring systems that can detect the presence of flammable vapours. 

There are many advantages to actuating the water spray or deluge system as soon as a flammable vapor or gas is detected ... 

Low levels of ammonia in ethylene, hydrogen, and other light hydrocarbons in industrial processes can canse downstream catalytic poisoning, and an IMS system with a limit of detection of 1 ppb was used to monitor NHj in hydrocarbons. A wide variety of process streams was tested without any evident interference from coexisting compounds. In another report, NH3 in ethylene was measured by an IMS with a silane polymer membrane, Ni ion source, H+fHjOln reactant ion, and nitrogen as the drift and source gas. Because ethylene has no noticeable effect on the analytical results, preconcentration or preseparation were unnecessary. Ethylene s flammability was made inconsequential by the nitrogen atmosphere inside the spectrometer. Response to NH3 concentrations between 200 and 1,500 ppb was nearly Unear, and a calculated minimum detection limit was 25 ppb. 

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