A flame arrester (also spelt arrestor) is a safety device that is fitted to the opening of an enclosure, or to the connecting pipe work of a system of enclosures, and whose intended function is to allow flow but prevent the transmission of flame in the event an ignition takes place. Flame arresters are generally passive devices and often the last or only line of defence against flame transmission and possible catastrophic explosions.
Flame arresters are typically used wherever there is the potential for an explosion arising from flammable gas or vapour being mixed with air. Accidental ignition of a flammable mixture will result in a flame that will travel through the unburnt mixture until the fuel is consumed by the reaction. Since a huge range of process applications are susceptible to this risk, flame arresters are utilised widely in order to protect people, plant & equipment and, of course, the environment. .
For a deflagration with no stabilised flame, the combustion products are cooled at the element surface by heat dissipation which prevents continuation of the combustion process through the flame arrester and into the protected line. In the event that a flame stabilises on the flame arrester element, a sudden increase in temperature will be detected by a monitoring system and secondary protection measures are activated to stop the flow of the flammable mixture.
Other types of flame arresters include devices that incorporate a small aperture that ensures the velocity of the gas being emitted exceeds the burning velocity of that gas thus preventing transmission of the flame. Alternatively, the gas may be bubbled through a liquid, often water, in a manner that provides a liquid barrier to flame transmission.
It is the flame arrester’s element that quenches the flame and the majority of designs incorporate a ‘flame filter’ comprising small apertures that allow the process gas to flow but which prevent flame transmission. The filter breaks down the flame front flame into smaller ‘flamelets’ which are cooled by the large heat capacity of the element, thus extinguishing the flame. Due to its construction, the element will cause a pressure drop or an obstruction to process flow. To mitigate this increased resistance to flow, the element area is usually larger than the cross-sectional area of the pipework.
When specifying a flame arrester, it is important to determine three important factors: (1) the possible source/s of ignition and what is to be protected; (2) the type of flame to be extinguished i.e. slow or fast deflagration to stable or unstable detonation; and (3) the most effective positioning of the specified flame arrester in the process – this is usually as close as possible to the ignition source. In many cases, a flame arrester is used in conjunction with other components to create a safety system. Flame arresters are designed to work in a wide range of operating and environmental conditions, so it is important to consider their installation and application limits. It is essential that the flame arrester is correctly specified to ensure that a flame is extinguished, or properly contained, and that an explosion is prevented from propagating through the equipment.
Flame arresters require regular inspection and maintenance. If a flashback is known or believed to have occurred, then the arrester should be inspected for damage. The small cells or components of the element are prone to collect dirt and become blocked thus increasing the pressure drop and reducing process gas flow. Damaged or dirty elements should be replaced. Often it is possible to clean the element for re-use. A correctly maintained flame arrester can provide many years of service.
The installation of detonation flame arresters is crucial to the safe working of offshore drilling rigs, storage & process tanks, vapour recovery units and combustion systems. They are specifically designed for pipelines with a significant distance between the ignition source and the arrester. They are also used where the pipe is rough, bent, obstructed or has section changes that result in turbulent effects and other sources of flame acceleration. This type of arrester is designed to accommodate extremely high ignition energies and the resulting pressure wave and flame front. Accordingly, in-line detonation flame arresters can be placed any distance from the ignition source and Elmac Technologies have a comprehensive range that are designed to offer the ultimate protection in these worst-case unstable detonation explosion scenarios.
End-of-line flame arresters prevent flames from entering a vessel. They are fitted to the end of a pipeline, or exit to a vessel, to prevent flames from entering and not, as is often thought, to prevent the flame exiting the pipe or vessel. Without a weather-hood, they may be mounted in almost any orientation, but inverted mounting is not recommended as this increases the risk of heat being trapped and causing a flashback. Weather-hoods should always be fitted where there is exposure to rain and snow and should be mounted in a conventional, vertical orientation.
Explosion depends on an atmosphere of a mixture of flammable material with oxygen. The best approach to prevent fires and explosions is to substitute or minimise the use of flammable material. If that is not possible, it is important to avoid effective sources of ignition. The manufacturing, processing or storage of explosives is not covered in this guide.
Pressure/Vacuum relief valves are specifically designed to protect tanks from under or over pressurisation. When correctly sized they protect against the fluctuations in pressure associated with the general operation of a storage tank i.e. thermal expansion and the normal filling/emptying cycles. Elmac Technologies also produce a full range of pressure-only and vacuum-only relief vents. Due to the process of pressure/vacuum relief vents, these products are also referred to as “breather valves”.
A vacuum relief vent is a protection device that allows a tank to ‘breathe’ thus preventing collapse or rupture due to overpressure of the tank. The vacuum condition of an atmospheric tank must be controlled by allowing air to flow into the tank. They can also be used as a primary vacuum relief for normal tank breathing as it is highly accurate and avoids unwanted interaction with relief valves. Elmac recommends that the atmospheric vent of an operating tank should never be covered or blocked and that tank vents are routinely inspected for plugging when in fouling or dirty service.