Understanding / Specifying Flame and Detonation Arresters
When properly applied, Protectoseal flame arresters and detonation arresters are effective in preventing the propagation and transmission of a flame or flame front in locations where flammable vapor/air or gas mixtures are present. In this section, an explanation of how flame fronts develop will be provided. The significance of different classifications of flammable chemicals will be described. The function of an arrester and the key parameters in sizing and specifying a device are explained. The importance of independent third party testing and approval of flame arrester and detonation arrester designs is documented. Definitions of terms commonly encountered when discussing arresters are provided, along with links to other useful and informative sites.
|Arrester Element –||The portion of a flame arrester or detonation arrester comprised of parallel spaced plates or crimped metal windings. The element provides the mechanical barrier to flame passage. The arrester element is mounted in the arrester housing.|
|Arrester Housing –||The portion of a flame arrester or detonation arrester that houses the arrester element and that provides the flanged or threaded connection to the pipe/tank being protected.|
|Flammable Liquid –||A liquid having a flashpoint below 100°F.|
|Combustible Liquid –||A liquid having a flashpoint at or above 100°F.|
|Confined Deflagration –||A deflagration (see below) propagating in a location where expanding combustion products are confined. A flame traveling within a pipe may be a confined deflagration.|
|Deflagration –||A flame front propagating through a flammable gas or vapor at a velocity less than the speed of sound in that gas or vapor.|
|Detonation –||(Also “Stable Detonation”) A flame front propagating through a flammable gas or vapor at a velocity equal to the speed of sound in that gas or vapor.|
|Detonation Arrester –||An arrester designed to prevent the propagation of unconfined deflagrations, confined deflagrations, stable detonations and overdriven detonations.|
|End-of-Line Arrester –||A flame arrester that is mounted at the end of a pipe (flanged or threaded inlet connection) and which vents directly to the atmosphere. The arrester is designed to stop unconfined deflagrations.|
|Explosive Range –||The range of values between and including the Lower Explosive Limit (LEL) and the Upper Explosive Limit (UEL) for any vapor/air mixture.|
|Flashpoint –||The minimum temperature at which a liquid gives off vapor in sufficient concentration to form an ignitable mixture with air near the surface of the liquid.|
|Lower Explosive Limit –||(LEL) The lowest volumetric concentration (expressed as a percentage) of flammable vapor in air that is capable of sustaining and transmitting a flame throughout the vapor mixture, at a specified temperature and pressure. Mixtures below the LEL are considered to be too “lean” to burn.|
|Overdriven Detonation –||An unstable flame front that propagates through a flammable gas or vapor at a speed in excess of the stable detonation velocity.|
|Stoichiometric Mixture –||The flammable liquid/air mixture where the fuel and oxygen are totally consumed if the mixture is ignited.|
Unconfined Deflagration – A deflagration propagating in a location where the expanding combustion products are not confined. A vapor cloud ignited in the open atmosphere is usually an example of an unconfined deflagration.
|Upper Explosive Limit – (UEL) –||The highest volumetric concentration (expressed as a percentage) of flammable vapor in air that is capable of sustaining and transmitting a flame throughout the vapor mixture, at a specified temperature and pressure. Mixtures above the UEL are considered to be too “rich” to burn.|
|Vent-Line/In-Line Arrester –||A flame arrester that may be mounted upstream of a pressure/vacuum relief vent, or that may be located upstream of a specified maximum length of vent piping to atmosphere. This arrester is suitable for stopping a confined deflagration that has propagated through a pipe for some specified maximum distance.|
If any flammable mixture of vapor or gas comes in contact with an ignition source, a flame front will develop. This flame will burn through the vapor or gas until:
- The supply of fuel (vapor or gas) is consumed.
- The heat necessary to sustain combustion is removed.
- The oxygen concentration becomes either too high or too low to allow continued burning.
If a flame front is propagating at a speed less than the speed of sound in the vapor, it is known as a deflagration. A flame front that propagates at a shock wave at the speed of sound in the vapor is known as a (stable) detonation. An overdriven detonation is a flame front propagating at a speed in excess of the speed of sound in the vapor. Such an overdriven detonation is a short lived phenomenon and usually occurs as the flame front is transitioning from a high speed (near the speed of sound) deflagration to a detonation.
A deflagration may develop in the atmosphere as an unconfined deflagration, or in an enclosed area, typically a piping system, as a confined deflagration. Detonations and overdriven detonations are most commonly encountered in closed piping systems.
An unconfined deflagration results in relatively low flame speeds and virtually no pressure increase. A confined deflagration (e.g. – an ignition in a run of pipe) starts at low speed and pressure. As the flame front propagates in the pipe, its speed and associated pressure increase. In long or complicated (multiple bends) pipe runs the flame accelerates until it transitions through an overdriven detonation state into a stable detonation. In a 4.3% propane/air mixture the stable detonation velocity is 5800 ft/sec and the associated pressure is approximately 300-400 PSIG.
Flame arresters and detonation arresters that are designed and tested to withstand and stop these various categories of flame fronts are available.
FLAMMABLE VAPOR / GAS CLASSIFICATION
Common flammable chemicals have been examined and arranged into groupings on the basis of their burning and explosion characteristics. In the National Electric Code (NEC) chemicals are categorized in Group A, B, C or D. Group D contains the least volatile flammable chemicals. Groups C, B and A contain, respectively, chemicals of increased volatility. Similar chemical groupings have been developed by the International Electrotechnical Commission. Their categories are designated as IIA, IIB, IIC, with IIA containing the least volatile and IIC containing the most volatile chemicals. In general terms, Group D is equivalent to Group IIA. Propane/air is a representative Group D (IIA) vapor. Group C is equivalent to Group IIB. Ethylene/air is a representative Group C (IIB) vapor. Group B is equivalent to Group IIC. Hydrogen is a representative Group B chemical. Group A contains only acetylene. The classification of the chemical in the flammable vapor is a significant parameter in the choice of a flame arresting device.
HOW AN ARRESTER FUNCTIONS
Flame arresters and detonation arresters are passive mechanical devices that are mounted to threaded or flanged connections on a tank or in a process piping system. In normal operation, vapors in the pipe are directed through the arrester. An arrester consists of a housing and an arrester element.
Arrester elements are available in a number of different configurations (parallel rectangular metal plate, wound crimped metal, parallel round metal plate). One common feature of all flame arresters is that the flammable vapor mixture is forced to pass through a series of small openings as it flows through the arrester. The size of the openings and their length of passage can vary, depending on the arrester style.
If the flammable vapor should ignite, the flame burns towards the arrester/element. As the flame attempts to pass through the element, it is slowed and cooled by contact with the metal walls of the small passages. Heat is transferred to the element until combustion cannot be maintained. The flame front is extinguished.
SIZING AND SPECIFICATION
The primary function of a flame arrester or detonation arrester is to provide protection against an approaching flame front. In their typical applications, however, they must also allow vapors and/or air to pass through the openings in their elements so that pressure and vacuum relief may be provided and so that normal processing of the vapors can be conducted. The resistance to flow through the arresters is based on their size and configuration. The arrester must be sized to allow the required flow rate at some acceptable resistance (pressure drop). Although the sizing procedure can be done manually, The Protectoseal Company has automated the calculation and specification process through the ProFlow® Sizing/Selection Software.
The optimum location for the arrester must be determined. End-of-Line flame arresters are mounted on outlet flanges and they vent directly to atmosphere. Vent-Line/In-Line flame arresters may be installed at some maximum distance (specified by the manufacturer) from the end of a section of open vent piping. Detonation arresters are designed so that they may be installed anywhere in a flammable vapors piping system. Specific information on the restrictions to location of any arrester is available from the manufacturer.
Flame and detonation arresters are rated for use with chemical vapors of appropriate groups defined by the National Electric Code (NEC) and the International Electrotechnical Commission (IEC). The suitability of the arrester for service with a particular vapor group must be verified. The initial pressure and temperature of the vapors in the system being protected are also significant factors that must be reviewed. The materials of construction of a flame arrester or detonation arrester must be selected to insure compatibility with the process vapors being handled. The possibility of corrosion of the arrester components or contamination of the process materials must be minimized.
APPROVALS AND LISTINGS
The Protectoseal Company has submitted their flame arresters and detonation arresters for inspection and testing by nationally recognized independent, third party approval agencies. We have been granted acceptance of our arresters by Underwriters Laboratories, Inc. (UL), Factory Mutual Research (FM), The United States Coast Guard (USCG), in the United States and by the Federal Institute for Physics and Technology (PTB), in Germany.
MATERIALS OF CONSTRUCTION
Protectoseal flame arresting devices are available in a wide range of materials (aluminum, stainless steel, ductile iron, hastelloy, etc.) The material must be compatible with the service conditions. Improper material choice can lead to contamination of the product being stored or reduction in the flame or detonation arrester’s ability to operate safely. Information on the corrosion resistance of materials under various service conditions is available in corrosion handbooks and chemical dictionaries.