Flame arrester types

Wliile all the flame arrester types disenssed above have a solid arresting element (matrix), the hydranlic (liqnid seal) flame arrester contains a liqnid, nsnally water, to provide a flame barrier. It operates by breaking np the gas flow into discrete bnbbles by means of an internal device to qnench the flame. A mechanical nonretnrn valve (check valve) is sometimes incorporated to prevent the displacement of liqnid dnring or after a flame event (deflagration or detonation). 

It also contains specific requirements for testing of flame arresters in equipment such as compressors, blowers, fans, and vacuum pumps. Procedures are presented for deflagration, detonation, and burning tests for all the flame arrester types in the standard. 

In certain exceptional cases, a specially designed deflagration arrester may be mounted in-line without regard to run-up distance. This can be done only where the system is known to be incapable of detonation. An example is the decomposition flames of ethylene, which are briefly discussed under Special Arrester Types and Alternatives. ... 

Alternatives to Arresters Alternatives to the use of flame arresters include fast-ac ting isolation valves, vapor suppression systems, velocity-type devices in which gas velocity is designed to exceed flashback velocity, and control of the flammable mixture (NFPA 69 standard, Explosion Prevention Systems ). The latter alternative frequently involves reduction of oxygen concentration to less than the limiting oxygen concentration (LOC) of the gas stream. 

One of the major reasons that this book was written is that nonspecialist chemical engineers know little abont DBAs. Althongh DBAs have been specified and installed for many years, qnite often they have failed becanse the wrong type of flame arrester was specified, or it was improperly installed, or inadeqnate inspection and maintenance were provided. 

This book covers many aspects of DBA design, selection, specification, installadon, and maintenance. It explains how varions types of flame arresters differ, how they are constrncted, and how they work, ft also describes when a flame arrester is an effective solntion for mitigation of deflagrations and detonations, and other means of protection (e.g., oxidant concentration rednction) that may be nsed. It also briefly covers some aspects of dnst deflagration protection. 

Kirby (1999) reports two snccessful applications of deflagration flame arresters. In one incident, a deflagration flame arrester was installed near the junction of a collection header from an ethylene oxide process nnit with a flare stack. Although this type of flame arrester was really inappro-... 

Fabiano et al. (1999) describe an explosion in the loading section of an Italian acetylene production plant in which the installed flame arresters did not stop a detonation. The arresters were deflagration type and the arrester elements were vessels packed with silica gel and aluminum plates (Fabiano 1999). It was concluded that the flame arresters used were not suitable for dealing safely with the excess pressures resulting from an acetylene decomposition, and may not have been in the proper location to stop the detonation. 

An in-line detonation flame arrester must be used whenever there is a possibility of a detonation occurring. This is always a strong possibility in vent manifold (vapor collection) systems, where long pipe runs provide sufficient run-up distances for a deflagration-to-detonation transition to occur. Figure 3-3 shows the installation of in-line arresters of the detonation type in a vent manifold system. 

As mentioned earlier, flame arresters can be classified as either deflagration or detonation type. Deflagration flame arresters on tanks are designed to stop a flame from propagating into a tank from an unconfined atmo-... 

FIGURE 3-7. Various types of detonation flame arresters. (Sources Protectoseal Company, NAO, Inc., and Westech Industrial, Ltd.)... 

Dry type deflagration and detonation flame arresters have an internal arrester element (sometimes called a matrix) that quenches the flame and cools the products of combustion. A great number of arrester elements have been developed and used. The most common types currently available are as follows ... 

Other types that have been used but are not currently commercially available in the United States are compressed wire wool and metal foam. However, compressed wire wool flame arresters are available in the United Kingdom. 

Other types of deflagration and detonation flame arresters that do not contain an arrester element (matrix) have been used snccessfnlly in various applications where conventional dry type arresters were not suitable or were very expensive. Among these are ... 

This section describes varions types of flame arresting elements (matrixes) that are nsed in fixed element (stadc) dry type flame arresters, as well as a nnmber of other types. Some of these arresting elements are often nsed in bodi deflagradon and detonadon flame arresters. 

The crimped metal ribbon arresdng element, shown in Fignre 5-1, is one of die most widely nsed types, especially for detonadon flame arresters. Crimped metal ribbon arresters are made of alternate layers of diin corm-... 

The concepts of the National Electrical Code (NEC) gronps and the Maxi-mnm Experimental Safe Gap (MESG) are important criteria in the selection and specification of dry type flame arresters. These are explained below. 

The flame propagation direction affects the type of flame arrester selected. An end-of-line or in-line deflagration flame arrester used for the protection of an individual tank may be of a unidirectional design because the flame will only propagate from the atmosphere towards the tank interior. A bidirectional flame arrester design, however, is needed for an in-line application in a vapor recovery (vent manifold) system because the vapors must be able to flow from the tank interior into the manifold, or from the manifold into the tank interior. Consequently, flame may propagate in either direction. 

Flame arrester pressure drops mnst he taken into consideration when selecting and specifying them, especially when they are installed on vent systems of low pressure storage tanks, snch as API-type tanks. If the total system pressure drop exceeds the design pressure of the tank, the tank will he overpressnred and may he strnctnrally damaged. Refer to Section 6.1 for additional discnssion of this topic. 

In many sitnations several types of flame arresters are snitahle for the application. Often pressure drop dnring normal operation and its possible increase over the flame arrester s service life may he the primary determinant in selection of a snitahle nnit. 

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