Flame arrestors can play a key role in an overall industrial explosion protection strategy, along with other techniques such as purging, gas detection, inerting, and implementation of correct operating procedures. Typical applications include:

- tank vents, which are intended to prevent any flames occurring outside a tank from entering the tank vapor space;

- process heater air intakes, which are protected by flame arrestors so that ingested flammable vapor-air mixtures are prevented from flashing back, and causing an external ignition;

- in-line flare stack protection, so that a loss of flow to the flare and subsequent flashback does not cause flame transmission to upstream equipment;

- marine vapor recovery systems, where flame arrestors are used to prevent the transmission of flame through pipes forming part of a vapor recovery system,

Because users, manufacturers, regulatory agencies, and other interests require a quantification of the minimum level of performance afforded by these devices, there has recently been an increasing amount of national and international interest in the development of flame arrestor performance standards. Performance standards allow for the equitable application of a set of performance tests which provide baseline performance data that can be used as part of an acceptance regime.

There are two general performance aspects involving combustion which must be addressed in any general flame arrestor performance standard: continuous burn performance and explosion performance. Explosion performance deals with the ability of a flame arrestor to prevent the transmission of a relatively rapidly-propagating deflagration or detonation combustion wave from its unprotected side to protected side. These tests have a duration of considerably less than one second. Continuous burn testing, on the other hand, deals with the effects of a flame which can be considered as having stabilized at an arrestor element and continues to burn, as a result of the gas flowing through the unit continuously providing adequate fuel for combustion to continue for minutes, or even hours. There have been reported instances of continuous burn situations leading to explosions at various industrial locations.

In either case, the intent is to assess these units so that a given level of performance can be determined for the product in these situations. This paper concentrates on the general area of continuous burn testing.

The first part of this paper (Part A) provides description of continuous burn tests that were performed on flame arrestors, the results obtained, a discussion of the mechanisms involved, and some conclusions concerning continuous burn performance. The second part of this paper (Part B) provides an analysis and discusses the technical validity of United States Coast Guard (USCG) continuous burn requirements, as stated in the United States Federal Register, Volume 55, dated June 21, 1990, Appendices A and B to Part 154, in light of the findings described in Part A of this paper.

In addition, an analysis is provided of International Maritime Organization (IMO) document MSC/Circ. 373/Rev.1, entitled ''Revised Standards for the Design, Testing and Locating of Devices to Prevent the Passage of Flame into Cargo Tanks in Tankers'', as well as on a proposed sixth edition for Underwriters' Laboratories document UL 525 ''Flame Arrestors for Use on Vents of Storage Tanks for Petroleum Oil and Gasoline''.

1. Objectives

The overall objective of this work is to further a general practical understanding of the flame arrestor continuous burn mechanism.

The specific objectives of this work were as follows:

- determine how gas velocity through a flame arrestor affects continuous burn performance;

- determine the effect of fuel-air mixture concentration on continuous burn performance by performing tests with lean, stoichiometric, and rich mixtures at a given velocity;

- obtain temperature profiles during burn tests to indicate temperature trends at key measuring points throughout the test;

- determine whether flow interruptions could cause failure prior to failure in the steady-state case;

- compare test data for various media types;

- make visual observations;

- arrive at some general conclusions about burn testing.

2. Introduction

A flame arrestor can be described as a device which connects one vapor space with another, and which allows gas vapor flow, but which has a certain level of capability to prevent the transmission of flames in at least one direction. Flame arrestors may be in-line devices to prevent a flame from propagating any further down a piping system, as in the system feeding a flare stack, or it may be a venting device such as a vent installed on the top of a fuel tank which prevents external flames from propagating into the vapor space inside the tank. There are many thousands of such devices in use today, with increasing numbers being installed in vapor recovery systems as a result of environmental legislation limiting the discharge of vapors into the atmosphere.

Internally, flame arrestors typically consist of some type of porous media, through which gas can flow. The media affords some measure of heat extractive capability and flame quenching by cooling, which in turn prevents flame propagation.

Although there are several different types of media, all of them have the characteristics of porosity and some measure of ability to absorb and dissipate heat; the essential flame-quenching mechanism in all of them is the same, The media breaks the single gas stream flowing into the flame arrestor into numerous smaller gas streams as it passes through the unit, and by breaking the gas flow into small streams, achieves efficient intermixing and cooling of the combusting gas with the media, which in turn can cool and extinguish the flame.

If the gas stream in an in-line flame arrestor system is ignited on the unprotected side, and the flame travels back to the flame arrestor element, the single flame in the gas stream will be broken into numerous small flamelets when it reaches the media.

These small flamelets will then continue to burn at the arrestor element if the gas flow velocity and heat output are sufficient to overcome the quenching effect of the media on the flame. If the conditions of mixture concentration and flow velocity are suitable, the flame may stabilize and continue to burn for minutes, or even hours at the arrestor. Eventually, if these conditions persist, flame arrestors can fail, allowing flames to be transmitted to protected side equipment.

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