Fail arrest

Deflagration Arresters The two types of deflagration arrester normally considered are the end-of-line arrester (Figs. 26-23 and 26-24) and the tank vent deflagration arrester Neither type of arrester is designed to stop detonations. If mounted sufficiently far from the atmospheric outlet of a piping system, which constitutes the unpro-tec tea side of the arrester, the flame can accelerate sufficiently to cause these arresters to fail. Failure can occur at high flame speeds even without a run-up to detonation. 

TOV is considered only to select the MCOV and the rated voltage, V, of the surge arrester. This is a reference parameter to define the operating characteristics of an arrester. It plays no part in deciding the protective level of the arrester, which is solely dependent on the transient conditions of the system, as discussed later. V, is used to make the right choice of an arrester and its energy absorption capability to ensure that it does not fail under the system s prospective traiisieut conditions. 

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. 

Flame arresters have often failed in pracdce. There have been significant advances in flame arrester technology over the last decade that explain many failnres as dne to misapplication. Plant inspections have shown that misapplication of flame arresters continnes to be common. 

The dual 20-mesh stainless steel arrester was effective in arresting flashback from all eight fnel-air mixtures tested except in some ethylene-air tests. It failed in three out of three tests where the flame speed was 4.86 m/s (15.94 ft/s) or greater. 

Hydranlic flame arresters may fail to stop high flame speed gas mixtures nnder certain conditions. Fundamental test work (Overhoff et al. 1989) demonstrates mechanisms whereby liqnid seal arresters may fail to... 

Velocity flame stoppers have been used for feeding waste fuel gas to furnace burners when the gas can become flammable due to contamination with air. They have also been used for feeding waste or depleted air streams to furnaces when the air streams can become contaminated with flammable gases (Howard 1982). It should be noted that a furnace pressure transient may render this device ineffective and consideration should be given to providing an upstream detonation flame arrester. In this arrangement a demand will only be placed on the detonation flame arrester when the velocity flame stopper fails. Therefore, detonation flame arrester maintenance should be minimal. 

A deflagration or detonation flame arrester fails hy definition if any flame propagates from the unprotected to the protected side. Failures can result for a numher of reasons, some of which are listed helow ... 

Bicarbonate is no longer used as the first line treatment during cardiopulmonary resuscitation following cardiac arrest. Recent evidence suggests little benefit, and the drug may actually be detrimental to resuscitation. According to the American Heart Association, bicarbonate is used when all other treatment options have failed. 

Perhaps of most significance, we have found that PCP use is relatively common in an arrestee population. Twelve percent of the male arrestees in Manhattan and about 30 percent of arrestees in Washington, D.C. have a urine test positive for PCP. In New York, this would translate into approximately 12,000 PCP-positive persons arrested and processed in Manhattan Central booking each year. Furthermore, these are probably minimum estimates of the number of users, since the urine tests failed to detect the many users who had not taken the drug soon before arrest. 

Hydraulic (Liquid Seal) Flame Arresters Hydraulic (liquid seal) flame arresters are most commonly used in large-pipe-diameter systems where fixed-element flame arresters are either cost-prohibitive or otherwise impractical (e.g., very corrosive gas or where the gas contains solid particles that would quickly plug a conventional arrester element). These arresters contain a liquid, usually water-based, to provide a flame barrier. Figure 23-62 shows one design. Realistic tests are needed to ensure performance, as described in EN 12874 [15]. Note that hydraulic flame arresters may fail at high flow rates, producing a sufficiently high concentration of gas bubbles to allow transmission of flame. This is distinct from the more obvious failure mode caused by failure to maintain adequate liquid level. 

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