Temperature, flame arrester

Monomer emulsions ate prepared in separate stainless steel emulsification tanks that are usually equipped with a turbine agitator, manometer level gage, cooling cods, a sprayer inert gas, temperature recorder, mpture disk, flame arrester, and various nossles for charging the ingredients. Monomer emulsions are commonly fed continuously to the reactor throughout the polymerisation. 

Consider upset conditions that could exceed the test conditions at which the arrester was certified. These include the gas composition with regard to concentration of sensitive constituents such as ethylene or hydrogen, maximum system pressure during an emergency shutdown, and maximum temperature. Under certain upset conditions such as a high-pressure excursion, there may be no flame arrester available for the task. 

The problem of flame arrestment, either of deflagrations or detonations, depends on the properties of the gas mixture involved plus the initial temperature and pressure. Gas mixture combustion properties cannot be quantified for direc t use in flame arrester selection and only general charac teristics can be assigned. For this reason, flame arrester performance must be demonstrated by realistic testing. Such... 

Decomposition Flame Arresters Above certain minimum pipe diameters, temperatures, and pressures, some gases may propagate decomposition flames in the absence of oxidant. Special in-line arresters have been developed (Fig. 26-27). Both deflagration and detonation flames of acetylene have been arrested by hydrauhc valve arresters, packed beds (which can be additionally water-wetted), and arrays of parallel sintered metal elements. Information on hydraulic and packed-bed arresters can be found in the Compressed Gas Association Pamphlet G1.3, Acetylene Transmission for Chemical Synthesis. Special arresters have also been used for ethylene in 1000- to 1500-psi transmission lines and for ethylene oxide in process units. Since ethylene is not known to detonate in the absence of oxidant, these arresters were designed for in-line deflagration application. 

Whether a flame is transmitted through a flame arrester depends on the length and aperture size of the arrester, the approach velocity of the flame, the pressure rise, and the temperature of the arrester (Wilson and Flessner 1978). Wilson and Flessner state that the evidence indicates that low-speed flames can be quenched by an array of small passageways placed in a duct, provided that the effective passageway diameter (critical diameter) meets the following criterion ... 

To predict die capability of a flame arrester to cool hot combnstion gases, the U.S. Bnrean of Mines has developed an equilibrium model and one- and diree-dimensional transient diermal models of a flame arrester, which are nsed to predict die heat losses from die arrester and the maximum temperatures developed (Edwards 1991). 

The ranges of mixture composition pins operating temperature and pressure are important. Unless a detonation flame arrester is used, it is essential to ensure that DDT will not occur after a flame enters the system. Even if a detonation flame arrester is used, it is important to use the correct type of arrester for the service. For example, if it is possible for a flame to approach from either direction, a bidirectional flame arrester must be selected. 

It may be necessaiy to position flame arresters away from heat sources that could cause the allowable operating temperature of the arrester to be exceeded. Positioning must be made with due consideration of DDT constraints. See Table 5-4, which shows the relationship of run-up distance to... 

Temperature monitoring of a flame arrester to detect a stabilized flame (endurance burning) should be provided if it is expected that this may occur, or if such an event has already occurred in the past. 

Acetylene may propagate decomposition flames in the absence of any oxidant above certain minimum conditions of pressure, temperature, and pipe diameter. Acetylene, unlike most other gases, can decompose in a detonative manner. Among the different types of flame arresters that have proven successful in stopping acetylene decomposition flames are hydraulic (liquid seal) flame arresters, packed beds, sintered metal, and metallic balls (metal shot). 

Capp, B. 1992. Temperature Rise of a Rigid Element Flame Arrester m Endurance Burning with Propane./. Loss Prev. Process Ind., 5(4), 215-218. 

Venting an explosion ahead of a flame arrester can reduce the thermal flux and the impulse to which the arrester is subjected. Test results indicate that peak side-on overpressure is halved, specific impulse is reduced by a factor of three, and the temperature is substantially reduced. However, overpressure and flame speed at the flame arrester do not appear to be changed significantly. 

An interlock system (sensors and valves) which isolates offgas flow to die process heater firebox and routes the offgas to atmosphere on detection of low nitrogen flow or high temperature at the detonation flame arrester outlet. 

Continnons monitoring of pressure drop and temperature should be carried out. Pressure drop should be monitored if fouling and subsequent plugging is suspected or has previously occurred. Temperature monitoring should be provided if it is possible for a standing flame to occur on the flame arrester face and subsequently destroy the element (see Chapter 7). 

Tests are needed to determine the effects of temperature (above and below ambient) on the performance of deflagration and detonation flame arresters. 

Endurance Burn Test A test in which a flame arrester is subjected to a stable flame on the surface of the arrester for a length of time (depends on whether UL 525 or the USCG protocol is used) until the highest obtainable temperature is reached on the ignited side or until the temperature on the protected side has a temperature rise of 100°C. 

Endurance Burn Under certain conditions a successfully arrested flame may stabilize on the unprotected side of an arrester element. Should this condition not be corrected, the flame will eventually penetrate the arrester as the channels become hot. An endurance burn time can be determined by testing, which specifies that the arrester has withstood a stabilized flame without penetration for a given period. The test should address either the actual or the worst-case geometry since heat transfer to the element will depend on whether the flame stabilizes on the top, bottom, or horizontal face. In general the endurance burn time identified by test should not be regarded as an accurate measure of the time available to take remedial action, since test conditions will not necessarily approximate the worst possible practical case. Temperature sensors may be incorporated at the arrester to indicate a stabilized flame condition and either alarm or initiate appropriate action, such as valve closure. It is very important to install an endurance burning flame arrester in the same way as tested to avoid malfunction [11]. 

Operating Temperature and Pressure Arresters are certified subject to maximum operating temperatures and absolute pressures normally seen at the arrester location. Arrester placement in relation to heat sources such as incinerators must be selected so that the allowable temperature is not exceeded, with due consideration for the detonation potential as mn-up distance is increased. Flame arrester manufacturers can provide recommended distances from heat sources, such as open flames, to avoid thermal damage to a flame arrester element. If heat tracing is used to prevent condensation of liquids, the same temperature constraint applies. In the case of in-line arresters, there may... 

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