Flame Arrester Decomposition Flames

 
By 3 April 2015
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A number of gases may propagate decomposition flames in the absence of any oxidant provided they are above minimum conditions of pressure, temperature, and pipe diameter. Common examples include acetylene, ethylene oxide, and ethylene.

The best known example, acetylene, will propagate decomposition flames under atmospheric conditions in large pipe although under typical conditions an elevated pressure is required. Acetylene, unlike the other gases mentioned, can also decompose in a detonative manner.

Neither ethylene oxide nor ethylene are known to detonate in the absence of oxidant whatever the run-up distance. This is based on practical experience in addition to experimental test work. Thus, deflagration flame arresters have been developed for in-line application. Ethylene oxide arresters consisting of tube bundles of specified length and diameter have been used for many years in process units (Britton 1990). Recklinghausen of Chemische Werke HuIs AG (1978) describes an alternative ethylene oxide arrester of the packed bed type. Ethylene can also propagate decomposition flames at elevated pressure. At room temperature this requires pressures in excess of 700 psig, but typical storage and transmission pressures exceed this value. Britton et al. (1986) review decomposition incidents with ethylene and mitigation methods including a U-tube bundle arrester developed and patented by ARCO. The latter arrester, mounted in a bath of water-antifreeze mixture, requires a flame detection device to indicate any flame stabilization at the arrester. Ethylene decomposition flames are exceedingly slow moving (about 1 m/s or less) and readily stabilize against gas flow. For this reason “detect, valve-in and vent down” systems featuring fast-acting valves maybe preferable.

Three types of arrester used for acetylene are sintered metal, packed bed
and hydraulic types. Types of dry and wetted packed bed designs using
Raschig rings are discussed by Schmidt (1971). Howard (1992) discusses
acetylene arresters associated with flares and recommends the use of dilution
to prevent the phenomenon of “burn down” from the flare tip. Per unit mass
of acetylene flared, dilution with about 1/3 this mass of natural gas or steam,
or about 1/2 this mass of nitrogen, can be effective. Suitable control and
monitoring instrumentation are required.