| Some flares are fitted with a device known as a Gas "Seal".
The Gas Seal is intended to manipulate the flow path at the top of the flare so as to inhibit air infiltration by buoyant decanting and, thus, reduce the amount of
continuous purge gas needed to protect the stack against atmospheric air infiltration.
In all cases, the device is not a "seal" in the sense that a plug is a seal, as it is a continuous, open passage, or series of passages to the tip.
The existence of such a physical device also makes it possible to define the position of measurement of permissible oxygen in the gas
and to set a purge rate which ensures that this is not exceeded. This rate is often set as a base flow which
I call the maintenance rate.
There are a number of different proprietary designs of Gas Seal [see individual descriptions] but these generally can be classified either as Labyrinth
style or Orifice style.
Mystery and Myth
Gas Seals are frequently misunderstood, in my opinion, and have acquired a degree of mystique regarding their abilities. I believe that this is partly due to
certain marketing techniques and also to the name "Molecular Seal", which tends to be applied generically but is really the trade name of a particular
equipment supplier. This names almost suggests a complex process, beyond our comprehension. The following comments are intended to help to
rectify these misunderstandings and explain the
things which gas seals cannot do.
- Gas "seals" are not effective in preventing oxygen entry into the flare under conditions of internal suction (negative pressure) caused by thermal
contractions of the process gases. Such matters must be considered separately.
- "Seals" do not have lower pressure drops than an equal length of nominal pipe. "Seals" do add pressure drop to
the overall system and the sizing and selection of the equipment becomes an integral part of system sizing as well as a purge consideration.
- "Seals" are not flame arrestors and should not be thought of as equivalents, even though the underlying issue in the use of a gas seal is the
prevention of flash-back. A flame arrestor works passively whereas a "seal" must be properly purged to have any effect. Additionally, a true flame arrestor or
detonation arrestor can be employed in situations where oxygen rich gas mixtures originate within the plant, whereas Gas "seals" have no beneficial effect for
this condition.
In the undesirable event of an actual flash back, and a subsequent explosion or detonation because of the pressure piling effect in the pipe, the use of a gas seal may
not be at all beneficial and some seals can be positively detrimental. As "seals" incur a significant flowing pressure loss when compared with the length of
plain pipe they actually replace, any flame passing (backward) through the seal can experience a dramatic change of pressure condition and turbulence. In many cases the
total equivalent distance traveled by the flame passing (backward) through the seal can approach or exceed the run-up distance limit to an explosive condition. The basis
for selection, location and sizing of the "seal" and its associated purge conditions must be such that the oxygen concentration on the input side is maintained
to be less than the UEL and will not support further passage of a flame.
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Molecular Seal® is a Trade Name used by the company John Zink
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