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A simplified gas "seal" may be provided in the form of a reduced diameter orifice, or a series of conical baffles in the upper part of a flare gas
riser or lower part of the tip.
Such integral seals are sometimes also known as Reactive, Orifice, Integral or Baffle Type seals. [typical sketch here soon ]
The action of such a seal is three-fold
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It presents a smaller cross section area to the upward moving gases thereby reducing the volume of gas needed in order to maintain a fixed purge velocity
or momentum.
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It acts to reduce the amount of purge because it simulates a length of stack somewhat greater than its true length by presenting an impedance to air flows
down into the stack. Typically, a seal of this type may simulate 2 to 20 stack diameters, depending on its design configuration and minimum diameter.
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It allows flows in the correct (upflow) direction to suffer lesser flowing friction losses than incoming (downward) air flows.
Because no flow path reversals are involved, orifice or baffle seals have no factor of safety for cases of loss of purge and the correct rate of purge must be
maintained at all times. On cessation of purge flow, light gases in the flare will immediately commence to decant.
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