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PURGE AND MINIMUM FLOW

ELEVATED FLARE
"SAFETY" PURGE

 

This is my definition, not an "official" definition

There are sometimes extreme  operating conditions when

  • no pre-purge sweep time is available AND
  • a start-up sweep must be completed by flammable gas

This condition could arise

  • if the flow of gas to the flare has been totally interrupted because of an operational oversight [ perhaps someone accidentally closed a valve or overfilled a water seal ]
  • if suddenly starting up a flare which has been deliberately lying dormant with no purge but with pilot flames which are already ignited.

In either of these conditions air can enter the stack from the top during the period with no gas flow.  If the flammable gas flow is simply recommenced, without any thought by the operator of the possible consequences, the mixing of gas and air in the stack might be sufficient to generate a flash-back or even a detonation once the mixture reaches the pilot flame. ....... Undesirable!! .........

In this case, immediately adopt a "SAFETY" purge rate.

To do this, establish a flow rate which is great enough to ensure that the velocity of the flow, at the flare tip discharge and in all line sections and vessels, exceeds the maximum possible flame speed which could be generated by a stoichiometric mixture of the gas. This is generally in the range of 1 to 10 fps at ambient conditions.
Note however, that published data usually covers ambient conditions determined at 25oC. Flame speed increases exponentially with temperature to a maximum (instantaneous) value at the auto-ignition temperature of the gas.
A suitable factor of safety should be applied to the actual velocity used.

  • DO NOT build-up gradually to the high flow.
  • DO open at full rate almost instantly.

I suggest that the engineer responsible for the Flare System should review this consideration, ahead of any possible event, to pre-determine

  • a suitable rate to which operators would default in such an emergency
  • a procedure to ensure a rapid  restoration of flow
  • conditions requiring alternative actions.