Plant Flare Header Emergency Purge Gas & Thermal Bowing Effect

Plant Flare Header Emergency Purge Gas & Thermal Bowing Effect

When the safety valve shall discharge very hot or cryogenic fluids the flare header could be affected by thermal bowing effect that could lead to damage and even pipe rupture.

What means very hot relief? All the relief with temperature higher than 400-424°C.

When thermal bowing occur? For temperature gradients between pipe top and bottom wall is higher than 40°C.

What is thermal bowing?

Bowing effect of the flare header: when safety valve pop-up the initial regime in the flare header could be of annular or slug type leading to a uniform distribution of the temperature in the piping cross section and consequent uniform longitudinal piping expansion. But, if the flow regime is stratified liquid, or residual slope, the presence of liquid in the bottom shall determine circumferential thermal gradient between the top and bottom of pipe. The circumferential thermal gradient between top and bottom of pipe leads to the bowing phenomenon of the pipe.

In case of high temperature release, being the flare header colder than discharged fluid, part of the vapor starts to condense bringing to a stratification of liquid and vapor due to the difference in density. Consequently, liquid and vapor start to exchange heat with pipe wall, each with a different film heat transfer coefficient. The difference in the efficiency of the thermal exchange with the pipe wall leads to a thermal gradient across the pipe that causes a different thermal expansion of the metal between the upper part and the bottom, leading to different stresses in the upper part - hot side subject to compressive stresses during straightening of the pipe when thermal gradient receded and, in the bottom part cold side subject to tensile stresses during straightening of the pipe when thermal gradient receded of the pipe. Excessive stresses on pipe could be high enough to lead to pipe damage or rupture.

Moreover, the bowing phenomenon would cancel the free draining capability of the line, leading to liquid pooling and associated HAZARD issues.

Other major concern is related to heavy condensation in flare header and vacuum formation in flare header when heavy molecular weight and higher temperature fluids are relief and shall enter in cold flare header where will start to condense.

Issues:
How the necessary fuel gas flow rate needed to be injected in flare header to avoid vacuum shall be estimated: the normal flow used to keep positive pressure in header for sure is not enough to avoid vacuum
Only one injection point is sufficient, or maybe more shall be foreseen?
Where the injection point/points shall be placed?
Redundancy is necessary for fuel gas considering the vacuum formation impact, maybe nitrogen?
What will be the trigger for fuel gas injection?
The safety analysis for injection system in critical considering the potential impact of failure, SIL?
The flare header shall be heat trace considering heavy pout point of released.
Maybe the flare header shall be FV? It’s possible?
A flare KO drum inside Plant battery limit could be sized to cut the liquids avoiding above measures?

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Mihail

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