I have never done the calcs, but I believe a few things determine the flare purge rate:
1. The rate should be sufficient to account for a rapid cooling of the flare header. If the purge is not sufficient then you can loss your DP to the surroundings leading to possible contamination or O2 in the header. You have to remember that you are dealing with gases that can be entering the header very hot and subject to cooling over time.
2. The rate should also be high enough to keep O2 for backing in the tip by diffusion. Molecular seal are sometimes used to lower the required purge.
The tip is generally sized to give sonic flow at a desired capacity. I am not aware of a direct relationship between the two.
1) I belive that API 521 will give you a rule of thumb. Flare tip vendors will tell you the purge rate for their specific design.
2) For a flare that burns there is generally noting except for the flame that prevents water (i assume that you are considering rain) to enter the stack.
purge gas is used to help prevent air from geting into the flare.
if air gets into the flare system an explosion can occure. Purge gas is injected into the flare header at the farthest point from the flare tip and sometimes in the mole seals. the type of seal used will effect the amount of purge gas required.
estimate of purge gas requirements:
John Zink states the velocity with no seal should be between 0.25 ft/sec and 50 ft/sec.
Open pipe flare Rate (Ft^3/hr)= 7 (flare Daimeter^3)/(MW^0.565)
I am working on a system that needs to have a continuous purge to keep the mateial in a silo O2 free. How do I know what velocity is needed to keep O2 from backwarks diffusing into the vessel.
