Flare headers often have a fixed back pressure caused by other sources using the flare header or sometimes a constant flow to maintain the flame. That pressure can rise if some other piece of plant dumps into the header.
Then you have your back pressure in your connection to the flare header. This is caused by fluid flow but is in addition to the static flare header pressure.
But the devil is in the details you haven't supplied....
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Also: If you get a response it's polite to respond to it.
Thanks LittleInch, i am more clear now regarding flare header. however, in general does the back pressure in a line same as the pressure drop in that line?
The back pressure can be seen as the specific point pressure along the line, i.e. P1 , P2, etc., and the pressure drop is the difference between two points, i.e. DP= P1-P2.