Dear Freinds,
In the refinery flare system, I have seen that the pipe that is connected to the flare stack have the same size of the flare stack. And in some cases the pipe has been enlarged to the size of flare stack before the connection.
My questions are:
1)why the pipe which is connected to the flare stack should be in the same size of flare stack?
2)what are the criteria for calculation of how far from the stack, the pipe should be enlarged?
3)any reference or procedure for calculating will be highly appreciated.
thanks

Calculations relevant to flare system piping are covered by "compressible flow" section in Crane technical paper 410.
Flarenet is an A$pentech s/w specialized for flare system hydraulics. Other few similar s/w are available on the market.
In the final part of the flare header flow, temperature and pressure of the fluid are very similar to the ones in the flare stack. Each single pipe in the system shall respect a max velocity criteria: since the velocities are very similar, the two sizes are usually the same. This is only a quick answer: things are a little bit more complicated....
Available for other discussions on this subject.