Flare piping hydraulic calculations

[Sawsan311]

Dear All,

I would like your views on thefollowing:

- the optimum pipe roughness to be considered for the pressure drop calculations for flare system hydraulics. Most of the design practices consider a roughness equivalent to corroded carbon steel pipe since a conservative approach is required for the critical flare lines experiencing high velocities driven by the low pressure compressible vapor flow as well as the fact that flare lines shall be designed safely enough to minimize/eliminate its replacement in case of any issue associated with high vibration (due to undersizing) or corrosion.

- API 521 considers the use of the isothermal/adiabatic flow equations for the compressible vapor flow hydraulic calculations of flare lines. However, section 5.4.4 also states that there can be options to consider the heat transfer with the ambient for the hydraulic calculations. Do you think that isothermal /adiabatic flow hydraulic equations shall be used with no credit for heat transfer between the flare line and the ambient since the relieved vapor would have a high velocity with strong forced convection heat transfer dominating the free convection with the atmosphere (in addition to the low heat capacity of the relieved vapor and hence small changes in temperature)??

Appreciate your views?

 

[georgeverghese]

Isothermal compressible flow assumes considerable heat transfer from the ambient, where applicable, and is the default expression used for flare line hydraulics calcs. It is more conservative in its backpressure estimate than adiabatic calcs which are somewhat more difficult to calculate.

 

[Sawsan311]

Thanks Mr. georgeverghese,

API 521 section 5.5.5 states that the isothermal flow equation considering ideal gas is more conservative than the adiabatic as well as the real gas behavior. However, we can deduce from its equation (equations 30 & 31) that it considers no heat transfer with the ambient.An option is provided in Flarenet to consider the enabled heat transfer option (between isothermal and adiabatic) asexplained in API 521 section 5.5.4.

Thanks

 

[georgeverghese]

It does assume isothermal flow conditions. In situations where the inlet temp is below ambient, with zero heat transfer ie adiabatic flow conditions, there is a temp drop which is identical to the isenthalpic temp drop. So, isothermal flow assumes a dQ which is positive; ie exit stream enthalpy is higher than that of the inlet stream.