consideration of the wetted area of S/T exchanger? 1

[DanielGong]

hi
when you calculate the relief load of PSV in fire case, there are vessel and exchanger in one fire circle. The normal operation condition of exchanger is both side liquid (naphtha)or one side two phase.
How to decide the wetted area of S/T exchanger? Because there is no NLL or HLL. Shall I assume the exchanger is full of liquid as the exchanger is on the grade below 25fts.
thanks for your help!

 

[Chance17]

For the shell side, the wetted area is 100% of the geometric shape.
For the tube side, the wetted area is the channel ends only (not the tubes).
Some organizations recommend 20% extra area to account for heat input from piping.

Yes , you fairly much assume the exchanger is liquid full.
There may be exceptions where a liquid level is controlled on either the shell or tube side.

It is common practice to interconnect heat exchangers with other vessels with safety valves - so that the heat exchanger does not require an individual safety valve.