It is envisaged to heat natural gas -on tube side- (at 675 psig/70 F) using saturated steam (at 100 psig) - on the shell side - in a shell and tube HX.

Is a relief valve required for protection against a fire hazard? If yes, are these mandatory requirements or engineering good practice. Pls. also indicate if the relief valve is required on the shell or tube side and the basis for sizing it.

Regards
Aaaron

Aaaron,
Welcome...

Only yourself can answer these questions.

Mandatory ?
Check with your Mechanical/Static engineer the design code for this S&T HX and check the relevant code. I am almost sure the code contain clause related to fire.

Good engineering practice ?
Analyze your system, do you find NO one scenario cause fire around this area ? E.g. absolutely no flange for leakage...
If you are absolutely sure, then you can avoid to have one overprotection device e.g. PSV, RD, etc for fire protection.

For fire relief load basis...three conditions are envisaged :
i) HX completely filled with liquid ==> liquid expansion
ii) HX partially filled with liquid ==> liquid boiling
iii) Hx filled with vapor or supercritical fluid ==> gas expansion