Dear Van,

Fig. 100.1.2(C)of Asme B31.1 (2007) clearly shows the code jurisdiction. i.e. If desuperheater is in the main steam line than upto desuperheater ASME Sec I bears the technical as well as administrative responsibility. Where as at downstream of desuperheater the B31.1 has the technical responsibility.

So you can only use B31.3 after the first STOP Valve (installed after desuperheater ofcourse).

Please refer to ASME B31.1, 122.4 section in which you'll see some requirements for desuperheater either in main steam or reheat lines (like between superheaters and bypasses etc).

Any desuperheater manufacturer will guide you for selection and type of desuperheater required. Mostly process engineers are incharge of this. There is also specific requirement of upstream and downstream distances which piping guyz have to maintain in order for spray water & steam proper mixing and achievement of sort of laminar flow. Also in order control the erosion (if in action as per velocities calculated) a sleeve is incorporated in the downstream of desuperheater piping.

Main analysis requirements are to consider the transient effects associated with water spraying on superheated steam. Also the temperature difference of spray water piping and steam piping is also main concern when evaluating expansion stresses.

Hope that helps.

Regards,
Salz