PSV Location in Vessels with Mesh Pads

[RRE]

In previous post (#) the question was asked about location of the PSV when a vessel contains a mesh pad. Although guidance was provided by several, the guidance (or better yet, the regulatory requirement given in ASME Section VIII D1 was not quoted. UG-35 discusses this requirement and states:

UG-135 INSTALLATION
(a) Pressure relief devices intended for relief of compressible
fluids shall be connected to the vessel in the vapor
space above any contained liquid or to piping connected
to the vapor space in the vessel which is to be protected.
Pressure relief devices intended for relief of liquids shall
be connected below the liquid level. Alternative connection
locations are permitted, depending on the potential vessel
overpressure scenarios and the type of relief device
selected, provided the requirements of UG-125(a)(2) and
UG-125(c) are met.
(b)(1) The opening through all pipe, fittings, and nonreclosing
pressure relief devices (if installed) between a pressure
vessel and its pressure relief valve shall have at least
the area of the pressure relief valve inlet. The characteristics
of this upstream system shall be such that the pressure drop
will not reduce the relieving capacity below that required or
adversely affect the proper operation of the pressure relief
valve.
101
(2) The opening in the vessel wall shall be designed
to provide unobstructed flow between the vessel and its
pressure relief device (see Appendix M).62

My question is around why this requirement is not addressed in API 520 or 521 but rather in a vessel code?? Any explanation ??

 

[don1980]

RRE - this guidance is also found in API guidance. I don't think the API guidance specifically mentions mesh pads, but neither does ASME.