What I understood is that the code allows to use multiple WPS backed by a PQR having single Processe(s) as Per 200.4(a), here there is no restriction on the thickness of the test coupon either for the root process or the subsequent process.This will require 2 WPS for each joint minimum. in case of Impact requirements, the number will go even higher as the thickness limitations as per B31.3 will apply. this will make WELDING too complex for the welder who is supposed to refer the WPS and weld accordingly.
So, Code allows an easy solution to this by allowing to combine 2 or more PQRs which are used in a joint which can be by different processes. only if one condition is fulfilled, The PQR used for the Proposed root welding of this Joint should be qualified using a test coupon of thickness 13 mm.
QW-200.4 Combination of Welding Procedures.
(b) As an alternative to (a) , when making root deposits only using GTAW, SMAW, GMAW, PAW, LBW, LLBW, and SAW, or combinations of these processes, a PQR for a process recording a test coupon that was at least 1/2 in. (13 mm) thick may be combined with one or more other PQRs recording another welding process and any greater base metal thickness. In this case, the process recorded on the first PQR may be used to deposit the root layers using the process(es) recorded on that PQR up to 2t (for shortcircuiting type of GMAW, see QW-404.32) in thickness on
base metal of the maximum thickness qualified by the other PQR(s) used to support the WPS. The requirements of Note (1 ) of Tables QW-451 .1 and QW-451 .2 shall apply.