I don't know if there's a standard or not, but I'd suggest a higher design torque level than that.
If I were able to fracture your valve with the amount of torque I could exert through a good pipe wrench's teeth, with a handle of arbitrary length, without stripping the outside surface off the pipe, I'd speak ill of your valve.
Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA