What is typically the criterion that sizes or defines the venting flow rate of an LH2 facility, for example, emptying LH2 in x minutes due to a possible BLEVE (boiling liquid expanding vapor explosion) of the LH2 storage tank?

Relief device sizing for liquid hydrogen tanks follow recognized standards such as CGA S1.3. The sizing criteria include a worst-case scenario of an engulfing fire with loss of vacuum integrity.

LH2 tanks are unlikely to BLEVE due to the vacuum insulation outer jacket (usually carbon or stainless steel) preventing direct impingement of fire onto the main pressure vessel, as well as the internal cryogenic contents maintaining the inner pressure vessel walls at a cooler temperature until the contents have been relieved by the relief devices.

While not required by Code, nearly all LH2 tank designs follow a best practice of having at least one non-reclosing relief device to better empty the tank of its contents during a fire.