Can you speak to liquid hydrogen accumulation in a vent and the accumulator design to catch this liquid?

Liquid hydrogen will almost never accumulate in a vent system since vent systems are typically designed without insulation. The extremely cold liquid hydrogen temperature of -420 F.

Additionally, vent stacks on an LH2 tank are connected to the vapor phase of the tank. Only in a few rare instances will LH2 be entrained in the gas stream.

Accumulators are recommended at the bottom of vent stacks to catch any moisture that might enter the stack from rain, snow, or condensation. Condensation will occur inside of the vent stack after cold GH2 has flowed through the stack and then stopped. Moisture will accumulate on the inside due to cryo pumping of moist air into the vent stack. 

The accumulator collects the water below the relief device inlets to avoid blockage and should be checked and drained each time the liquid hydrogen tank is filled.