Are there specific hazards to consider when mixing varied H2 temperature streams (gas and liquid) at a pipe junction, considering the following design features? (1) Cold hydrogen at the pump discharge is directed along two paths. One path uses ambient heat to warm the hydrogen, while the other path bypasses the heater to remain cold. Valves in each stream then adjust the flow split to achieve a controlled hydrogen mix temperature when the streams are recombined. The temperature-controlled stream is then directed to a dispensing station. (2) The hydrogen chilling system combines the cold hydrogen fluid with warmer hydrogen compressed gas to achieve the temperature necessary to provide a fast-fueling experience. The system will monitor the chilled fluid temperature and the vaporized warm gas and combine their streams using mixing valves and controls to provide -40 deg C° gas to the dispensers.

This is a fairly common arrangement and can be acceptable if properly designed. Some considerations:

1. The main concern is low temperature downstream of the mixing point, in case of mixing valve failure. Perform a LOPA (layer of protection analysis) on the safeguards. A shutdown table showing the action of each shutdown would help with understanding the layers of protection. The primary means of temperature control could fail if the temperature control valve fails open or if the sensing circuit fails high. The safeguards can then be evaluated to protect downstream equipment from getting too cold. The layers of protection should include two independent means of low temperature control: detection and shutdown. For example, the safeguards could consist of both stopping the pump and shutting an emergency AOV. 
2. Another low-temperature concern arises when the storage vessel AOVs are UPSTREAM of the normal temperature element (TE) to detect low temperature gas past the vaporizer. This is less of a concern for an ambient vaporizer since they can't fail "suddenly", but still it's best practice to install a TE upstream of any equipment that could experience brittle failure. It's possible that the buffer vessels will be stainless steel (or other material capable of low temps), but they are most likely carbon steel with a lower temperature limit of -20 deg F° or -40 deg F°. In this case, it is recommended that a TE be installed upstream of this connection. The temperature setpoint should be verified to be no lower than the rating of the vessel materials. It might also be possible (but unlikely) to backflow from the cold line if there is a leak on the storage vessels. The TE might protect against backflow depending on its operation. 
3. It might be possible to block in the vaporizers from the relief valve downstream. Verify that there is a relief valve that protects the vaporizer from a blocked-in liquid situation between shutoff valves and/or check valves. 
4. There could be sections of line on the vaporizer bypass where cold gas/liquid could be trapped between valves if closed when cold. For example, the double block and bleed valves. Verify that these lines can't be overpressurized in a trapped cold gas/liquid scenario. Install a relief valve(s) where needed or remove some of the valves to reduce the possibility of trapped product.
5. Failure of controls or control valves could "deadhead" the pump, leading to trapped liquid and/or an operating pump being blocked in. This could result in a relief valve lifting and/or an overpressure situation if no relief valves are present. Verify the intent of the AOV operation when the pump is running, verify that there are sufficient relief valves on the pump discharge, and determine if there is a blocked in or deadhead situation. 
6. Even if there is all stainless steel construction downstream, the dispensing hose (-40 deg F°) or the vehicle itself (-40 deg F°) still must be protected from cold temperatures.