For an electrolyzer building, is it better to provide multiple hydrogen vent stacks or to combine the vent lines into a vent system with one vent stack?

It depends on the facility and risk assessment, but generally multiple pressure and temperatures to one vent stack is not the best practice unless all are similar in pressure and temperature, and the streams have compatible composition and flow rate. Care must also be taken to prevent reverse flow and misdirected flow between portions of the system. Additionally, one vent stack can become a single mode of failure for an entire process or facility. Specific considerations for vent systems include the following: 

1. Design stacks for hydrogen fires at the vent stack outlets.
2. Locate to assure no harm to people or equipment from thermal radiation.
3. Hazard review should be completed for the venting node(s).
4. Potential single mode of failure should be analyzed.
5. System should not allow air to enter while exhausting H2 gas (venturi effect).
6. Vent outlet design should direct venting hydrogen to a safe direction meeting requirements for radiation and dispersion.
7. Vent stacks should be grounded.
8. Supports should be designed to resist reactions from high velocity flow.
9. Stacks and vent piping should be designed to resist overpressure due to internal deflagration.

Several codes and standards address vent systems, but not all topics are fully covered in each. Here is a list of codes and standards that address hydrogen vents: 

1. CGA G-5.4, Standard for Hydrogen Piping Systems at User Locations, and G-5.5, Hydrogen Vent Systems.
2. EIGA Doc. 75/07/E, Determination of Safety Distances; Doc. 211/17, Hydrogen Vent Systems for Customer Applications; Doc. 121/14, Hydrogen Pipeline Systems.
3. IFC 2209.5.4, Venting of Hydrogen Systems.
4. EIGA 211/17, Hydrogen Vent Systems for Customer Applications.
5. ASME B31.12, Hydrogen Piping & Pipelines.
6. NFPA 2, Hydrogen Technologies Code. 
7. ANSI/API 521, Guide for Pressure Relieving & Depressurizing Systems.

ANSI/AIAA G-095A, Guide to Safety of Hydrogen and Hydrogen Systems (formerly NASA Hydrogen Safety Standard).