Flare-less compression style fittings are commonly used. Choose tubing materials and tube wall thicknesses suitable for hydrogen and pressures you are using. Make sure all tubing joints are properly made, mechanically supported to minimize stress and vibration, are in a ventilated space, and are easily accessible for inspection and leak testing. 

Because hydrogen leaks frequently ignite, and because about half the time the ignition source is not identified, when evaluating hazards with hydrogen leaks, many people just assume the leak will be ignited. Note that consideration needs to be made for what may happen with immediate ignition (jet fire) and what may happen with delayed ignition (explosion). 

It is still important to minimize the probability of ignition and to minimize the consequences if it does ignite. We do that by properly characterizing hydrogen with all its unique properties, including flammability and low MIE, and by providing the appropriate safeguards prescribed by codes and standards specific to hydrogen. These safeguards include minimum quantities, using proper materials of construction, leak prevention practices, proper ventilation, proper disposal to safe areas, and ignition source control such as the use of non-sparking, electrical grounding, and classified electrical equipment.

In general, indoor storage should be limited and the use of hydrogen indoors should be the least necessary. Look to store flammable gases outdoors in dedicated protected area when practicable. Check to see what adopted building and fire codes in your jurisdiction say. NFPA 2, Hydrogen Technology Code, Sections 6.4.1 and 16.3 prescribe requirements to limit hydrogen storage and use in laboratories. NFPA also prescribes requirements for ventilation, gas cabinets, electrical classification, and fume hood operations. Consider outdoor or dedicated storage facilities if you need more than one standard-sized cylinder of hydrogen to support your work.

Purging is not recommended as a continuous part of vent stack operation. However, maintenance activity is a transient event and it’s prudent and recommended to purge a vent system prior to performing maintenance. It’s always possible that hydrogen could be leaking internally from a valve or other component and therefore create a hazard. Of particular note, care must be taken that proper isolation of the vent system is performed such that the vent system can’t be inadvertently used during maintenance. Since vent systems and stacks rarely have isolation valves to prevent unintended isolation of relief devices, proper maintenance on the vent system may require the entire system or plant to be taken offline.

We would not open the vent system to inspect the internal piping without a good reason.

It is recommended to check for water in the vent stack trap

1. At startup and daily during startup.
2. On LH2 tank system, every delivery
3. After the 1st rainstorm after a system is installed
4. LH2 vent stacks after establishing the baseline above
   1. After every large venting event
   2. Quarterly unless baseline requires more frequent
5. GH2 stack after baseline
   1. Check caps are still on quarterly for stationary tubes.

1. Understand any reactions the hydrogen can add to what is being vented. For instance, O2/H2 vented in the same stack would not be a good idea.
2. Understand all the flow and operating parameters of the streams to ensure no back flow into the hydrogen system or vice versa.
3. Ensure the venting/flaring system can handle the hydrogen flow parameters.

Recommended limits of heat flux for various exposures is provided in documents such as API Standard 521, the International Fire Code, the National Fire Protection Association and the Society of Fire Protection Engineers. Selection of a specific thermal radiation level is dependent upon a risk analysis. Some salient exposures are listed below.

• 1,577 W/m2 (500 Btu/hr ft2) is defined by API 521 as the heat flux threshold where personnel with appropriate clothing may be continuously exposed. This value is similar to the Society of Fire Protection Engineers “no-harm” heat flux threshold 540 Btu/hr ft2.
• 4,732 W/m2 (1,500 Btu/hr ft2) is defined by API 521 as the heat flux threshold in areas where emergency actions lasting several minutes may be required by personnel without shielding but with appropriate clothing. It is also defined by the International Fire Code as the threshold for exposure to employees for a maximum of 3 minutes.
• 20,000 W/m2 (6,340 Btu/hr ft2) is generally considered the minimum heat flux for the non-piloted ignition of combustible materials, such as wood.
• 25,237 W/m2 (8,000 Btu/hr ft2) is the threshold heat flux imposed by the International Fire Code for non-combustible materials.

NFPA 2 has also published Annex material which provides additional detail on the harm values used for the calculation of separation distances. 

The colors of hydrogen are not different hydrogen molecules. The colors represent the different methods to produce hydrogen. The colors are based on how much carbon is produced into the atmosphere during the manufacture of hydrogen. 

That being said there is no difference in hydrogen vent systems design by color, only by the design parameters (i.e. temperature, pressure, flow rate, etc.) 

It is normal for some air ingress to occur from the vent stack outlet. This is not a hazard if the stack has been properly designed to withstand an internal explosion or fire. Once hydrogen flow from a device is initiated, it will sweep out any air that might be in the stack. Generally, if the vent rate is insufficient to sweep the air out, then it’s also insufficient to freeze or liquefy air in the stack. However, it’s important to prevent air from being pulled into the stack from a venturi effect, so leaks or holes where air can enter the vent piping should be eliminated. 

Purging can be accomplished in several ways including by using pressure cycles, sweeping, or vacuum. Sweeping is the least reliable but can be effective on simple pipe runs. In most cases, vent systems are open to the atmosphere and the ingress of air from the outlet is likely. However, the vent system should be designed to handle fire or explosion internally. This
generally is not difficult and is a best practice that can also be found in several standards which are incorporated by reference in Codes. 

While air that enters from the vent stack outlet is expected to be within the vent stack, any leaks or openings in the vent system should be repaired such that they don’t act as a venturi and aspirate air while venting. A continuous source of air ingress should be avoided.