Low-pressure vents at mostly low hydrogen purity are not as large safety risk as high-pressure pure hydrogen vents. These vents should still go to a vent stack, but it will probably be small in diameter and thus the tee vent at the top can be small.

If the purge requires high flow, if purging horizontally, the reaction forces of the flow exiting and the hydrogen cloud should be modeled based on NFPA 2 to ensure the safety of the surrounding area. 

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. 

Several programs can predict this such as HyRAM or PHAST. The inputs are critical to a safe
answer.

This is not a simple answer due to the many types of flame lengths and flame orientations due to pressure and direction. NFPA 2 recommends that vent systems should be designed so that if the safety relief valve is relieving at capacity the radiative heat felt by an individual at grade does not exceed 500 Btu/hr/ft2 (5.68 MJ/hr/m2) (A.10.4.4).

NFPA 2, section E has a lot of good information on this subject.

As an example, let’s look at a gaseous H2 cylinder release through a rupture disc. The flow is straight up through a vent. If ignited, initially the radiation will be at it’s highest. As the pressure in the cylinder is reduced the flow rate is reduced, and thus so is the radiative area. At the same time there is a maximum time in which a person can be in the radiative heat flux. Distance to radiation heat flux level of 4732 W/m2 (1500 Btu/hr · ft2) with exposure to employees for a maximum of 3 minutes.

The heat flux location will define how transient flow vs radiation locations will be defined. The heat flux values in NFPA 2 include safety factors.

Auxiliary information

• 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 close to, but actually less than what the Society of Fire Protection Engineers determined to be the “no-harm” heat flux threshold (540 Btu/hr ft2), that is, the maximum heat flux to which people can be exposed for prolonged periods of time without experiencing pain.
• 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. This value is close to the heat flux level used by other standards (e.g., NFPA 59A, EN 1473) as the threshold for public exposure (1,600 Btu/hr ft2).
• 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. Other standards use somewhat larger values for heat flux damage to prevent damage to non-combustible construction.
  • API 521, Guide for Pressure Relieving Systems, 1997 Ed., Table 8, pg. 41
  • SFPE Engineering Guide, Predicting 1st and 2nd Degree Skin Burns from Thermal Radiation, March 2000, page 8
  • API 521, Guide for Pressure Relieving Systems, 1997 Ed., Table 8, pg. 41
  • 2003 International Fire Code, Sec. 2209.5.4.2(3)
  • According to literature, exposure to a heat flux of 1,600 BTU/ft2-hr will lead to 2nd degree burns over the exposed skin in approximately 30 seconds.
  • The Center for Chemical Process Safety )CCPS) book titled “Loss prevention in the process industry” lists 23,800 W/m2 as the minimum heat flux for unpiloted ignition of various kinds of wood
  • 2003 International Fire Code, Sec. 2209.5.4.2(3)
  • NFPA 59A sets a threshold of 10,000 Btu/ft2-hr for a property line that can be built upon.

Vent stacks and building ventilation systems are different and should be analyzed/designed differently. NFPA 2 has different location requirements for vent stack and ventilation system outlets. There are code requirements for elevation, distances from exposures, and between exposures. 

There are no specific regulatory or code requirements for vent system separation distances. These should be evaluated as part of a hazard assessment. A primary consideration is that a fire on one should not lead to ignition of another stack which might also be venting at the same time. Dispersion analysis can be performed to ensure that there is adequate separation. Additionally, the vent and ventilation system exhausts should not be able to be pulled into an air intake.

There are several levels of documents which can be used to assist with the design, sizing, selection, and installation of the pressure relief device settings for LH2 tanks. 

Pressure vessel design codes, such as the ASME Boiler and Pressure Vessel Code will provide minimum requirements for design of pressure vessels (including LH2 tanks), relief devices, and relief systems. However, these codes will not provide the sizing criteria nor anticipate all of the potential demand cases that might be imparted upon a vessel. 

In the US, the model fire codes require compliance with NFPA 2, which then references documents such as CGA S1.2 and CGA S1.3 for sizing criteria. These documents have been customized by the industrial gas business specifically for cryogenic fluids such as LH2. API Standard 520 “Sizing, Selection and Installation of Pressure-Relieving Devices in Refineries” of is also a helpful document to provide additional guidance. 

For LH2 storage tanks, usually the highest process demand is an engulfing fire with a loss of vacuum insulation to atmosphere. This failure mode can result in additional heat flux from air condensation in the annular space which must also be addressed. 

It is not required to proactively vent the contents of an LH2 tank when exposed to fire. Relief devices are required to prevent the accumulation of internal pressure to unsafe levels. Within the ASME BPV, this is 121% of Maximum Allowable Working Pressure for scenarios involving fire exposure. It is common practice, but not required, that at least one device be non-reclosing (e.g. a rupture disc) for both managing the high flow required as well as to relieve the contents of the tank. Reclosing relief devices will maintain pressure in a fire and are more likely to lead to a vessel rupture if the fire ultimately weakens the pressure vessel.

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 main pressure vessel walls at a cooler temperature until the contents have been relieved by the relief devices.