Heat tracing can be used as a safeguard against freezing. However, it must be understood that: 

• The heat trace system uses a utility supply so may not always be operational.
• The electrical equipment must be properly classified for the area, which can be challenging for the Div 1 classification for vent stack outlets.
• The heat tracing might be damaged by vent stack fires.
• For liquid hydrogen system vents
  • Heat tracing is nearly useless for stopping freezing with a higher flow cold GH2 stream due to the amount of heat needed.
  • The vent stacks can develop liquid air, so the equipment must be robust to survive cryogenic conditions without damage.
• Maintenance on the heat trace equipment might require a system outage for personnel to work on it safely.

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.