Low pressure tanks are storage vessels having a maximum allowable working pressure (MAWP) of less than 15 psig. Pressure relief devices (PRDs) for these vessels are sized using API Standard 2000. Sizing methods and constraints for PRDs in low pressure tank applications differ considerably compared to those used with pressure vessels.

Differences in ERS Design for Low Pressure Storage Tanks

1. Inbreathing must be considered to protect against vacuum conditions.
   • Latest edition of API Standard 2000 (March 2014) increased the requirements for
2. Storage tanks with an MAWP of 2.5 psig or less are covered under API Standard 650.
   • Treated as “atmospheric”
   • No overpressure is allowed
3. Storage tanks with an MAWP greater than 2.5 psig but less than 15 psig are covered under API Standard 620.
   • Treated as “low pressure”
   • Overpressure allowance of 10% permitted for process upsets
   • Overpressure allowance of 20% permitted for external pool fire
4. Sizing must take into account losses in the inlet piping (see sizing example).
   • Becomes significant when flame arrestors are installed beneath the relief valve