A consignor is preparing to ship a hazardous material that is a liquid with a vapor pressure of 25 psia at 100°F (37.8°C). This material is being loaded into a cargo tank. According to hazardous materials regulations, what is the maximum allowable working pressure (MAWP) that the cargo tank must be designed to withstand, at a minimum, for this material?
- A35 psi
- B8 psi
- C14 psi
- D25 psi
Show answer & explanationAnswer & explanation
Correct answer: A. 35 psi
For liquid hazardous materials with a vapor pressure exceeding 14.7 psi at 100°F, the cargo tank must be designed for a minimum MAWP equal to the vapor pressure of the material at 115°F, or 35 psi, whichever is greater. Since the vapor pressure is 25 psia at 100°F, it's above 14.7 psi. The regulation specifies a minimum 35 psi MAWP for these materials, unless the vapor pressure at 115°F is higher. Without the 115°F vapor pressure, the 35 psi minimum applies.
Why the other options are wrong
- B. 8 psi is the minimum MAWP for some materials, but not for those with such a high vapor pressure.
- C. 14 psi is too low; the vapor pressure is already above atmospheric pressure (14.7 psia).
- D. 25 psi is the vapor pressure at 100°F, but the minimum MAWP requirement for cargo tanks carrying materials with vapor pressures above 14.7 psi at 100°F is 35 psi (or the vapor pressure at 115°F, if higher).
Cargo Tank MAWP for High Vapor Pressure Liquids
Cargo tanks transporting liquid hazardous materials with a vapor pressure exceeding 14.7 psia at 100°F must have a minimum MAWP of 35 psi, or the vapor pressure of the material at 115°F, whichever is greater.
- Applies to liquids with vapor pressure > 14.7 psia @ 100°F.
- Minimum MAWP is 35 psi.
- Or vapor pressure @ 115°F, if higher.
- Ensures tank integrity under thermal expansion.
Memory trick: High vapor pressure means 35 PSI or more, for sure.