What is the cargo tank pressure of an LPG carrier?

flowchart TD
    A[Design vapour pressure Po] -->|≥| B[Cargo vapour pressure at 45°C]
    A -->|≥| C[MARVS]
    A -->|≥| D[0.7 bar for independent pressure tanks]
    C -->|Relief valve setting| E[Pressure relief system]
    E -->|Maintains| F[Operating pressure ≥ 0.007 MPa gauge during discharge]

• Definition: gauge pressure at the top of the cargo tank used for design and operation
• Design vapour pressure Po: maximum gauge pressure used in tank design; not less than cargo vapour pressure at 45°C and not less than MARVS
• Design floor for independent pressure tanks: not less than 0.7 bar gauge
• Operational minimum: during discharging, pressure ≥ 0.007 MPa gauge
• Governing instrument: IGC Code Chapter 4 §4.13.2 and Chapter 17 §17.18.14; IACS UR G1 §G1.2.5; UK survey instruction ch-3 §3.4.4
• Chief Engineer’s view: Po sets the structural limit; MARVS sets the relief limit; operating pressure is maintained above 0.007 MPa gauge during discharge to prevent air ingress