What is the difference between a P/V valve and a P/V breaker?
Short answer
| P/V valve | P/V breaker | |
|---|---|---|
| Level | primary, per tank | ultimate, system-wide |
| Where | each tank’s vent outlet | the common venting or inert gas main, a liquid-filled U-tube |
| Acts | normal breathing: relieves pressure, admits on vacuum | gross over- or under-pressure when the valves cannot cope or are isolated |
| Set points | low, of the order of a few kPa | higher |
| Extra | flame screen on a flammable-cargo tanker | the liquid seal blows out and re-seals |
- Why: a cargo tank breathes with temperature, loading, discharge and inert gas; too much pressure ruptures it, too much vacuum collapses it.
- P/V valve, primary and per tank: opens at a small set over-pressure to relieve and at a small vacuum to admit gas, air or IG, closed between; low set points of a few kPa; flame screen on a flammable-cargo tanker.
- P/V breaker, ultimate and system-wide: on the common venting or IG main, usually a liquid-filled (water and glycol) U-tube; relieves gross over- or under-pressure when the P/V valves cannot cope or are isolated, frozen or failed; higher set points.
- Distinction: everyday relief at low set points per tank versus the last line of defence at higher set points for the whole system; capacity sized per IACS Rec 25.
- Survey: SOLAS II-2/4 and the IGC and IBC Codes; DGMA or its RO checks valve settings, capacity and flame screens, and the breaker’s liquid level.
- As Chief Engineer: I keep both maintained and correctly set.
Full answer
The full answer explains each point step by step, with the regulations and figures behind it and the discussion from other candidates. It is in the MEO Class I section of the Marine One forum, open to members enrolled for the MEO Class I exam.