What to check on a PV Valves (Pressure/Vacuum).
An inert gas system has its own mast riser pressure/vacuum breaker, but that device only protects the common distribution main, not each individual tank once its branch valve is shut. PV valves sit directly on the tank dome or the venting header serving a small group of tanks, and they are the device that actually keeps a single tank inside its designed pressure envelope during loading, discharging, ballasting, or simple thermal expansion of the vapour space on a hot day. Where the mast riser breaker is a coarse, high-capacity safety…
What Sets Tank PV Valves Apart
An inert gas system has its own mast riser pressure/vacuum breaker, but that device only protects the common distribution main, not each individual tank once its branch valve is shut. PV valves sit directly on the tank dome or the venting header serving a small group of tanks, and they are the device that actually keeps a single tank inside its designed pressure envelope during loading, discharging, ballasting, or simple thermal expansion of the vapour space on a hot day. Where the mast riser breaker is a coarse, high-capacity safety net, the tank PV valve is a precision-set component whose cracking pressures directly limit how the cargo plan can be run.
Main Components
Pallet or Disc Assembly
A weighted or spring-loaded disc sits on a machined seat; rising tank pressure lifts it to vent vapour, and vacuum pulls a separate vacuum pallet open to draw in inert gas or air, preventing the tank from being sucked inward.
Pilot-Operated Valve Body
On larger tankers a small pilot valve senses pressure and triggers a much larger main valve to open fully once set point is reached, giving tight sealing at normal pressure but high flow capacity in an upset condition.
High-Velocity Vent Head
Where full free-flow venting is required, the vent outlet is shaped to discharge vapour as a high-velocity jet straight upward, dispersing flammable vapour away from the deck rather than letting it roll along the surface.
Flame Screens
Corrosion-resistant mesh across the outlet stops a stray spark or flame from travelling back down into the tank, and is one of the parts most often found clogged or corroded away during inspection.
Selection and Sizing
Set points follow the tank design pressure, commonly in the range of roughly +1400 mmWG for the positive side and around -350 mmWG for vacuum on a typical product or crude tanker, though the exact figures come from the ship's trim and stability and cargo system documentation rather than a fixed industry number. Sizing has to cover the worst-case simultaneous scenario: maximum loading rate plus the vapour generated by a sudden rise in ambient temperature, since undersizing a valve leaves the tank without adequate relief exactly when it is needed most.
Regulations and Class
- SOLAS Chapter II-2, Regulation 11 sets the venting requirements for cargo tanks, including the need for pressure and vacuum relief on every tank carrying flammable cargo.
- MARPOL Annex I and Annex II drive additional venting and vapour handling rules depending on whether the cargo is oil or a noxious liquid substance, affecting how PV valves interact with vapour return lines.
- Class requires each PV valve to be function-tested and its set pressure verified at a workshop or with a calibrated test rig at intervals defined in the survey scheme, and a valve that has been overhauled needs a certificate before it goes back in service.
Typical Faults
| Fault | Consequence |
|---|---|
| Pallet seat corroded or coated in cargo residue | Valve sticks shut, tank overpressures during loading, or sticks open, wasting inert gas |
| Set pressure drifted low from spring fatigue | Nuisance lifting during normal operations, inert gas consumption rises |
| Flame screen blocked with salt or paint overspray | Reduced flow area, effective set pressure rises well above the nameplate value |
| Pilot sensing line pinched or iced in cold trades | Main valve fails to open on demand, tank left without relief |
What to Look for in a Supplier
- Test certificates showing actual cracking and reseating pressures for each valve, not just a batch conformity statement.
- Materials suited to the specific cargo range the ship carries, since a valve fine for clean products may corrode quickly in a sour crude service.
- Availability of an exchange or workshop overhaul service near the ship's usual trading route, since PV valves are removed for testing on a running schedule.
Never isolate a PV valve for maintenance without confirming an alternative relief path is open on that tank; a blanked-off valve during tank cleaning has caused more than one crushed or overpressured tank top.