P/V Valves & Tank Venting — How a Cargo Tank Breathes Without Bursting

A cargo tank is not a pressure vessel — it is thin plate that must let its vapour space swell and shrink as the sun works on it and as cargo comes and goes. The venting system is how it breathes: a mast riser for the big flows of cargo work, and on every tank a pressure/vacuum valve — two weighted doors that hold the tank shut until it truly needs to exhale or inhale. Tap a step or press "Play" to follow the physics from why tanks breathe at all to the flame screen that keeps fire out of the vent — then take the simulator through a hot day, a night cool-down and a blocked screen.

tank breathing · vapour in, vapour out · pressure held near zero

Step 1 / 9

Why tanks breathe

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Key —

Tap a step

The one-minute tank — loading against a shut vent≈70 s to design pressure

Common mistaketrusting the P/V to cover a shut riser valve — if its flame screen is painted over or iced, the only other hole in the tank is choked too, and there is no relief at all.

Take a tank nearly topped off — 300 m³ of vapour space left — still loading at 3,000 m³/h (50 m³ every minute) with the vent line-up shut. The incoming cargo simply compresses the vapour space: pressure × volume stays constant, so as the space shrinks the pressure climbs on a curve that gets steeper by the second. It passes the P/V setting of +1400 mmWG at about 43 seconds and the tank's design pressure of +2400 mmWG at about 68 seconds. That is the whole venting lesson in one number: a mis-set valve during loading is not a slow problem you catch on the next round — the structure is in trouble in about a minute, which is why the vent line-up is proved open before the manifold valve, never after.

Try it — let the tank breathe

One cargo tank over a full day. The sun heats the vapour space and the night cools it; the cargo rate pushes vapour out (loading) or pulls the tank down (discharge). With the riser valve open the tank breathes freely up the mast riser; shut it and the tank is sealed — only the P/V valve stands between the pressure and the structure, at +1400 / −350 mmWG. Now block the flame screen and see what happens when that last hole is choked.

Cargo rate m³/h · + load / − discharge

+1500 LOAD

Sky drives the day/night swing

MILD

Riser valve open / shut

OPEN

Flame screen clean / blocked

CLEAN

Vapour pushed out1,500m³/h · vent taking1,500· tank pressureBREATHING

whatever the sun and the pumps put into the vapour space, the vent must take back out — when every path is shut, the plating takes it instead

riser open = free breathing near zero · riser shut = sealed tank riding the P/V settings · screen blocked = no relief at all

Tank press100mmWG Vapour temp24°C Ship’s clock10:00☀ Vent flow1500m³/h P/VSEATED

Manometer marks — P/V settings vacuum −350 · pressure +1400 · structural design limits −700 · +2400 mmWG. The P/V lifts at its settings; past the design limits the plating itself gives.

The day runs on its own (24 h in about a minute). Watch a sealed tank (riser shut, rate 0) ride the sun: up a few hundred mmWG by afternoon, then down through the night — a tropical night cool-down alone will pull it past −350 and lift the vacuum side. Load hard against a shut riser and the pressure side lifts at +1400. Then block the flame screen: the P/V is still trying to open, but its hole is choked — and the pressure walks straight past the setting toward the plating’s design limit.

READY — set the controls, then press Start

Press Start to run the day — then seal the tank (riser shut), load hard, or block the flame screen to see the pressure walk past the P/V.

Working principle — explained in our own plain words with our own schematic from general marine knowledge (not copied from any OEM manual). Conceptual; varies by ship/model. Reviewed and verified by a Master Mariner.