P3 – Air pipe closing devices

APPLICATION FIELD

IACS UR P3, “Air pipe closing devices,” applies specifically wherever air pipes are required, by class Rules or by the International Convention on Load Lines, 1966, to be fitted with automatic closing devices.

It’s a narrow, single-purpose UR: it doesn’t govern the air pipe itself, its diameter, or its wall thickness , those sit under P1 and P2, it governs the fitting mounted at the top of that pipe, the automatic closing head that keeps water out of the tank while still letting the tank breathe.

The trigger for P3 is the Load Line Convention’s requirement, carried through Regulations 19 and 20 as modified by the 1988 Protocol, that weathertight automatic closing be provided on air pipe openings that could be submerged at a small angle of heel.

PIPE CLASS / SYSTEM TYPE

P3 sits entirely outside the P2.2 Class I/II/III system, and this is worth stating plainly because it trips people up: an air pipe closing device isn’t assigned a pipe class at all. What governs it instead is Type Approval under P3.4, a component qualification process independent of whatever class applies to the piping it sits on top of.

The air pipe itself downstream is sized against P1’s Table 3, column B (vent, overflow and sounding pipes for integral tanks), but the closing head is a separate, independently approved fitting. Don’t go looking for a Class I/II/III designation on the device. The relevant question is whether that specific type and size of head has passed the P3.4 test program.

RELATED RULES

• Direct sibling: UR P1, Table 3 column B, sets the minimum wall thickness for the vent/overflow/sounding pipe that the P3 closing device is fitted to, the two rules describe adjoining parts of the same physical assembly.
• Statutory backdrop: The International Convention on Load Lines, 1966, as modified by the 1988 Protocol, specifically Regulations 19 and 20; is the source of the requirement that air pipes be fitted with automatic closing devices in the first place; P3 governs how those devices are designed and qualified once the Convention requires them.
• Cross-reference: UI LL49 interprets when weathertight, automatic-type closing is actually required, tying the obligation to whether the air pipe opening would be submerged at an angle of heel below 40 degrees (or a lesser angle where the ship’s own stability information calls for it).
• Cross-reference: UI LL36, cited alongside P1’s minimum wall thickness provisions, addresses related Load Line requirements on pipe strength for the same class of openings.

RESTRICTION

Devices are qualified for inclinations up to and including 40 degrees; use beyond that angle sits outside what the type test covers. Maximum leakage during the immersion cycling test is capped at 2 ml per millimetre of the inlet pipe’s nominal diameter, per cycle.

Float wall-thickness manufacturing tolerance is capped at ±10%. Non-metallic closures and seats are qualified only for ambient operating temperatures between -25°C and 85°C, and only where compatible with both the tank’s contents and seawater; using them outside that envelope or with an incompatible medium falls outside what P3 has actually tested for.

DESCRIPTION

An air pipe closing device has to do two contradictory jobs at once: stay open enough that a tank can breathe freely during normal loading and pumping, and shut fast and tight enough that a wave sweeping the weather deck doesn’t find its way into a double-bottom or ballast tank.

Whether a ball or float actually seats reliably depends on orientation, on how quickly it’s submerged, on how it behaves after years of exposure, none of which a designer can prove on paper the way a wall-thickness formula proves itself.

So P3 sets out an empirical qualification instead: measuring the flow characteristics of the open device, running immersion cycles at two different submersion velocities, and repeating the tightness test not just in the normal vertical position but at 40 degrees of inclination with the opening facing upward, downward and sideways, since the worst-case orientation isn’t always obvious in advance.

The float testing procedure, impact and compression loading, both dry and after soaking in water or fuel oil, across a temperature range from -25°C to 85°C exists for a  practical reason: a float that cracks or takes a permanent set under repeated water-hammer loading stops functioning long before anyone notices from outside, and a waterlogged or deformed float is, functionally, no closing device at all.

IMPORTANT NOTE

• The 2 ml/mm leakage point and the ±10% float wall-thickness tolerance are hard type-approval criteria; a device that exceeds either doesn’t receive a certificate, and finding an uncertified or non-conforming head in service is a survey finding (claims) that means a not suitable closing device. Therefore such elements must be carefoully choosen during the procurement and carefoully inspected during the onboard survey

• The supplementary hardened coating required at the erosion-prone zone above the standard galvanizing is frequently the point where devices actually fail in service.

• Substituting non-metallic closures or seats without re-verifying compatibility across the full -25°C to 85°C range, and with both the tank medium and seawater, is a compliance gap that tends to surface as an in-service failure rather than at document review.