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Course of eventsThe vessel was undergoing a periodical survey by a DNV class

surveyor. A total of 19 air vent heads (mostly from ballast tanks)

were found to be defective and the surveyor requested the

master to repair this before leaving port. This example is taken

from a recent case but defective vent heads (sometimes also

called air pipe closing devices) are frequently found in ship

surveys, so this example is by no means unique.

Extent of damageThe main findings for the 19 vent heads in the case discussed

were:n Guide pin broken in way of welding to float

n Guide pin broken at lower end due to corrosion

n Float shows signs of corrosion in way of weld seam, may crack

in future

n Internal corrosion of housing

n Rubber seat dislodged making the float stuck & unable to

operate

n Due to internal corrosion, rubber seat is dislodged

Defective air vent heads are a common finding during Port State

Control inspections and will in most cases result in a detainable

deficiency that must be rectified before leaving port.

Probable causeThe requirement of automatic closing devises on certain air

pipes (from tanks) can be found in Load Line Regulation 20.

This has been interpreted by IACS in Unified Requirements P3

(Air Pipe Closing Devices). TheIACS UR P3states some specific

requirements for the design, materials and testing in order to

have a common understanding between the Class societies for

the approval of air vent heads.

A vent head shall be approved (product approval or type

approval) by a recognised body (e.g. a Class Society) before

being used on board a ship. This should also be kept in mind

when replacing malfunctioning vent heads and for the general

maintenance of parts.

The purpose of the air vent head (as defined in IACS URP3.2.6) is:

n to prevent the free entry of water into the tanks

n to allow the passage of air or liquid to prevent excessive pres-

sure or vacuum coming on the tank

The most probable cause of defective vent heads is the daily

exposure to the outdoor environment on deck, occasional

sprays from green seas and ballast water being pushed through

the vent head during heavy rolling. For the case discussed, the

casing (house) was made of nodular cast iron and the float andguide pin were made of stainless steel (probably type 304).

There is a great variety of designs and materials for air vent

heads available in the market. IACS UR P3 is not so detailed

on the material requirements. It states Casings of air pipe clos-

ing devices are to be of approved metallic materials adequately

protected against corrosion. Further, it states that closures (e.g.

floats) and seats can be of non-metallic materials but should be

compatible with the media (in the tank) and ambient tempera-

tures of between 25C and 85C.

In DNVs experience, plastic floats have a tendency to

become brittle after some time and we see that they crack and

every so often break due to movements (hammering) towards

the seat during tank filling operations and in heavy seas.The challenge with the stainless steel floats is that not all

of them are so stainless after all. The material quality typically

used is stainless steel type 304, which is not considered sus-

tainable for marine applications. The less good stainless steel

Information from DNV to the maritime industry No. 3-11 November 2011

Casualty Information

Damaged air vent head a detainable deficiencyShip type: Bulk Carrier Size (GT): 31000 Year built: 2005

http://www.iacs.org.uk/publications/publications.aspx?pageid=4&sectionid=3http://www.iacs.org.uk/publications/publications.aspx?pageid=4&sectionid=3http://www.iacs.org.uk/publications/publications.aspx?pageid=4&sectionid=3

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Casualty Information is published by Det Norske Veritas,

Classification Support.

Det Norske Veritas

NO-1322 Hvik, Norway

Tel: +47 67 57 99 00

Fax: +47 67 57 99 11

The purpose of Casualty Information is to provide the maritime

industry with lessons to be learned from incidents of ship damage

and more serious accidents. In this way, Det Norske Veritas AS hopes

to contribute to the prevention of similar occurrences in the future.

The information included is not necessarily restricted to cover ships

classed with DNV and is presented, without obligation, for information

purposes only.

Queries may be directed to

Det Norske Veritas, Classification Support, NO-1322 Hvik, Norway.

Fax: +47 67 57 99 11, e-mail: experience.feedback@dnv.com

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stated as the source. DNV accepts no responsibility for any errors or

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We welcome your thoughts!

12-2011

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Casualty Information No. 3-11 November 2011

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makes the vent head cheaper but also requires a more frequent

maintenance programme to replace corroded floats and guiding

pins.

Some vent heads are also equipped with a wire mesh as a

spark arresting screen on top of the tank (it is a requirement to

have the wire mesh for e.g. heated fuel oil tanks, or if there are

anodes in a tank with a single air pipe, or if there are specificnational requirements). In reality, our surveyors often see the

wire mesh covered with paint. This will of course reduce the air

flow through the vent head substantially and shall be avoided. We

have seen a number of tanks, predominantly ballast tanks but also

fresh water tanks damaged due to over-pressurisation. We recom-

mend that the wire mesh is removed on fresh water tanks and bal-

last tanks were anodes are not fitted.

Lessons to be learnedn Malfunctioning air vent heads are a common finding, for both

the Class surveyor and the Port State Control inspector. Each

vent head is an essential safety feature on board and should be

kept in good condition. A broken air vent head will most prob-

ably result in a PSC deficiency which must be rectified beforeleaving port.

n The ship manager should ensure that there is a regular inspec-

tion and maintenance programme in place for the air vent

heads on board its vessels. Since we see vent heads broken after

only a few years in operation, we recommend inspecting each

vent head annually. Such a programme should include the gen-

eral condition (rust, dirt, functionality) and especially the con-

dition of the float/ball/disc, guiding pin, seat and wire mesh

(if installed). A vent head replacement shall be of an approved

type and any replacement parts shall be of the original or an

equivalent quality.

n Special attention shall be paid to air pipes and air vent heads

located in the fore deck (1/4 L) due to green seas and the

ships movement.

n A cheap air vent head can become an expensive experience.

The trend seems to be to buy the least expensive vent heads

for newbuildings. This often means stainless steel type 304 as

an accepted material quality for the floats according to present

requirements (Load Line and IACS UR P3). However, this does

not mean that such stainless steel is maintenance free. Manufac-

turers, ship designers, yards and ship managers are encouraged

to consider the implications for air vent heads during a ships

life of maybe 20 years.

n Air vent heads made out of aluminium or other grades of stain-

less steel have become increasingly popular in the last few years.

Such materials are options to be considered in order to achieve

a more maintenance-free vent head.