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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§ionid=3http://www.iacs.org.uk/publications/publications.aspx?pageid=4§ionid=3http://www.iacs.org.uk/publications/publications.aspx?pageid=4§ionid=3 -
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www.dnv.com/maritime
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: [email protected]
Det Norske Veritas AS. This publication may be reproduced
freely on condition that Det Norske Veritas AS (DNV) is always
stated as the source. DNV accepts no responsibility for any errors or
misinterpretations.
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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.