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Posted : August 30, 2008
what is the accepted percentage loss in thickness of compression chambers on ultrasonic testing. which AODC/IMCA rule?

Posted : September 3, 2008
Hi there,
for such a detailed and specific question as this I have asked Iain Middlebrook of HSM Engineering Technology Ltd to kindly furnish your answer - he is a leading manufacturer of Hyperbaric Chambers in the UK and has many eminent chamber installations to his credit (eg DDRC).... let me know if you need more information best regards Andrew.

[1]. AODC/IMCA don't make these rules they are only an association However they may make recommendations such as size.

Corrosion allowance is best described as a scratch skin it can be as much as the client wishes or non at all. But it is always on the vessel data plate.

Some vessels under the latest EU rulings for hyperbaric use have zero corrosion allowance, so any pitting is subject to test failure.

However for new diving chambers although not mandatory but recommended to have a 1mm or a 2mm corrosion allowance. Although some cheaper
chamber had none.

The rules for corrosion are found under the original design specification, and these are for the pressure vessel be it built in the UK to BS5500 category 1 2 or 3 for the UK. and ASME 8, division 1 or division 2 for chamber built for the US market.

For diving use category 1 is required

I have assumed BS5500 and not the latest PD5500 as this enquiry is for testing so the chamber will be 5 to 10 years old or older.

[2]. The loss of wall thickness is only allowed if within the stated corrosion allowance and was built into the pressure vessel in the first place.

[3]. The amount of loss allowed is stated on the pressure vessel data plate.

[4]. The area of corrosion should also be taken into consideration.

[5]. Hydro testing will NOT resolve any borderline case it must be assessed by an experienced diving vessel engineer.

In addition to ultrasonics you are required now to inspect the welds for defect or for undocumented modification such as replacement drain fittings. Or additional weldolets fitted, again without full documentation. The area compensation for the doors and manways is now required as is the internal headway weld. Some older chambers built under BS1500 have been scrapped due to attachment failure in the joint design.

[6]. This is the reason why all chambers must be supplied with the original drawings in order to clearly assess if any modification or additions have been made to the chamber thus invalidating the original data plate certs.

Iain Middlebrook
HSM Engineering Technology Ltd
Answer provided by ADM Diving

Andrew Dawson-Maddocks
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