needle gunning ?

[airtight]

I am wishing to remove some local corrosion from the inside of a pressure vessel. Although it is quite deep, it does not go past the corrosion allowance.

I wish in a few months time to have the vessel gritblasted, however we require to leave the unit in service until that time.

Is there any reason that I cannot remove the corrosion locally by needlegun?

The material is sa516 - 70.

It has been mentioned by a surveyor that this may not be allowed due to the impacting action of the needle gun -

Anyone had experience of this?

Thanks in advance ! Neville.

 

[gr2vessels]

airtight,
The needle gun is capable of removing thin rust on a lightly corroded surface, but deep scales might not be removed, the needles tending to flatten the scales and thus masking fine crevices, due to the impact type of work done by the gun needles. It does not affect adversely the internal corrosion or the intended blast cleaning operation, but will not replace the proper cleaning, the blast cleaning. For as long as you will follow up with blast cleaning, you should be right.
Cheers,
gr2vessels

 

[unclesyd]

Air chisels and needle air guns are used all the time to remove corrosion products on ships and vessels around the coast.

 

[robsalv]

Airtight, it's a month later, what did you do?

Needle guns can't bring metal back to shiny condition and cannot clean rust from deep pits. They certainly can remove bulk scale though.

There's a new power wire brush being marketed in Australia, which has uneven brush wires which will break, providing an aspect of "grit blasting" to the mechanical cleaning provided by the power wire brush. Might be the ideal answer. ?

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[airtight]

Thanks for all the input guys. The problem that we were worried about is the impacting damage that a needlegun may cause ie. hardening. I cant work out any reason why this should happen but there does appear to be a well heard rumour that it needle gunning may harden plate. I dont think there is much truth in this but I am interested in what others have to say.

The scaling is too thick for a wire brush and it will come off perfectly by gritbolasting to sa 2 1/2 . However the item in question is a diving bell which is in 24 hour use 7 days a week until we have a maintenance period which is not for a couple of months. It was then that we intended to blast it properly.

In the mean time we intend to locally needle gun the areas of corrosion and treat them.

Thanks once more. Neville