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Cleaning of welds internally on stainless steel pipework
A client of ours has recently asked how we are going to clean the inside of the thin wall grade 316 stainless steel pipework that has been site run and welded on site. The pipework (80 mm up to 450 mm) butt welds are inaccessible and we need to know what answer to give him. It is our belief that the internal welds, having been purged with Argon do not need cleaning internally as the pipework is self-cleaning during the running of the Water Treatment Plant as the process water is continually running. Are we right in our thinking? A point to note is that the industry norm in this case is that we clean externally and we clean any welds we can reach internally but do not clean the ones we cannot get at.David Yates
- Burton on Trent, Staffordshire, United Kingdom
A point to note is that the industry norm in this case is that we clean externally and we clean any welds we can reach internally but do not clean the ones we cannot get at.
If you're talking about potable water service, I believe you will find the industry norm is actually pickling followed by passivation. This is done by circulating the appropriate chemicals through the piping, usually in situ after all connections have been made. And, before you ask which chemicals to use, the answer is - if you need to ask, you're not qualified to do the job. Let a professional service do the pickling and passivation..
Jeffrey Holmes, CEF
Spartanburg, South Carolina
First of two simultaneous responses -- 2006
I remember a similar problem that a water cooler company had with their cooling coils about 10 years ago. They were getting a number of complaints from new customers about carbon flakes. To solve the problem we immersed the coil in a large ultrasonic tank with some mild slightly acidic chemical product.
retired business owner - Hatboro, Pennsylvania
Second of two simultaneous responses -- 2006
Jeffrey is correct. The British Stainless Steel Association's website, referring to the Drinking Water Inspectorate Code of Practice (Operational Guidelines and Code of Practice for Stainless Steel Products in Drinking Water Supply), mentions
"...if you can see a disclouration on the surface due heat tint, this must be removed for drinking water applications and should be adopted as good post weld cleaning practice for any stainless steel welded joints."
The absence of looking doesn't prove the absence of discoloration. There are fiber optic boroscopes for inspection.
A supporting document for the afore-mentioned OGCP is "Fabricating Stainless Steels for the Water Industry," NiDI 11026 2005. http://www.nickelinstitute.org
To avoid pickling of heat tint in the piping interior, argon purging must reduce oxygen to below 50 ppm. Also mentioned is that ASTM ASTM A380 [link is to the practice at TechStreet] provides information on pickling and passivation solutions.
- Goleta, California
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