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Interesting project. If I understand your description correctly, the water tubes will be under the boiler, like the Yarrow type?What exactly is an approved boiler design in the UK, who approves it?
The issue is not simply testing, it is approval. There are too many people making unapproved boilers out of bits of copper pipe these days. My message is to stop playing about with your own safety and build ONLY from approved designs and ensure that your pressure vessels fit within the PSSR-even if they don't have to.Serious injury could result without regards to this.