Why 15 and not 10, or 30? No reason I can think of. I've used 10 psi to test for leaks in fuel tanks and I've used 30 psi to test for leaks in pressure vessels.
The whole idea is to have some positive pressure to blow the soap water mixture so you can see it. If you have a very large leak and, say 30 psi, you can have the leak blowing the mixture so fast and far it doesn't have a chance to form a bubble and therefore not look like a leak.
Then again, if you have 10 psi and a very large vessel, you might miss a small leak.
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It is I believe because of a standards rut probably falling back to early steam systems. Or is it that about 15 psi is required to keep cooling systems below the boiling point? That may be it.
Leak Test
