API 650 - Vacuum Box Test Vs PT/MT (Penetrant Test/MPI)

The aim of this test, is to ensure sealing at the bottom of the tank. Not for weld quality itself. The welds must have been visually inspected before. Normally, for bottom plates, Laps weld sare used. With this test any discontinuity passing through the weld will be detected. With PT / MT is not guaranteed watertight. WP

Well i would like to keep is technically as simple as i can.

only Vaccum box testing (leak testing as API 653 would refer it as) has the capability of distingwish between leaks and non-leaks besides this advantage VBT can not detect non-leaking discontinuities for being detecting via VBT the discontinuity must be open to both surfaces in order to allow air pass through from either sides which eventually has to create bubbles in the soap solution which are then visible and are considered to be the evedence of leak.

on the other hand both MT and PT are lacking the capability of distinguishing between leak and non-leak just because of there principle of working.Either technically or visually operator will not be able to conclude either the indication is a leak or non-leak.

That is why VBT is prefered technique whwn it comes down to detect leaks.

Hope this answers the question.