Collapse of PVC casing pipe inside borewell


We are PVC pipe manufacturers. We often encounter a problem that the PVC casing pipe collapses inside the borewell after compressor wash or after some time of running. We want to know what test should be done beforehand to ensure that the casing pipe will not collapse inside the borewell. Moreover is there any formula for deriving the thickness of the pipe depending upon the depth of the borewell. Please suggest.