What should be the minimum wall thickness used on a buried pipeline? I've seen the parameter D/t <100 discussed, but I 've also spoken to a few pipeliners that limit the wall thickness to a minimum of .25 inches as a practical limit for sizes around 6 inches and above. Is there a written guideline available? This would be in a situation in which the basic pressure calculations yield a wall thickness less than .25 inches, say maybe .175 or so.

Thanks
sfol55