4.7.1 Suction Piping Systems
Piping that conveys product underground via suction must either:
have a precision tightness test conducted no less than once every three (3) years
be of double-wall construction with interstitial monitoring (refer to section 188.8.131.52).
Any below-grade piping that operates at less than atmospheric pressure, or that is sloped so that contents of the piping will drain back into the tank, if suction is released, does not require leak detection. However, only one check valve can be included for each suction line, and the check valve must be visibly located directly below, and as close as possible to the suction pump.
You must also provide a means by which these elements to be readily observed by authorized inspectors.