Pneumatic Testing of Piping Assemblies: Criteria for Stored Energy and Pinhole Leak Detectability

Abstract

Pneumatic testing is beneficial as an alternative to hydrotesting particularly in remote areas where access to hydrotest fluids becomes logistically difficult or impossible. In some cases, above-ground pipe supports cannot hold the water weight, or that the pipe is coated/lined with materials that would degrade with water/methanol/glycol mixtures, or the system cannot be fully dried and the remaining hydrotest fluids are detrimental to the service application once the system is commissioned. Additionally, the elevation profile of the piping is such that an excessive number of test sections would be required for hydrotesting according to most codes. The present work is aimed at addressing two salient questions often faced with pneumatic testing. The first is related to the appropriate piping volume to consider for calculating the stored energy in use with ASME Post Construction Committee (PCC)-2 calculation for determining the safe exclusion distance for a given piping geometry and test conditions. It was found that the 8D criteria specified in ASME PCC-2 cannot be generalized for all pipe sizes, different material toughness, grades, wall thicknesses, and test conditions. A criterion is developed based on the ductile fracture arrest length that considers all these factors combined. The second criterion is related to the ability to detect a pinhole leak from the pneumatic test data, again for a given geometry and test conditions, and what constitutes the minimum pinhole effective area in relation to the system total volume, measured uncertainties in the test pressure and temperature over the duration of the test. A seminormalized physics-based parameter is suggested that can be applied to determine the effective pinhole leak size. The methodology is applied to a pneumatic field test on DN200, 12.2 km pipeline lateral.