Retrofitting Crack Arrestors on a Pipeline Converted From Liquid to Gas Service

PETRONET undertook to evaluate the conversion of two liquid pipelines designed and constructed under ASME B31.4 to gas service under ASME B31.8. In a liquid pipeline, a rupture will result in small length of pipe (3 m–10 ft.) opening to release product and pressure. The same failure mechanism in a pipeline with gas service can be far more damaging. Under certain combinations of product, pressure, temperature and material, a rupture can result in a running fracture that can travel for miles. The releasing energy from the compressed gas can sustain the fracture mechanism until some barrier or material change can absorb the energy and allow the fracture to arrest. This can be the critical decision point in any planned service conversion. If the pipe material and operating conditions indicate that a running fracture may develop, then either the pipe has to be replaced or an arrest mechanism installed. The economics usually preclude pipe replacement so it becomes critical to develop a method of retrofitting suitable arrest units economically and safely. Clock Spring® Crack Arrestors have proven to be an economical solution to this challenge. This paper presents a case study of the conversion of a 457 mm (18-inch) pipeline, designed and constructed in accordance with ASME B31.4 for liquid service, to gas operation under ASME B31.8. The pipeline is located in South Africa and is operated by PETRONET. The use of Clock Spring® Crack Arrestors saved million dollars over pipe replacement and was a key issue in the economic justification of the conversion.