Obtaining Leak Free Bolted Joint Operation by Returning to Basics
Leakage of pressure vessel and piping bolted joints in refineries is an unnecessary hazard, with high associated cost, that can be easily rectified using currently available technology. There have been advances in gasket testing technology in recent years that have allowed great improvements to be made in the specification of gaskets for refinery applications. This minimizes the likelihood of joint leakage and results in reduce operating cost. In addition, there have also been advances in joint assembly procedures that have enabled significant reduction in joint assembly times, while resulting in a better final gasket stress distribution and therefore lower likelihood of leakage. This paper outlines a basis for justification of the implementation of improved joint and bolting technology in the refinery. It also details the critical calculation methods, assembly procedures and actions required to ensure leak free operation and ways of providing on-going quality assurance to a refinery leak-free program. An example of the recent implementation of a leak-free program in a refinery unit is followed in order to provide clarity on the required steps to achieve leak free operation.
A Brief Operating History of Pressure Vessel and Piping Bolted Joints
Pressure vessel and piping bolted joints have existed, in one form or another, since the introduction of pressure vessels during the industrial age. A design method for bolted joints was introduced into the 1934 ASME pressure vessel code, was updated in the 1943 code and has remained essentially unchanged since that time. The present design method, by self admission in Appendix S of ASME VIII, Div. 1 (ASME ), does not analyze most of the actual operational loads on the joint and, until the introduction of ASME PCC-1 (ASME ) in 2000, there was no official guidance on the assembly or operation of bolted joints. This is in spite of the fact that throughout history there have been major incidents involving the failure of bolted joints. The ICI risk management manual (ICI ) lists an estimate of the “average” number of joint leaks as 5 per year per plant. In most refineries, this is probably a low- ball estimate and probably actually only covers major leakage incidents. In a study of offshore platforms for the UK HSE (HSE ), bolted joint leakage was the leading cause of loss-of- containment reports with a total of 174 incidents versus the next important contributor (corrosion and erosion) at 171 incidents. Given that the process conditions offshore are generally kinder on bolted joints (lower pressure and temperature conditions), this is a fairly decent indication that estimates for bolted joint leakage on onshore equipment is an accepted occurrence that largely goes unrecorded.