Overpressure Protection of a Pressure Vessel By System Design through the Application of ASME VIII Ug-140 in Lieu of a Relief Device By an Appropriate Choice of Mawp and/or By Safety Instrumented System

 Abstract

This technical paper addresses the new provision of UG-140 of ASME VIII Division 1 and how it impacts on the traditional overpressure protection of a pressure vessel.  It offers the basics of a Safety Instrumented System (SIS), and explains the circumstances under which the user can replace the traditional relief devices to protect a pressure vessel from overpressure either by an appropriate choice MAWP so that the unmitigated pressures from all credible scenarios are less than MAWP of the vessel or through a safety instrumented system or a combination of safety instrumented system and traditional relief devices.  Overpressure protection by system design in lieu of a relief device is deceptively simple and deceptively economical.

The paper also explains the dependence of ASME B31.3 (process piping code) on ASME VIII Division 1(unfired pressure vessel code), and explains how the concatenation effect of the latter on the former implicitly mandates High Integrity Protection System (HIPS) -subset of the Safety Instrumentation System- to replace the traditional relief devices for the protection of a piping system.

Overpressure protection of a pressure vessel using ASME VIII Division 1, UG-140, has a place in industrial process safety. A safety instrumented system is worth considering when traditional relief devices are inoperable due to the hostile nature of the fluid or the required relief device is too large to install.  When traditional relief devices are replaced by a safety instrumented system, it may be the optimum choice if viewed through an optimization study with the traditional protection of a pressure vessel by relief devices as the base case, if the base case is a viable option. The cost of initial engineering, hardware and associated software cost, and life cycle cost of the safety instrumented system should be compared with the base case to justify the option of the safety instrumented system.