Inadvertent valve opening is a scenario that comes up time and time again during process hazards reviews. An example of why we spend the time to look at those scenarios happened at a facility a while back. A 12″ ANSI 1500# XV (actuated ball valve) was inadvertently opened with approximately 450 psi differential across it. This resulted in a downstream PSV almost immediately opening to relieve the excess gas as it was supposed to do. An operator nearby indicated that the PSV started chattering shortly after it opened. Then shortly after that the PSV failed catastrophically with the top cover being blown off (see pictures below) causing a large hole in the building where the equipment was. The operator was able to get the control room to close the XV.
RCE was brought in to conduct an incident investigation. We gathered the various plant data and started evaluating the equipment, valves, etc. We also performed various steady-state and dynamic simulations. We were able to match the incident using dynamic simulations which was important to understanding which equipment was affected and to what extent.
A few of the recommendations on how this can be prevented in the future:
- Use pressure differential permissives around large actuated valves to prevent them from being opened inadvertently.
- Utilize a high pressure shut-down that would close all upstream XV’s automatically instead of relying on board operator to take action.
- Use remote sensing pilots as the inlet line losses in this case were quite high (> 3% allowed by API-520) which was a contributor to the chattering that ultimately led to the PSV failure.
- Install adequately sized outlet piping to prevent high back-pressures. In this case the back-pressure exceeded the ANSI 150# limits.
- Develop procedures and train operators to make sure they cover and understand how to safely bring in a high pressure gas source to the system.