• Creation of Worst Credible Design Scenario – The WCDS is based on events that may occur during a process upset or emergency. Events that are commonly considered are:
  • Fire exposure – Pressure vessels exposed to fire may rupture with catastrophic results unless the vessels are relieved properly. National and international standards provide guidance for the minimum relief rate required.
  • Regulator, control valve failure – A regulator or control valve may move to full open position because of a hardware or software problem.
  • Deadheaded pump – The zero-flow pressure from a pump is usually much higher that the outlet pressure of that pump at the design point.
  • Thermal expansion – Pressure in a closed-in vessel or run of piping filled with liquid may rise above the Maximum Allowable Working Pressure (MAWP) of the equipment because of process heating, heat tracing, or ambient temperature changes.
• Selection of Relief Device – A variety of devices can be used for process relief, sometimes in combination. The most common devices are:
  • Relief valve – Relief valves are the most common devices for process safety venting because they will reclose after the process pressure upset has passed. They may be direct spring operated or pilot operated.
  • Rupture disk – These relatively low cost, non-reclosing devices are designed to vent a process by fracturing or tearing open. Polymer covered or high alloy rupture disks are often placed at the inlet of a relief valve so that steel or another low cost material may be used for the relief valve.
  • Rupture pin assembly – Rupture pin or bending pin devices are another non-reclosing relief device. They can be reset without a line opening unlike rupture disks.
  • Fusible plug – Fusible plugs are non-reclosing relief devices that are sensible to temperature upsets only. A low melting temperature alloy in a metal plug will vent the protected equipment if the plug is exposed to fire or another source of high temperature.
• Calculation of relief device inlet and outlet losses – Updates to relief device standards require that the pressure losses be calculated at the rated capacity of the device, not the required capacity of the relief scenario. As a result, many older relief device installations must be checked and updated.