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The Sizing of Pressure Control and Pressure Safety Valves Including Reaction Force Calculation

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The Sizing of Pressure Control and Pressure Safety Valves Including Reaction Force Calculation

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Pressure control valves (PCVs), pressure regulating valves and pressure safety valves (PSVs) are an important part of any plant design in the oil and gas industry. Gas products are typically transported at very high pressures to reduce pumping costs and reduce line sizes. Pressure control valves (and the subset of pressure regulating valves) are then used to reduce the pressures to the required levels at the point of consumption. However, if a pressure control valve fails, the plant design must make provision for safety systems to prevent catastrophic events from taking place. The proper selection and installation of pressure safety valves is one option. Alternatively, two pressure control valves may be installed in series in a so called active-monitor arrangement. Furthermore, a slam-shut valve may be installed that is able to shut down the plant in a short period of time.

This case study demonstrates the combination of pressure control and pressure safety valves where the latter is used as the means of ensuring safety should the former fail. The use of PSVs are commonplace in most oil and gas plants, in fact, all international design codes and standards in the oil and gas industry mandate their use at any position in the plant where pressures may exceed design pressures. Pressure increases beyond design values may occur due to mainly three causes:

  1. Process failure – a situation where faulty equipment is no longer capable of controlling the pressure to acceptable limits. This may include a failed control valve, a blocked outlet, a tube rupture, a loss of utility such as cooling medium or power, gas blow-by etc.;
  2. Locked-in thermal expansion – a situation where a vessel or length of pressure piping may be locked-in upstream and downstream and a resident heat source then causes the internal pressure to rise beyond design limits; and
  3. External fire relief – a situation where an external fire may add heat to a vessel or pressure piping, resulting in a similar scenario to the locked-in thermal expansion case.

A side-effect of the installation of a PSV is the resulting reaction force that may be created when the PSV opens. Since most PSVs “pop” open rather quickly, very high reaction forces may result and must be checked by the design engineer.

This case study demonstrates how Flownex® has been used to size a PCV as well as a matching full-flow PSV and easily calculate the resulting reaction forces. Checks for design code compliance are also performed.

The full case study discussion document is available on the Flownex® web page in the Case Studies section:

http://flownex.com/information/case-studies