Superimposed backpressure is the pressure at the outlet of the PSV before it opens i.e.
Piping should be free-draining to vessel.If two or more relief devices are on the same line, the common inlet piping internal cross sectional area must be equal to or greater than the combined inlet area of the individual relief devices.Gate valves must be configured such that the stem is in the horizontal position or upside down to prevent the possibility of the gate becoming detached and blocking the relief path.Gravity acts on (i) the disc and seat while closing, allowing potential issues with re-seating and (ii) the stem causing extra drag and wear. API 520 Part II Section 4 provides more details. If inlet line losses are greater than 3%, consideration should given to remote sensing pilot operated relief valves. High inlet pressure drop may result in (i) chattering of the PSV leading to premature failure (ii) reduced relieving capacity leading to higher accumulated pressure. Inlet line pressure drop or non-recoverable losses should be limited to 3% of the set pressure for all overpressure contingencies.When designing inlet piping to a relief valve, the following should be considered: Inlet Installation and Pressure Drop Considerations.Overpressure is the same as accumulated pressure (accumulation) only when the relieving device is set to open at the MAWP of the vessel. Overpressure is the pressure increase over the set pressure of the relieving device. Set Pressure is the pressure at which the pressure-relief device is set to open under service conditions. It should be noted that the MAAP is not permitted to exceed the test pressure of the pressure vessel. 21% of MAWP for external fire exposure contingency.Greater of 16% of MAWP or 4psi for process contingency with multiple relief device protecting.Greater of 10% of MAWP or 3psi for process contingency with a single relief device protecting.ASME Section VIII states the following criteria for MAAP:
This relieving event is also called an overpressure contingency. The maximum allowable accumulated pressure (MAAP) is the pressure increase over the MAWP allowed only during a relieving event. MAWP is typically the basis for the pressure setting of relief devices that protect the vessel. The MAWP is based on the nominal plate thickness used in fabrication it is always equal to or greater than the desired design pressure. The maximum allowable working pressure (MAWP) is the maximum gauge pressure rating stamped on an ASME pressure vessel after it is fabricated.
Cold Temperature Metal Embrittlement (CTME) on piping at PSV dischargeĪSME Section VIII is the industry code that governs the design and fabrication of pressure vessels.Acoustic Induced Vibration on piping at PSV discharge.Outlet installation and backpressure considerations.Inlet installation and pressure drop considerations.In determining the most suitable PSV for an application consideration must be given to the following: Conventional – A spring-loaded pressure-relief valve whose operational characteristics are directly affected by changes in the backpressure.Balanced-bellows – A spring-loaded pressure-relief valve that incorporates a bellows or other means for minimizing the effect of backpressure on the operational characteristics of the valve.Pilot-operated – Pressure-relief valve in which the major relieving device or main valve is combined with and controlled by a self-actuated auxiliary pressure-relief valve (pilot).API 520 Part I defines these PSVs as follows: The most commonly used PSV types are Conventional, Balanced-Bellows and Pilot-Operated. This article provides an introduction to common considerations for PSV selection and operation. PSV (Pressure Safety Valves) or PRV (Pressure Relief Valve) may never be activated, however they must be designed and maintained to function correctly on demand.