Technical Regulations and Scope of API RP 520
This recommended practice establishes engineering standards for the selection, design, and installation of pressure relief systems for oil refineries and related industrial facilities. The regulation applies to equipment operating at a design pressure of 15 psi (103 kPa) and above. The primary function of these devices is to protect unfired pressure vessels and auxiliary systems from emergency pressure increases caused by abnormal operating conditions or thermal exposure during a fire.
The document contains fundamental definitions, technical specifications for operating parameters, and calculation methodologies for flow capacity for Newtonian fluids under steady-state flow.
It is important to note: pressure relief devices provide protection exclusively against overpressure. They do not guarantee the structural integrity of equipment under critical heating (e.g., in a fire zone). For addressing heat exposure limitation and pressure relief, it is recommended to refer to the API 521 standard.
The scope of this standard does not include low-pressure tanks and atmospheric vessels regulated by API 2000, nor does it cover transport containers and specialized vessels for cargo transportation. Requirements for the protection of boilers and steam generators are governed by ASME (Section I) and ASME B31.1 and are also outside the scope of this document. EV Group China experts recommend strictly following these specifications when selecting equipment to ensure industrial safety.
Terminology and Device Classification
The definitions provided below are based on design and operational standards for pressure relief systems adopted in modern engineering practice.
Typology of Pressure Relief Devices
Pressure Relief Device — an automatic mechanism actuated by the static pressure of the working medium at the inlet. It is designed to prevent exceeding the design pressure within the system by forcibly discharging the working medium. It may include vacuum protection functions.
Pressure Relief Valve — a specialized device that provides automatic opening upon reaching a set point, relief of excess medium, and leak-tight closing after the system’s operating parameters are restored.
Safety Valve — a spring-loaded direct-acting mechanism that opens proportionally to pressure increase. It is optimal for working with incompressible media (liquids).
- Safety Valve: a device with a quick-acting mechanism (“pop” action), intended primarily for working with compressible media (gases, steam).
- Combined Safety Valve: a universal solution applicable for both liquid and gas media depending on operating conditions.
- Conventional Pressure Relief Valve: a standard spring-loaded design whose performance characteristics depend on the magnitude of backpressure in the discharge nozzle.
- Balanced Pressure Relief Valve: a design equipped with a bellows or other element to compensate for the influence of backpressure on actuation accuracy.
- Pilot-Operated Pressure Relief Valve: a high-precision system consisting of a main valve and an auxiliary pilot device that manages the relief process.