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Greyson Nguyen
Greyson Nguyen

What You Need to Know About API 520 and Its Applications

What is API 520?

If you work in the oil and gas industry, you may have heard of API 520. But what is it exactly, and why should you care?

Api 520.pdf

API 520 is a standard published by the American Petroleum Institute (API) that covers the sizing, selection, and installation of pressure-relieving devices (PRDs) in refineries, chemical facilities, and related industries. PRDs are devices that protect equipment from overpressure by releasing excess fluid when the pressure exceeds a predetermined limit. PRDs include pressure-relief valves (PRVs), rupture disk devices (RDDs), pin-actuated devices (PADs), and combinations thereof.

API 520 consists of two parts:

  • Part ISizing and Selection: This part provides methods and criteria for sizing and selecting PRDs based on steady state flow of Newtonian fluids.

  • Part IIInstallation: This part provides methods and considerations for installing PRDs for equipment that has a maximum allowable working pressure (MAWP) of 15 psig (103 kPag) or greater.

API 520 applies to unfired pressure vessels and related equipment that are exposed to operating and fire contingencies. It does not apply to atmospheric and low-pressure storage tanks, pressure vessels used for transportation, or fired vessels.

Why is API 520 important?

API 520 is important because it helps ensure the safety and reliability of pressure equipment in various industries. Overpressure can cause serious damage to equipment, property, environment, and human lives. By following API 520, you can prevent overpressure incidents, reduce operational risks, comply with regulatory requirements, and optimize your plant performance.

Some of the benefits and applications of API 520 are:

  • It provides a comprehensive and consistent approach for sizing, selecting, and installing PRDs.

  • It helps you choose the most suitable and cost-effective PRD for your specific application and service conditions.

  • It helps you avoid PRD instability, backpressure, and discharge piping issues that can affect the PRD performance and safety.

  • It helps you design and fabricate PRDs according to the relevant standards and codes.

  • It helps you inspect and test PRDs according to the appropriate procedures and frequencies.

  • It helps you maintain and repair PRDs according to the best practices and guidelines.

How to use API 520?

Sizing and selection of pressure-relieving devices

The first step in using API 520 is to size and select the appropriate PRD for your equipment. This involves determining the required relieving capacity, set pressure, overpressure, and blowdown of the PRD, as well as selecting the type, size, material, and trim of the PRD.

API 520 provides sizing equations and tables for various types of PRDs and fluids. The equations are based on the conservation of mass, energy, and momentum principles, and the tables are based on empirical data and correlations. You can use these equations and tables to calculate the required relieving capacity of the PRD, which is the minimum flow rate of fluid that the PRD must be able to discharge at a specified pressure.

API 520 also provides selection criteria and factors for different types of PRDs and fluids. These criteria and factors include the set pressure, which is the pressure at which the PRD starts to open; the overpressure, which is the pressure increase above the set pressure that is allowed during discharge; the blowdown, which is the pressure decrease below the set pressure that is required for the PRD to reseat; and the type, size, material, and trim of the PRD, which depend on the fluid characteristics, service conditions, and design specifications.

Installation of pressure-relieving devices

The second step in using API 520 is to install the selected PRD on your equipment. This involves determining the location, orientation, connection, isolation, discharge piping, and accessories of the PRD.

API 520 provides installation methods and considerations for various types of PRDs and fluids. The methods and considerations include the location, which is the position of the PRD on the equipment or piping; the orientation, which is the direction of the PRD inlet and outlet; the connection, which is the type and size of the flange or nozzle that connects the PRD to the equipment or piping; the isolation, which is the use of valves or other devices to isolate the PRD from the equipment or piping; the discharge piping, which is the pipe that carries the fluid from the PRD outlet to a safe disposal area; and the accessories, which are additional devices that enhance or monitor the PRD function.

What are the types of pressure-relieving devices?

Pressure-relief valves (PRVs)

A pressure-relief valve (PRV) is a type of PRD that opens automatically when a predetermined pressure is reached. It consists of a valve body, a valve seat, a valve disk or element, a spring or other actuator mechanism, a bonnet or cover, an inlet connection, an outlet connection, and optionally a lifting device or lever. A PRV can be used for compressible or incompressible fluids.

There are different types of PRVs based on their operational characteristics and applications. Some of them are:

  • Safety relief valve (SRV): A type of PRV that can be used for gas or liquid service. It has a rapid opening action that pops open when a certain overpressure is reached. It has a huddling chamber that creates a differential pressure across the disk to assist in opening and closing.

  • Relief valve (RV): A type of PRV that can be used for liquid service only. It has a gradual opening action that opens proportionally to the increase in pressure. It does not have a huddling chamber.

  • Pilot-operated relief valve (PORV): A type of PRV that uses a pilot valve to control the opening and closing of a main valve. It can be used for gas or liquid service. It has a modulating action that adjusts to maintain a constant pressure at its inlet. It can achieve higher set pressures and lower blowdowns than conventional SRVs or RVs.

  • Vacuum relief valve (VRV): A type of PRV that opens automatically when a predetermined vacuum is reached. It prevents excessive negative pressure in a vessel or system. It can be used for gas or liquid service.

Rupture disk devices (RDDs)



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