API 520/526

Overview

API 520 and API 526 define the requirements for pressure relief devices used to protect pressure vessels and piping systems from overpressure conditions. API 520 focuses on sizing and selection, while API 526 standardizes valve dimensions and orifice sizes.

Standard: API 520 / API 526

Function: Overpressure protection

Application: Pressure vessels & piping

Industry: Oil & Gas, Chemical

Key Features

Defines sizing methodology for relief valves
Standardizes valve orifice areas and dimensions
Ensures safety in overpressure scenarios
Applicable to gas, vapor, steam, and liquid service
Supports multiple valve types (conventional, balanced, pilot)

API 520 — Sizing & Selection

Part I: Sizing for gas, vapor, steam, and liquid flow
Relief valve orifice area calculation based on flow rate
Defines relieving scenarios (fire, blocked outlet, etc.)
Back-pressure correction factors included

API 526 — Valve Design

Standard orifice sizes from D to T
Orifice area range: 0.110 to 26.0 in²
Standard flange pressure and temperature ratings
Interchangeability between manufacturers

Relieving Scenarios

Fire exposure of pressure vessel
Blocked outlet or downstream isolation
Control valve failure
Thermal expansion of trapped liquid
External heat input or reaction runaway

Types of Relief Valves

Conventional spring-loaded valves
Balanced bellows valves
Pilot-operated relief valves

Applications

Pressure vessels and storage tanks
Process piping systems
Refineries and petrochemical plants
Steam and utility systems

Safety Importance

Proper sizing and selection of pressure relief devices is essential to prevent catastrophic failure of pressure systems. API 520 and API 526 ensure reliable overpressure protection, safeguarding personnel, equipment, and the environment.