Pressure Relief System Design

A:

Proper sizing ensures the valve can relieve the full rate of overpressure without the vessel exceeding its allowable accumulation.

Sizing is based on:

• Type of overpressure scenario: blocked discharge, fire exposure, thermal expansion, or control valve failure.

• Flow rate to be relieved: determined from the worst credible case.

• Fluid characteristics: gas, vapor, liquid, or two-phase flow.

• Relief conditions: pressure, temperature, compressibility, backpressure, and discharge piping design.

Key equations (from API 520 / API 521) determine orifice area using:

$$A = \frac{W}{C \cdot K_d \cdot P_1 \cdot K_b \cdot K_c}$$

A=C⋅Kd​⋅P1​⋅Kb​⋅Kc​W​

where:

• A = required discharge area
• W = mass flow rate
• C,

  $K_d$
  $K_b$
  $K_{\mathrm{c<span\; style="font-family:\; Georgia,\; \text{'}Times\; New\; Roman\text{'},\; \text{'}Bitstream\; Charter\text{'},\; Times,\; serif;\; font-size:\; 16px;">=\; coefficients\; accounting\; for\; fluid\; properties,\; discharge\; coefficient,\; backpressure,\; and\; combination\; effects</span>}}$

• P1​ = relieving pressure (set pressure + accumulation)

OSHA (Occupational Safety and Health Administration)

The Occupational Safety and Health Administration governs pressure relief systems primarily through its
Process Safety Management (PSM) standard —29 CFR 1910.119. This rule requires facilities handling highly hazardous chemicals to identify, design, and maintain pressure relief systems according to recognized and generally accepted good engineering practices (RAGAGEP). OSHA does not prescribe exact design equations but enforces compliance through documentation, inspection, and process hazard analyses. Pressure relief devices must be properly sized, tested, and maintained as part of each facility’s mechanical integrity and safe operating system. PSM applies broadly across chemical manufacturing, refining, and industrial process operations that exceed threshold chemical quantities.

API (American Petroleum Institute)

The American Petroleum Institute publishes the most widely used technical standards for designing, sizing, and installing pressure relief devices. The foundational codes are API 520 (sizing, selection, and installation), API 521 (pressure-relieving and depressuring systems), and API 526/527 (valve dimensions and seat tightness testing). These standards define calculation methods for valve orifice area, allowable accumulation, and considerations for fire and blocked outlet scenarios. API codes are considered industry RAGAGEP under OSHA PSM and are most applicable to refining, petrochemical, gas processing, and oil & gas facilities.

ASME (American Society of Mechanical Engineers)

The ASME Boiler and Pressure Vessel Code, particularly Section VIII, Division 1, establishes the mandatory design rules for pressure vessels, including requirements for overpressure protection and maximum allowable working pressure (MAWP). Related standards such as ASME B31.3 address process piping systems, ensuring each has relief provisions that prevent exceeding the MAWP.
These codes form the legal basis for vessel fabrication and certification throughout the United States and are often incorporated into
state boiler and pressure vessel laws. ASME standards apply broadly across chemical plants, manufacturing, energy, and industrial facilities where pressurized systems are present.

NFPA (National Fire Protection Association)

NFPA develops fire and explosion prevention codes that directly impact pressure relief system design, particularly in environments storing or processing flammable materials. NFPA 30 (Flammable and Combustible Liquids Code) and NFPA 58 (Liquefied Petroleum Gas Code) require emergency venting and relief devices for tanks and containers exposed to fire or overpressure. These standards guide relief sizing for fire scenarios and specify the types of vents suitable for flammable or combustible service. NFPA codes are most relevant to storage terminals, fuel distribution facilities, chemical warehouses, and process plants with combustible inventories.

Regional and Company-Specific Frameworks

Beyond national codes, state Departments of Environmental Quality (DEQ) and Departments of Natural Resources (DNR) may regulate secondary containment or emissions from pressure relief discharges, especially when the release involves hazardous substances. Companies also adopt their own internal engineering standards—based on API, ASME, and NFPA—defining relief valve inspection intervals, sizing methodologies, and materials of construction. These internal standards serve as RAGAGEP under OSHA PSM and form part of each facility’s pressure protection philosophy.