Rupture disc standards: ASME VIII, ISO 4126 and API 520

Short answer: rupture discs are designed, tested and certified mainly under the ASME Code (Section VIII for vessel protection and, since the 2021 edition, Section XIII for the devices themselves) and under the ISO 4126 standard (parts 2, 3 and 6). API 520 and API 521 are the recommended practices for sizing and installing relief systems in refining and petrochemicals. In Mexico, NOM-020-STPS-2011 regulates the safe operation of pressure vessels in workplaces.

By the P&V Control Devices technical team · Updated October 4, 2026 · Reading time: 9 min

Quick standards map

ASME Section VIII and Section XIII

Direct answer: Section VIII states when and within what limits a vessel must be protected; Section XIII states how the relief device must be manufactured, tested and marked to be accepted.

What Section VIII establishes

• Every vessel within its scope must have overpressure protection.
• Accumulation limits above the MAWP: 10 % with a single device, 16 % with multiple devices and 21 % in an external fire scenario (with absolute minimums for low pressures).
• Installation rules for the disc alone, for the disc upstream of a valve (including monitoring of the space between the two) and for the disc at a valve outlet.
• Up to the 2019 edition, these requirements were in paragraphs UG-125 to UG-140 (the rupture disc in UG-127). Since the 2021 edition they were moved to UG-150 to UG-156, which refer to Section XIII. Many documents and data sheets in plants still cite the old numbering.

What Section XIII establishes

• Burst tests per batch: from each manufactured batch, sample discs are burst at the coincident temperature to verify the marked pressure.
• Burst tolerance: the classic reference is ±2 psi up to 40 psig and ±5 % above; Section XIII also allows performance to be expressed through specific tolerances declared by the manufacturer.
• Capacity certification: the flow resistance factor (K_R) or the discharge coefficient of the model is certified through testing, and the certified values are published by the National Board.
• Certification mark: discs manufactured under the code carry the ASME mark with the UD designator.

ISO 4126-2, 4126-3 and 4126-6

Direct answer: the ISO 4126 series deals with safety devices for protection against excessive pressure. Part 2 is specific to rupture discs; Part 3 covers their combination with safety valves and Part 6 explains how to apply, select and install them.

• ISO 4126-2 defines the terms (specified burst pressure, performance range, burst temperature), the type and batch tests, and the marking of the disc and disc holder.
• ISO 4126-3 establishes how to determine the capacity of a disc + valve combination, either by testing or with a combination factor.
• ISO 4126-6 is the practical guide: location, operating ratio, effect of temperature and back pressure, and installation and replacement recommendations.

In the European Union, rupture discs are safety accessories under the Pressure Equipment Directive (PED 2014/68/EU), and conformity with EN ISO 4126-2 is the usual way to demonstrate it. For a project in Mexico that exports equipment to Europe, it is advisable to specify both frameworks from the start.

API 520 and API 521

Short answer: API does not certify rupture discs; it publishes recommended practices that the oil and gas industry uses to calculate how much must be relieved and how to install the devices.

• API 520 Part I covers sizing and selection: capacity equations for gas, vapor, liquid and two-phase flow; the flow resistance method for discs (K_R); the discharge coefficient of 0.62 where the "8 and 5 diameters" rule applies; and the combination factor of 0.9 for an uncertified disc + valve pairing.
• API 520 Part II covers installation: inlet and outlet piping, pressure drops, supports, reaction forces and block valves.
• API 521 helps identify overpressure scenarios (fire, utility failure, blocked outlet, exchanger tube rupture, reactions) and design discharge systems and flares.

The framework in Mexico: NOM-020-STPS-2011

NOM-020-STPS-2011, "Pressure vessels, cryogenic vessels and steam generators or boilers – Operation – Safety conditions", requires the employer to keep this equipment in safe condition, which includes having suitable pressure relief devices, keeping a file for each piece of equipment and applying inspection and maintenance programs. The standard does not replace design codes: in practice, the design of the equipment and its protection is documented under ASME or another recognized code, and NOM-020 regulates its operation.

For this reason, a pressure vessel file in Mexico normally contains the disc data sheet, its lot certificate, the reference to the design code and the replacement record. The maintenance and replacement guide explains which records are worth keeping.

Sanitary requirements

In food, beverage, pharmaceutical and biotechnology applications, in addition to the relief standard, hygienic design criteria apply, such as those of 3-A Sanitary Standards and ASME BPE: surface finishes, absence of crevices, materials and elastomers suitable for product contact, and clamp-type connections that are cleaned in place.

What the disc tag must state

Identification is the documentary proof that the installed disc is the specified one. It normally includes:

• Manufacturer and model.
• Nominal size.
• Marked burst pressure and coincident temperature.
• Disc material (and seal or support material, if applicable).
• Lot number.
• Certified capacity or K_R factor, depending on the code.
• Flow direction.
• Certification mark (for example, ASME UD) where applicable.

If the nameplate is missing or illegible, the disc should not be installed.

Frequently asked questions

Are ASME and ISO equivalent?

They pursue the same goal but are not interchangeable: the terms, tolerances, and tests differ. The one required by the vessel design code or the end customer is specified.

What about the UG-127 numbering on my old datasheet?

It remains valid for equipment designed under editions prior to 2021. For new equipment, the correct reference is UG-150 to UG-156 and Section XIII.

Is API 520 mandatory?

It is a recommended practice, not a law. It becomes mandatory when the customer, the process license or the contract requires it, which is common in refining and petrochemicals.

Note: this guide summarizes standards for educational purposes. For design and compliance, always consult the current edition of the original document.

Related guides: How to select a disc · Disc vs valve · Glossary