How to select a rupture disc step by step

Short answer: to select a rupture disc, start from the maximum allowable working pressure of the equipment (MAWP) and the actual operating pressure and temperature. With that data you set the marked burst pressure and its tolerance, choose a disc type whose operating ratio allows the working pressure with margin, select a material compatible with the fluid at the disc temperature, and check that the flow area is sufficient for the most severe relief case.

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

The data you need before you start

Without this data it is not possible to select a disc safely. It is best to collect it on a datasheet before requesting a quote:

• MAWP (or design pressure) of the protected equipment and design code (ASME VIII, EN 13445, etc.).
• Normal and maximum operating pressure, and whether there is cycling, pulsation, or water hammer.
• Minimum pressure: can there be vacuum? Is there back pressure downstream?
• Operating temperature and the temperature the disc will have at the moment of bursting.
• Fluid, state (gas, vapor, liquid, or two-phase), and corrosive contaminants.
• Required relief flow rate and the scenario that defines it (fire, control failure, reaction, blocked outlet).
• Line size, flange type, and whether the disc will protect a pressure relief valve.

Step 1: define the burst pressure

Basic rule: in a typical installation with a single device, the marked burst pressure must not exceed the equipment's MAWP at the coincident temperature. Codes allow somewhat higher values when there are several relief devices or when the scenario is an external fire, because in those cases more pressure accumulation is permitted; those cases must be justified under the applicable code.

For reference, ASME Section VIII allows an overpressure (accumulation) of 10% over the MAWP with a single device, 16% with multiple devices and 21% in the case of external fire, with absolute minimums for low pressures. More detail in the guide to ASME VIII, ISO 4126 and API 520 standards.

Step 2: understand the tolerance

Short answer: burst tolerance is the permitted variation between the actual opening pressure and the marked pressure. Under ASME, the usual tolerance is ±2 psi for marked pressures up to 40 psig and ±5 % above 40 psig.

Some manufacturers also offer reduced or asymmetric tolerances (for example, downward only) so that the operating pressure can be brought closer. Recent ASME editions (Section XIII, since 2021) reorganized the requirements for relief devices, including how the burst range is expressed; for this reason you must state which edition of the code the disc is specified to. ISO 4126-2 defines its own performance tolerances, which are declared on the disc marking.

The key point: the disc must be able to open at any point within its tolerance without putting the equipment at risk, and the operating pressure must be calculated against the lower end.

Step 3: verify the operating ratio

The operating ratio is the quotient of the maximum operating pressure to the burst pressure (marked or minimum, depending on the manufacturer's definition). Each disc type has a limit:

If the working pressure is above that limit, the disc will suffer fatigue or creep and will burst early. The solutions are: change to a type with a higher operating ratio, request a tighter tolerance or, if the equipment allows it, increase the burst pressure. The differences between types are in types of rupture discs.

Step 4: specify the correct temperature

Direct answer: the burst pressure of a metal disc drops as temperature rises, because the strength of the metal decreases. That is why the disc is calibrated at a specific coincident temperature, and that temperature must be the one the disc will actually see, not the process temperature.

• If the disc is on a long, uninsulated nozzle, it may be considerably colder than the process; in that case it would open at a higher pressure than expected if specified at process temperature.
• If the process has cold starts and hot operation, both points must be analyzed.
• In composite discs, the maximum temperature is limited by the fluoropolymer seal; in graphite discs, by the impregnation resin.

Step 5: choose the material

The material must resist the fluid on both faces (process and discharge) at the disc temperature. A corrosion of microns, which would be irrelevant in a pipe, can change the burst pressure of a thin foil.

The table is a guide: final compatibility must be confirmed with corrosion data for the actual fluid, including traces and impurities. Material details are in the materials for rupture discs guide.

Step 6: size and relief capacity

Direct answer: the disc must pass the worst-case flow rate without the pressure exceeding the allowable accumulation. Two methods recognized by ASME and API 520 are used for sizing:

1. Discharge coefficient method (K_D = 0.62): applies only if the disc discharges directly to atmosphere, is installed no more than 8 pipe diameters from the vessel, and the discharge line does not exceed 5 diameters (the so-called "8 and 5" rule).
2. Flow resistance method (K_R): the disc is treated as just another piping component, with its certified resistance factor, and the pressure drop of the entire relief system is calculated. It is the general method when the previous rule is not met.

The equation and a numerical example are in relief area calculation according to API 520. The disc is often selected at the same size as the line; even so, the capacity must be verified. If the disc protects a valve, the valve defines the size and a combination factor is applied (see disc vs pressure relief valve).

Step 7: special conditions

• Vacuum or back pressure: they require a vacuum support or a design that withstands reverse pressure. In the case of back pressure, remember that the disc responds to the pressure difference.
• Liquid service: confirm that the model is certified for liquid; some reverse-acting discs need gas under the dome.
• Cycles and pulsations: favor reverse-acting discs and a conservative operating ratio.
• Disc upstream of a valve: it must be non-fragmenting, and the space between the two needs a pressure gauge, indicator or vent.
• Sanitary: clamp connections, polished finishes, and materials and elastomers approved for product contact.
• Detection: if the process requires knowing when the disc has opened, add a burst sensor.

Worked example (indicative)

A vessel with an MAWP of 10 barg at 150 °C operates continuously at 8 barg with frequent cycling. The disc will be on an insulated nozzle, so its temperature will be similar to that of the process.

1. Marked burst pressure: 10 barg at 150 °C (does not exceed the MAWP).
2. With a tolerance of ±5 %, the disc may open between 9.5 and 10.5 barg.
3. Operating ratio against the lower limit: 8 / 9.5 = 84 %.
4. A solid forward-acting disc (≈ 70 %) is not suitable. A reverse-acting disc (up to ≈ 90 %) is, and it also tolerates cycling.
5. The material is chosen according to the fluid and the capacity is verified with the applicable method.

The example illustrates the reasoning; actual selection must be done with the manufacturer's code, edition and data.

Common mistakes

• Specify the process temperature when the disc is much colder.
• Calculate the operating ratio against the marked pressure and not against the lower limit of the tolerance.
• Forgetting vacuum during steam cleaning or cooling.
• Using a fragmenting disc in front of a valve.
• Choosing the material based only on the main fluid, ignoring traces of chlorides or moisture.

Related guides: Disc types · Standards · Glossary