Pressure relief · Buyer's guide
A rupture disc opens once to provide a relief path. Specify the pressure difference across it at the temperature when relief is required, then check the disc, holder and discharge arrangement together. The marked pressure, its allowed burst band and the maximum operating pressure are different quantities.
01
BS EN ISO 4126-6:2014 covers application, selection and installation; BSI identifies ISO 4126-2 as covering manufacture, inspection, testing, marking, certification and packaging. The public scope also separates relief-system capacity from disc-assembly flow resistance [1].
02
The membrane, holder and any required vacuum support perform different functions. This is a functional component reference, not an interchangeable spare-parts BOM. A flat disc has no dome, and a vacuum support is not universal.
| Part | Function | Material and configuration boundary |
|---|---|---|
| Disc membrane / dome | Provides the pressure-sensitive opening element. | AV-ST is a flat 316 stainless-steel design; its optional FEP/PTFE liner is part of the specified configuration. “Dome” applies to formed designs, not every disc [2]. |
| Holder / safety head | Seats, supports and clamps the disc in the specified assembly. | Fike’s compatibility table assigns RD500 to Atlas/Atlas-LO holders; its datasheet lists stainless, nickel-alloy and carbon-steel options. Match the ordered holder, flange and disc rather than selecting by DN alone [8] [6]. |
| Vacuum support · if required | Supports an applicable disc against reverse differential pressure. | BS&B describes support requirements by construction and pressure duty. Do not add an improvised support, assume stainless steel universally, or confuse support with a vacuum-relief disc [9]. |
03
Two allowances sit around the pressure you ask for: the manufacturing design range and the burst tolerance.
ZOOK explains MDR as the agreed range in which a manufactured lot’s marked pressure may fall, citing UG-127(a)(1). It advises considering MDR against the vessel’s MAWP. Treat that as the manufacturer’s explanation of the concept; check the requirements and clause numbering in the code edition governing your equipment [4] [5].
For AV-ST, BS&B lists ±2 psig at marked pressures up to 40 psig, and ±5% above 40 psig. This tolerance belongs to that product; it is not a category-wide allowance [2].
Do not reorder a disc only from the burst pressure on its tag; quote the original lot number so the same specification is made [4].
Illustration using an AV-ST marked at 50 psig: ±5% gives a minimum of 47.5 psig; 60% of that minimum gives 28.5 psig. These are arithmetic results from BS&B’s published tolerance and ratio, not a selected pressure for your vessel. A requested pressure before manufacture is not necessarily the marked pressure used in this calculation [2].
04
Makers state the maximum operating pressure as a percentage, but on different bases. Compare them on the same basis.
| Disc (published data) | Operating ratio | Basis stated |
|---|---|---|
| BS&B AV-ST · flat sanitary disc | Operating pressure to 60 % [2] | Of the minimum burst pressure [2] |
| Fike RD500 Atlas / RD520 Axius · G2 web summary | 95 % [3] | Of marked burst above 40 psig (ASME); of minimum burst tolerance at or below 40 psig (ASME and EN ISO 4126-2) [3] |
| RD500 / RD520 · ISO basis in the same summary | 100 % [3] | Of minimum burst pressure above 40 psig under EN ISO 4126-2 [3] |
| Fike RD540 · April 2025 datasheet | 90% [7] | The table does not explicitly identify marked-versus-minimum basis. Obtain that definition before calculating an operating limit [7]. |
The denominator matters as much as the percentage. The G2 webpage uses “minimum burst tolerance” at the low-pressure threshold; ask for the actual minimum pressure value and units. Do not apply its family headline to a different model.
05
Specify these fields for the ordered assembly. AV-ST and RD500 examples are separate products; their figures must not be combined into one order.
| Field | Source-backed example or requirement |
|---|---|
| Burst-pressure range | AV-ST 3-inch / DN80 positive-pressure version: 5–100 psig at 22 °C. This is a manufacturing capability range, not an operating band [2]. |
| Burst tolerance | AV-ST: ±2 psig through 40 psig; ±5% above. State the units and marked-pressure basis [2]. |
| Operating temperature | Material and liner limits differ; obtain the burst rating at coincident temperature [9]. |
| Nominal diameter | 3-inch / DN80 is the AV-ST example above. Also identify the holder, connection and flange standard [2] [8]. |
| Material | Specify membrane, holder and liner separately; a stainless disc does not identify the holder alloy or seal [2] [6]. |
| Minimum burst pressure | A model’s lowest available rating is different from the minimum of the delivered lot’s burst band. Ask which the quoted value means [2] [4]. |
| Maximum burst pressure | Use the selected size/material capability and the equipment’s protection requirements, not the largest number in a family brochure [2] [4]. |
| Flow coefficient / resistance | RD500 publishes resistance factors KRG 0.65 and KRL 1.50; these must not be substituted for a discharge coefficient Kd. Obtain the applicable certified assembly data and sizing method [6] [1]. |
| Leak tightness | RD500 lists 10⁻⁴ atm·cc/s helium without the O-ring option and 10⁻⁶ with it; the option is limited to 1–4 inches. Preserve the test unit and configuration [6]. |
| Cycle duty | Provide the full pressure/temperature history. A published cycle count depends on the stated profile and is not a replacement calendar [7] [8]. |
| Weight | No disc/holder mass is established by the cited tables. Request the selected assembly drawing and handling mass. |
BS&B’s AV-ST webpage contains inconsistent unit pairs: 85 psig is paired with 4.48 barg in the 8-inch row, and a ±0.5 psig vacuum range with ±0.4 bar. Those pairs are not used here; obtain a corrected quotation for the affected versions. [2]
06
Disc construction decides how it copes with pressure cycles and vacuum.
The G2 webpage says the concave face is toward the process; BS&B’s explanation of reverse buckling describes the convex face toward pressure. That conflicting shorthand is not an installation instruction. Fike’s current manual requires the disc and holder flow arrows to align with process flow, and the specified disc/holder combination to match. Follow the supplied assembly drawing and manual for the actual device [3] [10] [8].
07
Fike’s November 2025 installation manual covers the listed reverse-acting disc and holder families. It does not supply a universal procedure for other manufacturers’ assemblies [8].
08
Ask for evidence that ties the disc to your lot and code.
09
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Product: rupture (bursting) disc with holder [yes/no], size [DN / inch], quantity [units] Equipment code and edition: [ ]; relief-device requirements [ASME Section XIII edition / EN ISO 4126-2 edition / other] Protected equipment: MAWP [barg/psig], normal operating pressure [ ], maximum operating pressure [ ] Required burst pressure: [ ] at coincident disc temperature [°C]; manufacturing range [zero / maker's standard] Service: [gas / liquid / two-phase], medium [ ], corrosive [yes/no] Pressure profile: [steady / cycling n per day / pulsating], vacuum on process side [yes/no], back pressure [ ] Type preference: [reverse-acting / forward-acting / flat], non-fragmenting [yes/no], liner [none / PTFE / FEP] Connection: holder type [flanged insert / union / sanitary clamp], flange standard [ ] Documents: lot burst certificate, operating ratio basis, certified flow resistance / relief area, model-specific installation and replacement guidance Delivery to [country], target date [date]
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Discs are made within a manufacturing design range agreed with the user, and the marked burst pressure falls inside it; the range must be considered so the stamped pressure does not exceed the vessel's MAWP. [4]
BS&B’s AV-ST publishes ±2 psig through 40 psig and ±5% above that. Other models have different tolerances; obtain the ordered model’s table and temperature basis. [2]
Only with a disc rated for it and on the same basis. Published ratios range from 60 % of minimum burst on a flat low-pressure disc to 95 % of marked burst on reverse-acting designs. [2] [3]
EN ISO 4126-6:2014 covers application, selection and installation, including a purchaser checklist and replacement-period guidance; ISO 4126-2 covers manufacture, testing and marking. [1]
Check the exact model and burst rating. For example, the April 2025 RD540 sheet lists 10 psig reverse-pressure resistance below a 5 psig burst rating, and full vacuum at or above 5 psig. Do not infer vacuum resistance from a reverse-acting family label. [7]
Sources checked 2026-10-09. Standards bodies and regulations define the requirements; manufacturer pages show published ranges for those models only. CNFX does not certify products or suppliers.
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