The original rupture disk (bursting disc) manufacturer shaping the growth of pressure relief devices for over 80 years.



The Sigma EXL™ Reverse Buckling Rupture Disk (Bursting Disc) has been developed to provide the highest operating pressure capability available from a Rupture Disk (Bursting Disc) pressure relief device. Sigma EXL™ Rupture Disk (Bursting Disc) technology and the SRB-7RS™ Safety Head combine the accuracy and reliability of Reverse Buckling Disk technology with unique disk and safety head design features that enable the Sigma EXL™ disk to operate to 95% of the Marked Burst Pressure (100% of Minimum Burst Pressure according to CEN ISO 4126-2 standards). The Sigma EXL™ Rupture Disk (Bursting Disc) embraces SafT technology, Structural Apex Forming, the central feature on the disk dome enhancing burst pressure accuracy.

At its burst pressure, the Sigma EXL™ Rupture Disk (Bursting Disc) dome reverses and opens by shearing around a circular score line located near the perimeter of the dome. The score line engages with one or more points on the integral hinge down stream of the disk.

The disk opens around its circular score line with the central petal supported by the hinge, avoiding fragmentation.The Sigma EXL™ Rupture Disk (Bursting Disc) is offered with a simple burst pressure tolerance.

No Manufacturing Design Range needs to be considered for this high Operating Ratio disk (effectively a 'zero' range disk).

The performance of every Sigma EXL™ disk is assured by proof pressure testing each disk to its maximum recommended operating pressure. The Sigma EXL™ Rupture Disk (Bursting Disc) provides the user with the highest Operating Ratio of any Rupture Disk (Bursting Disc) pressure relief device.

For users who wish to obtain the enhanced quality assurance testing of this disk design, and can accept more traditional Reverse Buckling Disk accuracy, the Sigma™ Rupture Disk is offered with a -5% Manufacturing Design Range.

Sigma EXL™ and Sigma™ Disk Features

  • Reverse Buckling Disk in Sizes 1"-8" (25-200 mm)
  • High Operating Ratio: 95% of marked burst pressure
  • High Operating Ratio: 100% of minimum burst pressure (CEN ISO4126-2)
  • Standard 0% Manufacturing Design Range, optional -5%
  • Designed for non-fragmentation
  • Designed for Gas, Liquid, or two-phase flow conditions
  • Fail Safe: Damage- Safety Ratio <1
  • Saf™ technology
  • Vacuum resistant
  • Smooth process side of disk resists product accumulation
  • Long service life in pressure cycling or pulsating conditions
  • US Patent No.5167337 and worldwide patents pending
  • For installation in BS&B type SRB-7RS™ Pretorqued Safety Head

Sigma EXL™ & Sigma™ Disk Specificaton

Min/Max Pressure Rating at 72ºF (22ºC) PSIG (Bar)

Disk Size
Disk Material
Alloy 200
316SS Alloy 600 
Alloy 400
Hastelloy Alloy
Min Max Min Max Min Max Min Max Min Max
Psig Barg Psig Barg Psig Barg Psig Barg Psig Barg Psig Barg Psig Barg Psig Barg Psig Barg Psig Barg
1 25 35 2.4 500 34.5 35 2.4 500 34.5 50 3.5 500 34.5 58 3.5 500 34.5 55 3.8 500 34.5
1.5 40 30 2.1 400 27.5 30 2.1 400 27.5 45 3.1 400 27.5 45 3.1 400 27.5 45 3.1 400 27.5
2 50 25 1.8 400 27.5 25 1.8 400 27.5 30 2.1 400 27.5 30 2.1 400 27.5 30 2.1 400 27.5
3 80 20 1.4 400 27.5 20 1.4 400 27.5 22 1.5 400 27.5 22 1.5 400 27.5 25 1.8 400 27.5
4 100 16 1.1 400 27.5 16 1.1 400 27.5 18 1.3 400 27.5 18 1.3 400 27.5 20 1.4 400 27.5
6 150 15 1.0 225 15.5 15 1.0 225 15.5 15 1.0 225 15.5 15 1.0 225 15.5 20 1.4 225 15.5
8 200 15 1.0 125 8.6 15 1.0 125 8.6 15 1.0 125 8.6 15 1.0 125 8.6 20 1.4 125 8.6

Manufacturing Design Range
Manufacturing Design Range (MDR) is a range of pressure, always applied to the minus side of the user's requested burst pressure , that simplifies disk fabrication testing and provides an economic benefit to the user where such an added tolerance can be accommodated by the application.  The Sigma EXL™ Rupture Disk (Bursting Disc) has a standard "0%" MDR, and the Sigma™ Rupture Disk (Bursting Disc) less a "-5%" range.

Burst Tolerance
Burst tolerance is the range of pressure over which a rupture disk (bursting disc) is expected to burst, expressed as a "+/-".  Both Sigma EXL™ and Sigma™ disks have the same burst tolerance.

Marked Burst Pressure Burst Tolerance
Less or equal to 40 psig (2.76 barg)
above 40 psig (2.76 barg)
+/- 2 psig (0.138 barg)
+/- 5%

Damage Safety Ratio < 1
If a Sigma EXL™ or Sigma™ Rupture Disk (Bursting Disc) is accidentally damaged, it will relieve pressure by bursting at or typically below its marked burst pressure. This failsafe design feature is called the Damage Safety Ratio, and with a value of 1 or less, will ensure that the Sigma EXL™ or Sigma™ Disk will, if damaged, relieve pressure at a reduced burst pressure. A damaged Sigma EXL™ or Sigma™ Rupture Disk (Bursting Disc) will typically reverse and then open at a reduced pressure.

Operating Ratio
Sigma EXL™ Reverse Buckling Disks can sustain operating pressures to 95% of their marked burst pressure, or to 100% of their minimum burst pressure. (For disks certified under CEN ISO 4126-2 'Performance Tolerance', the Sigma EXL™ disk can sustain operating pressures to 100% of the burst pressure less the negative performance tolerance.) Sigma™ Reverse Buckling Disks having a -5% Manufacturing Design Range may also sustain   operating pressures to 95% of their marked burst pressure, or to 100% of their minimum burst pressure. However, this operating pressure may be lower than for a Sigma EXL™ disk by as much as the -5% MDR.

The Sigma EXL™ and Sigma™ Rupture Disks are available in a range of corrosion resistant materials (see table). For each material the upper temperature limit has been determined from the recommendations  of material manufacturers and user experience. While Hastelloy C-276 is the standard grade used, Hastelloy B and Hastelloy C-22 may be available upon request.

Fluorocarbon film liners are available as an additional corrosion barrier attached to the process side of  the  Rupture Disk (Bursting Disc). FEP is the standard liner material. PFA and PTFE may also be available.  See table for maximum temperature recommendations. Fluoropolymer materials are typically used.

Maximum Recommended Temperature

Nickel Alloy 200 750°F 399°C
Monel Alloy 400 800°F 427°C
Inconel Alloy 600 1100°F 593°C
316 Stainless Steel 900°F 482°C
Hastelloy Alloy C-276 900°F 482°C
Fluorocarbon Liner (FEP) 400°F 204°C
Fluorocarbon Liner (PFA) 400°F 204°C
Fluorocarbon Liner (PTFE) 500°F 260°C
Hastelloy ® is a trademark of Haynes International Inc., Monel® and Inconel® are trademarks of Inco Alloys International.BS&B may use equivalent materials from other sources

Testing at Temperature
The burst pressure of each lot of Sigma EXL™ or Sigma™ Rupture Disks is tested at the users requested temperature. If the disk is requested above or below ambient temperature, burst testing for product certification will be carried out at this temperature to ensure the best product accuracy. For applications with operating temperatures that may exceed this tested burst temperature, please review with BS&B Safety Systems, Inc. or BS&B Safety Systems Ltd.

Flow Performance - KR Values - Rupture Disk Sizing
Flow resistance factor 'KR' may be used to determine the relieving capacity of a system using calculations such as those described in Crane Technical Paper No. 410. Individual KR values have been established for both Gas and Liquid service for the Sigma EXL™ and Sigma™ disks. These are experimentally determined at the BS&B Safety Systems Inc. flow laboratory according to ASME procedures and witnessed by ASME representatives. Care must be taken to use the correct value for sizing purposes. When more than one 'KR' value is marked on the disk tag, 'KRG' is indicated for Gas service, and 'KRL' for liquid service. A single disk tag KR value is valid for gas service only. The KRg value is 0.38 and the KRI value is 1.25

When using KR value for sizing calculations, the nominal pipe size (assuming schedule 40 pipe) of the vent system is used. When using 'coefficient of discharge' sizing techniques, the Minimum Net Flow Area or Net Relief Area of the Rupture Disk (Bursting Disc) device is used.

Minimum Net Flow Area (MNFA) / Net Relief Area (NRA)

Disk Size
in  mm
1 25 1 1/2 40 2 50 3 80 4 100 6 150 8 200
in2 * 0.86 1.93 3.36 7.39 12.74 28.89 50.0
cm2 + 4.8 11.3 19.7 40.8 69.7 158.6 275.6

* Use MNFA in in2 when sizing according to ASME code, simple system case.
+ Use Net Relief Area cm2 when sizing according to European codes

The certification procedure for Sigma EXL™ and Sigma™  Rupture Disks exceeds those required by standard industry Codes and Standards that require limited destructive testing to validate burst pressure and permit acceptance of the lot with any burst test distribution and  uncentered test data.The Sigma EXL™ and Sigma™ disk's test data is graphically presented on their Burst Test Certificate (see picture). The curve includes 'in process' and final certification burst test data to build a clear picture of the burst pressure distribution. Only when the data is appropriately centered and distributed is the lot accepted for shipment. The Certificate  indicates also the burst test results of rupture disks (bursting discs) tested from the" lot "for Quality Department final acceptance - the quantity of test results will typically be determined by the certification Code or Standard chosen by the user.Sigma EXL™ and Sigma™ Rupture Disks are also validated for liquid service for each lot using a fully hydraulic burst test system and the Burst Test Certificate endorsed accordingly.

The combination of statistical control techniques for burst pressure, and proof pressure testing of all ordered Sigma EXL™ and Sigma™ Rupture Disks enables their application to the highest operating pressures available from the Rupture Disk (Bursting Disc) Industry.

Installation in SRB-7RS™ Safety Head

The Sigma EXL™ and Sigma™ disks are installed in an SRB-7RS™ Safety Head permitting the correct torque to be applied in the workshop before installation into the process. The asymmetric locating pins ensure centering and orientation of the disk within the Safety Head. The performance-proven metal-to-metal 'bite type' seal in the Safety Head (2"/50mm size and above) assures a leak tight assembly. When installed properly into the SRB-7RS™ Safety Head, the assembly can be removed, inspected, cleaned carefully then reinstalled without replacing the disk as the pretorque capscrews maintain the clamp load on the disk.

The SRB-7RS™ Safety Head is used with the other rupture disks (bursting discs) (Types SKR™, S90™, JRS™, RLS™, FRS™) of the STA-SAF®  System.

SRB-7RS™ Safety Head Specification

Nominal Size

Safety Head Flange Rating Safety Head Flange Thickness Dimensions
1 25 150 - - 1-1/2 38.0 2-5/8 66.7 2-5/8 66.7
1 25 300/600 10/16/25/40 10/16/20/30/40 1-1/2 38.0 2-7/8 73.0 2-7/8 73.0
1.5 40 150 - 10/16/20 1-5/8 43.0 3-3/8 85.7 3-3/8 85.7
1.5 40 300/600 10/16/25/40 30/40 1-5/8 43.0 3-3/4 95.2 3-3/4 95.2
2 50 150/300/600 10/16/25/40 10/16/20/30/40 1-3/4 47.5 4-1/8 104.8 4-1/8 104.8
3 80 150/300/600 10/16/25/40 16/20/30/40 2-1/8 55.0 5-1/4 133.3 5-1/4 133.3
3 80 - - 10 2-5/8 66.7 4-3/4 120.6 4-3/4 120.6
4 100 150/300 10/16/25/40 16/20/30/40 2-7/8 73.0 6-1/4 158.7 6-1/4 158.7
4 100 - - 10 2-3/4 69.9 Flower Petal
4 100 600 - - 2-5/8 66.7 7-5/8" OD/193.7 mm OD
6 150 150/300 10/16/25/40 10/30/40 3-11/16 93.7 Flower Petal
6 150 - - 16/20 4-1/5 106.7 Flower Petal
6 150 600 - - 3-1/8 79.4 10-3/8" OD/263.5 mm
8 200 150/300 - - 3-13/16 96.8 Flower Petal

US patent Nos 4,751,938, 5005722 and other international patents pending.

Products, specifications and all data in this literature are subject to change without notice.  Questions regarding product selection and specifications for specific applications should be directed to BS&B Safety Systems, Inc. or BS&B Safety Systems Ltd.

All sales are subject to BS&B Safety Systems, Inc. or BS&B Safety Systems Ltd. standard terms and conditions of sale.