This guide for rupture disk selection and sizing is made available by OSECO to assist those who are faced with this sometimes confusing task. Purposes and the historical background of rupture disks are reviewed. The different types of disks manufactured by OSECO are compared. Detailed guidelines for selection and sizing are then given. Useful related data are also included.

PURPOSES OF RUPTURE DISKS

A rupture disk is a sensitive relief device designed to rupture at a pre-determined pressure and temperature. It is a means of providing protection for personnel and equipment. As such, it must be a fail-safe device. Rupture disks are used where instantaneous and full opening of a pressure relief device is required. These devices are also utilized where "zero" leak-age is required of a relief device. These devices can also be used in series as "quick opening" valves.

Rupture disks may be used either in primary relief, in secondary relief, in series with a relief valve, or for other functions like "quick opening" valves.

PRIMARY RELIEF
If used for primary relief, the rupture disk is the only device utilized for pressure relief. As such, it has the advantages of being leak tight, an instantaneous response time, minimum pressure drop, lowest cost, very high re-liability, and minimum maintenance. It has the disadvantage that it must be replaced after each rupture occurrence, and allows venting until system pressure equals downstream pressure.

SECONDARY RELIEF
When used in a secondary relief capacity, the rupture disk provides a backup vent to a primary relief device, usually a relief valve. Its purpose here is usually to provide additional protection against an unlikely but possible major event that would exceed the capacity of the primary relief device.

IN SERIES WITH RELIEF VALVE
When used in series with a pressure relief valve, the rupture disk is usually installed upstream of the valve. The disk will protect the valve from process media that can corrode or plug it. The disk can also act as a seal, preventing any leakage through the valve unless the disk is ruptured. A table located elsewhere in this section lists the combination capacity factors established to date by OSECO.

The space between the rupture disk and the pressure relief valve must have a pressure gauge, try cock, free vent, or suitable telltale indicator. The normal configuration is an excess flow valve in combination with a pressure gauge. This arrangement is to eliminate the possibility of, or facilitate the detection of, a backpressure build up. Because a disk responds to the differential pressure across it, it will not burst at its rated pressure if a back pressure is allowed to exist in this cavity.

A low-pressure rupture disk, such as the FLCO, can be used on the downstream side of a relief valve that discharges into a common manifold to prevent exposure of the valve to process or corrosive media discharging through the common manifold. The space between the relief valve outlet and the rupture disk must be vented to prevent the accumulation of pressure, which could adversely affect the relief valve set pressure. An excess flow valve will suffice for this feature.

OTHER FUNCTIONS
Due to the small inertia characteristics of a rupture disk, the opening time, i.e., from a closed and sealed condition to a full open condition, is less than one half of one millisecond (0.0005 sec.). This characteristic allows a rupture disk to function as a "quick opening" valve. Some examples of rupture disks utilized in this manner are: 1) shock tube operations; 2) seismic testing; 3) simulation of large caliber gun discharges; 4) shifting of control mechanisms from a remote location; 5) injection systems for suppression of upsets within storage vessels or systems.

HISTORY AND BACKGROUND OF RUPTURE DISKS

Bursting of tanks and equipment was a serious problem in the early oilfield days for equipment utilized at the wellhead or tank storage facility. Rupture disks were developed as a means to overcome this problem.

The earliest disk designs, in the 1930's, consisted of a flat sheet of metal, commonly copper, clamped between a pair of piping flanges. It soon became apparent that the operating pressure caused bulging and stretching of the metal, resulting in premature failure at only 30% to 50% of the disk rating. Leakage between flat seat disk and flange was also a problem.

It was soon discovered that if disks were pre-bulged, they could operate at pressures of 50% to 70% of the disk rating. With the introduction of more suitable metals, such as aluminum, nickel, stainless steel, Monel, and Inconel, pre-bulged disks were developed that could be operated reliably at 70% of their rated pressure at a specified operating temperature

The leakage problem was largely solved by the development of a 30o angular seat rupture disk design, which required less companion flange bolting load for sealing than flat seat designs. These angular seats are still in wide use today on certain types of disks.

With the advent of more satisfactory operating to burst pressure ratios, use of rupture disks spread to refining and chemical processing. They also began to be used with relief valves. Before long, rupture disks began to be utilized in all industries for over-pressure protection.

The use of standard pre-bulged disks with relief valves presented the problem of fragmentation resulting in occasional blockage of the valve. The introduction of composite type rupture disks in the 1950's helped reduce this problem. Composite type disks can be operated at up to 80% of their rated pressure, and can also be manufactured to lower set pressures than standard disks.

Scored rupture disks were introduced in the 1960's. These designs are nonfragmenting and permit operation up to 90% of their rated pressure. Forward acting scored disks, introduced first, and operates in tension just as standard disks do. The burst pressure is controlled by the thickness of material under the score lines.

Reverse buckling scored disks are a compression type device. The dome reverses at the set pressure, causing the metal to part at the pre-weakened score lines. The reversal or set pressure is controlled by many variables such as the nominal disk size, material thickness, type of material, configuration and depth of scores and disk crown height.

TYPES OF OSECO RUPTURE DISKS

OSECO provides the following types of rupture disks.

STD - The standard domed rupture disk is a prebulged solid metal membrane disk designed to rupture in tension when pressurized from the concave side. Manufactured in both angular and flat seat designs, it is available in most sizes from 3/16 to 42 inch diameter. Burst pressures can be provided from 2 to 80,000 psig, depending on the size and material of construction.

Standard materials of construction are Aluminum, 316 Stainless Steel, Nickel 200/201, Monel 400, Inconel 600/625 and Hastelloy C-276. Other materials are available only upon consultation with the factory. Polymer membrane linings can be supplied on either or both sides, depending on size and rupture pressure.

The STD is an economical solution for many applications. Operating pressures should be limited to 70% or less of the rated burst pressure for good service life. Excessive pressure cycling beyond this operating limit and temperature variations beyond published material temperature limits will reduce service life. Because the standard type disk can fragment during rupture, it is not recommended for use under pressure relief valves.

A vacuum support may be required if the disk is to withstand vacuum conditions during process Operations. The vacuum support is fitted to the con-cave side of the disk and is designed to give full unrestricted relief area. The designation for this disk with a vacuum support is STDV.

CO - The composite disk is designed for applications that require a lower burst pressure than that obtainable with a standard disk. It is available in most sizes from 11/16 to 42 inch diameter, with either angular (30o) or flat seats, depending on size.

The burst pressure is controlled by the combination of a slit top section and either a metallic or polymer seal member underneath. The CO disk can be operated up to 80% of its rated burst pressure. Use of a teardrop to secure the center section of the top section minimizes the possibility of fragmentation in the smaller sizes (U.S. Patent 5,022,424).

The CO disk will require a vacuum support if the disk is to withstand vacuum conditions during process operations. The vacuum support is fitted to the concave side of the disk and is designed to give full unrestricted relief. The designation for this disk with a vacuum support is COV.

FAS - This is a forward acting scored disk. OSECO uses precision tooling to score the disk on the convex side, away from the process, after forming to a high crown. This permits operation up to 90% of rated burst pressure (U.S. Patent 4,655,070).

This type of disk has a flat seat and must be used in holders designed for it. It is available in sizes from 1 to 14 and 18 inch diameters.

Scored disks open at the score lines during rupture, thus no fragmentation. Because of the relatively thick material used for this type of disk, vacuum service capability is a "built-in" feature. This disk is fail safe in that if damaged or installed upside down, it will burst at or below its rated pressure.

A modified version of the FAS, known as the ICD, has metal rings, polymer gaskets, and a polymer liner on the process side. It is used primarily with intermodal containers.

PCR - This is a reverse buckling scored disk and is loaded in compression. Scores are on the concave side, with the convex side toward the process. It can be operated up to 90% of its rated burst pressure. This disk also has a flat seat.

The PCR has demonstrated longer service life than other types of disks when operated under pressure/vacuum cycling conditions and temperature fluctuations. No vacuum support is required. This disk will burst at higher pressure in the inward flow than in the forward flow direction, making it useful under pressure relief valves that will be tested in service. It will burst in the inward-flow direction at no more than 1.5 times its rated burst pressure. However, this ratio can be as low as 1.0 and is not controllable to a specific value.

Because of the "snap-thru" operating feature, this disk is not recommended for use in liquid service. The range of rated rupture pressures available is narrower than with other types of disks. Available sizes are from 1 to 8 inch diameters.


PCRS – This rupture disk design is the same as the PCR described above except that the component pieces are smaller to accommodate usage in sanitary applications involving clamped tubing joints. It is fabricated on the existing PCR tooling with trimming of the disk OD to fit the sanitary gasket. It is a holderless rupture disk in that it is sealed in a sanitary gasket and simply replaces the standard gasket seal at a tubing joint. This design is simply an extension of the PCR design for adaptation to sanitary tubing installations where the required burst pressures are above the PLRS, PLRSL and PLRSC maximums.

This rupture disk is available in sizes 1”, 1.5”, 2”, 3”, and 4” only. The nominal size is also the tubing clamp size for this product line. This is due to the fact that tubing I.D. is much smaller than corresponding piping I.D. Minimum burst pressures begin as low as 49 psig for Nickel, depending on the size. This rupture disk is available in the standard materials except for Aluminum and Hastelloy C-276. Please consult factory for minimum/maximum burst pressures for this design.

FLCO - OSECO can supply a flat composite rupture disk for low pressure usage. This style has polymer membranes on both sides of the slotted pressure resisting member. It will rupture at the same pressure in either direction. This disk is often used for isolation of the outlet side of relief valves connected to common headers. They are also used for the protection of low pressure vessels. They are installed between ANSI piping flanges, and are not torque sensitive. They typically must be operated at no more than 50% of their rated pressure. It is available in sizes 1 through 42 inch.

FLCOS – Oseco can also supply a flat composite rupture disk for low pressure usage with a built-in burst sensor when required by a particular application. This device is covered by US Patent 5,631,634. This style is identical to the FLCO, except that an additional gasket is incorporated along with electrical isolation features in the central metal membrane. It will also rupture at the same pressure in either direction, making it a bi-directional device. It is also used for isolation of the outlet side of relief valves connected to common headers. It may also be used for the protection of low-pressure vessels. It typically must be operated at no more than 50% of their rated pressure. It is also available in sizes 1 through 42 inch.

PLR - The Precision cut Low pressure Reverse buckling rupture disk, U.S. Patent 5,720,380, is a compression loaded, reverse buckling rupture disk. This rupture disk is a three (3) piece construction, i.e., hexagonal cutter, sealing membrane and a retainer. It is a designed to fit within a PRDI type holder. This disk has a flat seat and can operate up to 90% of the rated burst pressure.

This rupture disk is available in nominal sizes 1” through 8” with minimum burst pressures as low as 2 psig. The PLR has also demonstrated a service life comparable to the previously mentioned PCR. It is available in the standard rupture disk materials, except for aluminum. The retainer component is always on the discharge side and is available only in 316 stainless steel.

PLRS, PLRSL, PLRSC – This rupture disk design is the same as the PLR described above, except that the component pieces are smaller to accommodate usage in sanitary applications involving clamped tubing joints. It is a holderless rupture disk in that it is sealed in a sanitary gasket and simply replaces the standard gasket seal at a tubing joint. This design is simply an extension of the PLR design for adaptation to sanitary tubing installations.

This rupture disk is available in sizes 1.5”, 2”, 3” and 4” only. The nominal size is also the tubing clamp size for this product line. This is due to the fact that tubing ID is much smaller than corresponding piping ID. Minimum burst pressures begin as low as 5 psig depending on size. This rupture disk is available only in 316 stainless steel. For other materials, please consult the factory.

VAPRO - The VAPRO rupture disk is a dual acting pressure relief device, designed to protect against both a positive over-pressure and a vacuum relief in a single device. It is available in nominal sizes 3” through 10”. The vacuum relief pressures range from approximately three (3) inches of water column to thirty (30) inches of water column. Positive relief pressures range from four (4) psig to one hundred and fifty (150) psig depending on nominal size.

This relief device utilizes an insert hold, VRDI, for installation between companion piping flanges. It is also available for sanitary applications utilizing a VRDS holder which clamps into the sanitary tubing system. The vacuum relief is achieved through the reversal of a fluorocarbon sealing membrane on a knife-blade assembly located in the holder. Positive pressure relief is achieved through a composite top utilizing the same fluorocarbon sealing membrane.

POSIPRO - The POSIPRO is also a dual acting pressure relief device with a positive pressure relief range of three (3) inches of water column to thirty (30) inches of water column. Vacuum relief pressures are restricted from four (4) psig to full vacuum. This rupture disk is also available in nominal sizes from 3” through 10”.

This relief device also utilizes an insert holder, POSI, for installation between piping flanges. It is also available for sanitary applications utilizing a POSS holder which clamps into the sanitary tubing system. The vacuum relief is achieved through the reversal of a fluorocarbon sealing membrane on a knife-blade assembly located in the holder. Positive pressure relief is achieved through a composite top utilizing the same fluorocarbon sealing membrane.

FASS – The FASS rupture disk is a tension loaded, scored design which fits inside the same sanitary gasket utilized by the PLRS, PLRSL, PLRSC and PSRS rupture disk. This allows the use of a higher rated rupture disk design in sanitary applications than is capable by the PLRS, PLRSL, PLRSC and PSRS alone. It is a holderless rupture disk in that it simply replaces the standard sanitary gasket seal at a tubing point.

This rupture disk is available in sizes 1”, 1.5”, 2”, 3” and 4” only. The nominal disk size is also the tubing clamp size for this product line. Minimum burst pressure begins as low as 50 psig depending on size and material. This rupture disk is available in the standard rupture disk materials.

FST – The FST rupture disk is a tension loaded, scored rupture disk design which fits inside the FRDI holder type. This rupture disk utilizes a score on the process side via the PCR score tooling. This allows the scoring of materials that are difficult to form to the higher crown heights of the FAS as an example.

This rupture disk design is available in nominal sizes 1” though 8”. Minimum burst pressures begin as low as twenty-five (25) psig depending on size and material selection.

PSR – The PSR rupture disk is a compression loaded, reverse buckling rupture disk design. It is equivalent to the PCR except that the ratio of conventional burst pressure to reversal burst pressure is allowed to be as high as 2.0. This limits this design to relatively low pressures, generally below the published minimum burst pressures of the PCR. This rupture disk utilizes the same PRDI holder as the PCR disk.

This rupture disk is available in nominal sizes of 1” through 8”. Minimum burst pressures begin as low as fifteen (15) psig depending on size and material. The maximum burst pressure is restricted to 125 psig and is also size dependent. This rupture disk is available in the standard rupture disk materials except for aluminum.

PSRS – The PSRS rupture disk is a compression loaded, reverse buckling rupture disk design. It is equivalent to the PSR except that the OD has been trimmed to fit a sanitary gasket. This limits this design to relatively low pressures, generally below the published minimum burst pressures of the PCR.

This rupture disk is available in nominal sizes from 1” through 4”. Minimum burst pressures begin as low as fifteen (15) psig depending on size and material. The maximum burst pressure is restricted to 125 psig and is also size dependent. This rupture disk is available in the standard rupture disk materials except for aluminum and Hastelloy C-276.

PRO - This rupture disk is OSECO’s newest entry (Patent Pending) into the reverse buckling rupture disk field. It is currently available in 1” and 3” sizes only in Nickel material. It utilizes a PRDI holder for the larger sizes and is also utilized in the crimped OTU body (see OTC below). Minimum burst pressures are as low as 15 psig and maximum pressures restricted to 150 psig. Please consult factory for availability of additional sizes and burst pressure limits.

PROS – This rupture disk is OSECO’s newest entry (Patent Pending) into the reverse buckling sanitary rupture disk field. It is currently available in a 1”, 1.5” and 4” size and only in 316 ss material. It utilizes a sanitary gasket to fit with tubing secured with sanitary clamps. Minimum burst pressures are as low as 15 psig and maximum pressures restricted to 150 psig. Please consult factory for availability of additional sizes and burst pressure limits.

OTU - The One Time Unit (OTU) is a sealed unit consisting of a brass body and a STD/STDV series or CO/COV series rupture disk. The assemblies are an inexpensive bubble-tight pressure relief device. If an overpressure occurs, the entire assembly is replaced. OTUs are commonly used for air conditioning systems, gas cylinders, hydraulic accumulators, portable compressed air systems, and high pressure water power cleaning systems.

OTC – The One Time Crimped Unit (OTC) utilizes only the PRO rupture disk in Nickel, sealed within a brass body. The assemblies are an inexpensive bubble-tight pressure relief device. If an overpressure occurs, the entire assembly is replaced. OTCs are commonly used for air conditioning systems, gas cylinders, hydraulic accumulators, portable compressed air systems, and high pressure water power cleaning systems just as the OTU described above.

SDA, WDA - These are relatively small, high pressure assemblies in which the disk is soldered (SDA) or welded (WDA) to the holder. They are commonly used with extruders. Special configurations have been developed for low pressure applications concerning the protection of vacuum jacketed vessels or piping (U.S. Patent 4,682,619).

RC - OSECO manufactures a variety of rupture disks especially designed for use on railroad tank cars. The HP-RC is molded from Ryton plastic and can be polymer lined as an option. It fits standard tank car safety vents.

CRC - This is a composite disk with metal top section and vacuum support and a polymer sealing member. It also fits standard tank car safety vents.

HCL-RC - This disk is constructed of Ryton with polymer linings, is designed primarily for rubber lined tank cars used in HCl service.

Purposes of Rupture Panels

A rupture panel is a composite type pressure relief device designed to rupture at a predetermined pressure and temperature in accordance with the guidelines of the National Fire Protection Agency Bulletin No. 68. This device is a means of providing protection for personnel and property from deflagrations. This pressure relief device does not provide protection when an explosion occurs. The defining difference between a deflagration and an explosion is the fact that in a deflagration, the flame front has not achieved sonic velocity.

Rupture panels are used to protect pulverized coal ducts at electrical power generating plants, bag house filter installations on atmospheric exhaust discharges from processing plants, chemical or food processing buildings where explosive dusts can accumulate as well as those applications involving combustible gases or vapors within a confined space. These pressure relief devices are normally installed between simple structural angles or plates with sufficient bolting and gaskets to achieve a pressure seal sufficient to activate this pressure relief device.

History and Background of Rupture Panels

Bursting of duct work and associated equipment became a serious problem for applications dealing with combustible dusts and gases during the modernization of electrical power plants starting in the late 1970’s to the early 1980’s. Rupture panels were developed to address this problem. The initial configuration for these pressure relief devices was flat, rectangular or circular, three piece construction, i.e., metal membrane, fluorocarbon pressure seal membrane, metal membrane. The flat rupture panel has the same operating to burst pressure limits as any flat composite rupture disk, i.e., fifty (50) percent. Although the lower pressures achievable with rupture panels is attractive to plant operators, the lower operating limit is not. This disadvantage led to the evolution of a domed or crowned rupture panel.

The domed or crowned rupture panel is capable of operating at seventy (70) percent of the bursting pressure or higher depending on the design. This design innovation has gained wide acceptance from plant operators due to the improved operating features. All OSECO rupture panels are available with a burst sensor, SVT-03.

Types of OSECO Rupture Panels

OSECO provides the following types of rupture panels.

CRP - This is a flat rupture panel with a central slit in the metal membrane long direction, having additional slits running to each of the four corners from the adjacent area of the central slit end. The distance between the end of the central slit and the beginning of each slit towards a corner is designated a CAF or stitch. It controls the burst pressure of the rupture panel for a given metal membrane thickness. The construction of this type of rupture panel is an inner and outer metal membrane with identical bursting pressure and a fluorocarbon membrane seal between metal membranes. This makes this device a bi-directional pressure relief device. This type of rupture panel is available in square or rectangular sizes from 8” by 8” to 44” by 69” depending on the application. This particular rupture disk type is not available as a crowned or domed rupture panel. Burst pressures for this type of rupture panel range from a few tenths of a PSIG to 10 PSIG depending on size.

RND, RNDC, RNDCI – The RND rupture panel is a flat rupture panel which is of the same construction as the CRO except for the CAF or metal stitch controlling the burst pressure. The configuration of this particular rupture panel type is circular with sizes beginning at 8” and continuing through 36”. The controlling CAF or metal stitch is a circular hole pattern of six (6) holes in the exact center of the device. The distance between adjacent holes and the metal thickness determine the burst pressure. Slits are cut from the central six hole pattern to the rupture panel edge to provide an opening pattern sufficient to allow the required vent flow. Burst pressures for this type of rupture panel range from one half a PSIG to ten (10) PSIG depending on size.

The RNDC is a domed or crowned version of the RND. The RNDC is a one way pressure relief device which is vastly different from the RND type of rupture panel. The RNDC can operate at up to seventy (70) percent of the panel burst pressure. It can also withstand greater vacuum conditions than the RND.

Finally, the RNDCI is a duplicate of the RNDC except that is includes ceramic insulation fibers to protect the fluorocarbon seal from temperatures in excess of five hundred (500) degrees Fahrenheit.

CRV, CRVC, CRVCI – The CRV panel is identical in construction to the CRP type except for the CAF or stitch pattern used. The CRV stitch or CAF is a circumferential stitch pattern around the installed ID of the panel on three (3) sides with a hinge provided on the fourth side. This type of panel has the same operating limits as the CRP; however, the material thickness required for the same burst pressure is much thicker, providing a sturdier rupture panel. This is the chief advantage of the CRV over CRP. Burst pressures range from a few tenths of a PSIG to ten (10) PSIG depending on size.

The CRVC is a domed or crowned version of the CRV. The CRVC is a one way pressure relief device which is vastly different from the CRV type of rupture panel. The CRVC can operate at up to seventy (70) percent of the panel burst pressure. It can also withstand greater vacuum conditions than the CRV.

Finally, the CRVCI is a duplicate of the CRVC except it includes ceramic insulation fibers to protect the fluorocarbon seal from temperatures in excess of five hundred (500) degrees Fahrenheit.

MV – This rupture panel, Metal Vent, is OSECO’s newest rupture panel innovation (Patent Pending) and uses no sealing membrane. Instead it uses an all metal construction which overlaps the burst pressure control features in the primary sealing metal membrane. Resistance to vacuum service is provided in the same manner as the burst pressure control membrane. The MV is available in many of the same sizes and burst pressures as the other rupture panels mentioned above. For sanitary service, this panel utilizes gaskets from FDA approved materials.

Specials – OSECO has the R&D, Engineering and Manufacturing capability to develop, design and fabricate rupture disks and panels that do not fit neatly into any of the above classifications. Please feel at liberty to call OSECO if you have special applications. OSECO has had great success in providing special rupture disk assemblies in a timely fashion during our firms’ history.

HOW TO SELECT A RUPTURE DISK TYPE

The following steps can be used as a guide to select the proper type of rupture disk:

1) List as much of the information below as is available. All items should be specified if possible:
a) Maximum allowable working pressure (MAWP) of the vessel or system
b) Maximum operating pressure
c) Maximum temperature at the disk location
d) Desired rupture disk burst pressure and temperature
e) Backpressure or vacuum conditions, if any
f) Media: liquid or gas, corrosion characteristics
g) Static, cyclic or pulsating operation
h) Code requirements, i.e., ASME, ISO, BP, API, etc.
i) Any other special conditions.

2) Calculate ratio of maximum operating pressure to minimum burst pressure. Manufacturing range must be taken into account in determining minimum burst pressure. This is discussed below in the section entitled "Definition of Terms for Disk Selection". An example is also included.

3) Select a disk type that meets the pressure ratio calculated above. This ratio must be 0.9 or less. A lower pressure ratio often permits the use of a less expensive disk.

4) Select an appropriate material that meets the corrosion and/or temperature requirements.

5) Check the appropriate OSECO bulletin to assure that the burst pressure is within the available ranges for the material and disk type selected. Size will also be a factor here.

6) Select required holders and options, if any.