An Introduction To Rupture Disc. Types, Applications, And Calculations
1- Introduction
Rupture Disc Device. A rupture disc device is a non-reclosing pressure relief device actuated by inlet
static pressure and designed to function by the bursting of a pressure containing disc.It is used on low pressure machines and circuits
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| Fig 1 |
Function principle
The rupture disc is a plate that breaks open at a certain set pressure. The disc breaks open only once. When it is open It needs to be replaced. Rupture discs are found encoding corroding or contaminated
Processes. The rupture disc commonly is used to protect a safety relief valve from contact with the process fluids
The purpose of installing
a rupture disc upstream of a relief valve is to minimize the loss by leakage through the valve of valuable, noxious, or otherwise hazardous materials or to prevent corrosive gases from reaching the relief valve internals, see Figure 2. Typical bursting disc installation at a relief valve.
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| Fig 2 |
The installation of a rupture disc may be called for downstream of a relief valve. In this case, its purpose is to prevent corrosive gases from a common discharge line reaching the relief valve internals.
Rupture discs may also be installed as a sole relief device. See Figure 3
.
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| Fig 3 |
The original rupture disc consisted of a plain metal sheet that was clamped between two flanges. When exposed to pressure on one side, the disc would stretch and form a hemispherical dome before bursting. The predictability of the burst pressure, however, was poor. To improve the predictability, rupture discs were subsequently predoomed by applying pressure to one side of the disc that was higher than the normal operating pressure by some margin.
2- Terminology
Rupture disc device. A non-reclosing pressure relief device, consisting of rupture disc and holder, in which the rupture disc is designed to burst at a predetermined differential pressure across the disc.
Rupture disc. The pressure-containing, pressure and temperature sensitive element of a rupture disc device.
Forward-domed rupture disc. A rupture disc that is domed in the direction of the fluid pressure and designed to burst due to tensile forces.
Reverse-buckling disc. A safety disc that is domed against the direction of the fluid pressure and designed to buckle due to compression forces prior to bursting or to being expelled from the holder.
Holder. The component of the rupture disc device that holds the rupture disc around its circumference and consisting of the inlet and outlet holder parts.
Vent panel. A low-pressure venting device designed to vent the near instantaneous volumetric and pressure changes resulting from dust, gas, or vapor deflagrations.
Vacuum support. A device that supports the rupture disc against collapse due to vacuum pressure.
Back-pressure support. A device that supports the rupture disc against collapse due to superimposed back pressure.
Heat shield. A device that shields the rupture disc from the heat source in a manner that does not interfere with the rupture disc operation.
Specified temperature of rupture disc. The temperature at which the disc is rated and marked.
Burst pressure. The differential pressure across the rupture disc at which the rupture disc bursts at the specified temperature.
Specified Burst Pressure The customer-indicated burst pressure value at a coincident temperature that indicates the amount of differential pressure necessary to cause the rupture disc to burst. This value will be marked on the rupture disc tag.
range, unless otherwise specified by the customer.
Maximum Recommended Operating Pressure Continental Disc’s recommendation of the
maximum pressure to operate the rupture disc in order to maximize the life of the rupture disc.
This is used as one of the indicators in determining an appropriate type of rupture disc for an
PHOTO CAPTION application.
range, unless otherwise specified by the customer.
Maximum marked burst pressure. The marked burst pressure at the top end of the manufacturing range.
Minimum marked burst pressure. The marked burst pressure at the bottom end of the manufacturing range.
Burst tolerance:The maximum variation in burst pressure from the marked burst pressure.
Manufacturing range. A range of pressures within which the average burst pressure of test discs must fall to be acceptable for a particular application, as agreed between the customer and manufacturer.
Performance tolerance. A range of burst pressures comprising manufacturing range and burst tolerance at the specified temperature.
Operating ratio. The ratio of the maximum operating pressure to a minimum burst pressure
Damage ratio. The ratio of the burst pressure of the damaged rupture disc to the burst pressure of the undamaged rupture disc.
Reversal ratio. The ratio of the burst pressure of the reversed installed rupture disc to the burst pressure of the correctly installed rupture disc.
Lot. A quantity of rupture discs made as a single group of the same type, size, and limits of burst pressure and coincident temperature that is manufactured from material of the same identity and properties.
Deflagration. Burning that takes place at a flame speed below the velocity of sound in the medium.
Detonation. Propagation of a combustion zone at a velocity that is greater than the speed of sound in the unreacted medium.
Explosion. The bursting or rupture of an enclosure or a container due to the development of internal pressure from a deflagration.
Rupture discs do not reclose after bursting. The decision to install rupture discs may therefore have important economic consequences. How-ever, there are many applications where rupture discs are likely to perform better than pressure relief valves. These include:
• Under conditions of uncontrolled reaction or rapid over pressurization in which the inertia of a pressure relief valve would inhibit the required rapid relief of excess pressure.
• When even minute leakage of the fluid to the atmosphere cannot be tolerated at normal operating conditions.
• When the fluid is extremely viscous.
• When the fluid would tend to deposit solids on the underside of the pressure relief valve disc that would render the valve inoperable.
• When low temperature would cause pressure relief valves to seize.
4- Types Of Discs
No single type of rupture disc will meet all the numerous applications of an industry. Each type of rupture disc, Tension or Reverse Acting, has its own characteristics and capabilities.
The rupture disc thus produced is today's solid-metal forward-domed rupture disc. Flat metal rupture discs have also been reengineered for low-pressure applications. Both types of rupture discs are of the tension loaded type in which the fluid pressure stretches the disc material as the
fluid pressure increases.
The continuing effort to raise the operating ratio of rupture discs led to the development of reverse-buckling discs. This type of disc is domed against the fluid pressure so that the fluid pressure introduces a compression load on the convex side of the disc
There are two major types of rupture discs made of ductile metal:
• forward-acting types, being tension loaded
• reverse-acting types, being compression loaded
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| Fig 4 |
Forward-domed and flat rupture discs are the tension-loaded types, while the reverse-buckling disc is of the reverse-loaded type. The following describes a cross-section of these discs as offered by the industry
4-1 Tension Type rupture discs
Tension Type rupture discs are oriented in a system with the process media pressure against the concave side of the rupture disc (Figure 5, 30° Seat; Figure 6, Flat Seat). As the process pressure increases beyond the allowable operating pressure, the rupture disc starts to grow. This growth will continue as the pressure increases, until the tensile strength of the material is reached and rupture occurs. |
| Fig 5 |
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| Fig 6 |
4-1-1 Solid forward-domed rupture discs.
Solid forward-domed rupture discs are formed from flat discs by applying a fluid pressure to the underside of the disc of normally above 70% of the burst pressure. This method of manufacture gives the rupture disc a hemispherical shape, as shown in Figure 7 When operating pressure grows beyond the predoming pressure, the dome starts to grow. As the operating pressure approaches 95% of the burst pressure, localized thinning in the region of the dome center occurs that leads to rupture of the disc. This failure is accompanied by some fragmentation of the disc.
To guard the disc against further plastic deformation during service, normal operating pressure is commonly restricted to 70% of the rated burst pressure or less, depending on operating conditions.
Because the tensile strength of the construction material used for the manufacture of the discs is fairly high, solid forward-domed rupture discs for low pressures must be made of relatively thin foils. Periods of vacuum or superimposed back pressure will therefore tend to cause the disc to partially or fully collapse.
When these conditions exist, the disc must be provided with:
To guard the disc against further plastic deformation during service, normal operating pressure is commonly restricted to 70% of the rated burst pressure or less, depending on operating conditions.
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| Fig 7 |
Because the tensile strength of the construction material used for the manufacture of the discs is fairly high, solid forward-domed rupture discs for low pressures must be made of relatively thin foils. Periods of vacuum or superimposed back pressure will therefore tend to cause the disc to partially or fully collapse.
When these conditions exist, the disc must be provided with:
4-1-1-1 vacuum supports such as those shown in Figure 8, or in special cases.
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| Fig 8 |
4-1-1-2 back pressure, with supplementary permanent supports as shown in Figure 9. These supports must fit closely the concave side of the disc to prevent alternating collapsing and stretching of the disc
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| Fig 9 |
4-1-1-3 wrinkle pattern The deformation of disc manifests itself in wrinkle pattern identified as turtle backing, as shown in Figure 10, resulting in a poor service life.
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| Fig 10 |
4-1-2 Slotted and lined forward-domed rupture discs.This is a multi-layered forward-domed rupture disc in which the dome of the top member is slotted with pierced holes at each end. The second layer is the seal member,commonly made of fluorocarbon or an exotic metal. A vacuum support that may be required is the third component. Figure 11 illustrates the three layers of the disc.
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| Fig 11 |
For a given thickness and strength of the material, the burst pressure is controlled by a combination of slits and tabs. By this construction, the rupture disc for low burst pressures can be manufactured from thicker materials that permit the operating ratio to be raised to 80%.
The rupture discs are generally suitable in the lower pressure regions only. They may be used in gas and liquid service and permit pulsating pressure service. The discs are also non-fragmenting when used in conjunction with a fluorocarbon seal member.
Figure 12 and Figure 13 show two types of slotted and lined flat rupture discs that may be used for either one-way or two-way flow.
4-1-2-1 flat rupture disc for atmospheric vessels and isolating outlet port of relief valves; ready
with fiber gaskets; direct installation between companion flanges
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| Fig 12 |
The rupture disc shown in Figure 12 is a low-cost pressure relief device typically used as an environmental seal for transport and storage tanks and downstream of pressure relief valves. The disc does not require separate holders but can be mounted directly between class 150 flanges.
Variations are designed for vacuum relief only, or for pressure in one direction and for maximum double that pressure in the other direction. Highest operating ratio is restricted to 50%.
4-1-2-2 low pressure rupture disc with flat composite metal design that withstands full vacuum
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| Fig 13 |
The rupture disc shown in Figure 13 is designed for overpressure relief of low-pressure systems. The discs withstand full vacuum and maybe used in gas and liquid-full systems. Operating ratio of the disc is as high as 80%.
4-1-3 Scored forward-domed rupture discs.
These are solid forward-domed rupture discs that are cross scored on the convex side of the dome, as
shown in Figure 14. Scoring allows the disc to be made of thicker material that allows the operating ratio to be raised to 85%. The discs may be used in either gas or liquid-full service and offer a good service life in cycling service. The score lines provide a predictable opening pattern. so that the disc can be manufactured to be non-fragmenting. Within the lower burst pressure range, however, the rupture disc must be supported against full vacuum.
A special field of application of scored forward-acting rupture discs is in polymer service. The problem of polymerization is minimized by avoiding a crevice between the rupture disc and inlet holder component
in which growth of polymer could start. A rupture disc device that meets this requirement is shown in Figure 15. By implication, the rupture disc cannot be used in polymerization service when being fitted with a vacuum support.
shown in Figure 14. Scoring allows the disc to be made of thicker material that allows the operating ratio to be raised to 85%. The discs may be used in either gas or liquid-full service and offer a good service life in cycling service. The score lines provide a predictable opening pattern. so that the disc can be manufactured to be non-fragmenting. Within the lower burst pressure range, however, the rupture disc must be supported against full vacuum.
A special field of application of scored forward-acting rupture discs is in polymer service. The problem of polymerization is minimized by avoiding a crevice between the rupture disc and inlet holder component
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| Fig 14 |
in which growth of polymer could start. A rupture disc device that meets this requirement is shown in Figure 15. By implication, the rupture disc cannot be used in polymerization service when being fitted with a vacuum support.
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| Fig 15 |
4-2 Reverse Acting rupture discs
Reverse Acting rupture discs are oriented in a system with the process media pressure against the convex side of the disc (Figure 16), placing the rupture disc in compression. As the burst pressure rating of the disc is reached, the compression loading on the rupture disc causes it to reverse, snapping through the neutral position and causing it to open by a predetermined scoring pattern or knife blade penetration.
A reverse acting rupture disc provides some advantages, as compared to tension type rupture discs, which may warrant consideration when selecting a rupture disc. These advantages include:
. Zero manufacturing range allowing the rupture disc to operate to 90% of its stamped burst pressure
. Full vacuum capabilities without the need of an additional support member
. Longer service life under cyclic or pulsating conditions
. Constructed using thicker materials providing greater resistance to corrosion |
| Fig 16 |
A reverse acting rupture disc provides some advantages, as compared to tension type rupture discs, which may warrant consideration when selecting a rupture disc. These advantages include:
. Zero manufacturing range allowing the rupture disc to operate to 90% of its stamped burst pressure
. Full vacuum capabilities without the need of an additional support member
. Longer service life under cyclic or pulsating conditions
4-2-1 Reverse buckling disc with knife blades. Figure17 shows a reverse buckling disc in combination with knife blades that are designed to cut the disc open upon reversal. For this to happen, the disc must strike the knife blades with high energy. The disc may therefore be used in gas service only and in liquid service if there exists a substantial gas volume between the liquid and the disc. In totally full-liquid systems, reversal speed will be too slow to cut the disc. In this case, the disc comes initially to rest on the knife blades before being cut open after a substantial pressure rise. In the past, this situation has led to a number of recorded pressure vessel ruptures.
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| Fig 17 |
It is essential that the edges of the knife blades are kept sharp. The cutting edges must therefore be checked on a regular basis and must be resharpened if necessary. Care must be taken not to change the blade location or configuration. In most cases, the manufacturer should perform repair or replacement.
The advantages of this type of disc are that it can be designed for low burst pressures, it does not require vacuum support, it is excellent for cyclic or pulsating pressures, it is non-fragmenting, and it may be offered for 90% operating ratio, zero manufacturing range, and plus/ minus 2% burst tolerance.
Its disadvantages are the knife blades must be kept sharp, and it is not suitable for liquid-full systems.
4-2-2 Reverse-buckling disc with teeth ring. The reverse-buckling disc
shown in Figure 18 is provided with a teeth ring that pierces and cuts the rupture disc on buckling. The disc offers advantages and disadvantages similar to the reverse-buckling disc with knife blades, but is
designed for considerably lower burst pressures.
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| Fig 18 |
4-2-3 Reverse-buckling disc cross-scored. Figure 19 shows a rupture disc that is cross-scored on the concave side. Upon buckling, the disc will break open in pie-shaped sections along the score lines, with the base firmly held by the holder.
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| Fig 19 |
The cross-scored rupture disc has the advantage over the reverse-buckling disc shown in Figure 17 by functioning without the assistance of knife blades. The advantages and disadvantages of both types of reverse buckling discs are otherwise identical.
4-2-4 Reverse-buckling disc with partial circumferential score line.
Figure 20 through Figure 22 show three types of reverse-buckling discs with a partial circumferential score line around the rim of the disc. When buckling occurs, the discs shear open along the score line and fold
around a pivot.
In the case of the rupture disc shown in Figure 22, the score is perforated to permit the achievement of an extremely low burst pressure. The perforations are sealed with an O-ring that becomes energized as soon the fluid pressure is applied.
These rupture discs offer all the advantages of cross-scored rupture discs. In addition, the discs may be employed in liquid-full systems.
4-2-4-1 Reverse-Buckling Disc with Partial Circumferential Score Line Fig 20
4-2-4-2 Reverse-Buckling Disc Partial Circumferential Score Line, Low Pressure Series. Fig 21
4-2-4-3 Reverse-Buckling Disc Partial Circumferential Perforated Score, Designed for Lowest Pressures. Fig 22
4-2-5 Reverse-buckling disc of slip-away design.
around a pivot.
In the case of the rupture disc shown in Figure 22, the score is perforated to permit the achievement of an extremely low burst pressure. The perforations are sealed with an O-ring that becomes energized as soon the fluid pressure is applied.
These rupture discs offer all the advantages of cross-scored rupture discs. In addition, the discs may be employed in liquid-full systems.
4-2-4-1 Reverse-Buckling Disc with Partial Circumferential Score Line Fig 20
4-2-4-2 Reverse-Buckling Disc Partial Circumferential Score Line, Low Pressure Series. Fig 21
4-2-4-3 Reverse-Buckling Disc Partial Circumferential Perforated Score, Designed for Lowest Pressures. Fig 22
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| Fig 20 |
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| Fig 21 |
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| Fig 22 |
4-2-5 Reverse-buckling disc of slip-away design.
Reverse-buckling discs of the slip-away design, as shown in Figure 23, function by being expelled from the holder upon buckling. The relatively narrow flat seatings of the disc are mounted in a recess of the inlet holding part and sealed with an O-ring or a flat gasket. No special torque settings are required. To prevent the disc from traveling along the vent line, the holder can be provided with an integral or separate arresting device. This allows the disc to be mounted upstream of the pressure relief valve.
Advantages of this design are the operating ratio can be up to 95%; the reversal ratio in general is less than 1.0 with a maximum of 1.1 for smaller sizes; and the size range is from DN 25 (NFS 1) to DN 1200 (NFS 48).
The disadvantages are it is not suitable for use in liquid-full systems except in the larger sizes, and it may require vacuum support.
4-2-6 Reverse-buckling disc slotted and lined with buckling bars.
Advantages of this design are the operating ratio can be up to 95%; the reversal ratio in general is less than 1.0 with a maximum of 1.1 for smaller sizes; and the size range is from DN 25 (NFS 1) to DN 1200 (NFS 48).
The disadvantages are it is not suitable for use in liquid-full systems except in the larger sizes, and it may require vacuum support.
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| Fig 23 |
4-2-6 Reverse-buckling disc slotted and lined with buckling bars.
The rupture disc shown in Figure 24 consists of a slotted component that is the actual pressure sensitive element, and a seal element of either plastic or metal. In combination with a metal seal, the disc supports full vacuum.
The partial circumferential slot is teeth-shaped, while the tabs represent buckling bars that control the buckling process. On reaching the buckling pressure, the bars buckle and break off and allow the disc to hinge open. During this process, the teeth ring cuts open the seal member.
The disc is made in sizes DN25 (NFS 1) through DN 600 (NFS 24). The burst pressure range covers the lowest burst pressures up to 120 barg (1800 psig) at 22°C (72°F) operating temperature. Maximum operating temperature is 550°C (1000°F).
The advantages of this disc are zero manufacturing range, burst tolerance is down to plus/minus 2%, operating ratio is as high as 95%, and it may be employed in liquid-full systems
The partial circumferential slot is teeth-shaped, while the tabs represent buckling bars that control the buckling process. On reaching the buckling pressure, the bars buckle and break off and allow the disc to hinge open. During this process, the teeth ring cuts open the seal member.
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| Fig 24 |
The disc is made in sizes DN25 (NFS 1) through DN 600 (NFS 24). The burst pressure range covers the lowest burst pressures up to 120 barg (1800 psig) at 22°C (72°F) operating temperature. Maximum operating temperature is 550°C (1000°F).
The advantages of this disc are zero manufacturing range, burst tolerance is down to plus/minus 2%, operating ratio is as high as 95%, and it may be employed in liquid-full systems
4-3 Two-way rupture disc. In the case of domed two-way rupture discs, such as shown in Figure 25 and Figure 26, flow in one direction is forward acting and in the other direction reverse acting.
The rupture disc shown in Figure 25 consists of three components.
The upper perforated component is a forward-domed rupture disc that is provided with a partial circumferential score line and protects the pressure system against overpressure. The second element is a PTFE seal member that fails under overpressure and subnormal pressure conditions. The third element is represented by a reverse-buckling bar disc that fails on subnormal pressure. On rupture of the upper component, the buckling bars will fail in tension and allow the disc to open in unity with the upper disc.
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| Fig 25 |
The two-way rupture disc shown in Figure 26 consists basically of a perforated forward-acting rupture disc that is sealed by a Teflon® reverse-acting disc, followed by a girdle supporting the disc and knife blades. Under conditions of subnormal pressure, the Teflon® disc will be forced against the knife blades and be cut. Under conditions of excessive overpressure, the perforated disc and the supporting Teflon® disc will be put in tension and rupture.
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| Fig 26 |
4-4 Graphite Rupture Discs
Graphite is a valuable material from which low-pressure rupture discs can be produced.
The graphite commonly used for the manufacture of rupture discs is the resin-impregnated grade. The material is very brittle and ruptures almost without deformation. Its structure is very homogeneous and the strength of the material is low compared with the strength of metals.
Graphite rupture discs are therefore much thicker than metal rupture discs. This property permits graphite rupture discs to be made to small burst pressure tolerances.Pure graphite is a flexible material that is used for the manufacture of reverse-buckling discs. Because pure graphite is free of resin impregnation, pure graphite rupture discs may be exposed to higher operating temperatures.
Graphite rupture discs may be used in both gas and liquid-full services
4-4-1 Monoblock-type graphite rupture discs. Rupture discs made of resin impregnated
graphite are commonly produced in monoblock form, as shown in Figure 27 through Figure 29. These are one-piece devices that combine a flat bursting membrane with the mounting flange.
The rupture disc shown in Figure 27 is of non-armored construction made in sizes DN 25 (NFS 1) through DN 600 (NFS 24). Operating ratio is 90%. The discs are suitable for operating temperatures ranging from minus 70°C (minus 94°F) to 180°C (356°F). In conjunction with a heat shield shown in Figure 30, the operating temperature may be raised to 500°C (930°F). The rupture disc, however, requires a controlled torque loading of the flange bolts.
The rupture disc shown in Figure 28 is armored by a steel ring bonded to the disc circumference to prevent unequal piping stresses from reaching the pressure membrane of the disc.
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| Fig 27 |
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| Fig 28 |
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| Fig 29 |
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| Fig 30 |
The rupture disc shown in Figure 29 is a two-way version of graphite rupture discs. The discs are custom-produced in sizes DN 25 (NFS 1) through DN 600 (NPS24). The maximum burst pressure in either direction is 10 bar (150 psig). The minimum pressure differential between the two burst pressure ratings must be 0.7 barg (10 psig), depending upon diameter and rating of the rupture disc.
4-4-2 Rupture disc device with replaceable graphite bursting membrane.
The rupture disc device shown in Figure 31 represents a design in
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| Fig 31 |
which the graphite burst membrane is a replaceable component. The purpose of this design is to achieve a greater economy of graphite. The reverse pressure supports shown, however, reduce the available vent area to approximately 50% of full-bore area.
The disc is mounted in a controlled depth recess. By this arrangement, the installation of the disc is non-torque sensitive.
4-4-3 Reverse-buckling rupture disc of pure graphite.
The disc, such as shown in Figure 32, is made of pure graphite without the inclusion of resin. For this reason, the disc can be used for chemicals and high temperatures [typically 550°C (1000°F)] that would normally affect resin impregnated graphite rupture discs. Because the discs are thicker than the membrane of the equivalent resin-impregnated graphite rupture discs, reverse-buckling rupture discs of pure graphite can easily be handled.
Discs for burst pressures above 1.2 barg (17.4 psig) will support full vacuum. Below this pressure, an additional support may be required. For burst pressures higher than 1.0 barg (14.5 psig), the burst tolerance is plus/minus 5%.
5-Rupture Disc Types According to Burst Rating
Rupture disc burst ratings define how burst pressure requirements are specified, tested, and stamped on the disc. Manufacturers generally use three standard rating types: Rated, Specified, and Min/Max. The chosen type dictates the manufacturing range and compliance with pressure codes
The 3 Standard Burst Rating Types
5-1 Rated (Stamped Burst Pressure): The customer indicates a specific burst pressure at a coincident temperature. A manufacturing range is then applied strictly to this value, meaning the disc will burst somewhere within the resulting tolerance band. (Note: Required for ASME Section VIII Div. 1 compliance).
5-2 Specified: The customer defines an exact burst pressure required, and the disc is manufactured so that its entire manufacturing range falls at or below the specified pressure.
5-3 Min/Max: The customer establishes an absolute minimum and maximum burst pressure limit. The disc is designed and tested so that the marked burst pressure and its tolerance fall exactly within this defined window.
Discs for burst pressures above 1.2 barg (17.4 psig) will support full vacuum. Below this pressure, an additional support may be required. For burst pressures higher than 1.0 barg (14.5 psig), the burst tolerance is plus/minus 5%.
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| Fig 32 |
5-Rupture Disc Types According to Burst Rating
Rupture disc burst ratings define how burst pressure requirements are specified, tested, and stamped on the disc. Manufacturers generally use three standard rating types: Rated, Specified, and Min/Max. The chosen type dictates the manufacturing range and compliance with pressure codes
The 3 Standard Burst Rating Types
5-1 Rated (Stamped Burst Pressure): The customer indicates a specific burst pressure at a coincident temperature. A manufacturing range is then applied strictly to this value, meaning the disc will burst somewhere within the resulting tolerance band. (Note: Required for ASME Section VIII Div. 1 compliance).
5-2 Specified: The customer defines an exact burst pressure required, and the disc is manufactured so that its entire manufacturing range falls at or below the specified pressure.
5-3 Min/Max: The customer establishes an absolute minimum and maximum burst pressure limit. The disc is designed and tested so that the marked burst pressure and its tolerance fall exactly within this defined window.
6- Selecting a Type of rupture disk
The following factors need to be considered.
- Burst pressure
- Permissible overpressure or vacuum
- Working pressure
- Working process and system to be protected.
- Burst temperature of the fluid.
- Vacuum resistance
The selection of rupture discs is interdependent on the pressure differential between the operating pressure and the maximum allowable working pressure of the pressure system. Manufacturing range, burst tolerance, and recommended operating ratio for the selected rupture disc must be accommodated within this pressure differential. Figure 33 shows the pressure level relationships between the maximum allowable working pressure and the operating pressure resulting from the installation of a rupture disc as the sole pressure relief device.
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| Fig 33 |
Manufacturing range in this case is plus 10%/minus 5% and the burst tolerance plus/minus 5%. For an installation of this type, the ASME Code1 requires the rated burst pressure not to exceed the maximum allowable working pressure but permits the plus burst tolerance to do so. Under these conditions, the minimum possible burst pressure could amount to 82% of the maximum allowable working pressure. Allowing for an operating ratio of 70%, the operating pressure should not exceed 57% of the maximum allowable working pressure.
For burst pressures higher than 2.75 barg (40 psig), some rupture disc manufacturers deviate from the assessment of the minimum burst pressure as shown in Figure 33 by neglecting the minus 5% burst tolerance.
When the pressure differential is as short as 10%, as is common in pressure relief valve installations, the only available choice is a rupture disc with zero manufacturing range, an operating ratio of 90% or 95%,
and possibly a reduced burst tolerance.
7- Rupture Disc Sizing
Rupture Disc Sizing Methodologies
Three basic methodologies for sizing rupture disc devices are described below. These methods assume single phase, non-reactive fluid flow. Resources such as API RP520 Part 1, the DIERS Project Manual, and CCPS Guidelines for Pressure Relief and Effluent Handling Systems provide other methods for two-phase, flashing, reactive, and otherwise non-steady state conditions.
1-Coefficient of discharge method (KD) - The KD is the coefficient of discharge that is applied to the theoretical flow rate to arrive at a rated flow rate for simple systems.
2-Resistant to flow method (KR) - The KR represents the velocity head loss due to the rupture disc device. This head loss is included in the overall system loss calculations to determine the size of the relief system.
3-Combination capacity method - When a rupture disc device is installed in combination with a pressure relief valve (PRV), the valve capacity is derated by a default value of 0.9 or a tested value for the disc/valve combination
7-1 Coefficient of discharge method (KD)
Use this method for simple systems where the following conditions are true (8 & 5 Rule). This method takes into account the vessel entrance effects, 8 pipe diameters of inlet piping, 5 pipe diameters of discharge piping, and effects of discharging to atmosphere
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| Fig 34 |
7-1-2 STEAM SIZING
For viscous liquid sizing, first calculate AR using KV of 1.0. Apply the area A of the next larger size disc to the Reynolds number calculations to arrive at KV. Then re-calculate required area AV using the derived KV.
7-2-Resistant to flow method (KR)
Use this method when the 8 & 5 Rule does not apply and the rupture disc is not installed in combination with a pressure relief valve. This type of calculation is the responsibility of the system designer
The value of KR represents the velocity-head loss due to the pressure relief device. This head loss is included in the overall system loss calculations to determine the size of the relief system.
Types of KR
Because many rupture discs have different opening characteristics depending on whether they are opened with a compressed vapor or incompressible liquid, there are certified KR values that are denoted by the applicable service media. The KR values for different media are a result of differences in how the rupture disc opens with different media and test methods that have been standardized in ASME PTC25.
• Air or gas service – KRG
Use KRG when the media is a gas or vapor, or when the media is liquid but there is a significant vapor volume directly in contact with the disc at the time of rupture
• Liquid service – KRL
Use KRL when the media is liquid and the liquid is against the disc at the time of rupture
• Air or gas and liquid service – KRGL
KRGL can be used for any service conditions
The following examples will illustrate how KR values are used to establish the flow capacity of a pressure relief piping system.
7-2-1 VAPOR SIZING
The Darcy Equation defines the discharge of compressible fluids through valves, fittings and pipes. Since the flow rate into the example vessel is defined in SCFM, the following form of the Darcy equation is used:
To determine Y, first it must be determined if the flow will be sonic or subsonic. This is determined by comparing the actual
to the limitingfor sonic flow. Crane Table shows limiting factors for k=1.4 for sonic flow at the known value of KT. If ( )sonic < )actual, then the flow will be sonic.
For this example
From table A-22 at KT=7.33
Since () sonic = 0.754, then ∆P = 0.754 * P’1 = 0.754 * 1114.7 = 840.5 psig
The ASME Pressure Vessel Code, Section VIII, Division 1, paragraph UG-127(a)(2), also requires that the calculated system capacity using the resistance to flow method must also be multiplied by a factor of 0.90 or less to account for uncertainties inherent with this method.
Thus, the system capacity is greater than the required process capacity (20,000 SCFM)
Now that the fluid velocity is known, the volumetric flow rate can be calculated.
The Darcy Equation defines the discharge of compressible fluids through valves, fittings and pipes. Since the flow rate into the example vessel is defined in SCFM, the following form of the Darcy equation is used:
To determine Y, first it must be determined if the flow will be sonic or subsonic. This is determined by comparing the actual
to the limitingfor sonic flow. Crane Table shows limiting factors for k=1.4 for sonic flow at the known value of KT. If ( )sonic < )actual, then the flow will be sonic.From table A-22 at KT=7.33
Since (
) sonic = 0.754, then ∆P = 0.754 * P’1 = 0.754 * 1114.7 = 840.5 psigCalculating the system capacity is completed by substituting the known values into Crane 410 Equation
Thus, the system capacity is greater than the required process capacity (20,000 SCFM)
7-2-2 LIQUID SIZING
The friction factor used earlier in the calculations for piping frictional losses assumed that the flow in the pipes was fully turbulent flow. The value of the friction factor is related to the Reynolds Number (Re) of the resulting flow (Ref: Crane 410 pg 1-1). For Re< 2000, the flow is laminar and the friction factor is a function of Reynolds Number, only. For Re >4000, the flow is fully turbulent and the friction factor is also a function of the character of the piping wall (relative roughness).
The friction factor used earlier must be verified. First calculate the Reynolds Number:
Since the Reynolds Number is >4000, the flow is turbulent, and the friction factor is now a function of the relative roughness of the pipe. From Crane 410 Figure A-23, the friction factor, f, for 2” commercial steel pipe in fully turbulent flow is 0.019. This verifies the original assumption for friction factor.
Now that the fluid velocity is known, the volumetric flow rate can be calculated.
Therefore, the relief system can flow the required 50 ft3/min.
When a bursting disc device is installed in combination with a pressure relief valve (PRV), the valve capacity is derated by a default value of 0.9 or a tested value for the specific disc/valve combination. For specific application requirements when using bursting disc devices in combination with PRV’s, consult Fike. The table on next page provides certified combination capacity factors that are published in the National Board of Pressure Vessel Inspectors “Red Book”. To use these values, find the listing for the specific rupture disc and valve models. Multiply the rated valve capacity by the combination capacity factor to arrive at the capacity of the combination. Use the value of 0.90 for combinations in smaller sizes or lower pressures than listed
8- Components of a Rupture Disk Device
The rupture disk is made of two main components:
-- the rupture disk holder which provides the pressure boundary and clamps the disk into position,
-- and the actuating element or the disk itself.
 |
| Fig 37 |
7-4 Overpressure Allowance
When sizing pressure relief devices, the ASME Code defines the maximum pressure that may build up in the pressure vessel while the device is relieving. This pressure varies depending on the application of the device. The following table defines the various overpressure allowances.
References:
Fike Form No. TB8102-1 February, 2009 Specifi cations are subject to change without notice.
American Society of Mechanical Engineers, Boiler and Pressure Vessel Code Section VIII, Division 1
American Society of Mechanical Engineers, PTC25
American Petroleum Institute, RP520
Crane Valves, Technical Paper 410
Crane Valves, Crane Companion Computer Program
Fike Technical Bulletin TB8100 ASME Code and Rupture Discs
Fike Technical Bulletin TB8103 Certified Combination Capacity Factors
Fike Technical Bulletin TB8104 Certified KR and MNFA Values
Fike Technical Bulletin TB8105 Best Practices for RD & PRV Combinations
DIERS Project Manual
CCPS Guidelines for Pressure Relief Effluent Handling Systems
valve_selection_handbook_4e
8- Components of a Rupture Disk Device
The rupture disk is made of two main components:
-- the rupture disk holder which provides the pressure boundary and clamps the disk into position,
-- and the actuating element or the disk itself.
 |
| Fig 38 |
9- Installation is KEY
This is a big one. The vast majority of problems I see with rupture disks not being safe, or not performing properly is due to a bad installation, and it is easily avoided by following a few simple steps.
Firstly – check you are installing the right disk to the right location. You can check the tag for the part number/specification and as I mentioned above, make sure you are using the right holder. Usually, the disk tag will be customized to include a P&ID part no. so it doesn’t take long at all.
Secondly – Check the condition of the disk and holder. Sometimes disks can be damaged in storage if they have been there a while. Any dents, scratches or corrosion need to be checked out, as it can affect the disk performance. The holder is even more liable to damage as it has been sitting in the process for a long time, maybe even seeing corrosive substances! You need to check all the surfaces, legibility or the tag, and especially the bite seal (this is what makes the disk leak-tight).
Thirdly – USE THE INSTRUCTIONS! Every rupture disk will come with detailed instructions on how to install or replace that specific type of rupture disk, including important information like they type of holder it goes with, and what torque needs to be applied to cap screws and companion flange bolts.
If you have any doubts about this, ask the manufacturer to come to site and do a training session with your technician team. It doesn’t take long, and may just save the site from costly replacements due to bad installation. If the manufacturer won't support you with this, time to switch to one that will.
· ASME Section VIII, Div. 1: The most widely adopted U.S. standard for pressure vessels. It outlines the mandatory design, material, and testing criteria for rupture discs used in non-fired applications.
· ISO 4126: The international standard (often used alongside European EN standards like EN 14491) detailing performance, safety, and testing methods for overpressure devices.
· API 520 / 521: Standards that guide the sizing, selection, and installation of pressure-relieving devices, including specific limits on burst pressure relative to a vessel's Maximum Allowable Working Pressure (MAWP)
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