A rupture disc (RD) is a thin diaphragm (generally a solid metal disc) designed to rupture (or burst) at a designated pressure. The RD is used as a weak-point element for the protection of vessels and piping systems against excessive pressure (positive or negative). In contrast to pressure relief (PR) valves, a RD is non-reclosing. Thus, a RD provides a permanent open path into the discharge system. The discharge system may either be the atmosphere or a closed system such as a flare header.
Rupture discs can be used as the only pressure relieving device or can be used in conjunction with PR valves either as a secondary (parallel) relief device or in series with the PR valve itself (either at the inlet or the outlet of the PR valve). (If the RD is used at the inlet of the PR valve, it serves to reduce fugitive emissions or corrosion of the PR valve. When used at the outlet of the PR valve it can only serve to reduce fugitive emissions.) Rupture discs are generally installed in specially designed disc-holder assemblies (or safety heads). The most common type of RD assembly used, fits between standard flanges.
There are five major types of rupture discs available, as follows:
· Conventional tension-loaded rupture disc.
· Pre-scored tension-loaded rupture disc.
· Composite rupture disc.
· Reverse buckling rupture disc with knife blades.
· Pre-scored reverse buckling rupture disc: cross-score and semi-circular score.
The major characteristics of these are summarized in Table II-1. Other types such as solid graphite and machined discs are also available, but they may have more limited applications. Since graphite is chemically inert, solid graphite discs are sometimes used in chemical plants. In general, graphite discs normally behave similarly to metallic conventional discs.
7.8.1 Advantages
When compared with PR valves, rupture discs have the following advantages:
1. No simmering or leakage prior to bursting. Unless damaged or corroded, rupture discs are not subject to simmer or leakage at pressures below their burst pressure, Pb.
2. More effective against overpressure caused by deflagration. Rupture discs can open fully within 1 millisecond for vapor/gas systems, thus making them more effective than PR valves when the overpressure is caused by an internal deflagration or sudden pressurization (for example as a result of a tube failure in a high pressure heat exchanger).
3. Less expensive to provide corrosion resistance. Rupture discs can be made of, or coated with, a variety of corrosion resistant materials. This allows the RD manufacturers to provide the corrosion resistance at a lower cost than having to upgrade the materials in a PR valve.
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4. Less tendency to foul or plug. Rupture disc opening is essentially equal to the piping bore and results in a pressure relief path without changes in flow direction. This feature reduces the potential for fouling or plugging once the device has opened compared to a PR valve.
5. Can provide both depressuring and overpressure protection. Since it is a non-reclosing device, a RD offers the possibility of simultaneous overpressure protection and depressuring, if the RD is oversized. This feature is not possible with PR valves and may be advantageous in some services.
6. Lower initial cost than for an equivalent service PR valve.
Another reported advantage for reverse buckling type rupture discs is the possibility of on-line testing of a PR valve when the RD is installed upstream of the PR valve. However, this type of PR valve testing is not recommended since significant lift (“pop") can not be achieved on the valve (due to the small volume of test media employed), therefore, it is not possible to confirm PR valve opening. In addition, it is not possible to check for fouling, blockage or corrosion.
All of these are important to ensure the long-term adequacy of overpressure protection. Therefore, RDs should not be installed or used for the purpose of carrying out on-line PR valve testing.
7.8.2 Disadvantages
Disadvantages common to all rupture discs regardless of type include:
1. Non-reclosing pressure relief device. Replacement of the burst rupture disc is required to allow continued operation if it is the only protective device. Alternatively, if the RD is used in series with a PR valve, operations can continue without replacement of the burst RD, but the extra protection that is afforded to the system by the RD is lost until it is replaced.
2. Non-destructive testing of the RD burst pressure cannot be accomplished. Unlike PR valves which can be adjusted, the accuracy of burst pressure is solely based on the manufacturer's tests on discs from the same lot.
3. Require periodic replacement. Rupture disc life is finite even when discs are installed with adequate margin between operating and burst pressure. Thus, rupture discs must be periodically replaced. As a result of cycling, the disc will eventually be weakened by fatigue, although the reverse buckling discs normally are less susceptible to fatigue. In addition, rupture discs are subject to corrosion especially along the score line of pre-scored discs. Both these factors limit the useful life of the RD.
4. Greater sensitivity to mechanical damage. Any denting or distortion of the rupture disc will affect the RD burst pressure.
5. Greater sensitivity to temperature. Since the burst pressure depends on disc material properties, the temperature at the time of burst will cause the burst pressure to vary. Choice of material has a great influence on the sensitivity to pressure (see Figure II-11).
6. Some types, as shown in Table II-1 will not function in liquid service. Even when they function, the opening achieved is not as large as in vapor services, typically being about one-half the nominal rupture disc flow area.
Other disadvantages are specific to RD type and include among others: possible unsafe condition if installed upside down, possible fragmentation (which can restrict relief rates and also prevent PR valve closure if the RD is installed upstream of the PR valve), possibly having to provide a greater pressure margin to avoid fatigue failures, and a large manufacturing range which forces increases in equipment design pressure (or reduction in operating pressure).
7.8.3 Acceptable Types of Rupture Discs
To ensure the safety of the facility, rupture discs must be “fail-safe" and must be carefully selected and installed. To be “fail-safe," the rupture disc must not fragment and its design should limit the burst pressure when damaged or installed upside-down to less than 1.5 times the system's design pressure or proof test pressure whichever is lower.
Considering these requirements, only three RD types are generally acceptable/recommended:
1. Pre-scored (cross-score) reverse buckling RD for gas service. (Figure II-9)
2. Pre-scored (semi-circular score) reverse buckling RD installed in holder with capture bar for gas or liquid service.
3. Pre-scored tension-loaded RD for gas or liquid service (Figure II-10).
These RDs are available in a variety of sizes, burst pressures, burst temperatures and materials. Some of these are shown in Tables II-3 and II-4. Even though both these types satisfy the general “fail-safe" criteria, their installation and use must be carefully monitored to ensure adequate safety. For example, since these discs are only guaranteed
to burst at no more than 1.5 times the burst pressure when installed upside-down or damaged, they can not be used to protect systems that are pneumatically tested or hydrotested to less than 1.5 times their design pressure.
7.8.4 Rupture Disc Certification and Testing
In order to ensure adequate performance, rupture discs should be ASME certified. To ensure the “fail-safe"
characteristics of the RD, the manufacturer should also be required to stamp and rate the RD at the desired temperature and to test at least one disc upside-down. If this upside-down test shows the RD burst pressure to be higher than 1.5 times the stamped burst pressure (per the ASME certification) the complete lot of discs is unacceptable.
7.8.5 Rupture Disc Specification
Table II-3 presents a sample rupture disc specification sheet.
7.8.6 Manufacturing Range of Rupture Discs
Since it is not practical to make, or carry in inventory sheet metal in all possible thickness needed, the manufacturer uses the manufacturing range (MR) to compensate. Manufacturing range is defined as the allowable range of pressures, mutually agreed upon between the manufacturer and the buyer, within which the rupture disc can be rated and stamped. This range is generally defined as a percent of desired burst pressure (gauge). Thus, when specifying a RD it is not sufficient to list the desired burst pressure; the required manufacturing range must also be defined.
The magnitude of manufacturing range differs between manufacturers and is a function of RD type and burst pressure. The complete range must be applied on the low side of the protected system's design pressure. Typical manufacturing ranges are listed below. Lower manufacturing ranges may be available at additional cost. (If the manufacturing range is zero, the RD burst pressure is stamped with a burst pressure equal to the requested burst pressure, usually the system design pressure.)
RUPTURE DISC TYPE PRE-SCORED REVERSE BUCKLING
PRE-SCORED TENSION-LOADED
TYPICAL MANUFACTURING RANGE, % 0, -5, OR -0
Pb, psig (kPa gage) 20 to 45 (138 to 310) 46 to 90 (311 to 620) 91 to 270 (621 to 1860) 271 to 500 (1861 to 3450) above 500 (above 3450)
-7/+14 -6/+12 -5/+10 -4/+8 -3/+6
7.8.7 Rupture Disc Burst Pressure
For a new design, the type of RD to be used must be selected. The lowest allowed burst pressure is defined by using the appropriate spread between maximum operating pressure and RD burst pressure (Table II-1). The maximum possible burst pressure is defined by applying the corresponding total manufacturing range (positive plus negative manufacturing range) to the minimum allowed burst pressure. The system design pressure must be at least as high as the maximum possible burst pressure. To specify the RD itself, the lowest allowed burst pressure and the negative manufacturing range are used. After manufacture, each RD is stamped with its rated burst pressure (determined from burst tests of the lot in question). This stamped burst pressure will fall between the limits defined by the manufacturing range but will only match exactly the requested burst pressure if the manufacturing range is zero.
For example:
P = Maximum Normal Operating Pressure - 75 psig (515 kPa gage)
RD type = pre-scored tension loaded (from Table II-1; maximum operating pressure = 0.85 Pb) MR = - 6%/+12%
1. The minimum burst pressure is defined by the required spread between operating and burst pressure:
Pbmin = 75 / 0.85 = 88 psig
= 515 / 0.85 = 606 kPa gage
2. The system design pressure (DP) must take into account the full manufacturing range to be requested:
DP = Pbmin/ [1 – (| – MR| + |+ MR|) / 100] = 88 / [1 – (| – 6| + | + 12|) / 100] = 107 psig
= 606 / [1 – | – 6| + | + 12|) / 100] = 739 kPa gage
3. The RD specification must also take into consideration the manufacturing range. However, since the manufacturing range is defined in terms of desired burst pressure, only the negative manufacturing range is applied:
RD rating (Pb) = Pbmin /[1 – | – MR| / 100] = 88/[1 – | – 6| / 100] = 94 psig
= 606 / [1 – | – 6| / 100] = 645 kPa gage
4. The engineer requests a disc with a burst pressure of 94 psig (645 kPa gage) at a -6/+12% manufacturing range.
The resulting RD will be rated and stamped anywhere from 88 to 105 psig (606 to 722 kPa gage).
7.8.8 Rupture Disc Burst Temperature
The burst pressure of a rupture disc is a function of the temperature the disc experiences when it bursts. The sensitivity of the disc burst pressure to temperature depends on the material used and is illustrated in Figure II-11 for tension-loaded discs. Of the common disc materials shown, Inconel is the least sensitive to temperature followed by nickel and 316 SS. Aluminum is, in general, the most sensitive. Due to their design, reverse buckling discs are about half as sensitive to temperature as are tension-loaded discs.
Rupture disc temperature is a critical parameter since, in most instances, the disc is installed at the end of a piping section where there is normally no flow. Unless very well insulated, and especially when discharge is to the atmosphere, the normal rupture disc temperature can be significantly different than the normal process temperature.
(Heat loss from the static material below the RD is significant and can amount to 50 - 100°F (28 - 56°C) or higher depending on the length of piping). When an overpressure occurs, the RD will burst at its normal temperature rather than the ultimate relieving temperature. Thus, being able to accurately define the bursting temperature of the disc is vital to the safety of the system since at lower temperatures the RD may burst at higher pressure.
When the actual RD temperature is not known, use of Inconel as the disc (not the disc assembly) material should minimize the variability of burst pressure. Alternatively, the disc can be specified for the most conservative temperature which is normally the discharge system temperature. The discharge system temperature is ambient when the RD discharges directly to the atmosphere or to the inlet of a PR valve. Utilizing the ambient temperature is conservative since this is likely to be lower than the process temperature and the disc will normally experience a temperature somewhere between these two. Designing the RD for the lower temperature may result in a lower (and safer) actual burst pressure. Only in very few cases, will use of process or emergency temperature be appropriate for RD rating.
Whenever possible, the actual installation should be checked to verify that the actual disc temperature is reasonably close to the rated disc temperature. In cases where the rated RD temperature is higher than the actual temperature, the system must be further analyzed to determine if it is safe. Otherwise, a RD with the proper temperature rating should be retrofitted.
In specifying disc-holder assembly materials of construction, the relieving temperature is used, since the safety head will ultimately be exposed to these conditions.
7.8.9 Rupture Disc Sizing
The RD must be sized to prevent the pressure within the protected system from exceeding the limits allowed by code.
Once burst, the RD becomes a single component in the discharge piping and it is the complete piping system that has to be designed to allow the required relief without exceeding the accumulated pressure limits. Only limited tests have been carried out to define the frictional pressure drop across burst rupture discs. A conservative estimate of pressure drop can be obtained by assuming it has a pressure drop equivalent to the pressure drop in a pipe 75 nominal disc diameters in length. This approximation is used to compensate for the disc material protruding into the flow stream. Alternatively, if the RD is the only device and there is essentially no inlet or outlet piping (i.e., less than two pipe diameters), the RD can be sized as a PR valve with a discharge coefficient (Kd) of 0.62.
When used in liquid service, an additional pressure drop (as discussed above) is introduced by the fact that the rupture disc is not likely to open fully. To account for partial opening in liquid service, a pressure drop equivalent to the pressure drop through a restriction orifice with area equal to one-half the nominal rupture disc flow area should be used.
When the RD is used at the inlet of a PR valve, the RD must be at least equal in diameter to the PR valve inlet flange.
In this situation, the ASME pressure vessel code requires that unless they have been flow tested together, the RD-PR valve combination must be derated by 10% from the PR valve capacity (i.e., the combination has a capacity factor of 0.9 relative to the PR valve itself). In practice, most tested combinations have a combined capacity factor of 0.95 or higher. Thus, in general, tested combinations are recommended, (refer to the National Board of Boilers and Pressure Vessel Inspectors publications for information on the tested combinations and the measured capacity factor).
When rupture discs are used to relieve overpressure caused by an internal deflagration, equations such as those in NFPA-68 should be used to calculate the required relief area. In the case of reaction runaway, the relief area is usually determined from small scale tests.
7.8.10 Rupture Disc Installation
Rupture Disc Installed by Itself - A rupture disc not installed in series with a PR valve must comply with the following:
1. CSO block valves (when allowed by local codes) must be provided upstream of the RD (and also downstream if discharge is to a closed system) to allow isolation for inspection and replacement without shutting down the unit.
(Block valves would not be required if the equipment on which the RD is installed can be isolated or taken out of service with the remainder of the facility onstream.)
2. The discharge piping must comply with PR valve installation requirements including size, bracing and maximum/minimum support, velocity, drainage of piping away from the PR device, elevation of discharge point, and snuffing steam connections.
3. In general, inlet piping will be equal in diameter to the rupture disc nominal size. Outlet piping will be at least equal in diameter but may be of larger diameter than the RD size.
4. Rupture discs need not comply with the inlet piping pressure drop limitation since these are only required to prevent chattering of PR valves, which cannot occur in a rupture disc. However, if significant, the inlet line pressure drop must be taken into account in establishing the system design pressure. In addition, the discharge piping must limit the accumulated pressure in the system being protected to 10% of the design pressure (or 21%
in the case of fire contingency).
Rupture Disc Installed at the Inlet of a PR Valve - Any RD installed at the inlet of a PR valve must be located immediately beneath the valve. Only a small (less than 2 ft or .6 m) section of piping with a “tell-tale" assembly should be provided between the PR valve and the RD. This “tell-tale" assembly is required to detect and relieve pressure buildup between the RD and the PR valve. Unless this requirement is complied with, pressure buildup caused by any leakage through the disc will increase the system (upstream) pressure at which the disc will burst.
(The disc will continue to burst at its design differential pressure, but since the discharge side pressure has increased there is a corresponding increase on the upstream pressure at burst.)
The preferred method to detect/relieve pressure buildup between the RD and PR valve, is an installation which includes a bleeder valve, a pressure gauge, and an excess flow valve piped to a safe location (as per GP 03-02-04).
Under normal conditions any leakage through the disc is relieved through the excess flow valve. If leakage is large, the excess flow valve is forced closed and the pressure gauge will record a positive pressure. When the disc bursts due to overpressure, the excess flow valve will also close to prevent release of material. To make this “tell-tale"
design effective, the operator must regularly (at least once a shift) verify that the RD has not failed. If a small leak is suspected, the CSO valve downstream of the pressure gage may be closed. Thus, the pressure gauge readout and CSO valve should be brought to an easily accessible location. In addition, a high-pressure switch and alarm may be used to alert the operator to a problem.
Rupture Disc Installed at the Outlet of a PR Valve - A rupture disc can also be installed at the outlet of a PR valve to reduce fugitive emissions. This type of installation may also require venting of the volume between the PR valve and the RD to prevent pressure buildup. When used in this manner, the RD burst pressure must be very low (a few psig or kPa gage) to prevent a large back pressure on the PR valve prior to the disc bursting. In addition, the venting arrangement must go to a closed system to make feasible the reduction in fugitive emissions.