Glossary of Terms
ACTUAL BURST PRESSURE
This is the actual pressure in the system when the rupture disk bursts. This pressure would ordinarily be the same as the Stamped Burst Pressure, unless the disk was improperly installed or damaged. Back pressure on the disk would likely cause a deviation from Stamped Burst Pressure.
ASME (American Society of Mechanical Engineers)
A recognized standard to which most rupture disk manufacturers build their products.
COMPRESSION LOADED
A compression loaded rupture disk is installed into a system so that the normal operating pressure is on the convex or raised side of the formed crown. An example of a compression loaded rupture disk would be a reverse buckling style.
DAMAGE RATIO
A damaged rupture disk will burst at some pressure other than that predicted. This disparity can be reported by value called the "damage ratio". The damage ratio is equal to Actual Burst Pressure of a damaged disk, divided by the Stamped Burst Pressure. A damage ratio of 1 or less assures your client that the disk, even damaged, will burst at or below the stamped burst pressure while a value higher than one would indicate the actual burst pressure could exceed the stamped burst pressure. As an example, a damaged disk with a 100 psig stamped burst pressure and a damage ratio of 1.5 could have an actual burst pressure of 150 psig.
DIFFERENTIAL PRESSURE
A rupture disk is a differential pressure device. The disk will burst when the differential pressure across it exceeds the stamped burst pressure. If the system has back pressure, this must be added to the stamped burst pressure to calculate the true rupture pressure.
ENCLOSURE
A confined for partially confined volume such as a room, building, vessel, silo, bin, pipe, or duct.
ENVIRONMENTAL TESTING
A test break in ovens to simulate the operating temperature under which the disk will be expected to perform. For example, an Oseco FAS disk is ordered with a specific burst pressure at a specific temperature. One or more test breaks would be performed in an environmental oven to verify the disk would indeed break at that combination of events.
FAILSAFE
Refers to a disk with a Damage Ratio and a Reversal Ratio of one or less. If a disk is damaged or installed upside down, the disk will still open at or below the stamped burst pressure.
FINDING PROCESS
A controlled, experimental procedure that the craftsmen go through to achieve a burst pressure within the manufacturing range. The disk fabricator uses mathematical formulas, statistical process controls, and historical records to find a particular burst pressure.
FUEL CONSTANT
Fuel characteristic constant used in Low-Strength Enclosure equations.
HIGH-STRENGTH ENCLOSURE
Enclosure that is capable of withstanding pressures (Pred) higher than 1.5 psig or 0.1 bar.
Kg
Deflagration index for gases. Kg is essentially the maximum rate of pressure rise generated when tested in a confined enclosure. The manufacturer of the process material should be able to provide this value, it may be located on the MSDS sheet. Sometimes additional outside testing is required to determine these values.
Kst
Deflagration index for dusts. Kst is essentially the maximum rate of pressure rise generated when tested in a confined enclosure. The manufacturer of the process material should be able to provide this value, it may be located on the MSDS sheet. Sometimes additional outside testing is required to determine these values.
LOW-STRENGTH ENCLOSURE
Enclosure that is capable of withstanding pressures (Pred) of no more than 1.5 psig or 0.1 bar.
MANUFACTURING LOT
A "Lot" consists of all the disks on an order that are of like size and style with the same burst pressure and temperature requirements. In other words, they are identical.
MANUFACTURING RANGE
ASME describes manufacturing range as follows: "The manufacturing design range is a range of pressure within which the marked burst pressure must fall to be acceptable for a particular requirement as agreed upon between the rupture disk manufacturer and the user or his agent." (UG-127 Foot Note 46)
The manufacturing range is predetermined, allowable deviation from the REQUESTED burst pressure, within which the stamped burst pressure may fall and still be considered acceptable to the manufacturer and user. It's similar to tolerances on machined parts. Manufacturing ranges are published in the catalog by product type. Each disk style has its own table of manufacturing ranges.
An example of a manufacturing range for Standard or Composite disk might be as follows: Assume a requested burst pressure of 100#, and a manufacturing range between +10% to -5%. This order of disks could be produced with a stamped burst pressure anywhere from 110# to 95#, and would be considered "good parts" within the range. Keep in mind, every disk in the lot would be stamped at the same burst pressure.
Often the manufacturing range can be adjusted by shifting the entire range to the minus side of the requested burst pressure. Using our example above, the total 15% manufacturing range can be shifted to the minus side. Now the 100# requested burst pressure would be the maximum possible and the stamped burst pressure on the disk would fall between 85# and 100#. As before, every disk in the lot would be stamped at the same burst pressure. In some cases, 1/2 or 1/4 range disks are available.
The manufacturing range for pre-scored rupture disks, such as Oseco's PCR and FAS disks are usually expressed as 10%, 5%, or even 0% ranges. A 0% range rupture disk has a stamped burst pressure exactly as ordered without deviation. Scored rupture disk ranges are always on the minus side. For example, the stamped burst pressure for a 100# FAS disk with a 5% manufacturing range would fall from 95# to 100# inclusive.
MODEL NAMES
When describing an Oseco rupture disk with options, we usually use descriptive abbreviations such as COV or RSTDR. These abbreviations are assigned from the top down. Possible accessories include (R)ings, (L)iners, and (V)acuum supports. Using this system, you would know that a RCOV would mean a (R)ing on top of a (CO)mposite Disk with a (V)acuum support underneath.
NONFRAGMENTING
Some styles of disks are designed to burst or rupture without producing pieces. Others are designed to produce minimal fragmentation.
OPERATING RATIO
The operating ratio refers to the relationship between normal operating pressure and stamped burst pressure. Operating ratio is usually expressed as a percentage and varies with the style of disk. If the operating ratio is exceeded, the service life of the disk will be reduced. For good service life, the disk must be operated at or below its operating ratio. For example, an Oseco standard disk has an operating ratio of .7 or 70%. This means that the disk should not be operated at more than 70% of the stamped burst pressure for good service life. Other disks such as the FAS or PCR have a .9 or 90% operating ratio. It is important to consider the operating ratio when selecting a rupture disk.
Using an example, let's look at the relationship of these factors. Assume you are going to protect a vessel with a MAWP of 500 psig and a normal operating pressure of 410 psig. You have chosen the FAS scored rupture disk because it is nonfragmenting. Your requested burst pressure is 500 psig. Now we will consider the manufacturing range. If you ordered the disk with a 10% manufacturing range (10%, 5% and 0% available), your disks could have a stamped burst pressure of 450 psig to 500 psig. Let's assume the "worst case" in which the stamped burst pressure is 450 psig. Because the operating ratio is 90%, the normal operating pressure on this disk should not exceed 405 psig, which is 5 psig lower than the 410 psig your system requires. You would need to order a 5% manufacturing range to properly satisfy the requirements in this example.
PRE-BULGED
Forming the rupture disk at the plant into its traditional crowned shape.
Pmax
Maximum pressure developed in an un-vented vessel. The manufacturer of the process material should be able to provide this value, it may be located on the MSDS sheet. Sometimes additional outside testing is required to determine these values.
Pred
Maximum pressure developed during a vented deflagration. The pressure in an enclosure will continue to rise after a rupture panel releases until it reaches this value. Pred is generally based on the strength of the enclosure.
Pstat
Rupture panel release pressure.
REVERSAL RATIO
Is equal to the Actual Burst Pressure of a rupture disk installed in reverse divided by Stamped Burst Pressure. If the value is one or less, the disk will relieve at or below its Stamped Burst Pressure even when installed in reverse. If the value is greater than one, the Actual Burst Pressure will be greater than the Stamped Burst Pressure.
RUPTURE TOLERANCE
Applies to the amount of acceptable deviation between the stamped burst pressure and the actual burst pressure. ASME requires that the variance not be greater than +5% at the specified disk temperature for pressures above 40 psig. A pressure of 40 psig and lower requires a Rupture Tolerance of +2 psig.
SPECIFIED DISK TEMPERATURE
The temperature specified by the customer at which the disk expected to burst. The burst pressure at this temperature will be stamped on the tag of the disk.
STAMPED BURST PRESSURE
Often called the Set Pressure or Rupture Pressure. This is the pressure stamped on the tab which states at what point the disk is designed to open. A specified disk temperature will be stamped in the tab with the Set Pressure, as required by the ASME code.
STANDARD RUPTURE DISK MATERIALS
Oseco's standard rupture disk materials include 316 Stainless Steel, Nickel 200, Inconel 600, Monel 400 and Aluminum. Other materials suitable for the construction of rupture disks include various plastics like Ryton, Tantalum, Hastelloy C, Silver and Gold plated materials. Fluoropolymer is normally used for liners, slit slot covers and nonmetal seals.
SURFACE AREA
Internal surface area of enclosure.
TENSION LOADED RUPTURE DISK
The disk is installed in a system so that the normal operating pressure is on the concave or cupped side of the pre-bulged crown. When the material of construction reaches its yield point, the disk will burst open to relieve the trapped pressure.
VESSEL L/D
Length to diameter ratio of the enclosure. For non-circular enclosures, the equivalent value to be used for "D" is 2*(A / 3.14159)^0.5 where "A" is the cross sectional area normal to the longitudinal axis of the enclosure.
VESSEL VOLUME
Volume of vessel or enclosure that is to be protected.