Rupture disks are nonreclosing pressure relief devices designed to burst at a specified differential pressure, enabling flow through the area formerly blocked by the burst material. Rupture disks are manufactured in a variety of designs, for both liquid and vapor services, from a variety of materials. Consequently, they can have a variety of performance characteristics with respect to corrosion resistance, operating pressure margins, burst pressure ranges, and service lifetimes. See API Recommended Practice 520, Part I, the CCPS monograph Guidelines for Pressure Relief and Effluent Handling Systems, and manufacturer’s literature for further discussions of the various designs of rupture disk devices and their performance characteristics. Figure 1200-16 shows a diagram of a typical rupture disk and its holder.
1. Advantages of Rupture Disks
a. They open and allow flow more rapidly than valves.
b. They have a larger flow capacity than a pressure relief valve of identical inlet nominal pipe size.
c. They are less prone to leakage at pressures below their stated operating pressure limits.
d. They are simpler devices, and therefore generally cheaper to purchase. They require little or no maintenance, but, rather, are replaced periodically.
2. Disadvantages of Rupture Disks
a. Complete de-pressure with potentially large loss of inventory.
b. Need to replace before process can be re-started.
c. Need a larger gap between operating and set pressure, consequently the equipment has to be designed for higher pressure.
d. Subject to fatigue failure.
e. The recommended interval for replacement is generally one year.
Because of the short life, rupture disks are generally not recommended for refinery equipment to operate continuously for a number of years between shutdowns.
In light of their characteristics, rupture disk devices are selected for applications requiring rapid opening (such as for protection of the low pressure side of a heat exchanger against a tube rupture contingency) and/or high flow capacity. In these applications, rupture disk devices are used as stand-alone relief devices. In an alternative application, their positive closure and low cost lead rupture disks to be installed at the inlet or outlet of pressure relief valves to isolate the valve from fluids that may be toxic, corrosive, or that might cause fouling of the valve. In these applications it is important to note that the installation of a rupture disk in series with a pressure relief valve causes a small reduction in the valve’s flow capacity (see “Rupture Disks Upstream of Pressure Relief Valves” on page 1200-43 for further discussion of the combination capacity factor).
Note Retrofitting of an existing refinery relief valve with a rupture disk is not a viable option for two reasons: 1) it requires yearly replacement, and 2) it would generally require reducing the set pressure and could require increasing the relief valve size.
Rupture discs should be mounted in the holder provided by the maker to avoid premature rupture due to pipe stress. If back pressure or vacuum can cause pressure reversal, a rupture disc with adequate vacuum strength is required.
When ordering, specification data must include expected temperature at rupture conditions (not normal operating temperature). Disc rupture pressure is greatly affected by temperature; too many discs have ruptured prematurely because they were not specified correctly.