A single rupture disc protects a vessel or pipeline by bursting within a certified tolerance of its marked burst pressure, but between relief events it is also the only barrier between the process and the vent line. Where the media is toxic, aggressive or too valuable to lose, a leak through a corroded or damaged disc means product loss, an exposure risk, or an unplanned shutdown to change the disc. A double disc assembly is used to solve this problem by mounting two rupture discs in series with a monitored space between them, so a leak in the process-side disc shows on a gauge and is contained by the second disc instead of passing downstream.
How a double disc assembly works
The assembly holds two rupture discs in one flanged unit: an inlet disc exposed to the process and an outlet disc on the vent side, separated by a mid-flange. A pressure gauge fitted to the mid-flange monitors the space between the discs, and the ASME Code requires that gauge to be fitted. The assembly can be supplied with all standard materials, in all flange assemblies and ANSI ratings, with standard, composite or reverse buckling discs.
In normal operation, both discs sit closed and the relief path stays sealed. What happens in the space between them depends on the application, and that space is what gives the assembly its three distinct uses.
Reason 1: leak detection and containment for toxic or high-value media
Extremely corrosive conditions or mechanical damage can cause the process-exposed disc to leak. In a conventional single disc installation that leak passes straight to the vent line, and with toxic or high-value contents the usual response is an immediate shutdown.
In a double disc assembly configured for this application, the space between the discs is held at 0 psig. If the inlet disc develops a leak, the mid-flange gauge registers the rise while the outlet disc keeps the system sealed. Production continues, no product passes to the vent, and the leaking disc is changed out at an opportune time, for example the next planned stop. For chemical, pharmaceutical and gas processing applications where the media must stay in the pipe, this turns a forced shutdown into a scheduled maintenance task.
Reason 2: a quick-opening device with no moving parts
Pressurising the space between the two discs turns the assembly into a quick-opening device. Each disc holds against a reduced differential while the interspace is charged. When the interspace pressure is dumped on command, the discs burst and the full open area of the line becomes available within a few milliseconds.
Because the opening elements are rupture discs rather than a valve, the device has no moving parts to maintain or to stick. The arrangement has been used to activate pressure systems and to test systems for sudden pressure changes.
Reason 3: a reaction starter for media that ignite on contact
A related application keeps two reactive media apart until the moment they are meant to combine. In testing liquid media that ignite on contact, premature mixing could cause an unwanted explosion. The double disc assembly contains the fuels separately until it is activated to release them into the system for ignition. This suits research and test facilities working with contact-reactive fuels, where the release must happen once, on command, and not before.
Sizing the discs
Disc selection process starts from the operating pressure and the pressure that will be held between the discs, and the operating pressure must be pre-determined before the discs are specified. Continental Disc publishes the sizing rule as a working fraction: the pressure each disc sees in service must not exceed 70% of its rated burst pressure for a standard disc, 80% for a CDC or CDCV composite disc, or 90% for a reverse buckling disc.
The manufacturer’s worked example makes the arithmetic concrete. With an operating pressure of 1,000 psig and the interspace held at half that, 500 psig, each composite disc sees a 500 psig differential. At the 80% working fraction, both the inlet and the outlet discs are rated at 625 psig.
The design notes cut both ways. A lower interspace pressure combined with a lower outlet disc rating pushes the required inlet disc rating up, while a higher interspace pressure with a lower inlet disc rating pushes the outlet disc rating up. In leak-detection applications the interspace is held at 0 psig, so the inlet disc is selected directly against the full operating pressure at its working fraction.
Cost and delivery
A double disc assembly is priced at about one and three quarter times a single disc assembly of the same size, material and rating, and delivery matches a standard assembly of the same size and material. Set against a lost batch of high-value product, an exposure incident or an unplanned shutdown to change a leaking disc, the premium is a modest line in the relief system budget.
Specifying a double disc assembly in Australia
Measurement Solutions is the authorised Australian distributor for Continental Disc Corporation. Our application engineers size double disc assemblies against the relief case: send the operating pressure, the media, the temperature, the line size and the intended application (leak detection, quick opening or reaction starting) and we will recommend the disc types, the interspace pressure and the assembly configuration, then support the installation through commissioning.
Measurement Solutions Pty Ltd, A Bestech Company, has integrated into Bestech Australia, so the same application engineering is now backed by nationwide sales and support. Send us an email or call +61 2 9894 4300 to speak with an application engineer.