Rupture disc with isolation seal (IS): flat for low pressure

In short: The IS disc is a flat disc mounted directly between ASME Class 150 flanges and works as a low-pressure isolation seal. It is used mainly to relieve near-atmospheric storage tanks and to separate relief valves or headers from the process fluid.

How it works

Unlike domed discs, the IS has no dome: it is a flat sheet clamped between two standard flanges. As long as system pressure is below its burst point, the disc behaves as a leak-tight wall; when that point is reached, it yields and opens the path to relief. Since it needs no dedicated disc holder, it integrates easily into existing flanged piping.

There are two variants. The IS-1 bursts in one direction only, like most discs. The IS-2 is designed to burst in both directions, that is, on overpressure and also on vacuum, with the same rating in each direction. This dual action is useful in tanks that "breathe": they fill and empty, or undergo cooling that generates negative pressure.

The recommended operating ratio is conservative: the working pressure should not exceed 50 % of the minimum burst pressure. The operating ratio compares normal pressure with burst pressure; the lower it is, the more margin the disc has against fatigue, pulsations and manufacturing tolerances, although the usable range of the equipment is also reduced.

When it makes sense (and when it does not)

• It is suitable for atmospheric or very low-pressure tanks that need a simple relief path.
• Suitable as a barrier upstream of a pressure relief valve or a shared header, so that a corrosive fluid is not in permanent contact with the valve internals.
• Suitable as an environmental seal, when you want to prevent leaks to the outside or the entry of contaminants.
• Choose IS-2 if the equipment may be left under vacuum and you need the disc to open in that direction as well.
• It is not suitable if the process operates close to the set pressure: with a 50 % limit a lot of useful margin is lost, and a disc with a higher operating ratio may be a better fit.
• It is not suitable if the line does not use class 150 flanges.

Data to specify it

• Burst pressure and coincident temperature.
• Normal operating pressure, to check that it stays below 50 % of the minimum burst pressure.
• IS-1 or IS-2 variant and, in the latter case, the required burst pressure in the vacuum direction.
• Nominal diameter and confirmation of ASME Class 150 flanges.
• Fluid, compatible materials and main function: relief, valve isolation, corrosion barrier or environmental seal.

Check the flow-direction tag when installing: a disc installed backwards does not behave as designed.

To learn more: Types of rupture discs and Installation and disc holders.

Descriptive sheet for educational purposes. For availability and a quote for this model, consult P&V Control Devices.