Rupture Disks | Over Pressure Protection
Rupture disks may be simple safety devices, but they have an identity crisis.
Is it disk, or is it disc? Take your pick - both are correct.
Are they rupture disks, burst disks, bursting disks, pressure safety disks, pressure relief discs, frangible discs, blowout discs, or even burst plugs or burst diaphragms? Again, they are all correct.
Terminology tends to be based on geographical location of the user e.g. disk is used more in the US, disc more so in Europe.
What is a Rupture Disk?
A rupture disk is a non-reclosing pressure relief device used to protect vessels, piping, and other systems from excessive pressure and/or vacuum.
Rupture disks are thin, domed, devices engineered to produce a leak tight seal until called upon to "burst" at a predetermined differential pressure. Once the disk ruptures, the high pressure fluid vents through the new formed path, and in so doing relieves pressure.
They are fail-safe devices with a fast opening time; a disc will open fully from the closed position in a matter of milliseconds.
Rupture Disk Installation
Rupture disks are generally installed in a rupture disk holder. The holder clamps the disk into position and is designed to create a tight, metal to metal seal while clamping the rupture disk in place. There are many different designs of holder, constructed from many different materials.
Rupure disks can be installed with the concave side of the disk facing the process media, or the convex side facing the process media. Generally referred to as forward and reverse acting respectively.
What is a Forward Acting Rupture Disk?
When the concave side of the disk's dome faces the process media this is know as a forward acting rupture disk, or a tension type disc. In this orientation the disk ruptures when the weakest portion of the disk exceeds its tensile strength.
Forward acting rupture disks tend to have a finite fatigue life at higher operating differential pressures due to the stresses being tensile in nature.
What is a Reverse Acting Rupture Disk?
When the convex side of dome faces the process media this is know as a reverse acting rupture disk. In this orientation the disc ruptures when the pressure creates an instability in the dome, resulting in the dome buckling.
Reverse acting disks tend to have a longer life cycle than forward acting designs because they are compressive and do not promote crack propagation. Other advantages include:
- Available with maximum recommended operating pressure ratios up to 95% of the stamped burst pressure,
- Up to full vacuum capabilities without the need of an additional support member,
- Constructed using thicker materials providing greater resistance to corrosion.
Temperature Considerations
Changes in temperature will affect the bursting pressure of a rupture disk. In general, a rupture disk’s burst pressure will decrease as temperatures increases, and its burst pressure will increase as temperatures decreases. Therefore, the instrument engineer must consider the relieving temperature when selecting and specifying a rupture disk.
It is worth noting that the tolerance on the difference between actual burst pressure and specified burst pressure is typically around ±5% of specified pressure.
How are Rupture Disks Used?
Rupture disks can be installed alone or in combination with other types of devices, e.g. a relief valve.
Primary Relief
If the rupture disk is the only device used to provide pressure relief then it is said to give "primary relief". The major disadvantage of this installation is that it allows venting until system pressure equals downstream pressure, or in other words it allows the potentail loss of a lot of inventory.
Secondary Relief
In a secondary relief installation, the rupture disk provides a backup vent to a primary relief device, typically a relief valve. It provides additional protection against a major event that would exceed the capacity of the primary relief device.
In Series with Relief Valves
Rupture disks are often installed in series with a relief valves. When installed upstream of a relief valve the disk is used to protect the valve from process media that can corrode or plug it. The disk can also act as a seal, preventing any leakage through the valve unless the disk is ruptured. Occasionally, a low pressure rupture disk is used on the downstream side of a relief valve that discharges into a common manifold. Again this type of installation prevents exposure of the valve to corrosive media, in this case corrosive media in the common manifold.
Rupture Disk Tell Tale Assembly
The space between the rupture disk and the pressure relief valve should always be fitted with a "tell tale indicator". A tell-tale indicator monitors the pressure in the rupture disc assembly and lets you know if the disk has ruptured or if there is leakage in the rupture disk assembly - this is important to know because a rupture disk will not burst at its designed burst pressure if back pressure builds up in the space between the disk and the relief valve.
A typical tell-tale indicator installation consists of a pressure gauge (or/and a pressure switch), try cock, and free vent. The normal configuration is an excess flow valve in combination with a pressure gauge.
Technical Library
The following pages on Control and Instrumentation.com give more detail on Hazardous Areas, ATEX and other relevant information:
