According to ASME VIII Div. 1 every hermetic vessel operating at a pressure above atmospheric pressure must have a Pressure Relief Device (PRD), either a relief valve or a rupture disc.
The purpose of a PRD (Pressure Relief Device) in processing plants is to prevent damage to equipment, prevent injury to personnel or eliminate any risk to the welfare of the community at large. The function of a PRD is not to control and/or regulate the pressure in a vessel.
Rupture discs are one of the safest and most widely used solutions for pressure relief, they can work with liquid, gas and/or steam media. Nowadays there are two types of rupture discs, forward or direct acting and reverse or inverse acting, being the latter the most used and safest. When we talk about these types of action, we refer to their geometry. Rupture discs are dome-shaped, i.e. they are concave; when they are forward acting, the pressure of the vessel to be protected acts on the concave part and when they are reverse acting, the pressure acts on the convex part. They are also classified by the way they break, there are fragmentable and non-fragmentable discs; the latter being the safest, since when they break they do not disintegrate and only open, releasing and relieving the internal pressure of the vessel.
Fike, which has been manufacturing rupture discs for more than 50 years, has a wide variety of materials, the most common being 316L stainless steel, but also uses any special alloy such as Titanium, Monel, Hastelloy or Inconel. Within its portfolio of solutions there is also graphite, an application that is also very common. Carbon steel is not an ideal material for manufacturing rupture discs because of its high carbon content, which does not allow for proper deformation.
Poly-SD rupture disc
AXIUS rupture disc
It is important to know the meaning of the following terminology:
- Rupture Pressure: This is analogous to the seating pressure of a relief valve; it is the pressure at which the disc will rupture at a given temperature. Fike is an ASME accredited manufacturer, which means that, in order to manufacture a disc, it must test a minimum of 3 discs. The result of the pressures at which the discs broke will be the pressure marked on the final disc.
- Manufacturing range: It is a tolerance that is agreed between the customer and the manufacturer; it cannot violate the true tolerance that is given by ASME VIII, Div. 1.
- Tolerance: It is what is required by the standard, according to ASME, the tolerance allowed for burst pressures ≤ 40 PSI, the tolerance must be ± 2 PSI; for burst pressures > 40 PSI, the tolerance must be ± 5%.
- Operating radius: This is a ratio relating the maximum operating pressure divided by the burst pressure marked on the disc. This value is expressed in % and can vary between 50% and 95%, being 95% a value that shows the manufacturing quality of the disc.
Disc diameter and burst pressure should always be defined by process specialists. There are three mathematical methods to determine the sizing of rupture discs, these are:
- Discharge coefficient method Kd.
- Flow resistance method Kr.
- Combined capacity method.