API 520 Part I is a standard written and published by the American Petroleum Institute. It is the standard of preference in the selection of pressure relief devices in petrochemical applications. For rupture discs, this standard highlights the various types of devices, how to properly select them, and the sizing calculation used for selection.
When selecting a rupture disc for an application there are many process and device parameters to consider. As they all affect how a rupture disc will perform during a relief event, it is important to define them accurately.
· Maximum Allowable Working Pressure (MAWP) of the vessel
· Fluid state
· Operating pressures and temperatures
· Frequency and magnitude of pressure fluctuations
· Required relief capacity
· Process fluid corrosiveness
· Vacuum and back pressure conditions
· Upstream, downstream and parallel equipment, including PRDs
· Burst pressure and temperature
· Operating ratio
· Manufacturing design range
· Burst tolerance
· Disc type
· Materials of construction
· Disc/holder line size and class
One of the more confusing topics for designers is the selection of the manufacturing design range for the rupture disc. This is often confused with burst tolerance, which is a very different quality.
The manufacturing design range (also known as the manufacturing range) defines the maximum and minimum pressure limits within which a rupture disc’s burst pressure can be marked. The more common ranges are 0%, 5%, and 10%, though other standard ranges exist for various rupture disc designs. 5% denotes that the burst pressure will be marked at the average of the test breaks for a lot of rupture disc, which must lie between +0% and -5% of the requested burst pressure. 10% is similar to the 5%range but allows test breaks to fall between +0% and -10%. A 0% range allows the burst pressure to be marked at the requested burst pressure only. A 0%range is preferred as it prevents confusion when reordering or upgrading rupture disc equipment.
Burst tolerance refers to the accuracy of the rupture disc’s performance. It describes the amount of acceptable deviation between the marked burst pressure and the actual burst pressure in practice. ASME requires that the variance be no greater than +/- 5%at the specified disc temperature for pressures above 40 psig. Pressures at 40psig and lower require a rupture tolerance of +/- 2 psig.
API 520, Part I, section 18.104.22.168.3 highlights the process for proper selection of a rupture disc. This selection process is summarised below. OsecoElfab recommends following those guidelines and supplementing the process with support from our technical sales and engineering teams.
1. Select the upper limit of the manufacturing design range. This is typically the maximum allowable working pressure (MAWP) of the equipment being protected, but in some applications, may exceed the MAWP. When superimposed back pressure is present,more care must be taken in selecting the upper limit of the manufacturing design range.
2. Determine the specified burst pressure by subtracting the positive portion of the manufacturing design range from the upper limit of the manufacturing design range.
3. Determine the lower limit of the manufacturing design range by subtracting the negative portion of the manufacturing design range from the specified burst pressure.
4. Determine the operating ratio by dividing the maximum operating pressure by the lower limit of the manufacturing design range.
5. When calculating the operating ratio for rupture discs with burst pressures from 15to 40 psig, subtract the required burst tolerance of +/- 2 psig from the lower limit of the manufacturing design range prior to calculating the operating ratio. Contact OsecoElfab for support in selecting an operating ratio for burst pressures below 15 psig.
When selecting a rupture disc, we would always recommend working with an engineer from a specialist pressure safety company such as OsecoElfab. This is to verify correct selection, and ensure safety and compliance.