ASME Code Changes for Rupture Discs are Now Mandatory
Significant changes were made in 1998 to the ASME Code that have a direct impact on the use of rupture disc devices. The purpose of this article is to identify the significant changes and discuss the implications they may have on the manufacture and application of rupture discs.
The changes were published January 1, 1998 in Section VIII, Division 1, 1995 Edition, 97 Addenda. These changes are effective immediately and become mandatory for new construction beginning January 1, 1999. The sections concerning pressure relief devices are UG125 through UG137. Section VIII Division 2 and Section III have not yet incorporated the rupture disc certification requirements.
Manufacturer Certification [UG137(c)] (back to top)
A significant change to the code affecting rupture disc manufacturers is the application of the UD code symbol to rupture discs that meet the requirements of the ASME Code. Authorization to use the UD stamp is based on an audit by an ASME representative of various manufacturing, testing, and quality assurance systems. In addition, periodic validation tests are performed to ensure close agreement between production discs and certified discs. Rupture Disc Sizing [UG127(a)].
In the past, the coefficient of discharge method (K d =0.62) was specified to calculate the capacity of the rupture disc device. However, the validity of this method is limited to a disc mounted close to the pressure vessel and discharging to atmosphere. In today's processing plants this is the exception rather than the rule. The code now recognizes this situation and provides guidance for the limited use of this method.
- Disc must discharge to the atmosphere
- Disc must be installed within 8 pipe diameters of the vessel nozzle
- Length of discharge piping not greater than 5 pipe diameters The resistance to flow method (K R ) has been used by industry for years and has now been adopted by the ASME Code, for rupture discs.
Sizing is done on a relief system basis, not by capacity of individual components. The key elements of this method are:
- Rupture disc is treated as another component in the relief system that provides some resistance to flow
- System relief capacity must be multiplied by a factor of 0.90
- Based on Darcy formula (Crane 410) h L = f L/D v 2 /2g \ K R = f L/D
- API recommendation for rupture discs has been L/D = 75 \ K R = .02 (75) = 1.5
- ASME Code default resistance to flow factor K R = 2.4
- Certified resistance factors K R , can be provided by the rupture disc manufacturer for each disc model
- Certified values based on tests of specific disc models in accordance with ASME Section VIII and PTC25
- API recommendation for rupture discs has been L/D = 75 \ K R = .02 (75) = 1.5
Rupture Disc Device Certification [UG137(a)] (back to top)
In order to apply the UD code symbol, the rupture disc device family must be certified through testing. The testing involves bursting 3 discs from 3 different sizes within a product family. Each of these discs must burst within the ASME burst tolerance. Each of these discs is then mounted on an ASME certified flow test rig and flow tested to determine a K R value .
Rupture Disc Marking [UG129(e)] (back to top)
The marking requirements for rupture discs have changed to reflect the new sizing methodology and certification process. One change in the marking is that the marked burst pressure does not have to be the average of the tests. This means that zero manufacturing range can now be accomplished without taking exception to the code marking requirements.
Old Requirements
- Manufacturer's Name
- Model or Part Number
- Lot Number
- Disc Material
- Nominal Size
- Stamped Burst Pressure
- Specified Disc Temperature
- Capacity
New Requirements
- Manufacturer's Name
- Model or Part Number
- Lot Number
- Disc Material
- Nominal Size
- Marked Burst Pressure
- Specified Disc Temperature
- Minimum Net Flow Area
- Certified Flow Resistance K R
- ASME UD Code Symbol
Implications of Code Changes (back to top)
The changes by definition are limited to new construction, but local jurisdictions may impose these changes on existing installations at their discretion. How long will it be before the local inspectors begin looking for the UD stamp on every rupture disc? ASME certification requires additional testing on each rupture disc lot and will increase the cost of manufacturing. Also, the application of specialty rupture discs will become more difficult as the manufacturer must go through a testing and certification process before applying the UD stamp. Specialized product designs can be certified but it is not a quick or inexpensive process.
For assistance with rupture disc applications or additional information concerning the ASME Code pertaining to rupture discs, contact Fike Metal Products. Tel: 816-229-3405, fax 816-228-9277.