In 1999, the ASME Section VIII, DIvision 1, 1998 code established a new symbol stamp for rupture disks, called UD. While the Code recognized rupture disks as acceptable pressure-relief devices prior to this revision, there was no formal process for product certification. Very few manufacturers had performed flow testing of their products; therefore the methodologies for sizing relief systems reflected in the ASME Code and API Recommended Practices (RP) were estimates, at best.

The new UD stamp now requires any product carrying the stamp to be flow tested at an ASME PTC-25 accepted flow laboratory, in the presence of a representative from the National Board of Boiler and Pressure Vessel Inspectors. Results of the flow testing are communicated directly to the user via the certified flow resistance factor, K_R, and minimum net flow area (MNFA) is stamped on the disk tag. These values are also published in the National Board Red Book, which also covers relief valves.

Defining K_R

The concept of a loss coefficient, K, has been used for many years to define "minor" piping-system losses that result from elbows, tees, fittings, valves, reducers, and so on. Ref. [1] describes the methods of using the loss coefficient to calculate overall flowrates and pressure drops.

The loss coefficient, K, is defined as:

K = h_L/(V²/2g)

where h

K = ΔP/(0.5ρV²)

where ΔP is the pressure drop of the fluid flowing through the system and ρ is the density of the fluid. Therefore, K is the pressure loss expressed in terms of the number of velocity heads. While K is technically dependent upon the component geometry and the Reynolds number, the dependence is strongest to geometry in fully developed turbulent flow. ASME assumes in its Code that this dependence is strictly on geometry.

In 1999, the ASME Section VIII, DIvision 1, 1998 code established a new symbol stamp for rupture disks, called UD. While the Code recognized rupture disks as acceptable pressure-relief devices prior to this revision, there was no formal process for product certification. Very few manufacturers had performed flow testing of their products; therefore the methodologies for sizing relief systems reflected in the ASME Code and API Recommended Practices (RP) were estimates, at best.

The new UD stamp now requires any product carrying the stamp to be flow tested at an ASME PTC-25 accepted flow laboratory, in the presence of a representative from the National Board of Boiler and Pressure Vessel Inspectors. Results of the flow testing are communicated directly to the user via the certified flow resistance factor,