Either of two approaches may be used for sizing rupture disk devices: (1) nozzle flow and (2) pipe flow. Each method is described both in Section VIII of the ASME Boiler and Pressure Vessel Code (BPVC) and in API Recommended Practice 520 Part I.
1. Rupture Disk Sizing Using Nozzle Flow Equations
Rupture disks can be sized using the equations presented in “Pressure Relief Valve Sizing” on page 1200-39 for pressure relief valves. In this method, the equation applicable to the specific fluid phase is used, but the coefficient of discharge (Kd) is set to a value of 0.62 regardless of the phase. Paragraph UG-127 of BPVC Section VIII expressly restricts the use of this sizing approach to rupture disks discharging directly to atmosphere, and with inlet and outlet lines at least as large as the rupture disk and no longer than eight and five pipe diameters, respectively.
2. Rupture Disk Sizing Using Pipe Flow Equations (KR Method)
For rupture disk device installations in which the discharge is routed to a closed system or which include a significant inlet piping system, the disk is sized as one component in the entire inlet line – rupture disk device – outlet line system, using standard pipe flow equations. In this approach, each fitting contributes to the overall flow resistance factor of the piping system. The resistance factor for the burst rupture disk (KR) is that determined for each disk model as specified in paragraph UG-131 of Section VIII. Certified values of KR are available from individual disk manufacturers and are published by the National Board of Boiler and Pressure Vessel Inspectors in NB-18, Pressure Relief Device Certifications. In the absence of measured values of KR for a specific rupture disk device, a value of KR = 2.4 is required by Section VIII.
Once the rupture disk/piping system total resistance is defined, the system’s flow capacity is calculated by setting the inlet pressure equal to the relieving pressure and the outlet pressure equal to the constant back pressure. Alternatively, the required flow rate and the constant back pressure can be specified to determine the pressure in the vessel required to provide adequate relief. In the first approach, the calculated rupture disk/piping system capacity is compared to the calculated required relief rate, while in the second approach, the calculated pressure in the vessel is compared to the MAWP plus allowable accumulation to determine the adequacy of the flow capacity.