When a rupture disk device is installed upstream of a pressure relief valve, no capacity calculations are performed for the rupture disk. Rather, the effect of the rupture disk on the flow capacity of the device combination is represented by an additional capacity correction factor in sizing the valve. This “combination capacity factor” (Kc) may be determined experimentally by rupture disk manufacturers for each model of rupture disk in combination with individual pressure relief valve models, as specified in the ASME Boiler and Pressure Vessel Code, Section VIII, paragraph UG-132. These measured values of Kc are available from the disk manufacturers and are published in the National Board publication NB-18, Pressure Relief Device Certifications, for each combination of rupture disk model and pressure relief valve model tested. For disk/valve combinations for which no value of Kc has been measured, Section VIII requires the use of a value of 0.9.
ASME UG-127 outlines special mandatory requirements for installing rupture discs under PSVs. These requirements include:
• A means (i.e., a tell-tale) to check that the rupture disc is intact and that there is no liquid or pressurized gas in the cavity between the rupture disc and the PSV
• Derating relief valve capacity to 80% of valve design, unless the specific combination of rupture disc, disc holder, and PSV has been flow tested. This topic is covered in ASME UG-132. Note that it is necessary to include the rupture disc when calculating relief inlet pressure drop
• Reduction in operating pressure to match the larger bursting tolerance of the rupture disc
The rupture disc vendors make special holders which mount under the PSV and which include a tell-tale.