“Why did they have a Rupture Disc and a Relief Valve together?”
As I alluded to in my post regarding CBS’s update, this arrangement is fairly common but comes with some very SPECIFIC requirements. These requirements can be found in:
- ASME Section VIII, UG-127
- National Board, Part 4, 2.5.4
- API 520, Part II, 9.1
But the top four (4) requirements, we as safety engineers should be looking for in these designs are:
- The Rupture Disk (RD) must have a set point 90% of the Pressure Relief Valve (PRV)
- The RD MUST be of the “non-fragmenting” type
- The space between the RD and the PRV must have a “free vent” to atm; or if the gas/vapor is hazardous and venting to the atm is not safe, then the space must have a tell-tale indicator to INFORM personnel that there is pressure between the RD and PRV. This pressure can cause the RD to fail to function as designed and to OPEN at its set point.
- The RD has to be the same size as the PRV inlet.
Just because the image the CSB provided does not show these details, I have to assume this design basis is part of their investigation, as the pressure releif system failed to prevent the failure of the Pressure Vessel.