The Baker Panel Report - ABSA

3.1 Rupture Disks Under Relief Valves – Hazard Not Understood orAbatedA rupture disk is a device designed to relieve excessive pressure in a process. When thepressure on one side of the disk exceeds the design limit, which is based on a designateddifference in pressures on opposite sides of the disk, the disk bursts or opens, relievingthe pressure. Once a rupture disk opens, it cannot reclose.A relief valve is a pressure relief device that can reclose after opening. While there aredifferent types of relief valve designs, the basic principle is that when the pressure in theequipment exceeds the relief valve’s set pressure, the relief valve will open and remainopen until the pressure is reduced below the set pressure, after which the relief valverecloses to seal the system.A common design strategy uses rupture disks in combination with relief valves to preventdamage to the relief valve from exposure to process fluid during normal operation. 8,9 Ifthe system pressure on the process side rises above the rupture disk burst point, therupture disk will open, exposing the relief valve to the system overpressure. The reliefvalve will then open to relieve the system pressure. For this design strategy to workproperly (i.e., the rupture disk protecting the relief valve until the pressure reaches thedisk’s bursting point), the rupture disk must not leak or fail prior to the system pressureincrease. In a well-designed and maintained system, the space between the rupture diskand the relief valve is normally at atmospheric pressure.A rupture disk can fail in a number of ways: it can experience a pinhole leak or therupture disk can prematurely open or burst. If the rupture disk bursts (i.e., fully opens),the relief valve will still operate as intended as long as the process fluid against which itwas being protected does not degrade the integrity or operation of the relief valve. Onthe other hand, if the rupture disk experiences a pinhole leak, the pressure in the spacebetween the rupture disk and the relief valve can equalize with the system pressure. Inthis situation, the rupture disk will not burst at the system pressure at which it wasdesigned to burst because the rupture disk relies upon the difference in pressures on itsopposite sides. If the pressure on both sides is the same due to a small leak, the rupturedisk will open only at a pressure much higher than the designed system relief pressure.As a result, the system/vessel may be exposed to a much higher pressure than intended,which is potentially a serious process safety hazard. 10For these reasons, it is important to monitor the pressure in the space between the rupturedisk and relief valve to determine whether the rupture disk has failed prematurely. It is arecognized good industry practice to continuously monitor and alarm or frequentlymonitor and log the pressure of the rupture disk/relief valve space. If refinery staffdetects a higher-than-intended pressure in the space between the rupture disk and therelief valve, then the situation can be investigated, evaluated, and remedied. Whilerelatively few (less than 1%) of the relief valves in the BP refineries have rupture disksunder them, the instances in which vessels have the rupture disk/relief valve combinationare nevertheless important because of the potential consequences of a failed pressure17