02 Jun 10 INCH ACCU-SHEAR DEVICE – A SOLUTION TO AN OLD PROBLEM
Shear Pin and Failing Component Pressure Relief Devices
When there is repetitive loading below the material strength, fatigue failure is the result. This is an issue that is well known with component-failing pressure relief devices and among this type of device, rupture discs are one of the most popular. There is important reduction when the loading and unloading of a calibrated failure component in set pressure is experienced. As a result, most of the guidance in regards to these types of devices counsel to have 50% of set pressure for the operating pressure.
This fatigue issue has been addressed in the industry by turning to buckling pin devices. A buckling pin device will open when the pin buckles versus completely fails. With correct design, the buckling force can be set lower than the fracture limit of the material.
EnviroValve Solution: Accu-Shear
The engineering team at EnviroValve re-evaluated the problem and came up with a solution using their Accu-Shear. They determined that instead of avoiding part of the effects of fatigue on the set pressure, it could be stopped with a pre-load system. This is the same theory of bolts being pre-loaded to extend fatigue life; the shear pin is pre-loaded as well.
Maintaining the required seal and retaining the pin also served purposes of the pre-load, all of which allows for simple inline pressure relief devices that are modeled after a triple offset butterfly valve instead of 90 degree device or a complicated mechanism that translates the linear buckling motion to a rotary shaft motion instead.
A case study was performed of a 10” Accu-Shear device with different opening pressures at 5 consecutive times. From all 3 of these tests, there was no discernible effects on the opening pressure and the final set pressure was within either side of the 5% tolerance band. It is found that by utilizing a pre-loading method, the fatigue effects are eliminated on mechanical devices and a pressure relief device is reliable as well as economically manufactured and still meet’s the industry’s demands.