Failure in a tank of the compression system of compressed natural gas
The staff of a gas station of compressed natural gas (CNG) perceived a leak in one of the tanks of the compression system during a routine inspection of the preventive maintenance. The gas was leaking through the lower head and it was perceptible with a simple touch. It was necessary replace the component immediately for a new one due to the high risk that a CNG leak represents. This caused a non-scheduled stop in the productive system and an overrun since the component did not withstand the working time to which it was designed.
Kobaltum engineers inspected the lower head, extracted some samples and determined the cause of failure based on the material analysis, the liquid penetrant testing and using the scanning electron microscope.
The liquid penetrant testing allowed to identify the crack where the natural gas was leaking, in order to extract the samples for the analysis.
The sample allow to identify a crack that initiated at the inner surface of the tank and moved toward the outer surface through the heat affected zone (HAZ) of the weld that joins the lower head with the skirt. The failure was due to the plate used for the manufacturing of the tank presented precracking and an unusual event during operation accelerated the advance of the crack through a zone with low resistance, like the HAZ.
It is recommended to select suitably the raw material and the suppliers, as well as the design of a suitable welding procedure specification (WPS) for the particular application and operating conditions, in order to avoid this sort of failures. Also, it is recommended to carry out procedures for the technical validation like the inspection of the welds through Non-destructive testing (NDT) such as liquid penetrant, ultrasonic or radiography testing.
The technical thoroughness of the design, manufacturing and quality control process of the products and the verification of the raw material will help to mitigate the failures and reduce the non-scheduled stops of the productive system.