Pressure Vessels are pressurised containers designed to hold liquids or gas at a different temperature to the ambient temperature. They are found commonly in the oil, petrochemical, gas processing and energy generation industries.
The steel is forged at the mill and then shipped to a fabricator who welds the plates together to form the pressure vessel. Obviously any failure of the inner wall or welds in such a highly pressurised cylinder could prove catastrophic which is why the supply of material is governed by international law. The steel which is commonly specified by the customer is forged for its particular purpose - these purposes could include resistance to heat or resistance to corrosion or both.
One of the main problems which is found commonly in the oil and petrochemical industry is 'hydrogen induced cracking' also known as HIC. In sour service environments, hydrogen sulphide is produced which, above a certain level of pressure, can potentially corrode the steel from the inside. The corrosion rate is apportioned to the quality of steel used and failure to inspect the pressure vessel over a prolonged period could cause the cylinder to explode. Stress orientated hydrogen induced cracking (SOHIC) is a similar issue where hydrogen corrosion is present around the base metal at the weld point itself.
Pressure vessel steel can be supplied which is HIC tested to demonstrate the materials resistance to hydrogen induced cracking. Low sulphur content could be one stipulation whilst a sample could be tested under h2s conditions to measure how long the steel will last before replacement is required. The higher the resistance, the longer the steel will last.
High impact steel is tested using the Charpy V Notch Test which tests a specimen over a temperature range to show the change in ductile fracture at high temperatures to brittle fractures at low temperature revealed by a drop in impact energy. This test is especially important for steel material used in working environments which operate over a significant temperature variation.
The setting of technical standards for steel used in pressure vessels can be found in a number of organisations which include;
* ASME - American Society of Mechanical Engineers
* ASTM - American Society for Testing and Materials
* NACE - National Association of Corrosion Engineers
The quality of pressure vessel steel material has improved considerably in recent years and incidents which at one time were commonplace, are now relatively rare. This information however, does magnify the importance of procuring properly tested steel plate which is ideally suited to the working application.
No comments:
Post a Comment