Pressure Vessels

1Q. What is the biggest problem with pressure vessels?

1A. In the petrochemical industry pressure vessels of hazardous materials are designed to Codes of Construction. The code it is built to depends on the country. The ASME Pressure Vessel Code is pretty well recognized around the world, but there are other fine country codes as well. For this reason there are not usually many basic design problems. Most problems are either post construction deterioration problems, as discussed in the Life Extension section, or operating outside the design envelope.

2Q. What are high pressure vessels and what's the difference from just pressure vessels?

2A. Some say high pressure starts at 10,000 lb/in2 and higher. High pressure vessels and piping are usually much thicker wall and the conventional equations which are used when vessel thickness/vessel radius is 10% or more are no longer valid. ASME has a high pressure code which many now use on such vessels. Many times piping undergoes autofrettage treatment. This is a method to pre-stress the piping so that higher cyclic pressures can be tolerated.

The people who always seem to have a concerned look on their face in high pressure plants are the experienced hands who truly understand the equipment and what can happen. Everyone else is usually smiling. The vessels and machines require special care.

3Q. We have vessels that are pressured up and blow down several times and hour. They have been analyzed using ASME Section VIII, Division 1. Is this adequate?

3A. This is classified as a cyclic service and fatigue can control rather than strength. For this reason it should have been screened for the need for a fatigue analysis via ASME Section VIII, Division 2. A full fatigue analysis, considering stresses and temperatures, if the screening criteria says it is required, is expensive but necessary. Usually the weak link, high stress area is in the details only experienced engineers will recognize.