Why Process Plant and Machinery have a Design Life?

Not many end users appreciate that process plant and machinery have a specified “design life”. This is usually 20 years and is spelt out in the Design Basis. Special design considerations and codes have to be invoked if critical equipment has to be designed for a longer life. One obvious factor that limits the useful life is atmospheric corrosion. Atmospheric corrosion rates of carbon steel vary from 0.03 mm per year to 0.5 mm per year depending on the environment. Corrosion allowance applied for carbon steel is in the range of 1.5 to 3 mm.

The other reason for “design life” is even less appreciated. Each time a plant is started up and shut down it is subject to stress. Equipment and pipelines are heated up or cooled down, subjecting these to thermal stresses.  Pressure is raised and lowered. Metal undergoes fatigue and wear, grain morphology changes. Longer a plant operates more the number of start-ups and shut-downs, more the stress and fatigue it experiences. This explains why the failure rate of any equipment exhibits the characteristic “bathtub curve”.  Ageing plants are operating at the extreme right end of the curve making them very vulnerable to sudden failure with catastrophic consequences. Ageing was found to responsible for 28% of reported major loss of containment events in Europe between 1980 and 2006.