Crenger introduces Design Scope Group of P&ID items. It addresses the problem of engineering design never discussed openly. What happens after the guaranteed period will expire? Is the plant composed of the subsystems with similar life cycle? If the engineering contractor claims the plant has life expectancy of 20 years, does it mean that all its subsystems will last 20 years as well?

Design Scope Group defines the P&ID items subset with the roughly the same life expectancy; it is a risk-adding group. It should explain the measures adopted to extend the life expectancy, such as the equipment piece oversizing or selecting better material standards.