Final answer:
The exact sizes at which standard and Schedule 40 pipes have the same wall thickness are not specified in the question or the provided information. This information can typically be found in industry standards or detailed specification tables.
Step-by-step explanation:
When examining standard and Schedule 40 pipes, it is important to understand that these designations refer to the wall thickness of pipes, which is crucial in defining the pipe's strength and pressure rating. For 4-inch pipe, 8-inch pipe, and 16-inch pipe sizes used in the chemical industry, a common standard applied to assess their carrying capacity and cost-effectiveness is found within chemical engineering references, such as Table 7.15 which compares pipes and explores economies of scale in the chemical industry.
For the context of this question, standard pipe and Schedule 40 pipe tend to have the same wall thickness up to a certain nominal pipe size; however, the exact crossover size is not provided in the question or the referenced material. Typically, for smaller pipe sizes, the wall thicknesses follow industry standards closely before diverging for larger diameters. The specific crossover point can usually be found in technical specifications or standards, such as those published by the American Society for Testing and Materials (ASTM) or the American National Standards Institute (ANSI).