This paper explores the use of the B method as a formal framework for structuring and verifying process control systems engineering. In particular, it is shown how the B method can be used to define implementation independent modular specifications. Benefits are related to the re-use of verified and perennial specifications for control systems facing a fast evolution of implementation technologies. Limits are related to the compliance of formal methods with the other methods or methodologies involved in the development of a production system. This justifies the methodological framework needed for representing, reasoning and verifying the control system as interacting with other technological or human systems. The approach is illustrated and discussed using a level control system example.