Digital techniques applied to the analysis and synthesis of liquid pipeline control systems

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1968

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Abstract

The necessity and importance of modern liquid pipeline operations is a result of the world's demand for energy derived from petroleum products. This demand has resulted in many large and complex liquid pipeline systems both in this nation and abroad. An example of a "typical" modern liquid pipeline system; including pumping, terminal, and control facilities, and a brief description of their operation; is given in Chapter I. The control systems employed in such liquid pipeline operations are of paramount importance in economically and safely meeting petroleum transportation requirements. Most liquid pipeline control systems are both digital and sequential in nature. Accordingly, the operation and response of such control systems is best described by modeling them as sequential machines. Chapter II contains a brief discussion of applicable sequential machine theory. The input and output response of sequential machines is described; followed by a formal definition of a sequential machine in terms of sets of states, inputs, and outputs, and next state and output functions. Useful supplemental definitions are given. These are followed by examples and realizations of some simple sequential machines. Concepts of sequential machine decompositions are introduced and an example is given. Next, a sequential system is defined as a sequential machine with integral timing and sensing elements. The sequential system is then outlined in the most useful manner for use as a basis in the modeling of liquid pipeline control systems. Liquid pipeline control system models are presented in Chapter III to demonstrate modeling concepts for these rather complex control systems. Such models are ideal for defining and studying the structure of the control system and the information transfer between the components of the control system. The models developed are only for the "typical" system described in Chapter I. The methods of extending these modeling concepts to other control systems are obvious, however. The responses of the components contained in the control system models of Chapter III are not specified. In Chapter IV, models are developed to describe the response expected of various control components often found in liquid pipeline control systems. The devices modeled include relays and contactors, memory devices, counters, timers, reversing and non-reversing electric motor starters motor operated valves, and data acquisition devices. The extension of these modeling techniques to other control devices should be self evident. The method of representing the control response of timers and motor operated valves by clocks and counters is, perhaps, the most significant of the modeling techniques demonstrated. A control problem is presented and solved in Chapter V to illustrate the usefulness of the techniques and concepts developed in previous chapters in the synthesis of liquid pipeline control systems. The problem is solved by first defining the problem, then developing a control system model, and finally specifying the exact response of the components of the control system model. The components of the control system model are an output function, excitation function, valve system, interface handling programmer, and valve sequence programmer. The output and excitation functions are specified by Boolean equations. The valve system is composed of components previously specified with the addition of certain control interlocks. The interface handling programmer and valve sequence programmer are modeled as sequential machines. These sequential machine models are simplified and decomposed by formal techniques. Useful relay realizations are then obtained from the models.

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