A method of alarm system analysis for process plants

Abstract

The design and improvement of alarm systems in process plants has been given considerable attention recently. A methodology is presented in this paper which can be used as an aid in the design of new alarm systems or in the improvement of existing alarm systems. The methodology is incorporated in computer software into which expert knowledge of a given process plant can be entered and used to select alarm systems. Scope—Alarm systems play a very important role in process plants. They aid the operator in his primary tasks of detecting and interrupting progression of a failure, and diagnosing and providing corrective actions for fault conditions. There are, however, operator difficulties in handling alarms. A particular alarm system design can significantly affect the operator's success likelihood in receiving and processing alarms. Several factors are important in the design or improvement of a given system. Human likelihood of success using a given alarm system under a given fault condition is one obvious factor. Economical aspects of selecting alarm systems, given probable types and frequencies of fault conditions, should also be considered in the design or improvement of alarm systems. A systematic approach is necessary to consider the effects of all these factors in evaluating a set of proposed alarm systems and in selecting the most appropriate alarm system. The purpose of this paper is to provide a method for determining the worth of a given alarm system by considering all factors which influence that worth. In complex process plants, applications of the methodology presented in this paper can be very difficult without the aid of a computer. Therefore, a computer program is also described to carry out the methodology. Information regarding the process plant is entered into the program using the goal-tree concept. The goal tree contains the expert knowledge of the process which in turn is used in the design or modification of alarm systems. Conclusions and Significance—A methodology has been presented to perform systematic evaluation of alarm systems. The methodology is based on the goal-tree concept through which process plants can be modeled. Goal trees excellent tools for cause-consequence determination. The methodology makes use of decision trees which are constructed in parallel to goal trees to show all operator action(s) required to achieve each goal, and to show the consequences of operator's failure to achieve each goal. The decision trees can also model the progression of an initiating fault condition. The operator's likelihood of success of achieving each goal for a given alarm system design or alarm system modification can be estimated and used in the decision tree to estimate anticipated consequences of a given alarm design or modification. Anticipated consequence is defined as the probability of not achieving a goal times the consequence of not achieving that goal. Finally, alarm system designs or modifications with a low anticipated consequence and low implementation cost can be identified for further evaluation as potential alarm systems for the process. The methodology has been modeled in a system of computer codes called UMPIRE-I. Process knowledge is entered into the computer through a goal tree coupled with human success likelihoods. The rest of the analysis is performed by UMPIRE-I. The code is user-friendly and can be used with minimal training. An example is provided in the paper to further clarify the methodology. This method has been applied to several limited-scale engineering processes with rather significant success.