Failure experience of programmable logic controllers used in emergency shutdown systems

Abstract

Microprocessor-based programmable logic controllers (PLCs) are often used in applications to safety shut down a process or facility if unsafe conditions are detected. Increased use of quantitative reliability analysis requires additional and more detailed failure rate estimates for analyzing PLCs. This technical note presents failure rates derived from PLC performance in emergency shutdown (ESD) systems for natural gas compressor stations. The PLCs analyzed operate with roughly 40 inputs processed through three input modules and generate approximately 20 control signals through two output modules. Failure rates range from 0·072/year when the PLC alone was considered to 0·32/year when all types of failures were considered. (All failures include effects that are not necessarily attributable to the PLC microprocessors, such as human errors while working in the PLC cabinets and failures of common power supplies.)