CONTROL SYSTEMS SAFETY VS. RELIABILITY VS. COMPLEXITY

摘要

When selecting a safety control system for a combustion application, it is essential to recognize that many trade-offs will need to be weighed before making a decision. In simple terms, the reliability will decrease when adding safety to any system. This reliability decrease is related to the fact that adding safety features requires the addition of more ways to shut down the heater. It also requires adding sensors and logic solvers, which can fail. It is the job of the system designer to mitigate these potential failure points in a manner that does not decrease safety. The typical trade-off most often seen is a cost or complexity increase to the control system. Even considering the decrease in reliability, increase in complexity and higher cost, it is easy to see the value in adding safety controls to a system with the potential for loss of life, damage to the environment or damage to equipment.This article is focused on a burner-management application; however, the general principals would apply to similar safety systems with a "power-down" safe state. This means that to enter a safe state, the controller de-energizes all outputs that are connected to devices with a mechanical fail-safe default state. In the case of a burner-management system, the outputs are connected to fail-closed, fuel valves. The fuel valves are driven to a safe state by a mechanical method (typically, a spring) when power is removed.