4 Rules For Designing Safety into Control Systems
When you see a talk about safety, your first expectation is probably something on proper PPE, procedures or other aspects of safety that are typical fodder for safety “toolbox talks.” What I’d like to discuss in this post, at least in a very general way, is how to design safety into your process control system.
First off, a disclaimer: I am an engineer, although not (yet) a PE and I have no certification in any safety-related field. I do have roughly 30 years of experience in working around heavy equipment, much of it quite dangerous to life, limb, and property if the risks are not properly managed. In that time, a picture of what process safety is and how to achieve it has become clear.
That said, safety is not something that can be overlaid onto a process as an afterthought, at least not quickly, easily, or cheaply. For proper implementation of a safe process system, safety concepts must be designed in from the outset. Ideally, once the basic process design is complete and drawings are available, a deep review of them begins. This review has a number of names, but I’ll call it the process hazard analysis (PHA). This analysis looks at the hazards of the process, their scope, severity, and probable frequency of occurrence. From this, a hazard mitigation plan is developed. There are several standards developed, such as SIL, that have been developed to quantify these risks. Be sure to choose one applicable to your process and industry before initiating the PHA.
The first line of defense in any process is the basic process control system (BPCS), which should be designed and programmed to keep all process parameters within safe limits, and to alarm and/or take action when those limits are approached. The PHA, however, will almost certainly have shown that there are some risks in your system that have sufficient frequency, severity, or scope that they require mitigation that is more reliable than a standard BPCS can provide.
That is where the safety system comes in. A properly-designed safety system will examine inputs from the system (which may also include operator-initiated devices like E-stop buttons), and through logical analysis decide if a hazardous situation exists. Should such a condition be detected, the safety system will then shut down the process in a predefined, orderly manner designed to remove energy from the process and put it into a safe condition. Note that process design here is extremely important: valves, dampers, and other actuators must be designed to fail both electrically and mechanically in a safe condition.
4 basic rules for the safety system include:
1. It is usually separate from the BPCS. There are safety controllers that integrate both safety and non-safety devices, but their functions are still distinct. More common are systems that have completely separate hardware and/or software from the BPCS.
2. Redundancy is almost always a requirement. In all but the most benign and riskless processes, there will be hazards that require a high degree of reliability. To achieve this, redundant circuits, devices, and even controllers are implemented to avoid a single point of failure from allowing the safety function to fail when called upon.
3. The safety system is self-monitoring. Safety output devices (relays, valves, VFDs, etc.) are monitored by the safety system itself to ensure that they do indeed move to a safe state when called upon to do so. Should a safety device fail, its redundant partner will still bring the process to a safe shutdown state, and the safety system must then prevent the BPCS from allowing operation until the failed component is repaired or replaced. In addition, most safety systems have the ability to self-monitor for wiring problems that may prevent reliable operation, though they may require special wiring and/or programming to enable this feature.
4. Devices in the safety system must be rated for safety duty. Devices such as contactors, VFDs, pushbuttons, valves, transmitters, and so on, are available for duty in safety systems. Be sure to confirm that the devices you are choosing are so rated, as they are made with specialized materials and designed for high reliability.
Process safety has become a more critical focus of industry in the past twenty years, with many manufacturers marketing products and services intended to achieve a high degree of reliability in shutdown systems. As a result, prices for hardware and software have plummeted and it is no longer a difficult or expensive task to find vendors and support for your design efforts. It is therefore a high priority, in my mind, that engineers take the time to understand how safety systems are properly implemented to protect their employers’ and clients’ property, surrounding communities, environment, employees, and bottom line.