Changes in technology have led to a variety of recent developments in process safety, with many suppliers releasing new systems that depart from the past significantly. These do’s and don’ts can help you choose the right safety-instrumented system for your process.
Do Take Advantage of Scalable Systems
The first popular safety programmable logic controllers (PLCs) introduced in the mid-1980s were triplicated. The most economical implementation of such systems often was one large centralized system. However, not all applications need 1,000+ I/O. That’s why some vendors have developed safety PLCs ideally suited for small I/O applications. Still, using one system for small applications and a different system for large ones in the same facility is hardly an ideal solution. A number of suppliers recently launched systems that can scale from small and stand-alone to large and distributed — all using the same hardware platform.
Don’t Settle for a Single Level of Redundancy
Not all parts of a process safety system require triple redundancy. Depending on the level of safety risk, some applications require only dual redundancy. Three vendors have released safety PLCs that can be configured single, dual, or triple (one even offers quad). In one system, some modules can be single, others dual and others triplicated. Flexible redundancy within one system allows the system to more closely match your safety and reliability requirements for each loop, in a cost-effective manner.
Don’t Assume Two Vendors are Better Than One
The traditional approach for control and safety systems has been to buy two separate platforms from two separate vendors. The trend now is to have one supplier for both systems. That’s because the control and safety systems often look very similar (although they’re not interchangeable), and usually are programmed using the same software. This means users only have to attend one training course, and communication between systems is effortless.
Do Consider Using New Fieldbus Technology for Safety
The Fieldbus Foundation has been working on FOUNDATION™ Fieldbus for safety (FOUNDATION Fieldbus SIF) for several years with a consortium of users, safety PLC and field-device manufacturers. Early field-device products were demonstrated in the summer of 2008, and final products (both field devices and logic solvers) are nearing release. The primary benefit touted by safety fieldbus manufacturers is diagnostics: being able to better and earlier predict problems before they impact the process, and even lead to a shutdown.
Do Use Safety-Certified Field Devices
Dozens of new safety-certified field devices are available with much higher levels of internal diagnostics than devices of the past. Single devices with high levels of diagnostics usually offer similar safety performance to redundant standard devices, at much lower costs.
Don’t Overlook the Need for Employee Certification
A competent workforce is an essential defense against risk. Various organizations offer certification and certificate programs to help ensure employees understand what’s necessary to keep a plant operating as safely as possible. Organizations such as CFSE.org (Certified Functional Safety Expert), ISA International Society of Automation), and TÜV (both Rheinland and SÜD) offer a variety of programs.
Do Stay Up to Date on Standards for Fire and Gas Systems
Two factors have a dominating impact on the safety performance of fire-and-gas systems, and might prevent most systems from ever meeting SIL 1 performance levels: detector coverage and mitigation effectiveness. Despite these factors, it’s possible to apply performance-based concepts to fire and gas systems.
It’s also possible to assign risk reduction targets for fire and gas systems, and apply quantitative techniques in system verification. The ISA 84 committee published a technical report in 2010 on ways to account for detector coverage, mitigation effectiveness and other factors, thus allowing a quantitative, performance-based approach to fire-and-gas system design. Once the detector coverage and mitigation effectiveness limitations are better understood and addressed, focusing on the SIL rating of the hardware will be more meaningful.
