Using digital switches for critical safety systems


The following is an excerpt from “Digital switches with self-diagnostics can improve results and ease the implementation of safety systems” by Rick Frauton of United Electric, originally published in the September 2011 issue Pumps & Systems magazine. Rick is the product manager for UE’s One Series line of electronic pressure and temperature switches, and has been working with customers to identify applications where these switches can improve application and plant safety. Thanks, Rick, for being our guest blogger this week.

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Years ago, most switches were blind mechanical devices actuated electromechanically or by pneumatics. They offered no indication of reliability, such as success or failure in response to a command. This lack of feedback was particularly worrisome in safety applications. The result could be catastrophic, should a malfunction occur in place of the proper response to a tripped pressure or temperature alarm.

Partly because of this possibility, there has been a growing trend toward more effective solutions. One popular implementation has been to use a transmitter together with a dedicated control system—one that is separate and distinct from the basic process control system. Transmitters can convey a great deal of relevant process information that can be useful for safety, control, and later, optimization. This technology also ensures that connections are active and the transmitter is working, two critical requirements for any safety application.

In such setups, transmitters provide process continuous data, and an alarm or protection system acts on this information. Informal surveys have shown that somewhere between 25 and 33 percent of transmitters today are in such loops. Therefore nearly a third of the time, the result is a point safety solution offering binary, on-off action at the control room that is effectively equivalent to what had been provided by a traditional switch.

While transmitters have been evolving, switches have undergone their own revolution. Switches now are digital, with programmable setpoints and deadbands. They offer self-diagnostic, solid-state electronics, plug port detection and nuisance trip filtering. They also have fail-safe-open programming modes that eliminate the problem of undetected failure. Digital switch technology also satisfies requirements needed to achieve a given Safety Integrity Level (SIL), a measure of the relative risk-reduction provided by a safety function.

Point Safety Applications

What are some of the applications that demand a given SIL and a point safety solution?

The first group involves active processing in chemical and petrochemical plants, refineries and oil and gas facilities. In such situations, at least one, and likely several, critical process vessels exist where a reaction occurs that must be monitored for level, flow, pressure, temperature or a combination of these.

In petrochemical refining, incoming crude oil undergoes distillation with the output processed through an isomerization unit to alter its structure before emerging as a fuel. Isomerization often involves heating the product in the presence of a catalyst, such as platinum or another metal. The combination of heat and a chemical reaction can spiral out of control, ruining product and leading to a possible explosion. The same is true for sulfur-removing hydrotreater units found throughout a refinery. So it’s critical to monitor temperature and pressure at many locations, and if needed, the process stopped.

Other applications involve the transportation or storage of flammable materials, like those found in in grain elevators and power plant coal dust conveyors.

In grain elevators, the grain must be moved into and then within a structure. However, any fine airborne suspension of organic material is combustible. So, stones and metallic fragments are removed before grain is transported or milled. Still, the elevator itself can be a source of heat or sparks. Temperature within the mechanism must be monitored and transport halted if dangerous conditions develop.

The same situation prevails in power plants or other facilities that contain coal dust conveyors. If a conveyor bearing or roller begins to overheat and the safe threshold is exceeded, this must be detected and the conveyor shut down.

In several industries, eye wash or safety stations are installed to ensure worker safety and must function flawlessly when needed. The temperature of the eye wash is critical—cool enough not to scald and warm enough not to freeze. These stations are situated in industrial settings, where temperature extremes are possible. It’s critical to monitor the situation and signal a critical alarm, in the event that the wash temperature is too high or too low.

Considerations

In implementing a safety monitoring solution, engineers should keep the following in mind: Multiple technologies are better than one. Speed can save. And safety systems must be independent of the basic process control system. However, it is important that this independence not be total. That is, any safety instrument must be able to report on its own condition and interface with the rest of a plant network.

You can read the article in its entirety here, and learn more about United Electric’s offering of electronic pressure and temperature switches for safety applications.

Question to You

What areas of your plant should you be monitoring for improved safety? What’s stopping you? Complexity? Reliability? Price? Let’s talk about it.

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