Discrete and Process Safety Networks

Right now, the needs are different and so are the means. Can one network eventually do it all?

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By Dan Hebert, PE, Senior Technical Editor

Hybrid industrial plants are found in many sectors including food, beverage, pharmaceutical, and specialty chemical. Hybrid plants have two distinct types of operations: process and discrete.

Process operations include mixing, distilling, cooking, and other activities. These operations are monitored and controlled based on variables such as flow, temperature, pressure, and level. This is the world of process control, formally the domain of distributed control systems (DCSs), but now also inhabited by PLCs.

Discrete operations in a hybrid plant include tasks such as packaging, product movement, and air compression. These discrete operations typically are controlled and monitored by machines based on parameters such as presence, motion, and speed. Machines typically have been controlled by PLCs and custom-built controllers. Many machines also use PC-based control systems.

The similarities give hope that a hybrid plant can use one digital network for control and safety. The differences so far have kept this from being a reality—most hybrid plants feature multiple digital networks for process control, process safety, machine control, and machine safety.

But progress is being made and there is hope that hybrid plants can reduce the number of digital networks, perhaps someday to the ultimate end point of one network. Let’s first see what’s different.

Machine Shutdown Is Simple

Differences between process and machine control caused digital network standards to be created for each. Chief among them is that machine safety systems have simpler shutdown and recovery requirements.

When something goes wrong with a machine, usually it’s sufficient to shut down the entire machine immediately. Recovery typically consists of locating the fault, correcting the problem that caused the fault, and restarting the machine.

Because of simple shutdown requirements, many machine safety systems only require a few discrete contact inputs hardwired in series to a safety relay that stops machine operation.

 

Similarities Between Process and Machine Control Systems and Digital Networks

Both must:

  • have a high degree of reliability
  • operate at guaranteed speeds,
  • machine control systems generally faster
  • be certified, albeit to different standards
  • provide diagnostic information
  • be flexible enough to allow changes 
  • use digital networks to reduce wiring
 

Machine digital safety networks are useful when many events can activate a safety shutdown, as with machines made by Bosch Doboy. “Our machine safety networks must accommodate up to 17 guard switches and be easy to troubleshoot to a device level using normal tools and not a PC,” says Keenan Stahl, principal software engineer at Bosch Doboy, New Richmond, Wis., maker of packaging machines and product-handling equipment for food, pharmaceuticals, paper products, and other industries.

In cases such as Bosch Doboy’s and in other machine control applications, digital safety networks reduce the amount of wiring, speed diagnostics, and allow safety-related inputs to be easily added.

Process Shutdown Is Complex

Unlike machine safety systems, process safety systems often have very complex shutdown procedures. Different events trigger different shutdown sequences, and many sequences must proceed in a prescribed fashion to keep essential parts of the process running.

For example, overheating in a nuclear reactor often requires shutdown or at least slowdown of the nuclear reaction. But it would be disastrous to also shut down the pumps that provide cooling water to the reactor.

Dave Goodman, project manager at Cambrex Pharma, Charles City, Iowa, has another example of the complexity of process safety systems. “A high-speed compressor used for pressure control of a distillation system will need automatic shutdown systems based not only on its own health, but also on the current operating conditions of the distillation column,” says Goodman.

Process safety shutdown requirements are complex, and recovery from a shutdown also can be difficult if it doesn’t proceed according to plan. An extrusion machine process is a good example of a system that needs an orderly slowdown or shutdown to minimize recovery time.

Extruders heat and melt plastic with shearing forces and with heaters. If the extruder overheats, three courses of action can be taken. First, the extruder motor can be slowed or shut down to reduce heat from shear. Second, the heaters can be shut down or their heat output can be reduced. Third, the extruder cooling system can be activated.

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