Wired Versus Wireless Risk Analysis for Process Instrumentation Measurements

How does wireless communications compare against wired communications for process instrumentation? That was the subject of an Emerson Exchange presentation by Kenexis’ Ed Marszal and Emerson’s Gary Hawkins. Their presentation, Risk analysis of wired versus wireless transmission of process measurements was recently narrated by Ed as an encore presentation and uploaded to YouTube.

In this 30:31 video, Emerson Exchange 2014 Wired V Wireless, he discusses the strengths and limitations of using wireless communications for both basic process control and safety. He presents a typical fault tree analysis comparing the communication types along with case studies where wireless communications have been used successfully.

Ed opened some of the common perceptions among instrumentation and automation professionals who believe that wireless is not as reliable as wired transmission and not suitable for critical, safety-related services. Both perceptions he noted are not necessarily true. Nothing in the IEC 61508 and IEC 61511 global safety standards preclude the use of wireless devices. The risks must be accessed like every other component in the safety instrumented function for the SIL levels to be achieved.

He described tools that are used in risk analysis. Some, like layer of protection analysis (LOPA), are semi-quantitative and look at order of magnitude jumps in risk. Another qualitative tool that has been used for many years are probability versus severity matrices.

Beyond qualitative analysis is quantitative analysis. Simplified fault tree analysis can be used to provide different levels of accuracy and precision to the analysis to be performed. Outcomes of the analysis can be expected to differ from site to site due to differences in the sites.

At 5:35 of the video, Ed provides a general description of fault tree analysis and 7:20 he describes the safety nomenclature used for the wired vs. wireless risk analysis. This analysis begins at 10:45 looking at a fault tree analysis for wired transmission of instrumentation information. At 14:50 the wireless transmission analysis begins.

Wireless-Fault-Tree-Analysi

Ed provides some cases where a wireless solution was the preferable one. They involving timing to get installed, the speed of the process (process safety time), quicker recovery from an accident, and providing redundant routes (one wired, one wireless).

Take a look at the video to better understand how the fault tree analysis is performed for wired and wireless communications. You can also connect and interact with other safety professionals in the Safety Instrumented Systems group in the Emerson Exchange 365 community.

2 comments

  1. Jonas Berge says:

    I attended their workshop. It was great. At 9:30 in the video Ed explains: “failures in communication systems are generally overt” which, as shown in the slide is generally safe, and he continues “the same is actually quite true of a wireless system because with wireless systems the safety, if you will, is not in the medium by which the signal is sent, it is in the protocol for sending and receiving that signal”. He goes on: “if the data received by a final element or any receiving device is not timely, and is not valid, it is very easy to detect”. The ability to diagnose the digital communication to detect errors is an important advantage of digital protocols (fieldbus or wireless) over hardwired systems (4-20 mA or on-off).

    At 11:00 Ed explains partial circuit failures: there are also partial circuit failures which is kind of a wrong measurement as opposed to loss of measurement, this is going to be the result of loose or corroded screw terminals or ground faults that are partially dragging down the circuit, so for an analog circuit you might have a 4-20 signal that because of some sort of impedance on the circuit the transmitter might be trying to send 20 mA but as a result of excessive impedance you might only get 16 or 18 mA at the receiver. The slide shows partial circuit failures are undetected (covert) which as explained in the earlier slide is generally dangerous. Such analog signal problems are hard to detect because they are ‘on-scale’ meaning they are wrong, but still within the range of 4 mA and 20 mA so they appear to be correct to the system even though they are invalid. The slide shows such increased impedance may be due to loose or corroded screw terminals, or it could be ground fault. Water ingress creating a partial short is another common cause. That is, it is easy to detect a problem with a digital signal whereas a problem with an analog signal may go undetected. Fortunately logic solvers internally use digital communication with the I/O cards. And now we are talking about digital communication with sensors and actuators.

    Another great workshop along similar lines was the 2-1810 “On-Scale Failures – What You Don’t Know Can Hurt You” which also talks about the dangers of ‘on-scale’ failures such as Ground loop that causes the analog signal to wander, Water in the electrical house creating a partial short, and Loop over loaded – exceeding required maximum loop load etc. for which they have 3-4 great short little video clips. This includes voltage starvation due to too much loop load or unexpected loop load increase from poor electrical termination or corrosion, cable damage. This may result in that the transmitter won’t be able to go into alarm or higher mA value – which could be dangerous. They also note that this is the nature of analog and that use of a digital or wireless protocol can prevent the problem. The workshop also suggests diagnostics for electrical integrity such as “power advisory”, or it could be through digital communication statistics. It concludes that without diagnostics, On-Scale failures result in an undetected and inappropriate control actions

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