Maintain the Magic And Keep Good Data Coming

Without communication between the various components of a control system and, just as importantly, the control system and the corporate network (including manufacturing execution and enterprise resource planning systems), it’s impossible to make product. While plants realize that maintenance of field devices and production equipment is crucial, they don’t always pay sufficient attention to the monitoring and maintenance of the network used to interconnect these components. Yet this is essential for high reliability. Unfortunately for most people using these systems, “all the stuff” that makes them work is “magic” and this magic is becoming more complex (Figure 1).

Figure 1. Merlin at work -- Many users view data handling and the working of control systems and networks as magic. Click on image for a larger illustration.

As we all know, if we don’t have a reliable control system with availability as close to 100% as possible, production staff quickly lose faith in the system and circumvent it with jumpers, loops in manual, etc., that defeat the purpose of the investment. Even “finicky” analyzers have a 95% minimum acceptable availability.
A key part of any control system is the infrastructure that carries the signals from the field devices to the controllers — because without signals there won’t be data on which to control nor a means to manipulate the final control elements (valves, drives, etc.) to adjust process operating conditions. However, as the level of expected interconnectivity increases between the various systems, the degree of difficulty in maintaining them rises exponentially.

Essential monitoring
System reliability depends on proper design — which includes taking into account security and regular maintenance. Correct maintenance requires understanding the condition of the system. A single snapshot in time, say, when the system was started up and commissioned, isn’t good enough. Instead, you must gather and compare data over time so you can predict when one or more components are likely to fail.

Regularly manually capturing these data (at a minimum at the vendor-recommended intervals) forms the basis of a preventative maintenance program to verify everything is within tolerances and hopefully to spot when a component in the system needs to be replaced. The data collected this way normally then have to be manually keyed into appropriate software and, in most cases, interpreted by a knowledgeable end user. Unfortunately, this is labor- and time-intensive, and depends upon staff expertise, which is growing increasingly scarce as old timers retire.

Fortunately, the same systems that form the networks now have components, tools and software to monitor the networks in real time and provide analysis of the data to alert you that the system is degrading well in advance of an incident. In addition, because control systems are incorporating more and more “Commercial Off The Shelf” (COTS) technology as part of their infrastructure, many of the tools used for business networks can serve in the control environment as well.

Figure 2. Network architecture -- Plants usually layer communications into four levels from field devices to corporate business systems. Click on image for a larger illustration.

An “ideal” network-monitoring tool shouldn’t impact the network it’s watching. However, in practical terms this isn’t possible. So, we aim to minimize interruptions or traffic on the network by operating in passive or monitoring mode and simply recording the information flowing by or through the tool, which in many cases is software residing either on a computer or dedicated server/appliance on the network. One way to minimize the impact on the network is to transmit the data being gathered via a separate or parallel network. In the case of Level 0 and Level 1 (Figure 2), wireless networks are starting to handle some of this parallel data transfer. Wireless networks have a different range of conditions, constraints and considerations than conventional or wired networks (see Wireless comes with strings below).

Maintaining reliability
It’s often the “little things” that can create the greatest grief — this certainly is true for network systems. Surprisingly, terminations, which everyone takes for granted, are one of the largest causes of problems. Difficulties can develop if the terminations aren’t properly torqued, or through vibration, someone tripping over a cable, corrosion (because the unit isn’t properly vented or the vent becomes plugged), short circuit (this one will often manifest itself right away), surge and associated damage, or electromagnetic or radio frequency interference. So, it’s critical that this backbone of the system be properly installed and maintained.