Fewer sensors mean fewer process connections, fewer signal cables back to the control system and fewer potential emission points.
Typical cost savings per device include:
- process connection/nozzle, $2,500 each
- field device, $1,500 each
- cabling, $7,500 (based on $25/foot and an average cable length from field to control system of 300 feet)
- control system input/output (I/O) card and configuration, $1,000/point
This adds up to a total savings of $12,500 per field device.
Cabling and terminations. Since the introduction of digital networks, there’s been talk about the savings associated with reduced cable infrastructure. There definitely will be savings but maybe not as large as initially thought. Why? The majority of the cost associated with a cable isn’t the cable itself but its related labor and infrastructure such as tray and conduit. You will achieve savings from narrower cable trays, cables with fewer conductor pairs, and smaller termination boxes.
Figure 1. Using analog wiring requires a total of 76 terminations.
Figure 2. Using digital fieldbus cuts the number of terminations by more than half.
Predictive maintenance can provide greatest savings. Source: ARC.
The potential for a reduced number of field terminations promises significant construction cost savings. Consider the situation of an intrinsically safe (IS) installation — this is a “worst case” scenario because, due to an energy limitation of approximately 80 mA, IS networks only can support about four devices per network, not the eight to 12 devices per network typically used in most systems.
Figure 1 shows four field devices, three analog inputs (transmitters) and one analog output (valve – FCV-100). Traditional wiring practices bring all of these signals to a junction box so they can be commingled in a multi-conductor home run to the marshalling cabinet where the signals then are descrambled to go to the appropriate I/O card. However, in the Foundation Fieldbus case (Figure 2), instead of four wire pairs from the field junction box, there’s only one pair (shown in orange) and it only has one kind of signal, H1 communications. Because there’s no longer any need to descramble the signal types, the marshalling cabinet becomes optional. The IS barrier is replaced with a fieldbus power conditioner that contains the necessary IS circuitry; because there’s only one signal type, the I/O card is an H1 interface. Obviously the project also will require fewer I/O cards than a traditional system and consequently will have a smaller footprint, too.
Commissioning. Once the equipment is in the field and the units are ready to go, the next big savings opportunity arises. Several studies comparing commissioning times by British Columbia Institute of Technology, Southern Alberta Institute of Technology, Suncor Energy, Emerson Process Management and others have reported that digital systems start up four to ten times faster than analog ones. The savings in labor alone are noteworthy but what’s even more significant is the potential opportunity to start production sooner and so develop a positive cash flow earlier.
Once the facility is up and running, the plant manager constantly is seeking ways to reduce manufacturing costs. There’s usually only limited ability to control raw material prices. So, maintenance cost reduction becomes a top priority because on average 40% of manufacturing costs directly relate to maintenance.
Interestingly, despite all the experience we have with operating plants, 50% of the maintenance done in a facility is corrective/breakdown or unplanned maintenance. This is ten times more costly than preventive maintenance (Figure 3), which in its simplest terms is nothing more than scheduled maintenance — that is, checking each device on a regular basis just before it may fail. Doing so will forestall the devices failing at an inopportune time. Preventive maintenance is done 25% of the time. Of course, it shouldn’t be a surprise that 60% of such maintenance isn’t necessary — but it’s still a lot less costly than the alternative of breakdown maintenance.
Digital fieldbuses versus HART
While HART communications offer many of the advantages of all digital fieldbuses like Foundation Fieldbus and Profibus, it’s important to understand the differences. Although it can provide multiple outputs, HART in its standard arrangement supports in its definition up to four variables, and most control systems only handle the standard arrangement without custom tweaking. In addition, the primary variable in HART normally is assigned to the 4–20-mA analog signal; the other signals must all be polled. Because of this polling, the update time for the supplementary signals may not always be reproducible and, depending on network size, could take on the order of minutes to obtain. Full digital networks on the other hand update all the variables every scan cycle.
HART devices don’t support process alarms in the devices themselves, so all alarming must be done in the control system. Fieldbus devices, on the other hand, support multiple levels of high and low alarming in the field devices themselves.
Digital field devices have one other major difference compared to analog-based units: they don’t need to drive an analog signal. So, they place fewer restrictions on the memory and microprocessor. HART devices employ a fixed chip for any computations and diagnostic capability. Fieldbus devices can use an EEPROM, which can be upgraded while in service with new features and functionality as they become available. To add this same functionality to a HART device would require device replacement.
Preventive maintenance is five times more expensive than predictive maintenance (Figure 3) — in which devices continuously monitor their own health and report their condition to a central system, often a separate computer connected to the corporate maintenance planning tool. Fortunately, today’s smart devices truly make predictive maintenance possible. Note, however, that a digital fieldbus providing live diagnostic data isn’t enough to achieve predictive maintenance. So, before you try to use the savings possible from predictive maintenance as a justification, ensure that you have the essential maintenance software and other infrastructure in place. (More on predictive maintenance will appear in an upcoming article in Chemical Processing.)
Advanced device diagnostics also can lower costs during shutdowns. Rather than working on something because there won’t be another chance until the next outage (preventive maintenance mindset), you’ll be able to determine the actual condition of the equipment when planning the shutdown and decide whether the device needs replacement, complete overhaul, a minor adjustment or no work at all. The result is a reduced scope of work during the outage and hopefully a shorter outage as a result (or at least fewer technician hours). Just like for the initial commissioning, you also can expect a faster startup at the end of the shutdown.
Digital instruments also have better rangeability and accuracy than traditional analog devices. This provides two additional savings opportunities. First, spares inventory can be reduced because a single transmitter can handle a wider range of applications. For example, in the past it would have been necessary to keep differential-pressure analog transmitters used for orifice plate measurement (still the most common flow measurement technique) for each range (0–100 in. WC, 0–150 in. WC, 0–200 in. WC, etc.), but now a single transmitter can do all these signals and simply be ranged in the shop before being installed. Second, increased accuracy — resulting in part from fewer analog/digital conversions of the signal and in part because the sensors in modern instruments are better than those of their predecessors — promises greater throughput because process units will be able to operate closer to their operating limits with resulting higher product yields.
Consider the opportunities
Digital communications will be part of your plant’s future and, in fact, already likely are happening now without you realizing it. This article hopefully has given you a sense of the opportunities being lost with your existing infrastructure and the even greater opportunities possible by taking full advantage of a digital fieldbus.
Ian Verhappen is director, industrial networks, for MTL Instruments, and is based in Edmonton, Alberta. E-mail him at firstname.lastname@example.org.