For example, multi-parameter transmitters are difficult to implement with 4–20-ma wiring but a natural fit for fieldbus systems. Employing transmitters that measure multiple variables saves money and improves performance by delivering additional information about the process to the control system.
Adding new field devices to a 4–20-ma system often is impractical when new wiring from the control room input/output (I/O) rack to the device is required. In contrast, fieldbus systems are designed from the ground up to accommodate expansion. Because new devices can be added easily, more parameters can be measured and controlled.
Many fieldbus systems not only can add instruments to existing installations in a hard-wired daisy chain fashion but also through tightly integrated wireless links. Wireless connections can save substantial amounts of money and enable measurement of parameters at remote locations such as tank farms.
While a small-scale system is a good way to familiarize your plant with fieldbus, larger-scale projects will deliver added benefits when properly executed. Whichever approach your plant takes, it's important to understand and follow best practices.
Fieldbus implementation and support require a different set of knowledge and tools than the traditional 4–20-ma infrastructure. Like an office network of PCs, a fieldbus installation is a network of connected computing devices — so its installation and support call for the services of fieldbus network engineers and technicians.
A fieldbus network must be modeled during the design phase to check and verify performance. The fieldbus network from device selection to cabling to integration must be carefully engineered. Attention must be paid to grounding and electrical noise issues as a typical fieldbus network will have more wires connected over a wider area than a point-to-point 4–20-ma installation.
A common mistake is improper distribution of nodes and spurs along with poor network design of connections, bricks, isolators and other components. After installation, users must ensure continuing engineering and support because changes and adjustments always arise after initial design and implementation.
Each field device should be recognized as compliant with the fieldbus based on testing and certification, preferably by a recognized third party or by the fieldbus support organization. It's also important to ensure that integration testing defined by the control system supplier has been performed and that that the instrument in question is specifically listed by the control system supplier as compliant.
For expert assistance on fieldbus, there are two main avenues.
The first is a supplier. It may provide extensive advice and support, often on a free or low-cost basis. But advice is likely to be skewed towards its platform, with limited visibility into alternative solutions. It's often best to consult with suppliers that support multiple fieldbus solutions as they can provide a view that may be broader in scope.
Service providers such as system integrators and engineering firms can be a more independent source for advice and support. If you opt for this route, make sure to engage a firm that has extensive experience not only with fieldbus but also with your plant's particular processes and control system.
Fieldbus systems can provide tremendous benefits in terms of lower costs, increased uptime and better overall plant performance. But take care when selecting and deploying the fieldbus solution; it's often best to start small.
A field device usually is first connected to the control system via a fieldbus. The control system then is connected to the IT system through some type of middleware such as a database server.
However, for some field devices, particularly those involved in process monitoring as opposed to process control, direct connection between the field device and an IT technology platform may be a better solution (see Figure 1). This approach avoids the deterministic delivery, data management, safety and security issues associated with more-demanding control system environments.
Connecting directly from the field device to the IT system cuts costs substantially for three reasons. First, a direct connection eliminates the need for intermediate hardware components and software systems. Second, the field device in question may not need to be part of the control system's overall validation and maintenance program. Third, field device access can be controlled through existing IT security systems.
Many environmental applications such as EPA reporting and enterprise applications such as inventory management don't require the millisecond update speeds or deterministic behavior inherent to most control systems. Some facilities have found that up to 70% of their field devices don't have any associated real-time control functions.
Information from these noncritical field devices can be conveyed directly to standard networks and associated databases via wired and wireless field gateways. Not only is the primary information delivered in fully defined engineering units but device status also can be continually monitored and communicated on an event-driven or periodic basis.
Remote servicing tools and asset management applications working at the network level also can be used to configure and manage connected devices. Standard IT security and data management tools can be used to control access.
Craig McIntyre is chemical industry manager at Endress+Hauser, Greenwood, Ind. E-mail him at email@example.com.