Many newly built refineries, chemical plants, and other major process plants in Asia employ DCS systems connected by fieldbus. These control systems are mostly from Emerson Process Management, Yokogawa, Siemens, Honeywell, Invensys, and other control system vendors. The reason for adopting fieldbus systems is that they consistently promise users numerous advantages in operation and maintenance: more efficient process operation due to increased information; less system downtime due to predictive process diagnostics; and instruments that can tell us when system calibration or maintenance is needed. In a traditional plant, an instrumentation technician no longer spends approximately 65% ​​of their time troubleshooting instrumentation problems. With fieldbus, the location and specific details of problems can be easily identified from the HMI display. While fieldbus systems offer users many advanced features, several critical decisions still need to be made during project design, which will impact future system maintenance and operation. Long after the project implementation team completes the project and delivers it to the factory, many problems may still arise, even if they are not caused by the control system supplier. These problems include: • Installation and startup; • Short-circuit protection; • Instrument removal and replacement; • Installing intrinsically safe systems; • Setting up redundant systems. Many of these problems can be resolved by properly configuring the fieldbus devices as designed in the project. Users should pay attention to these issues, understand the latest hardware options, and request the DCS supplier to configure the hardware to minimize maintenance costs. A small survey shows that maintenance, startup, and installation issues are quite important when configuring the entire fieldbus system. Approximately 90% of fieldbus problems are caused by wiring issues, the biggest problems being: port, system conflicts, and improper grounding. Getting Started The most challenging problems always arise in the initial stages. Why isn't the instrument working? In many cases, this is an installation problem. Simply put, the technician did not configure the terminals correctly. All fieldbus segments require the correct ports to prevent communication errors. Unfortunately, many subcontractors pay little attention to the importance of terminals for operation, either ignoring them completely or simply installing them haphazardly. Typically, junction boxes and field control enclosures serve only to determine terminal positions, but this can be a significant factor affecting or delaying the commissioning process. Delays during commissioning translate directly into increased costs. According to an oil company's estimate, time delays at refineries can cost up to $400,000 per hour. This problem can be addressed from the initial system design stage by incorporating automatic segment terminals on newer device couplers, such as Moore Hawke's TrunkGuard. These automatic segment terminals greatly assist with segment commissioning. With various field device couplers, the automatic terminals respond at the furthest unit, automatically connecting any unconnected segments. Short Circuit Protection Short circuits are a common problem in any fieldbus installation. Maintenance personnel may accidentally knock over cables; corrosion can reduce connectivity; vibrations from pumps or motors can loosen cable or terminal connections. Maintenance personnel must be constantly vigilant for any potential short circuits in any instrument. Past fieldbus systems relied on current limiting methods for circuit protection. This method limits the current value to 40-90mA, but it has drawbacks in terms of continuous network segment connections. Traditional current-based methods can cause instruments with voltages below 9V to fail; when a device's voltage drops below 9V, it will be removed from the network segment. A typical fieldbus network segment has a current of 350mA and a voltage of 25V, theoretically sufficient to support 16 x 20mA fieldbus devices, but the system must account for all voltage drops. This can potentially cause some devices to have voltages below the limit, or even shut down the entire processing unit. There are two ways to address this problem. The first is to reserve a safety margin. That is, don't install too many instruments on a network segment based solely on theoretical values; instead, leave some buffer space. This will limit the number of instruments, but it ensures the network segment can still operate even if an instrument fails. However, considering the cost of fieldbus devices, this is a relatively expensive method. Another method is to select some device couplers with overload (foldback) short-circuit protection, where the overload circuit is isolated by the coupler. Fold-back technology's logic circuitry detects a short circuit in an instrument within the network segment when a short circuit excitation occurs with each current, disconnects it from the segment, and activates a red LED alarm. It also uses a small current to determine the timing of the short circuit. Removal and Replacement Maintenance personnel desire easy removal of a field instrument in hazardous environments without shutting down the entire network segment or requiring complex instrument disconnection procedures and mechanical interlocks. These headaches can be addressed by adding dedicated fieldbus device couplers. Intrinsically Safe Systems Intrinsically safe systems allow the use of instruments in hazardous environments. They ensure safety even if a connection cable experiences an open circuit in a hazardous environment. Intrinsically safe systems like FISCO are quite popular in Europe, particularly in Germany. Due to their high cost, their use is less common in other regions. An intrinsically safe system requires expensive isolation barriers and power supplies to support a portion of the bus instruments in the network segment. It's rare to find 4 to 6 fieldbus devices in an intrinsically safe network segment. FISCO can significantly reduce the system's MTBF and cut cable length by almost half. In recent years, a new concept has emerged that addresses some cost issues. In simple terms, the isolation barrier is divided into two parts: one at the safe/hazardous interface and the other at the device coupler. This efficient design allows each segment to withstand almost all 350mA current while maintaining standard intrinsically safe connections. Redundancy Setup Most factories have critical equipment whose failure can cause an entire process to shut down. The traditional approach is to install redundant instruments and control devices. Most fieldbus products use a field redundancy system that completely replicates the entire network segment, including H1 interfaces, power supplies, device couplers, fieldbus instruments, instrument-to-coupler connections, and fieldbus cables. However, replicating all components, from H1 cards to field instruments, is extremely costly. Not only does it double the hardware cost, but replicating complex field instruments, such as flow and pressure transmitters, is also very expensive. Newer fieldbus systems, such as TrunkSafe, include dual redundant field voltage regulators, two fieldbus cables, and a fault-tolerant device coupler. There is no need to replicate field instruments unless the user is concerned about the susceptibility of certain instruments. Moreover, the new system can reduce costs to half or one-third of traditional redundant systems. No More Headaches Fieldbus is an exciting technology, although maintenance issues have plagued people for nearly 10 years. Now, new solutions can address these problems. To avoid maintenance issues in your new fieldbus system, consider all design options from the initial system design stage. End users should understand that DCS vendors cannot provide all fieldbus hardware. For example, in an FF system, the DCS vendor typically only provides the H1 card. Other fieldbus devices, including power supplies, device couplers, etc., are provided by other vendors such as MooreHawke, Relcom, and P+F. End users have the right to request that their DCS vendors consider some new hardware and more options, rather than accepting a 10-year-old solution and then suffering headaches in maintenance and operation.