Control Systems

Fieldbus improves control system reliability

Profibus-PA and Foundation Fieldbus both reduce cabling cost by sharing wiring among several field instruments. They also save money by reducing the number of instrument interface cards required to connect field instruments to the control system. However, they all save money by being able to use the same field transmitter for a wide range of measured variables.

By Dick Caro

It’s well known that using any of the fieldbuses (HART, Profibus-PA or Foundation Fieldbus) can save money on new installations or even in major instrumentation and control system modernization projects. Profibus-PA and Foundation Fieldbus both reduce cabling cost by sharing wiring among several field instruments. They also save money by reducing the number of instrument interface cards required to connect field instruments to the control system. However, they all save money by being able to use the same field transmitter for a wide range of measured variables.

Wide operational range

Before HART, Profibus-PA or Foundation Fieldbus, it was necessary to purchase the field instrument specifically for the range of the specific process variable. For example, if the orifice of a flow transmitter were sized for 0 to 100 psig pressure drop, you’d purchase a differential pressure transmitter for that range. When process conditions changed, flow rates increased and the orifice now generated differential pressures to 200 psig, it was necessary to replace the transmitter with one that matched the new range, or to change the orifice diameter.

However, with HART transmitters, it’s only necessary to use the instrumentation console or HMI screens to change the instrument’s range settings dynamically. Range change also can be done with the HART handheld (275/375) terminal by attaching it to the instrument wiring wherever it may be accessible, such as the termination panel.

Profibus-PA and Foundation Fieldbus make it even easier. They both allow you to merely change the conversion settings to accommodate the higher differential pressure range and reset the alarm limits. This can be accomplished without ever going to the instrument or removing the cover. Because Profibus-PA and Foundation Fieldbus are only used with control systems that usually have extensive HMI support, access to the conversion settings and alarm limit values is available through the control system’s instrument detail display.

Smaller inventory

The primary savings from using any fieldbus instrument comes from reduction in spare instrument inventory. You need to keep only one instrument of any particular type (differential pressure, RTD temperature, pressure transmitter, etc.) at the local plant storeroom, because it can be used to replace any similar transmitter.

However, using fieldbus instruments doesn’t guarantee interchangeability. In the days of analog 4-20 mA instruments, identical mounting flanges meant the instruments were interchangeable, once they were set up for the range required. HART instruments are generally interchangeable, but if the application or instrument configuration uses any feature not required by the HART protocol, then the replacement must offer that same feature.

When using Profibus-PA instruments, the replacement must support the same Electronic Device Descriptions (EDDs), and must have the same Profibus address set into the interface switches before it’s connected to the network. Profibus-PA instruments are addressed by the switch settings, not by the location of the wiring connections, as are HART and analog 4-20 mA instruments. Once the instrument is connected to the Profibus-PA network, the control system must be told that it’s now active so that it can be downloaded with its configured database. This simple task must be done on the control system’s maintenance display. Some control systems are programmed to look for a reconnected node and require no notification.

Foundation Fieldbus field instrument instruments are even more complex, and generally should be replaced by the same brand and model number. Before connecting to the network, it’s necessary to write the instrument’s tag name in the shop using a simple maintenance terminal with configuration software. This operation also sets the instrument to its initial state. Because the Foundation Fieldbus instrument has an extensive database, the control system automatically downloads it when it’s recognized as being in its initial state after being connected to a network.

Calibration and drift

Components used in analog instruments drift from their initial values with time, especially under process conditions. Fieldbus instrument’s digital circuits compensate for drift automatically, or don’t drift with age. This is especially true for zero-drift. Thanks to digital circuitry, the long-term calibration stability of HART, Profibus-PA, and Foundation Fieldbus instruments of all types is far superior to pure analog instruments. This reduces the need for recalibration to once every five years rather than once per year.

While the calibration procedures for each fieldbus instrument are different, they all support fully-automated calibration. It’s a bench procedure using the instrument powered for operation and a PC equipped with appropriate interface and software for calibration and configuration. Rather than screwdriver adjustments typical of analog instruments, values are set through the PC interface when pressures or temperatures are applied with a standard source. These procedures greatly reduce calibration time.

Diagnostics

Fieldbus instruments usually provide both online troubleshooting diagnostics. Online diagnostics are useful for determining if there’s anything wrong with the elements connecting the basic sensor to the process. This includes the impulse lines normally used to connect pressure and differential pressure instruments to the process or the orifice used for flow measurement. Often, measurement problems come from obstructions in these small tubes. Fieldbus instruments detect such obstructions by sensing the normal pressure fluctuations in these lines. When these fluctuations stop, it indicates an obstruction, which usually results in an alert to the operator that the signal is questionable. If you find impulse lines that frequently become blocked, you might want to install a cleaning system that uses high-pressure air, process fluids or water to flush them out.

Temperature sensors also are expected to produce some minor signal variations, but a measurement without significant variation might indicate that the thermocouple or resistance bulb has burned out, or that the connection is open. Open thermocouple and resistance bulb detection usually is included in modern fieldbus instrumentation. Without this detection at the transmitter instrument level, the high-impedance of the sensor electronics can remain steady and incorrect for long periods of time. Sometimes, extra (redundant) sensors can eliminate the need for on-site repair.

Modern multipoint temperature instruments often are used to provide termination for redundant temperature sensors. A simple reconfiguration of the control system function block may be required to use the redundant temperature value, but Foundation Fieldbus function blocks can be configured to reject questionable signals and use only good signals.

Troubleshooting often is accomplished with the instrument still connected to the process. The results of the online diagnostics can be observed to determine the cause of the instrument problem. When the connection, impulse line or calibration problem is fixed, the troubleshooting diagnostics can be used to test the instrument before returning it to service.

Asset management

Fieldbus instruments are an integral part of a modern asset management system. Without fieldbus instruments, transactions with the asset management software must be manual — a procedure often ignored or delayed. Asset management software is connected directly to the control system network and monitors changes in fieldbus instrument status. When the instruments are restored to operation after maintenance, the asset management database updates automatically, including the name of the maintenance technician and the date of the last access.

Many process industry users configure their asset management software for predictive maintenance. By using the diagnostic features built into fieldbus instrumentation and control valve positioners, they track indications of impending failure. Some applications can produce a notice that results in a work order when repair is required.

Instrument or control valve repairs can be scheduled before failure without shutting down the process. This is far more economical than crisis repair, which is waiting for the failure and repairing the instrument or control valve on an emergency basis.

It’s also far more effective than preventive maintenance that schedules repair or replacement on the basis of instrument history. This is particularly true for one common application: control valve repair. Most fieldbus control valve positioners accumulate a usage statistic, often called “stem mileage,” that is effective for preventive maintenance. The continuous measurement of actual stem position versus desired stem position, known as hysteresis, can be used as a predictive factor of poor control loop performance. Hysteresis, commonly called valve stickiness, occurs when higher than normal air pressure is required to move the valve to the desired position just to overcome friction.

Many users have reported that predictive maintenance alone is the single largest savings resulting from using Foundation Fieldbus. Most of their savings result from being able to schedule maintenance only when necessary and convenient, which is less costly than fixing after it’s broken or servicing on a schedule when it might not need repair.

Conversion of instrumentation from pure analog to one of the three fieldbus options — HART, Profibus-PA or Foundation Fieldbus — can reduce the cost of maintenance, provided that their advanced diagnostic features are used. They all offer the opportunity to reduce instrument spares inventory by allowing one wide-range instrument to be used as a spare for many different ranges.

Dick Caro is CEO of CMC Associates, Acton, Mass. Contact him at RCaro@CMC.us and 978-635-9449.