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

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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.

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