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Pepperl+Fuchs Blog

6 Factors to Consider When Designing Fieldbus Infrastructure

Posted by Andreas Hennecke on Mon, May 06, 2019

Many of today’s process automation applications depend on fieldbus technology because of its efficiency in digital bilateral fieldconnex logocommunication. Fieldbus devices can communicate standard process values to the plant operator and the DCS; however, they also have the ability to provide additional information such as operating hours, process temperature, and status bytes for condition monitoring. This wide range of measurements allows plant operators to perform preventative maintenance, recognize field device faults, and predict outages to reduce downtime and increase availability.

Fieldbus enables easy connection to and integration of multi-variable devices to reduce device count, infrastructure, and overall capital expense. Less time and money are spent on setting up fieldbus systems than on standard communication systems due to decentralization of the distribution junction boxes. In fieldbus installations, the junction boxes are typically placed inside the process area, often within Zone 1 or 2 / Div. 2 areas, which simplifies installation and shortens cable runs to instrumentation. A fieldbus device can directly convert a signal and must do it only once for the digital value to transmit with higher resolution and absolutely no drift. When placed in the high-stress environments of many process plants, fieldbus communication remains reliable and is less prone to signal transmission error. Pepperl+Fuchs FieldConnex components have a long service life and are engineered to function under harsh conditions.

fieldbus topology

Due to cable voltage drop, the design of the fieldbus infrastructure requires planning. The most efficient design balances the following requirements: loop cycle time, maximum cable length, load from the instrument, and hazardous area protection for the lowest cost. The design should also take future expansions into consideration. We recommend following these six steps:

1.  Specify the environmental conditions

The work environment can affect the performance of fieldbus components. Shock and vibration, corrosive elements, and high temperatures are the most common elements that can hinder performance. Another element to consider is whether the environment is hazardous.

Fieldbuses can have the same simple design in both hazardous and non-hazardous areas, even though this requires fieldbus infrastructure design to implement ignition protection. Two ignition protection methods can be applied to the entire segment or part of it.

2. For hazardous areas, choose method of ignition protection

The first step in choosing the best ignition protection method starts with product selection. Hazardous area certified power supplies and device couplers can withstand harsh environments and high load conditions. “Increased safety” and “non-sparking” ignition protection guard the circuit against potential safety issues caused by mechanical damage, chemical influence, corrosion, and temperature. These methods carry the highest amount of power into the field and enable long cable lengths and a high device count.

fieldbus zones

Intrinsic safety has two methods for validation, the Entity concept and FISCO. Intrinsic safety allows live maintenance without a hot work permit in any hazardous area but with shorter cable runs. A combination of intrinsic safety and mechanical protection methods provides the best of both worlds: long cable lengths on the trunk and hot work at the instrument. Protection methods determine the types of power supplies and field distributors that are required.

3. Define loop typicals with maximum permitted bus cycle time and device count

For cost-effective segment instrumentation, finding the maximum allowed bus cycle times of control loops is essential. This is determined by the time required for getting the sensor data, processing the control algorithm, and sending the set-point to the actuator. A maximum bus cycle time is considered based on the characteristics of the control to be performed. Conversely, monitoring loops are designed with more devices and a lower bus cycle time for reduced infrastructure cost.

The number of field devices that can be placed per segment is determined by the bus cycle time of the loop. The longer the bus cycle, the more field devices can be connected per segment. The host system’s software can find what is influencing the bus cycle time.  

 feedback control loop

For future expansions, consider reserves in your device count. As a matter of choice, many system integrators make room for 20 % spares.

4. Select instruments and/or define the average current consumption per field device

The IEC standard specifies that at least 9 V of power should be available to each field device. The number of devices and the current they consume determines the voltage drop on a given cable.  Current consumption and device count should be engineered so that future fieldbus infrastructure expansions are not limited by a voltage reserve that is a too-low. 

A good starting point for average current consumption is 15 mA per field instrument. To account for some reserve of voltage drop, specify a minimum of 10 V at the field instrument.

When an application requires live work on field devices, it is recommended that Segment Protectors be chosen to prevent a short circuit in the segment. A Segment Protector is a device coupler that provides intelligent fault protection at each spur. This fault protection exceeds mere short circuit current limitation by including faults from day-to-day activities such as intermittent contact bounce during live work and device malfunctions, also known as jabber. FieldBarriers provide this protection by default.

5. Choose power supply, cable, wiring interfaces, and topology 

The power supply’s maximum voltage and load current determine cable runs, wiring interface, and the number of field devices. Cable type also dictates segment length. Cable type “A,” which is the reference cable type, permits segment lengths up to 1900 meters.

While any topology can be chosen, the trunk-and-spur topology has emerged as the de facto standard in the process industry. It combines a well-known concept from existing technologies with clear structure in installation, maintenance, and explosion protection. A trunk or master cable leads to a junction box installed in the plant for convenient access. Spur cables run from the junction box to the instrument.

6. Validate cable lengths 

The cable length and bus cycle time affect how many field devices can be connected to a segment. Since load characteristics of some wiring interfaces have non-linear behavior, using a planning tool when selecting wiring interfaces is recommended. Segment Checker is a free software tool that can be used to determine appropriate cable lengths. It enables the user to quickly validate the segment design, ensuring the fieldbus infrastructure will work as desired before even installing the first cable tray. Using the Segment Checker, users can:

  • Set up the topologies for typical control and monitoring loops
  • Set a good sized spur cable length as default, e.g., 60 m (this can be changed later).
  • Maximize the trunk length so that no warnings or error messages show.

These cable lengths should be sufficient for the majority of segments. Finally:

  • Check for the limited special requirements: Look for issues such as extra-long cable lengths, a very odd existing cable that needs to be re-used, devices with an unusually high device current, or an unusually high device count for a monitoring loop.

Check these limited, special cases individually.  These can even be saved with Segment Checker for a precise comparison and documentation for “as planned” vs. “as built,” which is available with FieldConnex advanced diagnostics.

The planner can now rest assured that the selected infrastructure will work properly. Most installations will work with the following two calculation examples:

Segment examples, Conditions, and results

High Temperature

Many Devices

Environmental temperature

70 °C

70 °C

Cable diameter (mm2)

0.8

1.5

Coupler type

Segment Protector

FieldBarrier

Device count

10

24

Device load current

20 mA

15 mA

Spur length

60 m

120 m

Possible trunk length

1150 m

725 m

Includes reserve for short circuits and connection of handheld monitors.

FieldBarrier spur length is always 120 m!


Learn more about fieldbus planning

 

 

 

  

 

 

 

Topics: Fieldbus

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