Wireless transmitter implementation: Design aspects

How to select wireless transmitters: With the advances in wireless technologies and development of communication standards, battery shelf life, ease of installation, and reliable maintenance, increased data use is attracting many customers in engineering, procurement, and construction (EPC) business.
By Ravi K Tata and Kalluri Anjaneyulu July 21, 2015

Figure 1: A firewall is shown as a standalone device, however in some configurations a firewall is part of the wireless gateway. The gateway is responsible for the security of wireless network assigned to it. Courtesy: BechtelCustomers are increasingly requesting engineering, procurement, and construction (EPC) contractors to provide optional quotes of wireless transmitters in their request for proposals (RFPs). EPC contractors often are presenting wireless solutions for lump sum turnkey (LSTK) jobs, as well, since they offer reduced construction and commissioning time more than cost savings.

Wireless transmitters offer cost advantage over conventional hardwired installation by using reduced hard cabling, cable trays, junction boxes, supports, field terminations, reduced distributed control system (DCS) or programmable logic controller(PLC) system panels and offer significant savings in construction and start-up efforts. Another advantage is that wireless transmitters are easy to add on to the existing system.

Wireless transmitter applications

The most common trend while offering wireless solutions during the RFP stage is to identify remote or inaccessible locations in the plant apart from those applications intended for monitoring only. The following systems and applications can be considered during the RFP stage:

  • Storm water ponds
  • Waste water ponds
  • Raw water storage tanks
  • Ammonia storage tanks
  • Demineralized water
  • Instrument air
  • Potable water, closed cooling water
  • Refinery tank-forms, columns
  • Boiler tube metal thermocouples
  • Analysis measurements.

In addition, wireless transmitters are being promoted for safety applications, such as obtaining feedback on relief valves, safety showers, actuators, etc. 

How to select wireless transmitters

To select wireless transmitters, consider the following key factors:

  • Outside battery limit (OSBL) (representing the facility or plant elements that support the main process). Usually OSBL units are considered as utility or other offsite facilities in refinery, petrochemical, fertilizer, or power plants and are located in the peripherals of a plot plan, at a distance from the main control rooms. Often these facilities are more than 984 ft (300 m) away from the main process and require #14/16 AWG gauge wires for power.
  • High installation costs. Running instrument /power cables from these facility instruments to DCS/PLC cabinets located in offsite locations or substations require cable trays. Running cable trays under sleepers or above ground requires additional supports, costs additional money, and obstructs crane movements during construction or maintenance. Labor costs for construction of cables trays, pulling, and terminating cables far outweigh wireless transmitter and gateway costs.
  • As wireless technology penetration is not high, and its acceptance to critical applications is yet to be received by end users and plant owners, it is important to consider those applications that are not part of any critical closed-loop control.
  • Some measurements, such as pressure, differential pressure, and temperature transmitters, are commonly available in wireless versions with battery power. A few other types of instruments, such as flowmeters and analyzers, require line power, but output can be converted to wireless with additional adapters at an additional cost. This aspect needs to be carefully evaluated and considered during design. Consider cost savings that may be available if measurements can be made in less expensive locations.
  • Based on contract or owner requirements some applications related to plant optimization and equipment condition monitoring can be considered for wireless applications. These can include safety valve leakage (wireless sensors based on acoustics) for leak monitoring and safety valve maintenance; cooling tower vibration measurements, which can be difficult to access; and steam trap monitoring to assess trap conditions. 

Wireless points identification, selection

Input and output (I/O) points are first identified in three main categories by using the following identification criteria:

Category 1

  • Indication points
  • Monitoring points associated with normal alarm.

Category 2

  • Noncritical, open-loop points
  • Monitoring points associated with critical alarms.

Category 3

  • Closed-loop control
  • Emergency shutdown (ESD) loops.

All Category 1 points are considered wireless. Under Category 2, noncritical open loops and monitoring points associated with warning alarms can be considered as wireless points. However, alarms that require immediate action shall not be considered as wireless points. Category 3 often has not been considered for wireless application because of the critical nature of closed-loop control and ESD systems. [Editor’s note: Redundant wireless systems have been used in certain safety critical applications with appropriate risk assessments.] 

Wireless communication standards

The following are the two most popular communication standards for wireless communication in the process industry:

  • WirelessHART
  • ISA 100.11a—Wireless industrial automation process control networks and related applications.

While not intending to compare advantages and disadvantages between WirelessHART and ISA 100.11a, from an EPC contractor’s viewpoint, selection of a particular standard depends on the following:

  • During the EPC phase of the project, the owner often seeks recommendations of various technologies available, such as WirelessHART and ISA 100.
  • Although client preferences prevail over the selection of technologies, EPC contractor perspective, integration with DCS, and trained start-up resources are key drivers for technology selection, since LSTK jobs and EPC projects come with schedule delay penalties or early completion bonuses. Often RFPs for brownfield projects express a preference for integration with existing DCS infrastructure, which is more difficult due to availability of spares, space, and other considerations.
  • Review of RFP requirements: Owners may require use of a particular DCS or brand of field instruments, which may favor use of one standard over another. 

Control system interface

When interfacing with a plant control system: 

  • Each wireless transmitter is associated with a wireless gateway by means of identification (ID) and a key.
  • Each wireless gateway can handle up to 100 points, typically.
  • Gateways can be configured in simplex or redundant configuration.
  • Gateways can ideally be located in a process interface building (PIB) or in a control room within distance limitations to the nearest wireless transmitters or repeaters with an antenna on top of PIB or control room.
  • Gateways can be located inside a DCS or PLC network cabinet.

A gateway communicates to DCS via Modbus (serial or TCP/IP) communications by hard connection to respective communication module through a network switch via a firewall.

Figure 1 shows a typical system overview diagram showing a wireless network integrated with a plant control system. 

Wireless I/O allocation

Wireless devices are grouped with wireless gateways as follows:

  • Respective unit or PIB—Allocate all wireless transmitters of respective units to the corresponding PIB or substation building where wireless gateways are located.
  • Scan rate—Assign a scan rate depending on criticality of the communications. Higher scan rates provide faster validation of the status of the asset.
  • Gateway loading—Depending on wireless scan rates and number of wireless points, one is required to calculate manufacturer wireless gateway loading. These tools are easily available on most manufacturers’ websites.
  • Spare philosophy—Consider 20% excess capacity for wireless gateway loading.
  • Consider separate gateways for wireless transmitters supplied by package vendors.

Wireless network design validation

It’s essential to have the wireless network validated with real, physical plant coordinates of the wireless devices, wireless gateways, and obstructions, such as pipes or structures. Wireless instrumentation vendors have software tools to validate the project wireless network by entering plant x, y, and z coordinates for each wireless device, location gateways, antennae, or physical obstructions and run the tool to check that wireless devices are covered through the respective gateways or to identify pinch points. Tools also can recommend installing repeaters to fortify the wireless network.

Figure 2: A typical validation output of a wireless network, which provides information for gateway validation. Courtesy: Emerson Snap ToolThis activity needs to be performed when the model is 60% completed and when instrument locations are modeled or available for validation. 

Figure 2 shows a typical validation output of a wireless network. The following data are required to validate gateways:

  • Plot area of the plant
  • Gateway location
  • Antenna location
  • Distance to the nearest wireless devices
  • Number of neighbors in percentage
  • Isolated devices
  • Obstructions elevations.

A graphical output shows pinch points where devices are not covered through an assigned gateway. Generally, pinch points are cleared by introducing repeaters at the strategic location. In some vendor-specific offerings, normal wireless pressure transmitters can be configured as a "repeater."

Wireless network best practices

  • Best practices for wireless network design include the following:
  • Wireless networks should be scoped to one process unit in a PIB.
  • Wireless gateways should be placed in the PIB area.
  • Within an effective gateway range, a minimum of five wireless instruments should be placed.
  • In a wireless network with more than 25 devices, five devices should communicate directly to the gateway. If devices number fewer than 25, repeaters may be needed. The design stage should validate the need for repeaters, their quantity, and location.
  • There should be 25% of wireless instruments on the network within range of the gateway for a large network. In some situations, the number may come down to 10%, which is still acceptable.
  • Each wireless instrument should have three neighbors. This leverages the self-organizing mesh technology implemented in IEC 62591, the WirelessHART standard.
  • Effective range is determined by the type of process unit and the density of infrastructure. In the engineering stage, the best practice is to feed in assumed ranges to a wireless field device based on its environment, instead of using the full the range of the device.

Broadly, the environment in a process unit is described under four types:

(1) Heavy obstruction—100 ft (30 m). This is the typical, heavy density plant environment, when a truck or equipment cannot be driven through.

(2) Medium obstruction—250 ft (76 m). These are less dense process areas, with lots of space between equipment and infrastructure.

(3) Light obstruction—500 ft (152 m). These are typical of tank farms. While tanks are big obstructions, lots of space between and above them makes for good radio frequency (RF) propagation.

(4) Clear line of sight—750 ft (228 m). The antenna for the device is mounted above obstructions, and the angle of the terrain change is less than 5 deg. Some WirelessHART vendors provide options and techniques for longer distance applications.

The wireless network design should be in a graphical format and deliver the following results:

  • Graphical wireless engineering document depicting the designed network complies with the design principle.
  • There should be no pinch-points in the network.
  • Each device must have at least two neighbors to ensure reliability by having redundancy in communication paths. In case of having only two devices in the network, then a repeater would be added. The validation of the requirement of the repeaters, their quantity, and location will be derived during the detail design stage.
  • Number of repeaters should be noted when needed to reinforce the network or to avoid pinch-points.

Wireless transmitters are a great choice for remote and inaccessible locations and can reduce installation and commissioning times. Maximum benefits from using wireless technologies are observed in projects located in areas where labor is expensive. As a rule of thumb, all monitoring applications are good candidates for wireless transmitters and technology implementation. Understanding customer requirements through upfront discussions can help in selecting an appropriate technology and host system interface. Vendor interactions with preliminary piping and instrumentation diagrams (PIDs) during the design stage can help in identifying suitable applications for wireless technology so that wireless design can be part of initial instrumentation designs.

Midway during design stage, it is essential to involve wireless instrument vendors to validate the wireless network using a 3D model to remove any pinch points and finalize wireless gateway, antenna, and repeater locations.

– Ravi K Tata is chief engineering and Kalluri Anjaneyulu is engineering group supervisor, Bechtel India; edited by Eric R. Eissler, editor-in-chief, Oil & Gas Engineering, eeissler@cfemedia.com.

Key concepts

  • Wireless networks should be scoped to one process unit in a PIB.
  • There should be a minimum of five wireless instruments within effective range of the gateway.
  • Keep 25% of wireless instruments on the network within range of the gateway.

Consider this

How can you improve your plant’s wireless connectivity by modeling the facility and finding optimal placement for wireless devices?

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