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.

07/21/2015


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. 


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