Engineers at Emerson Process Management recently studied and compared the costs of installing wireless and wired networks at a large hydrotreater facility. In addition to determining if wireless is a good value, the study asked: How should wireless instrumentation technology be used in capital projects? How does it affect it the cost of those projects? What other applications and process improvements open up when implementing wireless?
Emersons John Dolenc, PE, principal consulting engineer, and Dan Daugherty, fieldbus consultant for process systems and solutions, provided detailed results of the study in their presentation, Economic and Technical Considerations for Deployment of Wireless in Capital Projects, on the second day of Emerson Global Users Exchange 2008 at Gaylord National Resort and Conference Center near Washington, D.C.
Wireless cut costs from $8.8 to $5.2 million. An analysis by Emersons John Dolenc (pictured) and Dan Daugherty demonstrated an installed cost savings of 40% for a typical capital project.
To learn if wireless could reduce capital project costs for all monitoring points, the investigators considered three main areasmaterials, labor and engineeringat a multi-unit refinery hydrotreater controlled from a central rack room. The rectangular site is approximately 150 ft x 600 ft, and the rack room is 100 ft from the end of the hydrotreater area. Average wired distance was 400 ft for each device. This includes 350 ft average wire length from the equipment room to the junction boxes, and 50 ft average wire length from the junction boxes to the field devices.
Labor costs assumed for the study were $120 per hour for engineering and $65 per hour for field labor. It was decided that it would take three hours to commission a device and four hours per loop drawing.
Though the study included only wireless devices used for monitoring and not part of a control loop, this still left the investigators with a variety of solutions to consider. These device types included pressure, differential pressure and differential pressure (level); temperature (RTD or thermocouple); discrete input (hand, level, flow, pressure and limit switches); vibration monitor; corrosion monitor; pH/ORP; position monitor; and HART upgrade module for stranded diagnostics.
To start the study, investigators exported the hydrotreaters database into Excel and sorted it by unit and loop. Dolenc reported they only considered device types for which a wireless product exists. Next they assigned field wireless gateways to each unit, but didnt allow networks to cross units and limited size to 50 devices per gateway to enable the fastest speed.
To address temperature, they used single-point devices where the wired system did so and placed all temperature points on four-input multiplexer (MUX) devices. They also maintained a measurement update rate of four seconds or longer to achieve six years of battery life. Finally, they used existing wireless backhaul access points.
Once these preparations were in place, Emersons researchers began gathering data for their comparison. First, they found the hydrotreater had 5,882 total signals and 3,268 control signals and other signals that couldnt go wireless. That left 2,614, or 44%, that could go wireless. This meant the application would need 39 gateways at 50 devices each.
The largest differential costs were conduit and labor between the junction box and each field device, and this was followed by labor costs for all the terminations, said Dolenc.
The investigators quickly determined that a wired network for the hydrotreater application would cost $8.8 million, including $141,000 for cabinets, $1.6 million for cable trays and conduit, $1.17 million for wire and terminations, $84,000 junction boxes, $3.7 million for field devices, $0 for gateways, $327,000 for I/O cards, $408,000 for rack-room footprint and $1.36 million for engineering labor.
They also found that a no-MUX wireless network for the hydrotreater would cost $7.2 million, including $0 for cabinets, $0 for cable trays and conduit, $0 for wire and terminations, $0 for junction boxes, $6.48 million for field devices, $272,000 for gateways, $0 for I/O cards, $0 for rack room footprint and $442,000 for engineering labor.
Meanwhile, a wireless network with MUX would cost $5.2 million, including $4.64 million for field devices, $175,000 for gateways and $392,000 for engineering labor.
Consequently, the hydrotreaters cost per signal was $3,369 for wired, but only $2,753 for no-MUX wireless and $1,993 for wireless with MUX. This translates to per-signal savings of $617 and $1,376, respectively. For the whole network, this meant savings over wired of 18% or $1.6 million for no-MUX wireless, or 41% or $3.6 million for wireless with MUX.
Besides these initial savings, Dolenc adds that one wireless gateway can replace six or more I/O cards, shrink the footprint of a system cabinet by 20%, cut the footprint of a marshalling cabinet in half and eliminate the need for two or more field junction boxes.
Wireless communication technology can reduce the total installed cost of monitoring instrumentation, concluded Dolenc. However, as with any new technology, its important to use best engineering practices when implementing wireless technology to ensure a successful start-up.
