By Alice McWilliams, PE, Chevron Phillips
The application of new technologies seldom goes as planned and can be an outright adventure. But the potential for significant cost savings can make overcoming the hazards and pitfalls well worth the journey.
That was the case at our plastics plant in Pasadena, Texas, where we needed to overcome obstacles and create a wireless system to monitor certain data readings.
Initially, the impetus for the project involved the need to obtain two readings, but things escalated quickly. The equipment was installed in August 2007. Design for the installation had started earlier, in February.
The Application: a Wireless Opportunity
We have a 40-ft-tall API tank used when we unload a barge. Each time, the operator would climb the tank to record temperature at the vent and vent valve position after the tank's condenser unit. The plant wanted to automate the recording of measurements to record the vent temperature and vent valve position to provide improved documentation for our environmental records and improve operator efficiency by eliminating the time-consuming, lengthy, repeated trek to the top of the tank (Figure 1).
Because the process variables we wanted to measure were at the very top of this tank, it would have required a long cable and conduit run up the side of the tank. It would have required a 40-ft-tall scaffold to run the conduit and cable. The costs to obtain just two new readings did'nt seem justified compared to a traditional hardwired method.
"We began to look at a wired solution," says John Scott, senior account manager from the Rosemount division at Emerson Process Management (www.emersonprocess.com), "but we came up with a 65% installation savings by going wireless. One of the bigger costs for a wired solution would have been the scaffolding.”
Wireless obviously was the best choice. We looked at several wireless network solutions but determined that Emerson was a good choice for this plant. Regular-style wired Rosemount transmitters already were used at this unit. Training on the handheld was up-to-date. This meant that training only needed to cover troubleshooting changes due to the wireless hardware and setup. This would minimize our training costs, as well.
Wireless Network Design
We began to seriously explore implementing a wireless solution to this problem. One concern was how to provide valve position. Emerson didn't offer any direct valve-positioning transmitters except as an add-on to the digital valve positioner. The controller for tank pressure was a local pneumatic controller with a pneumatic valve positioner. I eventually realized that I could use a pressure transmitter on the controller output and provide an implied valve position.
Designing a wireless network, to my interpretation, meant that cable runs were kept to a minimum, reducing both installation and material costs. It seemed, therefore, that the best location for the wireless gateway would naturally be on top of the rack room (Figure 2). I could easily interface with the old Provox DCS equipment via the intelligent device interface module set to accept Modbus and run only three short cables—Modbus, Ethernet and power—to the roof.
This older version of the wireless network required that the distance to the gateway be less than 500 ft. The temperature and valve-position-measurement points were 600 ft away from the rack room. Additional transmitters between the API tank and the rack room were needed to act as repeaters for the API tank data. These additional wireless applications between the API tank and the rack room would improve both distance and stability needs. Mesh wireless networks also become more stable as the number of nodes to the network increases. Fortunately, it was easy to find more readings that the plant wanted. The production engineers for our unit also were excited about trying a wireless network. Two additional measurement applications, the bearing temperatures for two pumps and the barge unloading line pressure, fell into our laps (Figure 3).
One of the pumps on a tank closer to the rack room had experienced bearing failure earlier that year and caused a hydrocarbon leak into the electrical housing. Monitoring the pump bearing temperature would improve reliability and safety in this operation.
Also, our barge unloading line was experiencing pressure excursions in excess of what we would like. It was not easy for the production engineer to tie the higher pressures with specific events during the unloading cycle. Most of the unloading valves were hand-operated, and the barge unloading information also was isolated to the dock area. The recorded pressure indication could help determine the point in the cycle when the high pressure occurred so we could develop better procedures for preventing recurrence. Both of these applications were a great opportunity to bring information that would provide improved plant operation to the DCS. In the end, we planned to develop a wireless network consisting of five transmitters: tank vent temperature, vent valve position, two pump-bearing temperatures and barge unloading line pressure.
Transmitter Update-Time Decisions
Wireless transmitters don't provide data on a very fast update rate. Emerson generally recommends update times around a minute. Pressure is a fast-changing variable and I was particularly concerned about the slow update rate for the two pressure transmitters on the unloading line. What if, at a 1-min update rate, we totally missed recording the event that signaled the pressure excursions on the unloading line?
In the end, I picked 15-sec update rates for all the transmitters except the barge unloading pressure. I found out at installation time that, although the recommended lower limit was 15 sec, the system actually could perform at much faster rates. I selected a 3-sec update rate for the barge unloading line pressure. The down side to changing to a faster update rate was sacrificing some battery life and gateway capability.