Tank farms were among the first applications for wireless instrumentation, in no small part because long distances mean long wires—and high installed costs. And if a "normal" tank farm doesn't sound challenging enough, imagine running new wires at Chevron's Richmond, Calif., refinery tank farm, where 230 storage tanks are terraced like rice paddies 600 feet up forested hillsides.
So when Chevron wanted to bolster the safety of its tank farm operations through the installation of a high-level tank alarm network, wireless was an obvious candidate, according to Greg LaFramboise, wireless technology lead, Chevron Energy Technology Company. LaFramboise discussed the company's early and ongoing efforts to test a WirelessHART network to provide IPL (independent protection layer) risk reduction at last week's gathering of users of Invensys Operations Management's Triconex safety systems in Galveston, Texas.
Beyond the challenge of running new conduit in this challenging environment, wireless proved appropriate in several respects, including the fact that its power and network function independently of the tank farm's primary instrumentation. The weight-based switches used to track movement of the tank farms' floating roofs feature "independent power and independent communication path," LaFramboise said.
The operator is in the loop, and there's no automatic executive action [based on the wireless alarm]," LaFramboise explained. Instead, the wireless device provides a back-up signal if the primary device fails or the trend is not moving in the anticipated direction. And while LaFramboise would have preferred a continuous level signal, this particular application pre-dated availability of truly wireless, guided wave radar tank gauges, which only now are poised to enter the market. A downside of using switches is that they have to be tested monthly by manually lifting the switch weight, LaFramboise said.
Early on, some "flat spots" in data were encountered, which after considerable troubleshooting turned out to be issues with an OPC-based integration with a legacy system, as well as a single malfunctioning transmitter. "It's working well now, but it took a while," LaFramboise said. "Reliability and availability are on the order of three nines [99.9%]."
LaFramboise is active on the ISA's S84 committee's Wireless Safety working group 8, which currently is at work on a technical report on the use of wireless for safety alarming to the operator. Chevron's wireless tank farm experiences no doubt will help shape the conclusions of that working group's report, but the WG is actively seeking new members and perspectives, LaFramboise said. "We want to communicate the issues and provide guidance on how to apply it, just what to do."