How interesting can thermocouples be, anyway? If you’re building a gasification plant, they can be very interesting indeed, especially if they are a frequent point of failure.
Jack Jones of Shell Global Solutions gave a quick tutorial in “Gasification 101” this morning at Emerson’s Global Users Exchange at the Gaylord Texan Resort and Conference Center in Grapevine, Texas. “There are two kinds of gasification systems,” he said, jabbing at his competition, “even though other people don’t think so. There are liquid systems that take liquid feedstocks, such as bottoms and liquid byproducts, and make syngas from them. We’ve been building them since 1956 and currently have 87 gas/oil units operating. There are also solids gasification systems, which are newer, and we have five units in operation. These units use coal, lignite, pet coke, oil residue, biomass and other relatively low-BTU solids for their feedstock.”
Temperature is the important controlling parameter for a gasification system. Emerson’s Steffen Langner, director of temperature for Emerson Europe, took over the tutorial at this point to talk about thermocouples.
Jack Jones of Shell Global Solutions discussed Shell’s pursuit of temperature instrumentation that could reliably withstand the nasty temperature and pressure extremes of the company’s gasification processes.
How interesting are thermocouples and thermowells?
Well, when you want to control a gasifier at 1700 °C, they can get very interesting, Langner said. Not to mention that gasifiers work at up to 65 bar, with high concentrations of hydrogen and with high soot and metal content.
“Traditional thermocouples and thermowells don’t work well at that temperature,” Langner reported. “Metal thermowells work at temperatures under 1300 °C quite well,” he said. “Sintered metal thermowells die quickly at 1700 degrees, but ceramic thermowells work quite well with a margin for safety.”
What produces thermowell and thermocouple failure? “Mechanical or physical stress does,” Langner said, “As does chemical stress. Carbon penetration is a real problem, as is the reduction of the platinum melting point when the thermocouple is poisoned by hydrogen intrusion or other chemical diffusion.”
Langner showed pictures of thermocouples that exhibited a failure mode he called “bamboo structure.” Instead of a solid thermocouple, there were bamboo-like breaks all along the length of the rod.
“What customers need,” Langner reported, “is a long operating sensor life, with long term stability and reliable measurement. Using our ceramic thermowell, sapphire tube thermocouple construction, and our dual gastight process pressure seals, we have been able to maintain proper operation in excess of 16,000 hours (667) days with no contamination.”
“This allows us to predict the sensor life, schedule maintenance and provide better measurement,” he said.
So even with the emphasis we all place on advanced process control, wireless, distributed control and asset management, we can never forget the lowly thermocouple and thermowell…one of the basics of process automation.