Some like it hot

Dr Peter Saunders calibrating
a radiation thermometer.

Dr Peter Saunders, of IRL’s Measurement Standards Laboratory, uses infrared radiation thermometry to measure the high operating temperatures of these furnaces. He has just completed contracts for Shell Eastern Petroleum in Singapore and the New Zealand Refining Company at Marsden Point near Whangarei.

Shell had just commissioned 10 new ethylene-cracking furnaces which ‘crack’ heavy hydrocarbon molecules into lighter molecules used in the petroleum refining process.

Dr Saunders says the furnace temperature is critical. “If it’s too high, the process tubes may fail prematurely due to increased carburisation, and they’re expensive to replace. If it’s too low the plant doesn’t run at the optimum temperature, resulting in incomplete cracking and reduced efficiency.”

Dr Saunders was in Singapore for a week in November carrying out measurements and providing measurement training to Shell staff for future in-house use. He then returned to IRL to develop a computer model that would allow him to provide software to Shell to manage the furnace.

“The problem that needs to be overcome in these furnaces is that the furnace walls are much hotter than the tubes, and the tubes partially reflect the radiation from the walls. The thermometers used to measure the radiation can’t distinguish between radiation emitted from the tubes (which tells us the temperature) and radiation reflected from the walls.

“This reflected radiation can cause errors of 20-60°C, or even more, depending on the geometry and operating temperature of the furnace. The error can be compensated for by developing a radiation exchange model for each furnace.”

He developed a software application based on the model, and sent this with a full report of the measurements to Shell in January. His contract for the New Zealand Refining Company, also in November, presented similar problems to solve, this time with two different types of furnace. One was a reformer furnace that converts natural gas into hydrogen for use in the refining process. The company has a 315-tube reformer, measuring approximately 15 metres on each side, and its staff could see that two of the tubes were glowing significantly hotter than their neighbours.

If they operate just 20°C higher than the optimum temperature, their design life of 11 years can be halved. “That’s not a lot of room for error – 20°C out of about 900°C.”

Dr Saunders found the two tubes were running at 912°C, about 60°C hotter than their neighbours, and were at risk of premature failure. He also assessed a high-vacuum furnace used to distill oil. These furnaces run at a cooler temperature of 600°C but because they are cooler the reflected radiation is even harder to eliminate.

The outsides of the tubes are prone to oxidisation which causes patches of temperature disparity resulting in wear of sections of the tube. Because the high-vacuum furnace is a bottleneck in the refining process, if one of these tubes had prematurely failed and cracked, it could have shut down the plant for a week.

While the petrochemical industry tends to be conservative when it comes to altering operating conditions, there can be considerable return through good management of the measurements.

Dr Saunders believes the precision of IRL’s software models in calculating critical operating temperatures is unmatched in the world. IRL receives a lot of referral work from US-owned company Quest Integrity Ltd, which is based at IRL’s Lower Hutt site. Over the years, IRL has also had contracts in Thailand, Indonesia, Malaysia, the US, Chile and Australia for many types of industrial furnace.

IRL’s expertise was seeded with the help of research grants from the Foundation for Research, Science and Technology through the 1990s and first half of the 2000s.