Storage tank failure has plagued the oil and gas industry for a long time. However, fluids there are not kept at high temperatures by daily heating and cooling cycles, as is the storage of concentrated solar thermal energy.
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One of the failure mechanisms in the steel used in the thermal energy storage tanks of the CST and CSP plants has been isolated, and a steel formulation from Finnish stainless steel firm Outokumpo has now been successfully tested by the Colorado School of Mines.
Today’s commercial CSP technology depends on storing the thermal energy of an extremely high-temperature fluid in a giant open tank. These tanks can hold thousands of tons of molten salt, an extremely hot liquid at high pressure that varies between 300°C and 600°C every morning and evening as it is heated daily and cooled.
The industry standard metal for these tanks has been grade 347H austenitic stainless steel. However, failures in some high-profile storage tanks have been attributed in part to this choice of steel.
How the steel and weld behaves under the heat cycles as it goes through the welding process is where it fails. The welding process changes metallurgy. Under a certain stress and temperature, some alloy components migrate to the grain boundaries, causing the material to become brittle and crack at the boundaries. This phenomenon is known as Stress Relaxation cracking.
“Diagnosing stress relief cracking in real project storage tank failures is extremely difficult,” said Kurt Drewes, technical director at Australian solar firm Vast Energy.
Although it is impossible to conduct failure tests in giant tanks containing molten salts, the Colorado School of Mines conducted laboratory tests on this 347H steel to understand the causes. They showed that welds fail due to stress relaxation cracking (SRC) in the steel.
Post-weld heat treatment (PWHT) can reduce some of this tendency, but is almost impossible to achieve on the scale of a typical storage tank built by industry.
Alternative stainless steel formulation; Therma 4910
When the entire industry experience is built around one material, it’s very hard to say that you need to go back and find a more robust alternative. But now Vast, Outokumpu and the Colorado School of Mines have found and demonstrated an alternative. Stronger steel for tanks.
An old formula from the Finnish high-temperature stainless steel company Outokumpu has now been successful in preventing heat storage tank failure in weld joints.
“The logical answer is to try and find a material that you don’t need to PWHT to achieve structural integrity. And tests of this new steel formulation have shown it; that it’s possible,” Drewes said.
Led by the Colorado School of Mines, Vast tested the Therma 4910, now known as the 316 LNB. It is an alternative steel formulation supplied by the Finnish company Outokumpo, one of the world’s largest stainless steel producers.
“Of course, some other steels are not prone to stress relaxation cracking, but they have lower creep and fatigue strength. 316LNB has similar or better creep fatigue properties than 347H and is less prone to stress-relief cracking,” added Bruce Leslie, head of product development at Vast.
This alternative steel formulation is slightly more expensive than the traditional one. However, Vast is sounding the alarm that a thermal storage tank failure will cost the entire industry its reputation for reliability. Therefore, they feel an urgency to inform the industry about a new option in material selection.
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“This formula is not really a newly developed new material,” Leslie noted.
“It was developed for coal-fired power plants in the 80s and 90s. There was a big push to move to very high temperatures and pressures in coal-fired plants. So 316LNB was used quite a lot, but then cheaper stainless steels were made that worked for industrial needs. But there is a lot of test data and information about it. So, this is a tried-and-tested material.”
The transition to this “new old” storage tank material has become even more urgent today, given that a new solar sector generating heat for industrial processes and solar fuels is just starting up: CSP may be required to generate only solar energy. Up to 600°C, these new processes should guarantee continuous heat production around the clock and at higher temperatures up to 1500°C.
It is now being tested at the Colorado School of Mines
The Colorado School of Mines now performed the same tests they did on the old steel formula, the new 316LNB steel.
“We were aware of the previous tests and thought it appropriate to repeat the same tests on the new material,” Drewes said.
“It’s a structural test of the structural failure of materials and what happens during welding. We simulate the effect of welding on materials in those tests. What they show is that the 316LNB has solved these issues. It has low levels of carbon and increased nitrogen and boron, which give it these properties. And we argue that it is the more preferred material for the CSP industry.
Testing has shown that this formulation is resistant to this failure mechanism. They will present their results at the SolarPACES Conference in Rome later this year. Vast and Outokumpo co-funded the trials with Vast, along with construction partners CyD.
Drewes noted that Vast is interested in having reliable thermal storage tanks and that Outokumpo, which offers special materials for this application, is interested in promoting the use of its high-temperature steel capabilities. He said that they can meet these requirements and believe that thermal energy storage will be the future as a necessary component of the New Energy Transition.
In previous work, Vast focused on strengthening the floor of these giant tanks to prevent buckling, which is just one aspect of improving the stability of thermal storage tanks. The firm is working on other tank improvements for its projects with the DOE, but Drewes and Leslie see the solution as important information for the industry as a whole.
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