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Save to Reading List Hydrocarbon Engineering, Thursday, April 28, 2016 11:30

Sealing at high temperatures (usually 400°C and above) is challenging because standard materials such as expanded graphite cannot function reliably for a long time under such conditions. This can lead to failure of the bolted connection and leakage. So far, compromises have to be made in material selection, which will adversely affect the leakage performance of bolted joints over time at high temperatures.

The chart shows the comparison between Thermiculite and suppressed graphite and standard graphite. Note that although the oxidation of inhibited graphite is slightly delayed, it will still degrade, and Thermiculite will not be affected over time.

High-temperature processes can be seen in many industries, including oil and gas, chemical processing, and power generation. Specific applications include fluidized catalytic cracking, ammonium nitrate fertilizer production, ethylene production, flare systems, steam and gas turbines, exhaust systems, and more recently, solid oxide fuel cells (SOFC) and concentrated solar heat using molten salt heat transfer fluids Power system. Molten salt works at high temperatures and is chemically corrosive, thus adding further sealing challenges.

Thermal spiral wound gasket with mica on the outer and inner diameters. Note that even with a mica "barrier", graphite will be lost from the spiral due to oxidation. A leak in the gasket caused the shutdown of a titanium dioxide plant.

Traditionally, the options available to gasket users are to use graphite or mica, or a combination of the two materials, to compensate for their inherent weaknesses. Although graphite seals well at ambient temperature because it is an organic material, at medium to high temperatures, carbon will oxidize. Over time, the seal will lose integrity and performance will decrease. Even at mild temperatures, this can happen very quickly, and accelerate as the temperature rises. Even if the graphite is treated with oxygen-inhibiting chemicals, their effect is only temporary.

Graphite is carbon, and carbon will oxidize and cause the seal to degrade. In the most extreme cases, graphite will be completely oxidized, causing the container to fail completely-which can be catastrophic.

Comparison of the leakage rate between the mica spiral wound gasket (blue) and Thermiculite 835 spiral wound gasket (orange). Even under very high surface stresses, mica gaskets will leak significantly.

Another option for delaying the onset of graphite oxidation is to use barrier protection seals. Mica exhibits excellent heat resistance, but because it is porous, it does not serve as a good seal.

This means that although mica theoretically provides heat resistance and protects the graphite sealing element, in practice it does not provide an effective airtight seal, so the graphite is still corroded and eventually these high operating temperature (HOT) gaskets fail.

In some very high temperature applications, or where graphite is chemically incompatible or promotes corrosion, companies have tried to use mica alone, but these gaskets cannot provide adequate sealing and therefore cannot be considered as a viable option. Other technologies have been tried, such as hydrophobic talc-based materials. Although these technologies claim to provide good performance, serious failures in use have been recorded because the materials cannot provide reliable long-term integrity.

The process temperature is getting higher and higher, and the operator wants to extend the maintenance interval. This requires gasket materials that can withstand these high temperatures while also providing long-term reliability. Safety is paramount, so choosing a proven and reliable solution is paramount.

To solve these problems, Flexitallic has developed a new material that has the same sealing properties as graphite but has the heat resistance of mica. This material is called Thermiculite®, which was born out of the industry's demand for gasket materials, which can work at higher temperatures for longer periods of time while maintaining the integrity of the seal.

Thermiculite achieves this performance through the innovative use and manufacture of uniquely processed vermiculite. By combining specially prepared thermal peeling and chemical peeling vermiculite, the crystal plate structure in Thermiculite ensures an airtight seal even under the most extreme process conditions.

The innovative manufacturing technology that has been developed means that many different forms of Thermiculite gaskets-sheet, spiral wound and Kammprofile, as well as the new Change™ gasket can be provided. This means that choosing a material technology in a site is completely achievable.

Change is a highly elastic metal wound heat exchanger gasket that provides the most dynamic seal. Most importantly, the performance of Change is 60% longer than other heat exchangers, CGI spiral wound, double jacketed, corrugated metal or Kammprofile gaskets.

Since Flexitallic created spiral wound gaskets more than 100 years ago, one of the most important innovations in the sealing industry has been the introduction of Change to directly solve customers' long-term heat exchanger problems. The heat exchanger has gone through multiple cycles involving extreme temperature and pressure ranges. Because of the replacement of gaskets that cannot operate under these harsh conditions, it is easy to perform regular maintenance and shutdowns.

The patented Change gasket is manufactured using proprietary equipment, and its metal spiral profile is five times thicker than standard gaskets. By using a unique laser welding process that completely penetrates the winding, no inner or outer ring is required. The function of the gasket has been confirmed through a series of studies, including radial shear testing (RAST), extended thermal cycling of the housing, leakage and compression testing, all of which have produced very positive results.

For example, in the 24-day, 24-cycle pressure and thermal cycle test under 302°C replicate industry conditions, Change only lost 1.5 PSI, which is better than all other gaskets that have been tested for at least 9 days. Compression tests have shown that, based on its high level of stored energy, Change's recovery capacity is almost five times that of Kammprofile and double-jacketed gaskets.

Scanning electron microscope image of Thermiculite's very thin flexible board structure, which provides an elastic hermetic seal over a wide temperature range.

The radial shear test was originally developed by PVRC/ASME and Ecole Polytechnique of the University of Montreal to simulate the different expansion and contraction of heat exchanger flanges. The test is performed with 100 thermal cycles, with nitrogen pressurized to 40 bar, and leak tests are performed every 20 cycles.

The flange is a tongue and groove type, a typical heat exchanger. The gasket size is 453 x 427 mm (nominal 3.2 mm thick). The lower flange circulates to 300°C, and the upper flange is water-cooled to maximize differential growth in the system and generate approximately 0.8 mm of radial shear on the gasket. However, the key measurement is bolt slack, as this is considered a measure of the resistance of the washer to radial shear.

The washer survived the test, visible damage was negligible, and the bolt stress was only relaxed by 15% after 100 cycles. In contrast, the second-best performance is spiral wound gaskets, with a bolt slack of about 25%. The replacement gasket showed no major leakage during the test, which is in sharp contrast with the jacketed gasket.

However, the best evidence comes from successful applications, where Change performs consistently in harsh environments. For example, in refinery applications, Change operates effectively in a cycle from ambient temperature to 379°C, has not encountered any problems so far, and has better performance than all previously used gaskets. Before installing a 63-inch diameter gasket with a pressure of 510 PSI, the refinery will need to replace multiple gaskets between major shutdowns after 28 thermal cycles.

Change can be supplied with PTFE, graphite fillers, and various metals, but it performs well in high temperature applications when combined with Thermoculite.

Since vermiculite is an inorganic silicate, Thermiculite material has excellent resistance to very high temperatures and corrosive chemicals. It also passed the strict API 6FB fire test, making it ideal for hydrocarbon applications. Due to the electrical insulating properties of the material, it is now used in applications where crevice corrosion is a problem, such as offshore oil and gas and seawater applications.

Thermiculite provides a single solution, replacing the need for a range of materials, none of which can provide a lasting solution at temperature. Real savings can be achieved in terms of reduced maintenance costs, reduced downtime, inventory rationalization, emission compliance and, of course, improved safety through better joint integrity.

Since 1997, Thermiculite has been successfully used in thousands of applications throughout the industry.

A state-owned oil company introduced Flexitallic product innovation after facing leaks in the high temperature and corrosive environment of its refinery, which may have an impact on productivity and efficiency.

The head flange in the refinery heater system has a pressure of 7 bar and operates at a temperature of 520 to 540°C. Previously, the company had been using spiral wound gasket CG 304 with graphite filler recommended by the equipment manufacturer.

However, this gasket material combination can cause leakage under the conditions of use, thereby increasing corrosion in the heater system. In order to overcome this problem, Flexitallic recommends the use of CGI 316 ring and winding spiral wound gasket containing Thermiculite 835 filler.

Flexitallic's Thermiculite gasket material is developed for critical service applications, from low temperatures to temperatures exceeding 1000°C. Therefore, the use of spiral wound gaskets in combination with Thermiculite eliminates system leakage, thereby preventing future corrosion problems.

Written by Alex Lattimer, Product Line Director of Flexitallic Ltd.

Read the article online: https://www.hydrocarbonengineering.com/special-reports/28042016/overcoming-the-challenges-of-sealing-at-high-temperatures-3152/

Tecnimont SpA has signed three EPC contracts with Borouge concerning the fourth phase expansion of the Ruwais polyolefin complex in Abu Dhabi, UAE.

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