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Lessons from a Gas Holder


Lessons from a Gas Holder

When Austrias VoestAlpine Steel was troubled by excessive wear in the operation of its converter gas holder, it called on experts at AC2T Research in Vienna for help unraveling the issue. The answer, a recent industry meeting heard, was not simply to pick a different lubricant, but to pick apart the gas holders unique needs and use them to set benchmarks for the oil selection.

A gas holder is a large, hollow-shell cylinder or above-ground tank used to store gas at near-ambient pressure, AC2Ts Rainer Franz explained at the 21st International Colloquium Tribology in January. Inside the cylinders walls, a tight-fitting piston floats on top of the gas phase, and slides up and down as the gas is pumped in and then drawn out.

Lubrication helps assure the gas-proof operation of the piston, which needs to be able to move up and down freely, said Franz, who heads the chemistry lab at AC2T. The lubricant provides the critical seal assuring that gases cant escape through any minute gaps. The circulating oil is fed continuously to the rim of the piston and then slides down the walls to the bottom of the tank, where it can be collected, cleansed of water and contaminants, and recycled back to the top of the system for reuse.

Gas holders fall into two main types. In older telescoping versions, the walls are made up of concentric rings or lifts. Just like the sections of a telescope, the interlocking segments slide in and out as needed, adjusting to the volume of gas inside.

The second type of gas holder-the kind involved in AC2Ts research project-is a rigid or piston type with walls of a fixed height; often these walls are polygonal in shape rather than perfectly round. Inside, youll find a piston that rests on top of the gas, riding up and down as the gas fills and empties. Rollers around the pistons edge keep it level and aligned by gliding smoothly along vertical guiderails set at regular intervals inside the rigid tank.

Gas holders date to the coal-gas era and were used by cities to store gas for lighting and other uses for almost 200 years. Most public utilities moved on to other storage options in the past 30 years, leaving their gas holders to rust away, be demolished or adapt to new uses. The 384-foot gasometer that towers over Oberhausen, Germany, has become an arts complex, Romes gazometro is a steampunk mecca, and at Kings Cross, London, the cast-iron guide frames from a trio of Victorian-era gas holders now encircle 145 swanky apartments (starting price: 810,000 pounds).

Even if gas utilities have deserted them, the tanks are still highly useful for heavy industry. Iron and steel producers use them to store gases generated in their blast furnaces and coking ovens. Such gases are rich in carbon monoxide and hydrogen, and rather than being flared off can economically fuel other operations in an integrated steel plant, observes a 2015 report for Tata Steel by researcher Abhishek Mohata. As well as a buffer to store gas temporarily, he noted that gas holders also are used for balancing purposes-ensuring that gas pipes remain within a safe range of pressures.

VoestAlpines rigid gas holder is used to store converter gas generated from the pig-iron works at its steel plant in Linz, Austria. Inside it, sealing oil forms a thick bead between the walls and the piston to keep the gas tightly inside. The oil comes down the cylinder walls, is collected at the bottom, and then pumped back up to the top again, AC2Ts Franz narrated.

The oil must be thick enough to provide good sealing properties, he continued, but also has to satisfy other needs. The inlet temperatures for the gas reach about 70 degrees Celsius (158 Fahrenheit), and as they cool the stored gases release condensing moisture, subjecting the lubricant to water, reactive gases and dust.

This application demands an oil that has high aging ability, good water-separation properties, and low pour point to keep moving in cold weather, he told the colloquium, which was organized by the Technische Akademie Esslingen in Ostfildern, Germany. Theres also a need for anti-stick-slip properties to reduce stress and wear on the component parts.

The ideal lubricant should last a long time, but the steel producer was unhappy with the life and wear protection of the sealing oil it was using before. The phenolic and ester based antioxidants in the oil were being consumed too quickly, leading to viscosity gains that made the oil too thick after only 18 months of use. They saw a loss of performance and increase of water content, too, leading to failure of the machinery, Franz added.

AC2T was asked to help set up a procedure to benchmark sealing oils for the gas holder. The laboratory started by testing fresh samples of the incumbent oil, which showed almost no evidence of oxidation or acidification. It would have been a good oil, if not for the conditions in the gas holder, he said. As it was, the equipment only lasted two years despite regular oil changes.

Probing the issue of longer service life, AC2T examined four classic sealing oils: two naphthenic based products and two paraffinic. As youd expect, the naphthenic based products had naturally low pour points, but also lower viscosity indices (see Figure 1). Oil 3, a paraffinic oil, had much higher V.I., but also much higher pour point. Oil 4, also paraffinic, was formulated with a pour point depressant, bringing this property down to -42 C, more in line with that of the naphthenic products.

In alpine Austria, low temperatures are a given, Franz reminded. You need good pumpability in winter, so the pour point and low-temperature viscosity are critical properties.

AC2T also took a hard look at water-separation ability, which was limiting the incumbent oils life. As the oil aged, its ability to shed water began to fall. The temperatures in the gas holder and the oxygen and water contact were stress factors.

Using the Turbine Oil Oxidation Stability Test method, AC2T tested the oil samples for changes in viscosity, oxidation and base reserve, an indicator of acid-neutralizing capacity. The TOST test involves both oxygen and water contact, so researchers also did dry TOST tests with no water present, to further tease out any performance differences between the oils. This helped to illuminate how each of the stress parameters (oxygen and water) contributed to the failure pathways.

In a five-week screening test, artificially aged oil samples were created and analyzed at one-week intervals for both chemical changes and water separation. When an oil started to show significant degradation, Franz pointed out, the water separability also tended to falter. Over the five weeks of the test, Oils 1 and 2 failed within three weeks and two weeks, respectively, while Oils 3 and 4 did not show any signs of degradation and hence can separate water well, even after five weeks of TOST.

We also looked at the tribology of the oils, he continued. Stick-slip generates noise, increases wear, and leads to failure of the machinery. The question was how to do a lab test for this.

The researchers settled on a Falex micro-tribometer test that uses an oscillating ball-on-disk configuration to measure friction force and vibration, over 500 cycles of strokes. Each cycle involved constant-speed sliding and turning points where the ball reversed at the end of the stroke.

In Oils 1, 3 and 4, friction force was rather constant over the whole stroke length: good stick-slip prevention. But Oil 2, he noted, was characterized by high fluctuation over the whole stroke length, indicating the oil had low ability to prevent stick-slip.

By calculating the friction force variation over stroke length for each cycle and then adding up those values for all the cycles, AC2T was able to assign a cumulative value for each oils stick-slip performance (Figure 2) and rank the products. These tests also showed that the ester addition has a positive effect, Franz confirmed.

The steel producer currently is deep into a field trial with Oil 3, a paraffinic product, which to date has been very successful. We are now at 81 weeks of operation. The sealing oil is showing stable water separability and aging, as seen in the antioxidancy, and theres been zero viscosity increase. We expect now to see five to 10 times the life cycle as before, Franz predicted.

In closing, he emphasized that naphthenic based sealing oils have well-known performance, but all sealing oils need a balanced additive formulation if theyre not to fail. In this particular case, sealing oils based on paraffinics showed superior stability as well as stick-slip prevention, but they do have higher pour points; that can be improved by pour point depressants if needed.

What we did was highly specific for this application, Franz cautioned in closing. There are many types of failure modes, depending on the gas involved and the system.

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