【Daily Question】Chemical Engineering Principles 283: Decrease in hydrogen purity (January 3)

Hydrogen contains air; oil leakage occurs on the hydrogen side of the seals, and due to frequent refilling or draining of the seal oil tank, water vapor is present in the hydrogen.

1. Since the air-side seal oil and the hydrogen-side seal oil are two relatively independent systems, the air-side seal oil comes from the oil used for lubricating the generator bearings; its return oil mixes with the return oil from the generator bearing lubrication system before returning to the main oil tank. The hydrogen-side seal oil system has a separate oil tank, and the oil replenishment for this tank comes from the air-side system, while the waste oil also goes to the air-side oil system. Theoretically, it is required that the two oil systems be independent; during operation, the oils on the air side and the hydrogen side must not mix with each other, in order to prevent the oil on the air side from contaminating the quality of the oil on the hydrogen side. However, in actual operation, oil leakage from the connection doors, improper piping layout that leads to uneven fluid resistance losses, poor design quality of the balance valves and differential pressure valves, impurities in the oil that cause these valves to operate inflexibly or get stuck, excessively large gaps in the sealing surfaces, as well as significant fluctuations in hydrogen pressure that make it difficult for the balance valves and differential pressure valves to adjust properly, all contribute to oil leaking from one side to the other. If oil leaks from the hydrogen side to the air side, the oil level in the seal oil return tank on the hydrogen side drops; the automatic oil replenishment float valve then opens, allowing oil to be supplied from the air side to the return tank on the hydrogen side ； If the aforementioned phenomena occur continuously in the system, then oil replenishment will also be continuous: as the air-side seal oil tank contains a large amount of air and moisture, when the oil containing air comes into contact with hydrogen through the seal pads, according to the law of partial pressures, the gas or vapor separated from the oil will enter the generator, resulting in a decrease in hydrogen purity and an increase in humidity. If oil from the air side leaks to the hydrogen side, the purity of hydrogen decreases and its humidity rises more rapidly. 2. Another reason for the decrease in hydrogen purity and the increase in humidity is the shutdown of the hydrogen-side seal oil pump. At this time, the seal oil on the air side flows not only toward the air side but also toward the hydrogen side; it functions like a single-flow oil seal and can still prevent hydrogen from escaping outside the machine. Although this is permitted by the manufacturer, since the hydrogen side and the air side share a single seal oil source, hydrogen dissolved in the oil on the hydrogen side can escape through the oil circuit on the air side, while gases dissolved in the oil on the air side can enter the machine through the oil circuit on the hydrogen side, resulting in a decrease in the purity of the hydrogen inside the machine and an increase in humidity. To ensure hydrogen purity, a large amount of hydrogen must be replaced, resulting in unnecessary waste. So, this approach should not be used for a long time. 3. Under normal operating conditions of the steam turbine generator, due to the mechanical effect of oil being flung off as a result of the high-speed rotation of the shaft, as well as the thermal effect of increased oil return temperature, the seal oil containing air and moisture is trapped in the oil return chamber on the side of the seal bearings

1. Oil from the air side leaks to the hydrogen side; 2. The seal oil pump on the hydrogen side has stopped operating

What are the reasons for the decrease in hydrogen purity? 1) The liquid level in the hydrogen-oxygen separator is too low, resulting in poor gas-liquid separation. 2) The analyzer is inaccurate. 3) The concentration of the alkaline solution is either too high or too low. 4) The circulation volume of the alkaline solution is too large. 5) The diaphragm is damaged

What are the reasons for the decrease in hydrogen purity? 1. Failures of the seal oil differential pressure valve, balance valve, and related instruments. 2. During the system’s automatic oil replenishment process, seal oil from the air-side, which contains a large amount of air, is introduced into the hydrogen-side seal oil tank; this increases the air content in the hydrogen-side seal oil, thereby reducing the purity of the generator. 3. Fault in the float valve of the hydrogen seal oil tank. 4. Issues related to the method of refilling the seal oil. 5. The gap between the seal pads of the generator is above the specified limit. 6. Insufficient attention is paid to oil quality monitoring. 7. The exhaust fan in the air-side seal oil tank (i.e., the explosion-proof fan) has problems with inadequate oil extraction capacity or insufficient power output.

What are the reasons for the decrease in hydrogen purity? 1. Failures of the seal oil differential pressure valve, balance valve, and related instruments. 2. During the system’s automatic oil replenishment process, seal oil from the air-side, which contains a large amount of air, is introduced into the hydrogen-side seal oil tank; this increases the air content in the hydrogen-side seal oil, thereby reducing the purity of the generator. 3. Fault in the float valve of the hydrogen seal oil tank. 4. Issues related to the method of refilling the seal oil. 5. The gap between the seal pads of the generator is above the specified limit. 6. Insufficient attention is paid to oil quality monitoring. 7. The exhaust fan in the air-side seal oil tank (i.e., the explosion-proof fan) has problems with inadequate oil extraction capacity or insufficient power output.

Answer: 1. Is the compressed air system properly isolated, and are the gauges in good condition? 2. An imbalance in pressure between the air side and the hydrogen side exists; the seal oil carries gas from the air side into the generator, which causes the oil level in the seal oil tank to rise. 3. Air may enter the generator from the lubricating oil system to the seal oil replenishment system, but the rate of entry should not be very fast. 4. There may be problems with the measuring gauges. 5. The generator is under high load and at high temperatures, leading to severe contamination of the seal oil; in addition, the cleaning of the hydrogen purity detector is not carried out promptly, resulting in contamination of the probe. 6. Fresh oil is frequently added to the air-side seal oil. 7. The dehumidification unit is not operating properly. 8. The explosion-proof fan is not functioning correctly

1. Since the air-side seal oil and the hydrogen-side seal oil are two relatively independent systems, the air-side seal oil comes from the oil used for lubricating the generator bearings; its return oil mixes with the return oil from the generator bearing lubrication system before returning to the main oil tank. The hydrogen-side seal oil system has a separate oil tank, and the oil replenishment for this tank comes from the air-side system, while the waste oil also goes to the air-side oil system. Theoretically, it is required that the two oil systems be independent; during operation, the oils on the air side and the hydrogen side must not mix with each other, in order to prevent the oil on the air side from contaminating the quality of the oil on the hydrogen side. However, in actual operation, oil leakage from the connection doors, improper piping layout that leads to uneven fluid resistance losses, poor design quality of the balance valves and differential pressure valves, impurities in the oil that cause these valves to operate inflexibly or get stuck, excessively large gaps in the sealing surfaces, as well as significant fluctuations in hydrogen pressure that make it difficult for the balance valves and differential pressure valves to adjust properly, all contribute to oil leaking from one side to the other. If oil leaks from the hydrogen side to the air side, the oil level in the seal oil return tank on the hydrogen side drops; the automatic oil replenishment float valve then opens, allowing oil to be supplied from the air side to the return tank on the hydrogen side ； If the aforementioned phenomena occur continuously in the system, then oil replenishment will also be continuous: as the air-side seal oil tank contains a large amount of air and moisture, when the oil containing air comes into contact with hydrogen through the seal pads, according to the law of partial pressures, the gas or vapor separated from the oil will enter the generator, resulting in a decrease in hydrogen purity and an increase in humidity. If oil from the air side leaks to the hydrogen side, the purity of hydrogen decreases and its humidity rises more rapidly. 2. Another reason for the decrease in hydrogen purity and the increase in humidity is the shutdown of the hydrogen-side seal oil pump. At this time, the seal oil on the air side flows not only toward the air side but also toward the hydrogen side; it functions like a single-flow oil seal and can still prevent hydrogen from escaping outside the machine. Although this is permitted by the manufacturer, since the hydrogen side and the air side share a single seal oil source, hydrogen dissolved in the oil on the hydrogen side can escape through the oil circuit on the air side, while gases dissolved in the oil on the air side can enter the machine through the oil circuit on the hydrogen side, resulting in a decrease in the purity of the hydrogen inside the machine and an increase in humidity. To ensure hydrogen purity, a large amount of hydrogen must be replaced, resulting in unnecessary waste. So, this approach should not be used for a long time.

The seal of the programmable valve is damaged, which leads to air leakage or contamination of the instrument air, causing the programmable valve to get clogged； The design margin is small; after use for a period of time, the molecular sieve becomes pulverized, resulting in the penetration of raw gas ； Or if the adsorption time is short, it can be appropriately extended.