Functions of compressed air storage tanks and precautions for drain valves

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Draining the compressed air storage tank is an essential task in the maintenance of air compressor systems, yet many companies neglect this task, resulting in issues such as tank corrosion, reduced tank capacity, and frequent damage to the equipment in the air compression system. Based on the author’s experience, this article offers some suggestions on how users of compressed air storage tanks can carry out drainage in a scientific manner. Functions of compressed air storage tanks: Compressed air storage tanks serve several purposes: First, to stabilize air pressure. The air pressure of a compressor is very unstable during operation, with significant fluctuations. Using a gas storage tank allows the air pressure to be maintained within an appropriate range, eliminating pulsations in the airflow within the pipelines. With a gas storage tank, the compressed air output by the air compressor has a place to be buffered, allowing the air supply to be maintained at a set value and ensuring that the air-using system receives a constant pressure. Second, store air. There is no place inside the air compressor to store compressed air; once it is generated, it must be used immediately. This mode of operation is not ideal. With a gas storage tank, compressed air can first be pumped into the tank until a certain pressure is reached; once the pressure drops to a certain level, the compressor starts up again. This approach allows for meeting the increased demand for air from equipment that suddenly requires more air, thereby ensuring a more stable supply of air. Third, the energy-saving effect. Frequent starting and stopping of the air compressor results in very high current consumption by the motor. With a gas storage tank, it is possible to ensure the automatic shutdown of the air compressor; once the tank is filled to the set pressure, the compressor shuts down automatically, preventing it from running continuously and wasting electrical energy. Fourth, cooling and filtration. The temperature of the compressed air entering the air storage tank is reduced, achieving an initial cooling effect on the compressed air; at the same time, this process allows water and oil particles in the compressed air to settle, resulting in higher quality compressed air. Be sure to drain water from the air storage tank. Outside air usually contains moisture, especially in rainy areas in the south, and the air produced by air compressors contains a large amount of water vapor. Once air enters the air storage tank, the air molecules with higher temperatures come into contact with the cooler walls of the tank and condense into water. This water flows down along the tank walls to the bottom, resulting in a large amount of water accumulating inside the air compressor’s storage tank ； It also reduces the moisture content in the compressed air discharged from the tank, thereby improving the quality of gas used in the company’s production processes. Since moisture and oil in the compressed air settle at the bottom of the compressed air storage tank, if these substances are not removed, the amount of moisture in the tank will increase over time. The rising water level can cause various problems; therefore, the compressed air storage tank should be drained regularly. The methods of draining compressed air storage tanks mainly include manual drainage and automatic drainage. Manual drainage refers to the act of manually opening the drain valve on the gas storage tank to carry out drainage at regular intervals. A common problem with this method is that operators are susceptible to external influences and may forget to drain the gas storage tank; some poorly managed companies are not even aware of the need to do so. Automatic drainage refers to the draining of the air storage tank using an automatically opening drain valve. Automatic drain valves are divided into electronic and mechanical types. The principle of the electronic automatic drain valve is that a solenoid valve controls the opening and closing of the valve, and the timing and frequency of automatic drainage can be adjusted by setting the switch on it. This is mainly used on precision filters, oil-water separators, and air receivers. However, this type of installation is rather troublesome, as it requires electrical connections. The physical automatic drain valve (also known as the mechanical automatic drain valve) operates on a principle similar to that of a steam trap. The installation is relatively simple; all that’s needed is to connect an extended connecting tube. Its lifespan is longer than that of electric actuators. The valve opens automatically at regular intervals to perform periodic draining. A common problem with this method is that, due to the small inner diameter of the valve, rust formed in the air storage tank can easily clog the valve, causing it to fail and preventing waste discharge. Problems caused by unsound drainage from storage tanks include corrosion. Ordinary outdoor air always contains moisture, especially in the southern regions where there are many rainy days; the air produced by air compressors contains a large amount of water. The water molecules in compressed air generally contain a certain amount of acidic or alkaline molecules. If the water that accumulates at the bottom of the gas storage tank is not removed over time, it can cause corrosion of the tank, **reducing its service life and threatening its safe operation. Failing to clear accumulated water over time causes the water level to rise, which reduces the volume of the air storage tank. This affects the stability of the air supply for production purposes, and it also introduces a large amount of water vapor into the air being used, inevitably affecting the quality of the compressed air. Certain precision pneumatic valves have high requirements regarding the moisture content in the air; an increase in moisture content can cause these valves to malfunction or even get damaged. Failure to clean up accumulated water over time can cause damage to other components in the air compressor system. When the water level rises to a certain height, it can enter devices such as the oil separator, dryer, and compressor within the air compression system. Excessive water in the oil leads to emulsification of the lubricating oil, as well as poor lubrication of the compressor itself, which prevents the air compression system from functioning properly. A company failed to drain the air storage tanks regularly, which allowed water to enter the compressor and caused poor lubrication; this ultimately led to damage to the compressor, requiring repairs every two to three months. Scientific methods for draining compressed air storage tanks: The entities that use compressed air storage tanks should determine the interval between drainings based on the humidity and temperature of the ambient air, as well as the amount of air required by the enterprise. This determination should take into account both conditions prior to and subsequent to the dryer. The air storage tank located after the dryer does not require regular drainage, as the dryer has already removed the moisture; whereas the air storage tanks before the dryer and those in air compressor systems that lack a dryer must have their water drained regularly. Taking a common air compressor with a capacity of 20 m3/min and a pressure of 0.7 MPa, and a 2 m3 compressed air storage tank located before the dryer as an example, the drainage cycles for the two different drainage methods are explained. When manual sewage discharge is used, it is advisable to drain the water every 12 hours during spring and autumn, until all the water has been removed. In cases of rainy or hot weather, or when the compressor operates at intervals of less than 30 minutes, it is recommended to drain the waste water every 8 hours until the water level in the tank is completely drained; more frequent drainage is advisable. Automatic sewage discharge: When automatic sewage discharge is used, it is necessary to check regularly every two weeks whether the sewage discharge valve is functioning properly. If it fails to discharge sewage correctly, it should be repaired or replaced immediately. The typical service life of an automatic drain valve is about 3-5 years, and it should be replaced in a timely manner when its lifespan is reached. It is advisable to add a bypass line to the automatic drain valve pipeline; the bypass should be activated each time it is inspected to determine whether the water content in the tank is excessive. Before installing the drain valve, copper shavings, rust, and other impurities in the air storage tank should be removed first. It is recommended to empty the system under full pressure for 3–5 minutes before installing the automatic drain valve. During installation, simply install a drain valve at the bottom (lowest point) of the air receiver tank. Also, ensure that the drainage direction is consistent with the direction indicated by the arrow on the valve body. Special requirements for installing electronic drain valves: 1. The supply voltage must match the voltage of the electronic drain valve. 2. The TEST membrane switch on the timer of the electronic drain valve is a manual testing button; each time it is pressed, the drain valve releases water once. This button is used in daily operations to check the drainage status at any time. The two knobs on the timer are used to adjust the duration of each discharge (how long each discharge lasts) and the interval between discharges (how often discharges occur). 4. The small screws on the wiring box of the electronic drain valve not only serve to make connections but also function to compress the sealing gasket, preventing water from entering the timer and the coil; therefore, they must be tightened securely. 5. For the air storage tanks in high-pressure systems, high-pressure electronic drain valves should be used. Recommendations: Drawing on years of experience in inspecting air compressors, the author offers the following recommendations to organizations that use compressed air storage tanks: 1. For those organizations that rely on manual sewage discharge, it is recommended that they establish a regular sewage discharge system for the storage tanks in accordance with the requirements outlined in this article, and that they adhere strictly to this system. This will help prevent severe corrosion of the storage tanks as well as damage to the machinery resulting from improper sewage discharge. 2. For organizations that use automatic sewage discharge systems, it is recommended to establish a regular inspection system in accordance with the requirements outlined in this document, in order to conduct periodic inspections of solenoid valves and mechanical valves. Avoid situations where valve damage prevents sewage discharge, which in turn can lead to severe corrosion of the storage tank as well as damage to the machinery. 3. For organizations that use automatic sewage discharge systems, it is recommended to request from the manufacturer that the diameter of the sewage outlet pipe be increased at the time of purchase. Currently, most manufacturers of compressed air storage tanks use sewage pipes with a diameter of DN15mm to DN25mm; when an automatic sewage discharge system is used, the diameter of the sewage pipe after installing the discharge equipment drops to DN8 to DN15. When there is rust in the sewage, this can easily cause blockages in the pipe outlets. Therefore, it is recommended to purchase compressed air storage tanks with sewage discharge ports of DN40 to DN50, to prevent improper sewage discharge due to an excessively small diameter of the sewage pipes.