breather valve

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Introduction to breather valves: According to the **Standard ‘Code for Fire Protection Design of Petrochemical Enterprises’ (GB50160), fixed-roof tanks storing Class A and B liquids shall be equipped with flame arresters and breather valves**. It is evident that breather valves and flame arresters are essential safety devices for storage tanks. A breathing valve not only maintains pressure balance in the storage tank, preventing it from being damaged under conditions of overpressure or vacuum, but also reduces the evaporation and loss of the medium inside the tank, meeting the ventilation requirements associated with the tank’s size. Structure and Working Principle: The breathing valve is an important accessory for ensuring the safety of storage tanks. Installed at the top of the tank, it is used to automatically control the opening and closing of the gas passages inside and outside the tank, thereby maintaining pressure balance within the tank. It protects the tank against overpressure or undervacuum conditions, and it can also reduce the evaporation loss of the medium contained within the tank to a certain extent. A breathing valve uses the weight of a valve disc (sometimes with weights attached to it) to control the exhalation pressure and the vacuum level inside the tank. When the pressure of the gas inside the tank exceeds the allowable pressure level for the tank, the pressure valve is pushed open, allowing the mixture to escape from the tank (be exhaled), thereby preventing the pressure inside the tank from rising any further. When the vacuum level of the gas inside the tank exceeds the allowable vacuum level for the storage tank, the vacuum valve is pushed open to draw in air (be drawn in), thereby maintaining pressure balance in the tank. When the pressure remains within a certain range, the storage tank does not breathe; therefore, the breathing valve reduces the evaporation loss of the medium to a certain extent. The values of pressure and vacuum controlled by the breather valve should be determined based on the allowable limits set by the structure of the storage tank itself. When selecting a vent valve model, it should be determined based on the maximum discharge volume during the transfer of the medium. When the diameter of the breather valve is greater than 250 mm, two breather valves not exceeding 250 mm each can be used. When the gas pressure inside the tank exceeds the allowable pressure for the tank, the medium vapor escapes through the pressure valve; at this time, the vacuum valve is in a closed state ； When the gas pressure inside the tank is lower than the allowable vacuum level of the tank, fresh air enters the tank through the vacuum valve; at this time the pressure valve is closed, and the allowable pressure (or vacuum pressure) is controlled by the weight of the adjustment disc. Table for selecting mechanical vent valves: Flow rate of media in and out of storage tanks (m3/h), number of vent valves, and specifications of vent valves. For flow rates <251 m3/h, use DN50 vent valves; for 26–100 m3/h, use DN100 vent valves; for 101–150 m3/h, use DN150 vent valves; for 151–250 m3/h, use DN200 vent valves; for 251–300 m3/h, use DN250 vent valves. For flow rates >300 m3/h, use DN300 vent valves. Key technical parameters of vent valves: operating pressures ranging from +355 Pa to –295 Pa, +1750 Pa to –295 Pa, +980 Pa to –295 Pa, as well as other special pressures. Materials used include HT, 20#, Q235, 304, 304L, 316, 316L, etc. Explosion protection ratings: BS5501: HA, IIB, IIC. Seal material: polytetrafluoroethylene. Flange standards: HG20592-97, SH, GB, JB/T81-94, etc. Specifications range from DN50 to DN350. Manufacturing and testing standards: Production and inspection are carried out in accordance with standards such as SY7551 (Vent valves for petroleum storage tanks) and GB5908 (Fire suppression performance and testing methods for petroleum storage tank flame arresters). Use and maintenance of breather valves: The common faults of breather valves include air leakage, jamming, sticking, blockage, freezing, as well as improper operation of the pressure valve and vacuum valve. 1. Air leakage: This is generally caused by rust, hard objects scratching the contact surface between the valve and the valve disc, deformation of the valve disc or valve seat, and an inclined valve disc guide rod. 2. Jamming: This usually occurs when the breathing valve is not installed correctly, or when the oil tank deforms, causing the valve disc’s guide rod to become misaligned or the rod to rust. As a result, the valve seat cannot move up and down along the guide rod properly, and the valve disc gets jammed at a certain point on the guide rod. 3. Adhesion: This occurs as a result of chemical and physical changes that take place when vapor, moisture, and dust and other contaminants deposited on the valve disc, valve seat, and guide rods mix together; over time, this leads to the adhesion of the valve disc to the valve seat or guide rods. 4. Blockage: This is mainly caused by the long-term lack of maintenance of mechanical breathing valves, which leads to the accumulation of dust, rust, and other debris inside the breathing valve and its air ducts. It can also be caused by bees or birds building nests at the opening of the breathing valve, resulting in blockage. 5. Freezing: Due to changes in temperature, moisture in the air condenses on parts such as the valve body, valve disc, valve seat, and guide rod of the breathing valve, eventually turning into ice and making it difficult to open the valve. Some of these faults prevent the breathing valve from operating when it reaches the control pressure, resulting in overpressure in the oil tank and posing a risk to its safety ； Some of them render the breathing valve ineffective, leading to uncontrolled breathing processes on both large and small scales; this increases evaporation losses during feeding, reduces the quality of the medium, exacerbates air pollution in the area, affects the health of operators, and increases the risk factors in that region. During routine inspections and each operation, it is necessary to conduct thorough visual and observational checks to identify problems promptly and address them right away. Check for any abnormal changes in the tank body and the breather valve body ； When materials are being loaded or unloaded from the storage tank, is the breathing valve functioning properly? ； Is the gauge of the U-type manometer functioning properly? ； Is the sealing net damaged, and is it unobstructed? ； Check for leaks in the body of the pipeline-type breather valve of the storage tank, etc. Inspection and maintenance items: ① Open the top cover, check whether the valve disc, valve seat, guide rod, guide hole, spring, etc. inside the breather valve are rusty or contaminated, and clean them; if necessary, use kerosene for cleaning. ②Check whether the valve disc moves smoothly and without any sticking, and ensure that the sealing surface (the contact surface between the valve disc and the valve seat) is in good condition; repair it if necessary. Since the material of the sealing surface is a colored soft metal, a finer abrasive should be used when grinding it. ③Check whether the sealing mesh of the valve body is in good condition, free from freezing or blockages, and wipe away any rust or dust on the mesh to ensure smooth gas flow in and out. ④Check whether the gland gasket is tight; replace it if necessary, and lubricate the bolts. Inspection frequency for breathing valves: For flame-arresting breathing valves that are used in winter, when the temperature is below 0 ℃, they should be inspected at least once a week to prevent the valve disc from sticking to the valve seat due to freezing and thus causing malfunction. ②For the all-weather explosion-proof vent valves of above-ground tanks and semi-underground tanks, inspections are conducted twice a month in January and April of each year, and once a month in February and March. ③Weatherproof vent valves in the oil depot area should be inspected every six months. To maintain the safety of the breathing valve, it is necessary to conduct comprehensive inspections and maintenance on it at regular intervals over a certain period of time. 1. First, the breathing valve should be disassembled, and its corrugations should be thoroughly cleaned to prevent dirt buildup on them from interfering with the smooth opening and closing of the breathing valve. II. The small holes in the fire barrier layer also need to be checked; each hole must remain unobstructed, as otherwise it will affect safe operation. III. The valve disc of the ventilation opening is the most flexible and commonly used component in breathing valves; therefore, during maintenance it is necessary to check the flexibility of the valve disc as well as the stability of the connecting rod. IV. The sealing performance of the valve disc is also very important for breather valves; it is necessary to check whether the gland gasket is tight, and any signs of damage to the sealing mechanism should be addressed by replacing it promptly. During the inspection process, care must also be taken to avoid causing new damage. Make sure to lubricate the bolts. V. For each disassembly, corresponding records must be kept to prevent the loss or damage of critical components. VI. Maintenance cycle: Check twice a month in Q1 and Q4 (to prevent freezing) ； For the other months in the second and third quarters, inspections are conducted once a month.