Working principle of plastic butterfly valve

Update:18 Sep 2020

The plastic butterfly valve ( originated from the American invention of the butterfly valve in the 1930s. The plastic butterfly valve was developed in Japan in the 1950s and was only widely used in Japan in the 1960s. In the world, butterfly valves above DN300 mm have gradually replaced gate valves. Compared with gate valves, butterfly valves have shorter opening and closing times, small operating moments, small installation space and light weight. Take DN1000 as an example, the butterfly valve is about 2T, and the gate valve is about 3.5T, and the plastic butterfly valve is easy to combine with various driving devices, which has good durability and reliability. The disadvantage of the rubber-sealed butterfly valve is that when it is used for throttling, it will cause cavitation due to improper use, causing the rubber seat to peel off and damage. For this reason, metal-sealed butterfly valves have been developed internationally to reduce the cavitation zone. In recent years, my country has also developed metal-sealed butterfly valves. In Japan, comb-shaped butterfly valves with cavitation resistance, low vibration and low noise have also been developed.

Working principle of plastic butterfly valve:
The relationship between the opening of the plastic butterfly valve and the flow rate basically changes linearly. If it is used to control flow, its flow characteristics are also closely related to the flow resistance of the piping. For example, two pipelines are installed with the same valve diameter and form, but the pipeline loss coefficient is different, and the flow rate of the valve will also be very different. If the valve is in a state with a large throttle range, the back of the valve plate is prone to cavitation, which may damage the valve. Generally, it is used outside 15°.
When the plastic butterfly valve is in the middle opening degree, the opening shape formed by the valve body and the front end of the butterfly plate is centered on the valve shaft, and the two sides form different states. The front end of the butterfly plate on one side moves in the direction of the flowing water, and the other side is against the direction of the flowing water Therefore, one side of the valve body and the valve plate form a nozzle-like opening, and the other side is similar to a throttle opening. The nozzle side has a much faster flow rate than the throttle side, and negative pressure will be generated under the throttle side valve. Rubber seals often fall off.
Plastic butterfly valves and butterfly rods have no self-locking ability. For the positioning of the butterfly plate, a worm gear reducer must be installed on the valve rod. The use of worm gear reducer can not only make the butterfly plate have self-locking ability, make the butterfly plate stop at any position, but also improve the operation performance of the valve.
The operating torque of the plastic butterfly valve has different values ​​due to the different opening and closing directions of the valve. The horizontal butterfly valve, especially the large diameter valve, due to the water depth, the torque generated by the difference between the upper and lower water heads of the valve shaft cannot be ignored. In addition, when an elbow is installed on the inlet side of the valve, a bias current is formed and the torque will increase. When the valve is in the middle opening, the operating mechanism needs to be self-locking due to the action of the water flow torque.
The plastic butterfly valve has a simple structure, consisting of only a few parts, and saves material consumption; small size, light weight, small installation size, small driving torque, simple and rapid operation, only need to rotate 90 ° to quickly open and close; and At the same time, it has a good flow adjustment function and closing and sealing characteristics. In the application field of large and medium caliber, medium and low pressure, the butterfly valve is the dominant valve form. When the butterfly valve is in the fully open position, the thickness of the butterfly plate is the only resistance when the medium flows through the valve body, so the pressure drop generated by the valve is small, so it has better flow control characteristics.
The butterfly valve has two sealing types: elastic seal and metal seal. Elastic sealing valve, the sealing ring can be embedded in the valve body or attached to the periphery of the butterfly plate. Valves with metal seals generally have a longer life than valves with elastic seals, but it is difficult to achieve a complete seal. The metal seal can adapt to higher working temperature, while the elastic seal has the defect of being limited by temperature.

Development prospects of plastic butterfly valves:
If the plastic butterfly valve is required to be used as a flow control, the main thing is to select the correct size and type of the valve. The structural principle of the plastic butterfly valve is especially suitable for making large diameter valves. Plastic butterfly valves are not only widely used in general industries such as petroleum, gas, chemical, and water treatment, but also used in cooling water systems in thermal power stations. Commonly used plastic butterfly valves include wafer butterfly valves and flange butterfly valves. The plastic wafer butterfly valve uses stud bolts to connect the valve between two pipe flanges. The flange type butterfly valve has a flange on the valve, and the two ends of the valve are flanged to the pipe flange with bolts. The strength performance of the valve refers to the ability of the valve to withstand the pressure of the medium. The valve is a mechanical product that bears internal pressure, so it must have sufficient strength and rigidity to ensure long-term use without cracking or deformation.