3. Classification according to sealing form
(1) Forced sealing butterfly valve. 1) Resilient sealing butterfly valve. The sealing specific pressure is generated by the elasticity of the valve seat or valve plate when the valve plate squeezes the valve seat when the valve is closed. 2) Applied torque sealed butterfly valve. The sealing pressure is generated by the torque applied to the valve shaft.
(2) Pressurized and sealed butterfly valve. The sealing specific pressure is generated by charging the elastic sealing element on the valve seat or valve plate.
(3) Automatic sealing butterfly valve. The sealing pressure is automatically generated by the medium pressure.

2. The pipeline has micro flow inspection flow chart item 2. Subjectively, it is believed that there is no flow in the flow sensor, but there is actually a trace flow. This kind of fault is mainly caused by the poor tightness of the stop valve of the original flash foot pipeline, and the small leakage detected by the electromagnetic flowmeter is misunderstood as zero point change or instability. The valve is often used for a long time or the valve is not fully sealed due to liquid dirt, especially for large valves. Another common reason is that there are several branch pipes in addition to the upper pipes of the flow instrument, and the valves of these branch pipes are forgotten or ignored to close. Sometimes, it is difficult to confirm that there is no flow in the pipeline system on site. At this time, as shown in Figure 4, a small caliber leakage monitoring valve 3 can be set between the stop valves 1 and 4 in front and behind the flow sensor 2 to observe whether there is enough leakage.