2、 There are many kinds of butterfly valves, and there are many classification methods.
1. Classification by structure
(1) Center sealing butterfly valve
(2) Single eccentric sealing butterfly valve
(3) Double eccentric sealing butterfly valve
(4) Triple eccentric sealing butterfly valve

What inspection is required for bellows compensator before delivery
The compensator is composed of many links such as design, manufacturing, installation and operation management. For the selection of materials for bellows used in heat supply network, in addition to the working medium, working temperature and external environment, the possibility of stress corrosion, the influence of water treatment agent and pipeline cleaning agent on the materials should also be considered, and on this basis, the welding and forming of bellows materials and the function cost ratio of materials should be connected to select the economic and applicable bellows construction materials. The materials of bellows compensator shall meet the following conditions:
(1) Excellent plasticity facilitates the processing and forming of bellows, and sufficient hardness and strength can be obtained through subsequent processing technology (cold hardening, hot processing, etc.).
(2) High elastic limit, tensile strength and fatigue strength ensure normal operation of bellows.
(3) Good corrosion resistance, meeting the requirements of bellows working in different environments.
(4) Excellent welding function can meet the welding process requirements of bellows during construction.
For the heat pipe network laid in the trench, when the pipeline where the compensator is located is in a low terrain, the corrugated pipe will be soaked by rainwater or random sewage, so it is necessary to consider selecting materials with stronger corrosion resistance, such as iron nickel alloy, high nickel alloy, etc. Due to the high price of such materials, when manufacturing bellows, it can be considered that only a layer of corrosion resistant alloy may be added roughly when contacting with corrosive media. The fatigue life design can be seen from the failure type and cause analysis of the corrugated compensator that the plane invariance, circumferential invariance and corrosion resistance of the bellows are all related to their displacement, i.e. fatigue life. Too low fatigue life will lead to the invariance and corrosion resistance of metal bellows.