The disadvantages are as follows:
1. Its regulating performance is worse than that of the stop valve, especially the pneumatic valve;
2. Because the main seat sealing ring material of pneumatic ball valve is polytetrafluoroethylene, it is inert to almost all chemicals. However, the physical characteristics of PTFE, including high expansion coefficient, sensitivity to cold flow and poor thermal conductivity, require that the design of valve seat seal must focus on these characteristics. When the sealing material becomes hard, the reliability of the seal will be damaged. Moreover, the temperature resistance grade of PTFE is low, and it can only be used at less than 180 ℃. Above this temperature, the sealing material will age. If long-term use is required, it is generally recommended to use it at 120 ℃; Advantages of pneumatic butterfly valve: 1. Convenient and quick opening and closing, labor saving, small fluid resistance, and frequent operation. 2. Simple structure, small size and light weight.
3. It is capable of transporting mud and storing little liquid at the pipe mouth.
4. Excellent sealing can be achieved under low pressure.
5. Good conditioning performance.

At present, in addition to austenitic stainless steels (such as 304, 304L, 316, 316), 254SMO, 904L, AL6XN and other super austenitic stainless steels have also become important application data for most consumer manufacturers.
As for the heat pipe network laid in trench, when the contraction joint of corrugated pipe compensator is located in a low terrain, rainwater or accidental sewage will be soaked. Therefore, it is necessary to consider the data with stronger corrosion resistance, such as high nickel alloy, iron nickel alloy, etc., such as INCOLOY800, INCOLOY825, INCONEL625 and Hastelloy C-276.
With the vigorous implementation of the national policy of energy conservation, emission reduction and cogeneration, many small boilers and small heat and power with large energy consumption, low efficiency and poor environmental protection have been shut down. The 300MW units in large and medium-sized cities across the country have completed the centralized heating of cities and towns through the heat supply transformation of units, which has reduced the loss of cold source heat to power plants, improved the economic benefits of enterprises, and made the 300MW unit power plants out of the dilemma.
With the vigorous development of central heating in cities and towns, the first problem encountered is the laying method of steam pipes. The management committee of municipal administration and industrial parks, starting from urban planning, city appearance and environmental protection, is not allowed to cross the steam pipes passing through the enterprise gate overhead, and requests to go through the ground, and some even request to be buried along the whole line. It is a failure to adopt trench laying for publicly laid steam pipelines according to the traditional practice in the past. First, the pipelines under urban roads are dense and intertwined vertically and horizontally, and the trench is too large to deal with the interpenetration with other pipelines; Second, it is unable to deal with the drainage problem of the trench. The pipeline is soaked in water, the insulation is invalid, and the heat loss is large.