The valve is used to open and close the pipeline, control the flow direction, adjust and control the parameters of the transmission medium (temperature, pressure and flow) of the pipeline accessories. According to its function, it can be divided into shut-off valve, check valve, regulating valve and so on. The valve is a control component in the fluid conveying system, which has the functions of shutting off, regulating, diverting, preventing countercurrent, regulating pressure, diverting or overflow pressure relief. Valves for fluid control systems range from the simplest globe valves to valves used in extremely complex automatic control systems in a wide variety of varieties and specifications. Valves can be used to control the flow of various types of fluids such as air, water, steam, various corrosive media, mud, oil, liquid metal and radioactive media. Valve according to the material is also divided into cast iron valves, cast steel valves, stainless steel valves (201, 304, 316, etc.), chromium molybdenum steel valves, chromium molybdenum vanadium steel valves, biphase steel valves, plastic valves, non-standard valves and so on.

Classification by function and use (1) truncation class: such as gate valve, globe valve, plug valve, ball valve, butterfly valve, needle valve, diaphragm valve, etc. Cut-off valve, also known as closed-circuit valve, cut-off valve, its role is to connect or cut off the medium in the pipeline. Check class: such as check valve, check valve is also known as check valve or check valve, check valve is an automatic valve, its role is to prevent the media in the pipeline to reverse flow, to prevent the pump and drive motor reversal, as well as the leakage of container media. The bottom valve of the water pump suction close also belongs to the class of check valve.

When the temperature is lower than -29 ° C, low temperature resistant shell materials and internal materials should be selected. In the low temperature range of -29 ~ -196 ° C, the shell material and the interior material are required to have sufficient impact toughness. Special means must be used to maintain the heat capacity of the valve, so that it is protected from the cooling load, but also to keep the temperature of the stuffing box part above 0 ° C.

Measures should be taken to prevent abnormal pressure rise. After the valve is closed, some liquid will remain in the valve chamber. With the increase of time, the liquid remaining in the valve chamber will gradually absorb the heat of the atmosphere, rise to normal temperature and re-vaporize. After vaporization, its volume expands rapidly, increasing by about 600 times, so it will produce extremely high pressure and be used inside the valve body, which is called abnormal pressure boost. This is a unique phenomenon of low temperature valves.

First, the working process of thermostatic trap
According to the temperature difference between the steam and the condensate, the working condition of the trap is controlled by the thermostatic induction element, and the condensate in the trap is cooled to below the saturated steam temperature.
1, when the machine starts, the sealing bag is cold, the valve is fully opened, and the cold air and condensate are discharged by the trap.
2. When the condensate temperature is close to the steam saturation temperature, the filling liquid evaporates, the resulting pressure acts on the diaphragm, overcomes the pressure, pushes the spool close to the seat before the steam enters the trap, and closes the valve.
3. The condensate is cooled to a set temperature lower than or close to the steam saturation temperature, the pressure of the sealed bag drops, the valve is opened again, and the condensate is discharged.
Second, the working process of mechanical trap
Depending on the density between the steam and condensate, the operation of the trap is controlled mechanically through the movement of the "float ball" or "float".
1, in the starting stage, thermostatic emptying device (TV) can discharge air, otherwise air accumulation in the trap will cause gas lock;
2, when the condensate reaches the trap, the float rises, the valve opens, and the hot condensate makes the exhaust valve close. Condensate is discharged at saturation temperature;
3, when all the condensate is discharged, the steam enters the trap. The float drops and the valve closes. The spool is always below the water level to prevent direct steam leakage.