What is the purpose of valve preliminary selection?
The selection of valve selection is often not achieved overnight, especially for beginners. In order to calculate the discharge coefficient, estimate the noise, etc. It is necessary to select the structure of the valve body to obtain some calculation parameters. If the calculation results do not meet the requirements, the valve type should be re selected. The actuator, the form of upper valve cover and the material of valve internals can be ignored temporarily.
What is the basis of primary valve type selection?
According to the nature of the medium, process parameters, use requirements of the primary selection of valve type.
1、 Which of the following media properties should be specified
1. General fluid;
2. High viscosity, suspended solids, fibrous and other fluids;
3. Toxic or expensive fluids;
4. Corrosive fluid.
2、 Which of the following types of process parameters should be specified
1. Low pressure, small pressure difference and large flow rate;
2. Medium pressure or low pressure, large pressure difference;
3. High temperature, high pressure and high pressure difference;
4. Temperature and temperature difference;
3、 The use requirements are one or more of the following
1. The leakage is small or cut-off is required;
2. Good stability and convenient maintenance;
3. Requirements for the weight, size and volume of the control valve;
4. Action speed requirements.
What should be known about valve control medium? What else should be mastered about process parameters?
The specific name, property and main physicochemical parameters of controlled medium, density, kinematic viscosity, molecular weight, thermodynamic critical pressure, valve inlet temperature, saturated vapor pressure, specific heat capacity ratio, compression coefficient, etc. Process parameters should be generally understood: flow, pressure before valve, pressure after valve, inlet temperature, etc.
What else should be mastered about valve service conditions?
It is also necessary to master the piping form of the control valve, the pipe diameter before and after the valve, and the system resistance. The main parameters and main interference factors should be understood in the type and characteristics of the automatic control object, and the leakage and stability requirements should be understood in the regulation performance requirements.
Basic composition of control system (1) Manual control Take the liquid level control shown in Figure 1-1 as an example