1. Confirm the selection criteria
(1) Process fluid conditions: fluid name, flow rate, inlet and outlet pressure, pressure difference during normal operation, pressure difference when valve is closed, fluid temperature, specific gravity, viscosity, whether there are suspended solids, fibrous, muddy, etc. In some cases, it is possible to separate the various process conditions for the maximum flow rate, normal flow rate, and minimum flow rate.
(2) Adjust the appearance condition: flow characteristics, effect mode, adjust the output signal of the appearance, azimuth feedback signal, etc.
(3) Pipeline connection conditions: nominal pressure, connection method, pipe size, pipe wall thickness, pipe material, etc
2. The way of primary selection of valves
According to the process conditions, choose the most suitable valve method.
(1) The nature of the medium
A. General fluids
B. High viscosity, fluids containing suspended solids, fibrous and other fluids
C. Highly toxic or valuable substances
D. Corrosive fluids
(2) Process parameters
A. Low pressure, small differential pressure, large flow
B. Medium and low pressure, large pressure difference
C. Some of the above conditions are grouped and other
(3) Application requirements
A. When the valve seat is closed, the leakage is large or small, or the switch type requires tight blocking
B. Good stability and easy repair
C. Air defense, low noise
D. Light weight and small size
3. Reconfirm the type of control valve
(1) According to the calculated Cv value, confirm the caliber and additional Cv value of the control valve. According to the conditions of operation of the control valve, and taking into account the accuracy of the control valve, it is worth selecting the valve caliber and additional CV according to the following conditions: when the maximum flow rate, the maximum opening of the control valve should not exceed 95% (about equal percentage flow) and 90% (about linear flow characteristics); When in normal operation, the opening of the control valve should be between 50%-80%; The opening of the control valve is not less than 10% at the minimum flow rate.
(2) Determine the material and pressure level of the valve body and valve trim according to the operating pressure, medium temperature and medium characteristics; The resolution is room temperature type, high temperature type, low temperature type, that is, the structure of the packing box, the material of the filler, and whether the bellows is required; IN ADDITION, IF THE ESTIMATED NOISE EXCEEDS 85DBA, A LOW-NOISE REGULATING VALVE IS REQUIRED.
Confirm the valve method, confirm the type of actuator according to the power source, and confirm the number (output example or torque) and method of the actuator according to the operating pressure difference and the valve seat closure pressure difference.
4. Confirm the accessories of the control valve
(1) According to the output signal of the regulator to confirm the variety and standard of the valve positioner;
(2) Data sheet or on-site requirements to confirm the level and standard of protection of electrical accessories;
(3) Some control valves, wearing a single-effect actuator, spring scale of 20-100KPa, withstand 20-100KPa gas signal, but if the owner proposes, the design institute also recognizes, it can be without a valve positioner, of course, the adjustment accuracy is much lower than that with a valve positioner.
(4) According to the data table or the owner's on-site requirements, other control valve accessories are selected.
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