Views: 5690 Author: Site Editor Publish Time: 2020-03-17 Origin: Site
Regulating valves are very important in the automatic control of modern factories. By accepting the control signal output from the control unit, the process parameters such as medium flow, pressure excitation, temperature, and liquid level can be changed. It can be said that the control valve is the last gate in the process loop. Therefore, how to properly select the control valve in order to achieve the best control effect has become a most critical issue.
There are four general selection principle.
1. The valve body pressure rating, operating temperature range, corrosion resistance and material should not be lower than the requirements of the process connection pipeline material. And preference should be given to manufacturers of shaped products. Generally, cast steel or forged steel valve body is used.
2. It is not suitable to use cast iron valve body for water vapor or wet gas with high water content and flammable fluid.
3. Cast iron body should not be used in the environment temperature lower than -20 ° C.
4. The valve trim should be resistant to corrosion, fluid erosion, and cavitation damage to the valve trim when the fluid is cavitated or flashed through throttling.
As for trim material selection, here are four tips.
1. Non-corrosive fluids generally use 1Crl8Ni9, 1Crl8Ni9Ti or other stainless steel.
2. Corrosive fluids should be selected according to the type, concentration, temperature and pressure of the fluid, as well as the fluid containing oxidants and flow rates. Commonly used corrosion-resistant materials are 1Crl8Ni9Ti, 0Crl8Nil2M02Ti, 20 # alloy, Hastelloy grade titanium steel.
3. For working conditions with large flow velocity and severe erosion, wear-resistant materials should be selected. Such as heat-treated 9Crl8 and 17-7PH and chrome-molybdenum steel, G6X and other materials with fastening oxide layer, toughness and high fatigue strength.
4. Material selection for severe wear occasions
When flashing, cavitation, and fluids containing particles are present, or when the temperature of the fluid is too high and the difference is too large, the valve core and valve seat should be surface hardened.
Valve types with different levels of leakage are selected according to the requirements of the process for leakage.Generally, the leakage of a single-seat valve should be less than or equal to 0.01% of the rated C value, and the leakage of a double-seat valve should be less than or equal to 0.1% of the rated C value.
1. There is no requirement on the flow direction of ball valve and common butterfly valve, any flow direction can be selected.
2. Three-way valve, Venturi angle valve, double-sealed sleeve valve with balanced holes have specified a certain flow direction, and generally cannot be changed.
3. Single seat valve, shape valve, pressure valve, single sealed sleeve valve without balance hole and small flow regulating valve, should choose the flow direction of the regulating valve according to different working conditions.
1) For high-pressure valves with DN≤20, because of high static pressure, large pressure difference and serious cavitation erosion, flow-closed type should be selected; when DN> 20, good stability should be selected as the condition to determine the flow direction.
2) For high-viscosity, solid-particle-containing media that require a "self-cleaning" performance, a flow-closing type should be selected.
3) The single-seat valve and small flow regulating valve are generally flow-open type. When the erosion is severe, the flow-close type can be selected.
4) Single-sealed sleeve valve is generally flow-open type; when self-cleaning is required, flow-closed type can be selected.
5) Two-position control valve (single-seat valve, angle valve, sleeve valve, quick-opening flow characteristics) should be flow-closed; when water hammer or surge occurs, flow-open should be selected.
6) When the flow-closed type is selected and ds <d, the stability of the valve is poor, the following points should be noted: the minimum working opening is greater than 20% to 30%, the spring with a high stiffness is selected, and the flow characteristics of equal percentage are selected.
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