Questions&Answers About Valves

1.Specify the purpose according to the working conditions of the valve: applicable medium, working pressure, working temperature.     

2.Make sure the nominal diameter&connection mode of the pipeline: flange, thread, welding.

3.Determine the mode of valve operation: manual, electric, electromagnetic, pneumatic, hydraulic, electrical linkage or electro-hydraulic linkage.

4.Be sure of the materials of the shell and internals. They are according to the medium, working pressure and working temperature.

5.Determine the type of valve: gate valve, globe valve, ball valve, butterfly valve, needle valve, safety valve, plug valve，etc.

1. The output of the actuator shall be greater than the load of the valve and shall be reasonably matched.   

2. Make sure the allowable differential pressure meet the valve requirements. When the pressure difference is large, the unbalanced force on the valve core should be calculated.

3. Consider whether the response speed of the actuator meets the process operation requirements, especially the electric actuator.

Ball valves have a stem and ball, which turns horizontally, and are commonly referred to as “rotational” valves. They are best suited to applications requiring on/off control without pressure drop. Gate valves open by lifting a round or rectangular gate (or wedge) out of the path of the fluid.

The advantage of the two seat valve core is the force balance structure, which allows a large differential pressure. Its prominent disadvantage is that the two sealing surfaces cannot be in good contact at the same time, resulting in large leakage.

The electric drive source is simple and convenient, with large thrust, torque and stiffness. But the structure is complex and the reliability is poor.It is often used in places without air source or without strict explosion-proof and flame-proof.

Output forms: angular stroke, straight stroke, and multi rotation.

Diaphragm valve, butterfly valve, O-type ball valve (mainly cut-off), V ball valve (with large adjustment ratio and shear effect), eccentric rotary valve are all valves with adjustment function.

The valve core of the straight stroke valve is vertically throttled, and the medium flows in and out horizontally. The flow channel in the valve cavity must turn upside down, which makes the flow path of the valve quite complex (such as the inverted “s” shape). In this way, there are many dead zones, which provide space for the sedimentation of the medium. In the long run, it will cause blockage. While the throttling direction of the angle travel valve is the horizontal direction. The medium flows in and out horizontally, which is easy to take away the dirty medium. At the same time, the flow path is simple. And there is little space for medium sedimentation, so the angle travel valve has good anti blocking performance.

Valve positioners are devices used to put a valve in the correct position by increasing or decreasing the air load pressure on the actuator. Instead of the process controller sending a signal directly to the actuator to move the valve, a valve positioner senses the exact position of the valve and communicates to the actuator to move the valve.

1. Determine the calculated flow – Qmax, qmin.   

2. Determine the calculated differential pressure – select the resistance ratio s value according to the system characteristics, and then determine the calculated differential pressure (when the valve is fully open).

3. Calculate the flow coefficient – select the appropriate calculation formula chart or software to calculate the max and min of kV;

4. Kv value selection – according to the max value of Kv, the kV closest to the first gear in the selected product series, the primary diameter is obtained;

5. Opening checking calculation – when Qmax is required, ≯ 90% valve opening; At qmin, ≮ 10% valve opening;

6. Actual adjustable ratio checking calculation – General requirements should be ≮ 10; R actual ＞ r requirement

7. Caliber determination – if it is unqualified, reselect the Kv value and verify it again.

This is because the sleeve valve only improves the throttling form, stability and maintenance.  The weight, anti blocking and leakage indicators are as same as those of the single and double seat valve.