Selection Technique of Common Valve in Chemical Engineering Design
With the rapid development of science and technology, chemical production is also moving towards the direction of automation and intelligence. As the key structure of chemical system, valve plays an important role in the whole system. The products produced by chemical enterprises as ZJCLV full port ball valves, are highly prone to accidents due to their inflammable, explosive and corrosive properties. In the past, most production processes used by chemical enterprises were closed processes to prevent chemical accidents. But only enhanced valve selection can solve such problems at a high level. In this process, due to the variety of valves, different valves are different in function, and the valve as an important component of the chemical system, the quality and function of the valve will have a significant impact on the operation of the chemical system.
In order to ensure the stable operation of chemical system, it is necessary to master the selection skills of common valves（gate valve, ball valve, globe valve etc.）and select appropriate valves on this basis.
From model selection to use, valve buyers may have all kinds of questions. Here are some information collected for you.
Q1: How to select model?
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.
Q2: What are the main factors that should be considered in selecting an executing agency?
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.
Q3：What is the difference between gate valve and ball valve ?
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.
Q4: Why can’t the double seal valve be used as a shut-off valve?
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.
Q5: What are the characteristics and output forms of electric actuator?
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.
Q6: In addition to single, double seat valves and sleeve valves, what other valves have regulating functions?
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.
Q7: Why is the anti blocking performance of the angle stroke valve better than the straight stroke regulating valve?
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.
Q8: What is the valve positioner ?
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.
Q9: What are the seven steps to determine the diameter of the regulating 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.
Q10: Why did the sleeve valve replace the single and double seat valve, but it didn’t work?
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.
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As we all know, chemical products are generally corrosive, flammable, explosive and other characteristics, which put forward higher requirements for the selection of valves. And the choice of the valve is very important in the chemical pipe design, which to a certain extent, determines the quality of pipeline design work, so on the concrete when the choice must be to the characteristics of the valve in common understanding, on the basis of careful to choose. From previous passage we’ve known the characteristics of three valves. In this paper, We continue to learn the characteristics of butterfly valve, plug valve and ball valve.
1. Butterfly Valves
A butterfly valve is often used in the design of chemical pipeline. It is characterized by small volume and light weight, and its structure is also very simple. These characteristics bring great convenience to the application process. In the process of practical application, workers tend to use it more in some work with large flow. In the process of practical work, it also shows some advantages and has been widely used accordingly. But at the same time, we can also find that in the process of using the butterfly valve, the temperature is often not too high, because it is sensitive to temperature, once the temperature is higher, it will affect its normal working process, make it poor sealing.
2. Plug Valves
In the chemical pipeline design work, compared to other types of manufacturing costs, this valve is relatively high. At the same time, it also has many advantages. First, the plug valve in the working process of sealing is very good; Secondly, it is very convenient to close the valve after opening; And the use of the valve cycle is very long. Finally, the valve in use process can better realize the bidirectional closed. It also makes the superiority characteristic of its application range is wide, and on some polluting the stronger material transport can also achieve good sealing work has strong value and function. But it cost expensive sex is affected to some extent it spread more widely and use.
A ball valve is a valve type very similar to a plug valve. According to its body structure, ball valves can be divided into three types: one-piece ball valve, two-piece ball valve and three-piece ball valve. Ball valves to a certain extent followed the advantages of plug valves, can be better for some temperature being not high, strong pressure and strong viscosity of the material to transport. But its cost is much lower than the plug valve, maintenance is also more convenient than other valves. Therefore, it has been more widely used. It is worth noting that if some requirements for flow control aspects of the work, the use of ball valves is not very convenient, because it is not a good flow to regulate and control.