Regulating valve body type selection of regulating valve selection

Regulating valve body type selection of regulating valve selection

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There are many kinds of regulating valve body, commonly used are straight single seat, straight double seat, Angle, diaphragm, small flow, three-way, eccentric rotation, butterfly, sleeve type, spherical and so on. In the specific selection, the following considerations can be made:
(1) spool shape structure
Mainly according to the selected flow characteristics and unbalanced force and other factors.
(2) Wear resistance
When the fluid medium is a suspension with a high concentration of wear particles, the internal material of the valve should be hard.
(3) Corrosion resistance
Because the medium is corrosive, try to choose a simple valve structure.
(4) medium temperature, pressure
When the temperature and pressure of the medium are high and the change is large, the material of the spool and seat should be selected by the small change of temperature and pressure of the valve.
(5) Prevent flash and cavitation
Flash evaporation and cavitation occur only in liquid media. In the actual production process, flash and cavitation will form vibration and noise, shorten the service life of the valve, so the valve should be prevented from flash and cavitation when selecting the valve.
Adjustment valve selection Experience:
The key to selecting a regulating valve is to evaluate the dynamic characteristics. For many years, traditional factors such as pressure rating, pressure drop, flow medium, temperature and cost have been considered in the selection of regulating valves. However, much has changed in the last 10 years, as valve design has progressed and the cost-effectiveness characteristics of the production process have changed significantly, diminishing the importance of many of the traditional considerations that must be taken into account in the selection of valves.
Adjust the valve dynamic characteristics:
While some traditional factors are still important, they tend to favor the “static” performance of the valve. They are actually the result of measuring the valve on the “bench”, but such results hardly indicate how the valve will behave under actual operating conditions. The traditional theory of valves is that careful adjustment of static factors will result in good performance for the valve (and thus the entire loop). Now, however, Taichen is learning that this is not always the case.
Thousands of performance checks by researchers and manufacturers have demonstrated that as many as 50% of valves in use, many of which were selected with traditional considerations in mind, do not have much effect on optimizing the performance of the control loop. Subsequent studies have shown that the dynamic characteristics of the valve play an important role in reducing the flow variability. In a number of critical processes, even a 1% difference in the amount of process variability reduced by different valves can significantly increase productivity and reduce waste, resulting in more than $1 million in economic benefits. Obviously, such economic benefits allow us to reject the traditional approach of only making a decision based on the initial purchase price of the valve.
Second, the conventional wisdom has always been that improvements in process optimization always come from upgrading control instruments in the control room. However, the test data show that the dynamic characteristics of the valve can have the greatest influence on the loop performance under the same control instrument conditions. Spending a lot of money on an advanced instrumentation system with a control accuracy of 0.5% is not going to do much good if the control valve is only 5% accurate.
Type of regulating valve:
When looking for a valve to match the use of the situation, we should first look at the four basic types of throttling regulating valves, namely cage ball valve, rotary float valve, eccentric valve and butterfly valve.
The wide variety of adjustment disc forms available for cage ball valves allows them to meet the needs of most applications, making them a priority among valves. There are many kinds of cage ball valve adjusting pieces, including balance adjusting pieces, non-balance adjusting pieces, elastic seat adjusting pieces, constrained adjusting pieces and full-size adjusting pieces. In many cases, the various regulator configurations of one body are interchangeable.
Cage ball valves also have several disadvantages. One is that the valve is limited in size (usually 16 inches); Second, compared with line valves of the same specifications (such as float ball valves or butterfly valves), its capacity is relatively low; Third, the price is higher, especially the large diameter cage ball valve. However, the excellent performance of cage ball valves in reducing process variability often more than compensates for these shortcomings.
Eccentric valves have less friction and lower prices than float valves. The unique structure design makes it more accurate to control the process variability. This can be seen in Taichen’s new BV500 product. In addition, the advantages and disadvantages of eccentric valves and floating ball valves are not much different.
According to the performance of the valve to measure, butterfly valve belongs to the low – grade valve. Butterfly valve flow is large, low price, and there are a variety of different calibers. However, the butterfly valve characteristic curve is only proportional characteristic curve, which limits the performance of the butterfly valve to reduce the flow variability. For this reason, butterfly valves can only be used in situations where the load is fixed. Although butterfly valves are available in a variety of different calibers and can be manufactured in most cast alloys, they do not meet ANSI requirements for face to face sizes and are not suitable for use in vacuum-prone fluids or noisy environments.
Selection of regulating valve actuator:
★ Simple pneumatic film type regulating valve, followed by piston type, is electric.
★ The main advantages of electric actuator are convenient driving source (power supply), but the price is high, reliability, waterproof and explosion-proof as pneumatic actuator, so pneumatic should be preferred.
★ Old electric actuator bulky, Taichen company has electronic fine small high reliability electric actuator to provide (the price is correspondingly high).
★ The old ZMA, ZMB thin film actuator can be eliminated, replaced by multi-spring light actuator (performance improvement, weight, height drop about 30%).
★ Piston actuator varieties and specifications are more, the old, big and stupid suggestions no longer choose, and choose the new light structure.
Selection of regulating valve according to:
To determine the nominal pressure, it is not to use Pmax to set PN, but to find the corresponding PN from the table by the temperature, pressure, material three conditions and meet the PN value of the selected valve.
The leakage of the valve type can meet the technological requirements.
The working pressure difference of the valve type should be less than the allowable pressure difference of the valve, if not, it must be considered from a special Angle or choose another valve.
The temperature of the medium is within the operating temperature range of the valve, and the ambient temperature meets the requirements.
Consider the problem of valve blockage prevention according to the unclean condition of the medium.
Consider the corrosion resistance of the valve according to the chemical properties of the medium.
According to the pressure difference and the medium containing hard material, the erosion and wear resistance of the valve are considered.
Comprehensive economic effect to consider the performance, price. Three questions to consider:
1, simple structure (the simpler the higher reliability), easy maintenance, spare parts source;
2. Service life;
3. Price.
Butterfly valve, single seat valve, double seat valve, sleeve valve, Angle valve, three way valve, ball valve, eccentric rotary valve, diaphragm valve.
Selection of regulating valve material:
Valve body pressure grade, temperature and corrosion resistance should not be lower than the requirements of the process connection pipeline, and should be preferred to choose the manufacturer’s fixed products.
Water vapor or water containing more wet gas and flammable and explosive medium, should not choose cast iron valve.
When the ambient temperature is below -20℃ (especially in the north), cast iron valves should not be selected.
For the medium with serious cavitation and erosion, in the rectangular coordinate composed of temperature and pressure difference, the temperature is 300℃ and the pressure difference is 1.5MPa outside the two point line, wear-resistant materials should be selected for the throttle sealing surface, such as cobalt-based alloy or surface surfacing Stetley alloy.
For strong corrosive media, the selection of corrosion resistant alloy must be based on the type of media, concentration, temperature, pressure, the choice of appropriate corrosion resistant materials.
The valve body and throttle parts are treated separately, the throttle speed of the inner wall of the valve body is small and allowed to have a certain corrosion, the corrosion rate can be about lmm/year; Throttle parts by high-speed erosion, corrosion will bow [leakage increase, the corrosion rate should be less than 0.1mm/year.
The selection of lining materials (rubber, plastic), the temperature, pressure, concentration of the working medium must meet the scope of the use of the material, and consider the valve action on its physical and mechanical damage (such as shear damage).
Vacuum valve body lining rubber, plastic structure should not be selected.
Rubber lining should not be used for the two position shut-off valve of the water treatment system.
Selection of typical corrosion resistant alloy materials for typical media:
Sulfuric acid: 316L, Hastelloy, alloy No. 20.
Nitric acid: aluminum, C4 steel, C6 steel.
Hydrochloric acid: Hastelloy B.
Hydrofluoric acid: Monel.
Acetic acid, formic acid: 316L, Hastelloy alloy.
Phosphoric acid: Inco nickel, Hastelloy alloy.
Urea: 316L.
Caustic soda: Monel.
Chlorine gas: Hastelloy C.
Seawater: Inco nickel, 316L.
So far, s corrosion resistant material is tetrafluoroethylene, known as the “king of corrosion resistance”. Therefore, the first choice should be tetrafluorocorrosion-resistant valve, under the circumstances (such as temperature 180℃, PN1.6) to choose alloy.
Selection of regulating valve flow characteristics:
The following is a preliminary selection, detailed selection in the special materials:
S0.6 time selective logarithm feature.
Logarithmic characteristics are selected when the opening degree is small and the unbalance force is large.
The required adjusted parameters reflect the speed of fast choose straight line, slow choose logarithm.
Optional linear characteristics of pressure regulation system.
Optional linear feature for liquid level adjustment system.
Mode selection of regulating valve:
In foreign countries, the open or close is often expressed by fault, namely, fault open, fault close, and our country’s gas open and gas close expressions are exactly the opposite, fault open corresponds to gas close valve, fault close corresponds to gas open valve.
The new light valve and fine small valve do not emphasize the positive and negative effects of the execution machinery, so it must be marked on the endnote. B(gas closed)K(gas open)
Selection of regulating valve flow:
At the throttle port, the medium flow in the open direction of the spool is the flow open, and the flow in the closed direction is the flow closed.
The choice of flow direction is mainly single seal type regulating valve, single seat valve, Angle valve, single seal sleeve valve three categories. It is based on the specified flow direction (such as two seat valve, V ball) and arbitrary flow (such as O ball).
When dg > 15, the flow is usually selected to open, when dg≤15 small caliber valve, especially the high pressure valve can be selected to close the flow, in order to improve the life.
Optional flow closure for two – position on-off valve.
If the flow-closed valve oscillates, change it and the flow-open valve can be eliminated.
Selection of adjusting valve accessories:
Adjusting valve accessories are: positioner, converter, actuator, booster valve, holding valve, pressure reducing valve, filter, oil mist, travel switch, position transmitter, solenoid valve, hand wheel mechanism.
Accessories provide supplementary functions and ensure the operation of the valve. Increase what is necessary and do not increase what is unnecessary. Adding accessories when unnecessary raises prices and reduces reliability.
The main function of the locator is to improve the output force and speed of action, do not need these functions, do not take, not with the locator.
For fast response systems, do not valve action fast, optional converter.
Strictly explosion-proof occasions, optional: electrical converter + pneumatic positioner.
Solenoid valve should choose reliable products, to prevent it from action when it does not act.
It is recommended not to use hand wheel mechanism on important occasions to prevent human misaction.
It is better to be supplied by the manufacturer and the assembly is supplied on the valve to ensure the reliability of the system and assembly connection.
To reiterate: please pay attention to the importance of these “little things”, especially reliability, if necessary, can be equipped with pneumatic components, such as solenoid valve.


Post time: Dec-02-2022

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