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How to correctly choose the control valve Dynamic evaluation characteristics are the key

Several traditional factors have been considered in the selection of control valves over the years, such as pressure ratings, pressure drops, flow media, temperature, and cost. However, the situation has changed a lot in the past 10 years, and the valve design has made a lot of progress. The cost-effectiveness of the production process is quite different from before, which makes many traditional factors that must be considered when selecting valves. The importance has been greatly weakened.
Dynamic characteristics
While some traditional factors are still important, they only focus on the "static" performance of the valve. In fact they are the result of measuring the valve on the "workbench", but such a result is difficult to tell what kind of performance the valve will perform under actual operating conditions. Traditional theory suggests that careful adjustment of static factors will result in good performance of the valve (and thus the entire circuit). However, now we realize that this is not always the case.
Thousands of performance checks conducted by researchers and manufacturers have demonstrated that up to 50% of in-service valves (many of which are selected by considering traditional factors) do not have much effect on optimizing control loop performance. Subsequent studies have shown that the dynamic characteristics of the valve play an important role in reducing process variability. In many key processes, different valves reduce the variability of the process even if the difference of 1% can significantly increase production efficiency and reduce waste, resulting in more than $1 million in economic benefits. Obviously, such economic benefits make it entirely possible to deny the traditional practice of deciding whether to buy only based on the initial purchase price of the valve.
Second, the traditional view always believes that the improvement of process optimization always comes from the upgrade of the control room control instrument. However, the test data shows that the dynamic characteristics of the valve can have a significant impact on loop performance under the same control instrumentation. in case
The accuracy can only reach 5%, then it takes a lot of money to configure a set of advanced control instrumentation systems with a control accuracy of 0.5%.
Valve type
When looking for a valve that matches the application, first consider the four basic types of throttle control valves, namely cage ball valves, rotary float valves, eccentric valves and butterfly valves.
Cage ball valves are available in a wide variety of tab styles to meet the needs of most applications, making them the preferred choice for a wide range of valves. There are many types of cage ball valve adjustments, including balance adjustment tabs, unbalanced adjustment tabs, elastic seat tabs, constrained tabs, and full-size tabs. In many cases, the various tab configurations of a valve body are interchangeable.
Cage ball valves also have several drawbacks. First, the size of the valve is limited (usually 16 inches); second, compared with the same size sight valve (such as float valve or butterfly valve), its capacity is relatively low; third is the higher price, especially large Cage ball valve with caliber. However, cage ball valves have excellent performance in reducing process variability and are often sufficient to compensate for these deficiencies.
The flow rate of the rotating float valve is larger than that of the cage of the same diameter. Although the control range of the rotary float valve is larger than that of the cage ball valve, but the 诖蠖嗍渌嘈诖蠖嗍渌嘈 А А А А А А А А 寨寨寨寨寨寨攘 攘 攘 攘 攘 攘 攘 ǔ ǔ ǔ ǔ Difficult to use the sideboard?.0x105kg/m2, suitable for use at temperatures below 398 °C. Float valves are not suitable for liquids that are prone to cavitation, and often use large amounts of noise when used in gases with high pressure drops.
The eccentric valve has less friction than the float valve and is less expensive. The unique structural design makes it more precise to control process variability. This is evident from Fisher's new BV500. In addition, the advantages and disadvantages of the eccentric valve are not much different from those of the float valve.
The butterfly valve is a low-end valve as measured by the performance of the valve. The butterfly valve has a large flow rate, the cheapest price, and a variety of different calibers. However, the characteristic curve of the butterfly valve has only one proportional characteristic curve, which greatly limits the performance of the butterfly valve to reduce the process variability. For this reason, butterfly valves can only be used in applications where the load is fixed. Although butterfly valves are available in a variety of different sizes and can be fabricated from most cast alloys, butterfly valves do not meet ANSI requirements for face-to-face dimensions, nor do they apply to fluids that are prone to cavitation or where noise is high. .

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