Over the years, people have considered a number of traditional factors when choosing a control valve, such as pressure ratings, pressure drop, flow media, temperature, and cost. However, the situation has changed dramatically in the past 10 years and much progress has been made in valve design, and the cost-effectiveness of the manufacturing process is quite different from what it was before, making many of the more traditional factors that have to be taken into account before choosing a valve The importance has been greatly diminished. The key to choosing a valve is to evaluate the dynamic rather than static characteristics. Dynamic characteristics are the key to choosing a valve: Although some of the traditional factors are still important, they only emphasize the "static" performance of the valve. In fact they are the result of measuring the valve on the "workbench," but the result is hard to say what kind of performance the valve will perform under actual operating conditions. Traditionally, it has been suggested that careful adjustment of the static factor will give the valve (and hence the entire circuit) good performance. However, we now recognize that this is not always the case. Thousands of performance tests conducted by researchers and manufacturers demonstrate that up to 50% of valves in service, many of which are selected by considering traditional factors, have had little effect on optimal control loop performance. Subsequent studies have shown that the dynamic characteristics of the valve play a very important role in reducing process variability. In many key processes, different valves reduce process variability by a factor of 1 to dramatically increase productivity and reduce waste, resulting in more than $ 1 million in economic benefits. Obviously, this economic benefits so that we can completely deny the traditional approach, that is, only according to the initial purchase price of the valve to decide whether to buy. Second, the conventional wisdom has always been that improvements in process optimization always come from upgrades in control room control instrumentation. However, the test data shows that the dynamic characteristics of the valve can have a significant impact on circuit performance with the same control instrumentation. If the accuracy of the control valve can only be as low as 5%, it can not do much to spend a lot of money configuring an advanced control instrumentation system with a control accuracy of up to 0.5%. 2, the valve classification: Looking for a match with the use of the occasion of the valve, we should first look at the four basic types of throttle control valve, that cage valve, rotary ball valve, eccentric valve and butterfly valve. Cage ball valves are available in a wide variety of trim forms to meet the needs of most applications, making them the first choice of any valve. There are many kinds of cage ball adjusting pieces, including balance adjusting piece, unbalance adjusting piece, elastic seat adjusting piece, constrained adjusting piece and full size adjusting piece. In many cases, the various trim configurations of a valve body are interchangeable. Cage ball valves also have several disadvantages. First, the size of the valve is limited (usually 16 inches); second sight valve with the same specifications (such as float valve or butterfly valve) compared to its capacity is relatively low; third is the higher price, especially large Cage ball valve. However, cage ball valves have excellent performance, often enough to compensate for these drawbacks, in reducing process variability. Rotating float valve flow than the same diameter cage valve larger. Although the control of the rotary float valve is larger than the cage ball valve, it is still superior to most Other types of valves. Rotary float valve allows the pressure drop and allowable temperature range smaller than the cage valve. Normally their pressure drop is limited to 7.0x105kg / m2, suitable for use at temperatures below 398 ° C. Float valves are not suitable for liquid that is easy to empty and often emit more noise when used in gases with higher pressure drops. Eccentric valve friction than the smaller ball valve, the price is lower. Unique structural design makes it more precise for the control of process variability. This is evident from the new Fisher product bv500. In addition, the advantages and disadvantages of eccentric valve float valve is not much difference. By the performance of the valve to measure, the butterfly valve is a low-end valve. Butterfly valve flow, the cheapest price, but there are many different caliber. However, the butterfly valve characteristic curve is only a proportional curve, which greatly limits the butterfly valve to reduce flow variability performance. For this reason, the butterfly valve can only be used in fixed load situations. Although butterfly valves come in many different caliber sizes and can be made from most cast alloys, butterfly valves do not meet ansi's requirements for face-to-face dimensions, nor do they apply to fluids or noises that are prone to cavitation .
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