Minimum flow electric control valve

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Small flow electric control valve

in the application of many industrial systems, it requires accurate control of a small amount of material under normal pressure or a slightly larger amount of material under extreme high pressure difference. At this time, the calculated CV value is very small. This can be used in intermediate experimental devices with very small production or in industrial equipment that adds a small amount of reagent to a large amount of process liquid. The theoretical minimum CV value is 0.00001. The CV range included in the small flow rate increases the compressive strength girth ratio, which is equivalent to the range included in the size of ordinary valves from 1 inch to 16 inches

when considering the use of these small flow valves, the selection criteria are very different from those of conventional control valves. For example, the friction loss connected in series with the control valve is very small, and there is no adverse pump head characteristics. No real equal percentage characteristic is required. The main design problem is to carefully study the actual required flow range. The smaller the CV value becomes, the more it becomes a theoretical value. They are useful as indicators of flow capacity, but their accuracy and repeatability are questionable, because in many cases, the fluid passing through the throttling part may be laminar flow, or a mixture of laminar flow and turbulent flow, which changes with the change of valve stroke or flow conditions

from the design point of view, the actual size of fluid throttling is indeed very small; However, the fact that many small flow applications involve high differential pressure and wear brings more problems

in the design of standard valves, the flow capacity is mainly determined by the diameter of the valve seat hole. When the flow capacity is reduced, the actual minimum diameter reaches 1/16 inch. In this way, the theoretical CV value of the hole is approximately 0.06 or close to the upper limit of the small flow range. It is mainly reduced by reducing the stroke of the valve element or limiting the opening of the channel port

the short stroke actuator is very suitable for the balanced throttling effect. The pneumatic actuator is used to drive the arm, and a little rotation will drive the threaded valve stem. In this system, two threads with different screw moments are used to further limit the stroke. With this method, the stroke can be limited to less than 0.005 inch

when the linkage mechanism is variable and the stroke ratio is greater than 10, the maximum stroke of the valve core is 0.188 inch. For actuators with a rated stroke of 3/4 inch, the proportion of their stroke to the exports of emerging countries will also be greatly increased and can be reduced to 1/4. In this way, the travel can be reduced to a minimum of 0.010 inch. This allows the flow capacity to be adjusted on a large scale on site, while providing high operating force for high differential pressure operation. Both special valves use a simple conical spool. For very small flow, use a delicate taper pin and adjust it in the taper hole

for a few strict applications, taper can be used to facilitate the opening of the sealing cover needle or the adjustment of the plunger with milling in the hole. The hole diameter ranges from 1/8 inch to 1/4 inch. The stroke of this type of valve is about 1/2 inch. This kind of small flow valve trim can be installed in the valve body of ordinary ball valve in factory application, but it can significantly reduce the size of both valve body and actuator. These valves are light and compact, and are very suitable for the operation of experimental devices

when the size is reduced, the CV value of the trim of the small flow valve has little significance. Fortunately, a large effective flow range is obtained in the valve seat hole. In addition to the adjustable valve stroke, the valve trim is easy to replace. Moreover, if many of the same valves are used, the cost of repairing several sets of valve trim is not high, because the same valve body is interchangeable

calculate the required CV value with conventional formula, and then select the valve trim close to the maximum size for the specific valve used. If these valve internals are too small in actual operation, the flow capacity can be improved by simply increasing the valve stroke or slightly modifying the valve core

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