Diaphragm valve flow characteristic selection
Pneumatic Diaphragm Valve flow characteristic refers to the relationship between the relative flow of the medium flows through the valve and displacement (valve relative opening degree). the ideal flow characteristics are mainly straight lines, equal percentage (logarithm), parabola, and quick open, characteristic curves and spool shape. The ideal flow characteristics that commonly used are only straight line, equal percentage (logarithm), quick open three kinds. Parabolic flow characteristic is between the equal percentage and straight lines, generally can take the equal percentage features to replace, while the quick opening feature is mainly used for two-conditioning and process control, and therefore the choice of pneumatic diaphragm valves feature is actually straight lines and equal percentage flow characteristics selection.
Rubber lined diaphragm valve, fluorine lined diaphragm valve flow characteristic selection can be calculated theoretically, but the methods and equations used are very complex. At present, mainly using experience criteria, specifically from the following aspects to consider: ①analysis and selection from the conditioning system to regulate quality; ② considered from the process piping; ③analysis from load change situation.
Choose right diaphragm valves flow characteristics, we can according to its flow characteristics to identify core shape and structure, but to the valves like the diaphragm valves, butterfly valves, etc., because of their structural characteristics, it is impossible to change the spool surface shape to achieve the required flow characteristics, so we can achieve it through the changing of the equipped valve positioner feedback cam shape.
BS diaphragm valves diameter option and identification are mainly based on the valve flow capacity Cv. When in a variety of engineering instrument design and selection, we need to calculate Cv of diaphragm valves, and provide diaphragm valve design specifications. From diaphragm valve Cv calculation to the valve diameter determination, generally need the following steps:
1)The determination of calculating flow. The existing production capacity, equipment load and medium situation decide the Qmax and Qmin of calculation flow.
2) The front and after valve pressure differential determination. According to the selected valve flow characteristics and system characteristics to select S (drag coefficient), and then determining the calculation of pressure differential.
3) The calculation of Cv. According to the regulative media to choose the right formula and charts, obtained Cmax and Cmin.
4) The selection of Cv. According to Cmax, in the standard series of products chosen, select> Cmax and the closest level C.
5) Diaphragm Valve opening degree checking. Generally require maximum calculated flow opening degree ≯90%, the minimum calculated flow opening degree ≮ 10%.
6) Diaphragm Valve actual adjustable ratio checking. Generally require the actual adjustable ratio ≮10.
7) The seat diameter and the nominal diameter determination. Verified suitable, according to C to determine.
Diaphragm Valves selection is a very painstaking work, not only with a solid professional theoretical knowledge, but also need rich practical experience. Slecting well not only helps to balance control loop PID parameter tuning, making the transferred parameters to get better control effect, and also make the service life of diaphragm valves increased greatly. Diaphragm valves selection should according to local conditions, not set in stone, we must constantly sum up and innovate in the practice process, in particular with the mechanical and electrical integration technologies, computers and digital information technology applications, diaphragm valve structural function has become better, more comprehensive, providing a great convenience for the diaphragm valveselection.
