Characteristics of control valves

http://bbs.hcbbs.com/viewthread.php?tid=119748&page=1#pid484981 Learn to search on the forum: Linear flow characteristics – A valve with ideal linear inherent flow characteristics will produce a flow rate that is directly proportional to the amount of travel of the valve element within its range of motion. For example, at 50% of the rated stroke, the flow rate is 50% of the maximum flow rate ； At 80% of the rated stroke, the flow rate is 80% of the maximum flow rate ； Wait. As the valve spool stroke changes, the flow rate remains constant. Valves with linear characteristics are often specified for level control and fluid control applications that require a constant gain. Equal percentage flow characteristic – ideally, for equal increments in the valve stem displacement, the change in flow rate as a function of this displacement can be expressed as a constant percentage of the flow rate at that point. The observed flow variation with stroke is relatively small when the valve spool is close to the valve seat, while it is relatively large when the valve spool is almost fully open. Therefore, valves with inherent equal percentage flow characteristics provide precise proportional control at the lower end of their stroke range, while the flow capacity increases rapidly as the valve element approaches the fully open position. Valves with equal percentage flow characteristics can be used in pressure control applications, in situations where most of the pressure drop is normally absorbed by the system itself with only a relatively small portion remaining for the control valve, and in cases where significant variations in pressure drop are expected. In most practical systems, the inlet pressure decreases as the flow rate increases, and a constant percentage characteristic is appropriate. For this reason, equal percentage becomes the most common valve characteristic. Valves with a quick-opening flow characteristic provide the greatest flow rate change at the low stroke position. The flow curve remains essentially linear within the first 40% of the valve stem travel, and then flattens out significantly, indicating that the increase in flow as the travel approaches the fully open position is minimal. Control valves with quick-opening flow characteristics are commonly used in on/off applications. During start-up/shutdown, as the valve begins to open, a large flow rate must be established quickly. As a result, they are often used in applications involving release valves. The quick-opening valve can also be used in many of the same applications where a linear flow characteristic is recommended, as its quick-opening behavior remains linear up to 70% of the maximum flow rate. When the flow area generated by the valve spool’s stroke becomes equal to the flow area of the valve seat, the linearity decreases sharply. For a typical quick-opening control valve, this phenomenon occurs at a position where the stem travel equals one-quarter of the seat diameter. This post was last edited by XJTZWKDPJ on 2007-12-14 15:25.]