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1、 VSSTR-9/05 VALVE SIZING a further decrease of the outlet pressure does not increase flow. Choked flow is also called critical flow. Cavitation: Cavitation is a two stage phenomena with liquid flow. The first stage is the formation of vapor bubbles in the liquid as the fluid passes through the trim
2、and the pressure is reduced below the fluids vapor pressure. The second stage is the collapse of the vapor bubbles as the fluid passes the vena contracta and the pressure recovers and increases above the vapor pressure. The collapsing bubbles are very destructive when they contact metal parts and th
3、e bubble collapse may produce high noise levels. Flashing: Flashing is similar to cavitation except the vapor bubbles do not collapse, as the downstream pressure remains less than the vapor pressure. The flow will remain a mixture of vapor and liquid. Laminar Flow: Most fluid flow is turbulent. Howe
4、ver, when the liquid flow velocity is very slow or the fluid is very viscous or both, the flow may become laminar. When the flow becomes laminar, the required CV is larger than for turbulent flow with similar conditions. The ISA sizing formulas adjust the CV when laminar flow exists. THE SIZING PROC
5、ESS The first sizing step is to determine the required CV value for the application. Next determine if there are unusual conditions that may affect valve selection such as cavitation, flashing, high flow velocities or high flow noise. The valve sizing process will determine the proper valve size, va
6、lve trim size , valve trim style and actuator size. Warrens Valve Sizing Program will accurately calculate the CV, flow velocity and flow noise. The program will also show messages when unusual conditions occur such as cavitation, flashing, high velocity or high noise. The results from Warrens Valve
7、 Sizing Program are only one element of the valve selection process. Knowledge and judgment are also required. This overview will give the user some of the sizing basics. The liquid, gas and steam CV calculation methods, in this manual, are in accordance with ISA S75.01 and the gas and steam flow no
8、ise calculations are in accordance with ISA S75.17. These two ISA Standards are in agreement with IEC-534. These standards have worldwide acceptance as the state of the art in CV and Flow Noise determination. Operating Conditions The most important part of Valve Sizing is obtaining the correct flowi
9、ng conditions. If they are incorrect or incomplete, the sizing process will be faulty. There are two common problems. First is having a very conservative condition that overstates the CV and provide a valve less than open at maximum required flow. The second is stating only the maximum flow conditio
10、n that has minimum pressure drops and not stating the minimum flow conditions with high-pressure drops that often induce cavitation or have very high rangeability requirements. 5Fluid Properties Table 2 lists many fluid properties needed for valve sizing. These fluid properties are in Warrens Valve
11、Sizing Programs database and do not need manual entry. Rangeability: Rangeability is the ratio of maximum to minimum controllable CV. This is also sometimes called CV Ratio or Turndown. The maximum flow for Warren Controls valves is at maximum travel. The minimum controllable CV is where the Flow Ch
12、aracteristic (CV vs. Travel) initially deviates or where the valve trim cannot maintain a consistent flow rate. This is partially a function actuator stiffness as well as valve “stiction”. The Trims rangeability is not always the useable range as seat erosion may be a governing factor with respect t
13、o erosive fluids and high drops in the near-closed position. A valve with a significant pressure drop should not be used to throttle near the seat for extended periods of time. The rangeability values, listed in Table 1, apply to the rated CV, not the required CV. For example, an application may req
14、uire a maximum CV of 170. A 4” Equal Percentage Trim may be selected that has a maximum CV of 195. Using the rangeability value for this trim, the minimum CV is 195/100=19.5, not 170/100=17. Valve applications subject to pressures from nature, such as gas and oil production, are usually sized for fu
15、ll flow at about 80% open as the pressure may be unknown when the valve is sized and the pressure may vary with time. Those valve applications with fairly consistent inlet pressures, such as process control and power applications are usually sized at full travel. The valve specifier usually includes
16、 a fair margin of safety in the stated sizing conditions. If the valve supplier includes additional safety, such as full flow at 80% open, the valve may be at full flow at less than travel giving poor performance. TRIM RANGEABILITY Table 1 Valve Trim Rangeability Globe Valves. Equal Percent All Equal %, Full Port Trim styles 50:1 Linear Flow and Reduced Port Trim styles 30:1 Rotary Valves Eccen
