选择恰当的通径 -
确定压降
Selecting the Proper Size –
Determining Pressure Drop

　　由于截止止回阀具有一个浮动阀瓣构件，因此重要的是要确定在平常的流体环境下及阀门的使用寿命期限内，能够为阀瓣提供足够提升力的通径。如果阀门太大，则阀瓣浮动只能达到部分开启位置，并且可能导致阀瓣的振动和迅速磨损。相反，如果阀门太小，则压降就会太大。
　　Since stop-check valves have a floating disc member, it is important the valve be sized to provide full disc lift under flow conditions prevailing during the major portion of the service life. If the valve is too large, the disc will float in a partially open position and may cause fluttering of the disc and rapid wear. Conversely, if the valve is too small, pressure drop will be excessive.
　　下一页的图表表示经过测试后获得的流量参数。它提供一个简单的方法来确定最适合的截止止回阀通径，以及不同流体状况下的流体压降，不需要任何的计算。
　　The chart on the opposite page is a graphic representation of flow data determined by test. Its use offers a simple method of determining the best size of stop-check valve, as well as the pressure drop under varying conditions of flow, without any computation.

如何使用下一页显示的图表
How to Use the Chart Shown on the Opposite Page

　　已知：蒸汽压力-温度...300 psig 750^oF
　　流量...典型的日需求量曲线
　　Given: Steam pressure-Temperature...300 psig 750^oF
　　Flow Rate...Typical Daily Demand Curve

典型的日负荷-需求曲线
Typical Daily Load-Demand Curve

时间
TIME OF DAY

　　找出：阀门样本编号和上述安装下的最恰当的通径。
　　Find: Valve Catalog No. and the best size for above installation.
　　解：参阅第35页所需要的压力等级Class 300阀门的压力-温度额定值。因此，可能需要采用下列阀门：
　　截止阀，型号28XU，法兰；或型号28¹/₂XU，对焊
　　角阀，型号30XU，法兰；或型号30¹/₂XU，对焊
　　Solution: Reference to the pressure-temperature ratings on page 35 indicates a Class 300 valve will be required. Therefore, the following valves may be used:
　　Globe...No. 28 XU, Flanged or No. 281/2 XU, Butt-Welding
　　Angle...No. 30 XU, Flanged or No. 301/2 XU, Butt-Welding

1. 进入750^oF处的温度表。垂直向上移动到300 psi曲线位置，然后水平向右移动到指定的流量点。从这个点上，划出一条直线，通过事先调查过的流量(100,100 Lb/H)，在索引1上建立一个点。
   Enter the Temperature chart at 750^oF. Move vertically upward to the curved line for 300 psi, then horizontally to the right to establish a point on the specific volume scale. From this point, draw a line through the flow rate being investigated (100,000 Lb/ H) and establish a point on Index 1.
2. 从这个点，划出另一条直线通过阀门通径，例如8英寸(DN200)通径，然后在索引2上建立一个点。现在水平移动到右边压降对角线上。在这些线相交的点上，8英寸(DN200)、压力等级Class 300截止阀的压降是9 psi；8英寸(DN200)、压力等级Class 300角阀压降是10 psi。由索引2上的点到处的图表解位于压力等级Class 300阀门直线A-A的下方，或在压力等级Class 600阀门直线B-B的下方，这表示在所应用的流体状况下，阀瓣并无法完全提升。不推荐这种状况下的操作，但是，有时在低负载的状况下，可以容忍短期运行。
   From that point, draw another line through the valve size, for example the 8-inch size, and establish a point on Index 2. Now move horizontally to the diagonal pressure drop line on the right side. Where these lines intersect, the pressure drop is 9 psi for the 8-inch, Class 300 globe valve and 10 psi for the 8-inch Class 300 angle valve. Chart solutions resulting in a point on Index 2 that falls below the Line A-A for Class 300 valves or below Line B-B for class 600 valves indicate the disc will not be fully lifted under the flow conditions used. Operation under such conditions is not recommended but, at times, must be tolerated for short periods during the low loads.
3. 找出图表中直线A-A与压力等级Class 300阀门索引2相交点，或压力等级Class 600阀门的直线B-B下方。沿着对角线向上移动，通过所要求解的(8英寸)，在索引1上建立第二个点。在这个点上，将线段延长到第1步建立的指定流量上，在与流量曲线相交点，查出42,000 Lb/H，这就是能够使阀瓣完全提升所需的最小流量。在这个流量状况下，截止阀的压降是1.9 psi，角阀的压降是2.1 psi。
   Enter the chart where Line A-A intersects Index 2 for Class 300 valves or below the Line B-B for Class 600 valves. Move diagonally upward through the size being investigated (8-inch) and establish a second point on Index 1. From this point, extend a line to the specific volume established in Step 1 and at its intersection with the flow rate line, read 42,000 Lb/H as the minimum flow rate at which the disc will be in the fully lifted position. The pressure drop at this flow rate is 1.9 psi for globe and 2.1 psi for angle valves.
4. 对于其它可能的阀门通径，重复第2步和第3步，将结果制成表格，然后根据压降和阀瓣部分提升持续时间等因素，来确定阀门通径。
   Repeat Steps 2 and 3 for other possible valve sizes, tabulate results, and make size selection on basis of pressure drop and duration of partial disc lift considerations.

　　需求曲线上的虚线表示DN200和DN250阀门敞开状态下的最小流量。
　　Dotted lines on Demand Curve indicate minimum flow rates for wide open 8" and 10" valves.

5. 这个例子最好选择是DN250通径，因为压降太低，而部分提升阀瓣的时间比DN200通径只是稍微长一些。
   The best choice for this example would be the 10" size because pressure drop is much lower and duration of partially lifted disc is only slightly greater than for the 8" size.
6. 任何中间流体条件下的压降都可以按照第2步和第2步进行确定。
   Pressure drop for any intermediate flow condition can be determined as outlined in Steps 1 and 2.