阀座端口流速计算
calculating seat port velocity

　　下列公式可以用来计算任何阀门的阀座流速：
　　the following formulas can be used to calculate seat port
velocity in any valve:

流量(磅/小时): 流速: (英尺/秒)=	f()2 25a
flow (lbs/hr): vel: (fps) =	f()2 25a

f	=	流量(磅/小时)
f	=	flow (lbs/hr)
	=	比容积的平方根
	=	square root of specific volume
a	=	阀座端口面积 (英寸2)
a	=	seat port area (in2)
q	=	流量 (加仑/分钟)
q	=	flow (gpm)

例：
　　dn300型号58809 (压力等级class 900)止回阀，流体介质为温度500 ^of，压力200 psig的蒸汽，流速为90,000磅/小时，求阀座端口的流速？
example:
　　what is the seat port velocity in a 12" figure 58809 (class 900) check valve with a flow rate of 90,000 lbs/hr of steam at 500 ^of and 200 psig?

解：
　　从蒸汽表得为1.61，从阀座端口面积表a得阀座端口面积为78.5英寸²。
answer:
　　from the steam tables is 1.61, and from the seat port area table a is 78.5 in².

流速: (英尺/秒)=	f()2 25a	=	90,000 (1.61)2 25 (78.5)	= 118.8 英尺/秒
vel (fps) =	f()2 25a	=	90,000 (1.61)2 25 (78.5)	= 118.8 fps

例：
　　dn300型号180 (压力等级150)旋启式止回阀，流体介质为温度500^of，压力200 psig的蒸汽，流速为90,000磅/小时，求阀座端口流速？
example:
　　what is the seat port velocity in a 12" figure 180 (class 150) swing check valve with a flow rate of 90,000 lbs/hr of steam at 500^of and 200 psig?

解：
　　查表得为1.61，a为 113.1英寸³.
answer:
　　from the tables is 1.61 and a is 113.1 in³.

流速: (英尺/秒)=	f()2 25a	=	90,000 (1.61)2 25 (113.1)	= 82.5英尺/秒
vel (fps) =	f()2 25a	=	90,000 (1.61)2 25 (113.1)	= 82.5 fps

局限性
limitations

　　流速公式在下列条件下无效：
　　velocity formulas are not valid under the following conditions:

1. 对于可压缩性流体，如果压降超过进口压力的一半。
   for compressible fluids, where pressure drop exceeds half the inlet pressure.
2. 对于不可压缩性流体，如果压降产生空穴或闪蒸。
   for non-compressible fluids, where pressure drop causes cavitation or flashing.
3. 对于双相流体，如蒸汽-水混合流体。
   for dual-phase flow such as steam-water mixtures.

注意
note

　　在确定止回阀通径时，最适宜的工作流速应该位于这些范围内：
　　when determining sizing for check valves, the optimum operating velocities should fall within these ranges:

bb旋启式止回阀：55 至115或135至240
bb截止止回阀：40 至120
ps提升式阀瓣止回阀：55至200
psy形止回阀：148 至200
bb swing check valves - 55 to 115 or 135 to 240
bb stop check valves - 40 to 120
ps tilt disc check valves - 55 to 200
ps stop check valves - 40 to 180
ps y- stop check valves - 148 to 200

球形止回阀，截止止回阀，提升式止回阀的流量特性曲线
characteristic flow curves for globe stop, stop-check, lift-check valves

小于全开状态的估计流量系数cv
estimating cv for less than fully open valves

图2
球形止回阀，截止止回阀和提升式止回阀
figure 2
globe stop, stop-check and
lift-check valves

　　表中所列流量系数仅适用于全开状态下的阀门。不是全开状态的流量系数可以采用适用于提升式阀瓣止回阀和截止阀的流量特性曲线进行估算。不建议将闸阀用在节流应用中。
　　the flow coefficient (cv) listed in the tables apply to fully open valves only. flow coefficients at other than fully open may be approximated by use of the characteristic flow curves for tilting disc check and globe valves. gate valves are not recommended for throttling service.

例：
　　dn300，压力等级class 900的美国太平洋压力密封截止阀在开气度为1/4时的流量系数是多少？
example:
　

图3
y形球形止回阀，截止止回阀和提升式止回阀
figure 3
y-globe stop, stop-check and
lift-check valves

解：
　　从上列流量系数表中得到，dn300，压力等级class 900截止阀在全开状态下的cv是1820。
solution:
　　from the flow coefficient table above, the fully open cv for a 12 inch, class 900 globe valve is 1820.

　　查询图2，全开状态的1/4时阀门的比率是52%.
　　consulting figure 2, the percent of fully open cv at 1/4 valve open is 52%.

　　因此计算1/4全开的流量系数cv为：
　　0.52 x 1820 = 946.
　　the cv at 1/4 open is then computed as:
　　0.52 x 1820 = 946.