如何使用流量系数
How to use Cv
法门流量系数(Cv)是表示阀门通过流体能力的数值。Cv越大，在给定压降下阀门能够通过的流体就越多。Cv值1表示当通过压降为1 PSI时，阀门每分钟流过1加仑15oC的水。Cv值350表示当通过压降为1
PSI时，阀门每分钟流过350加仑15oC的水。
Valve coefficient (Cv) is a number which represents
a valve's ability to pass flow. The bigger the Cv, the more flow a valve
can pass with a given pressure drop. A Cv of 1 means a valve will pass
1 gallon per minute (gpm) of 60oF water with a pressure drop
(dp) of 1 PSI across the valve. A Cv of 350 means a valve will pass 350
gpm of 60oF water with a dp of 1 PSI.
公式1
FORMULA 1
流速：磅/小时(蒸汽或水)
FLOW RATE LBS/HR (Steam or Water)
| dp |
= |
压降，单位：PSI |
| dp |
= |
pressure drop in PSI |
| F |
= |
流速，单位：磅/小时 |
| F |
= |
flow rate in lbs./hr. |
 |
= |
比容积的平方根，单位：立方英尺/磅
(阀门下游) |
|
= |
square root of a specific volume in ft3/lb.
(downstream of valve) |
公式2
FORMULA 2
流速：加伦/分钟(水或其它液体)
FLOW RATE GPM (Water or other liquids)
| dp |
= |
压降，单位：PSI |
| dp |
= |
pressure drop in PSI |
| Sg |
= |
比重 |
| Sg |
= |
specific gravity |
| Q |
= |
流速，单位：加伦/分钟 |
| Q |
= |
flow rate in GPM |
局限性
LIMITATIONS
上列公式在下列条件下无效：
Above formulas are not valid under the following
conditions:
- 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.
- 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.
- 对于双相流体，如蒸汽-水混合流体。
For dual-phase flow such as steam-water mixtures.
例子
EXAMPLE
- 通过DN350, 压力等级Class 900的压力密封闸阀，水温400oF，压力为1500 PSIG，流速为5,000,000磅/小时的压降是多少？
What is the pressure drop through a 14" Class
900 pressure seal gate valve with 400oF water, 1500 PSIG,
and 5,000,000 lbs/hr flow rate?
解：采用公式1
Solution: Use formula 1.
| F |
= |
5,000,000 |
|
= |
0.137 (从水温为400oF的表中得到) |
|
= |
0.137 (from chart for water at 400oF) |
| Cv |
= |
9150 (来自本页) |
| Cv |
= |
9150 (from page 26) |
|
|
- DN350 x DN300 x DN350，压力等级Class 900缩喉管压力密封闸阀，其它条件与例1相同，求压降。
What is the pressure drop through a 14"x12"x14"
Class 900 Venturi pressure seal gate valve with the same conditions
as example 1.
解：采用公式1
Solution: Use formula 1.
Cv = 6285 (来自本页)
Cv = 6285 (from page 26)
- 温度900oF, 压力1200 PSI，流速500,000磅/小时的蒸汽应用中压降小于5 PSI的压力等级Class
2500闸阀的最小通径是多少？
What is the smallest Class 2500 gate valve that
will have less than a 5 PSI pressure drop in 900oF, 1200
PSI steam service at a flow rate of 500,000 lbs/hr?
解：采用公式1
Solution: Use formula 1.
| F |
= |
500,000 |
|
= |
0.785 (来自900oF, 1200 PSIG蒸汽表) |
|
= |
0.785 (from chart for steam at 900oF,
1200 PSIG) |
 |
= |
 = 2.24 |
| |
 |
最接近的、较大的Cv是DN250压力等级Class 2500闸阀，其Cv是3130。这里是压降小于5 PSIG的最小的阀门。
The closest higher Cv is a 10" Class 2500
gate with a Cv of 3130. This is the smallest valve with a pressure drop
less than 5 PSIG.
水和蒸汽的备选特性
Selected Properties of Water and Steam |
水的(下游)
For
Water (Downstream) |
所有压力等级
All
Pressures |
温度(oF)
Temperature (oF) |
| 100 |
200 |
300 |
400 |
500 |
600 |
| 0.127 |
0.129 |
0.132 |
0.137 |
0.143 |
0.154 |
蒸汽的(下游)
For
Steam (Downstream) |
压力
PSIG
Pressure
PSIG |
温度(oF)
Temperature (oF) |
| Sat |
500 |
600 |
700 |
800 |
900 |
1000 |
1100 |
100
200
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
1600
1800
2000
2500
3000
4000
5000
6000 |
1.97
1.46
1.21
1.06
0.950
0.865
0.801
0.747
0.701
0.662
0.628
0.598
0.570
0.545 |
2.20
1.61
1.30
1.11
0.979
0.895 |
2.32
1.68
1.38
1.19
1.06
0.960
0.880
0.815
0.759
0.710
0.670
0.629
0.590
0.559 |
2.44
1.77
1.46
1.26
1.12
1.02
0.941
0.875
0.821
0.773
0.732
0.695
0.662
0.633
0.58
0.54
0.50
0.41
0.31
0.17
0.16
0.16 |
2.54
1.85
1.52
1.32
1.18
1.08
0.994
0.926
0.870
0.816
0.781
0.745
0.710
0.683
0.63
0.59
0.55
0.48
0.42
0.32
0.24
0.19 |
2.64
1.95
1.60
1.38
1.23
1.13
1.04
0.972
0.914
0.865
0.823
0.785
0.752
0.723
0.67
0.63
0.59
0.52
0.46
0.38
0.32
0.28 |
2.74
2.00
1.65
1.43
1.28
1.17
1.08
1.01
0.955
0.917
0.872
0.813
0.789
0.752
0.70
0.66
0.62
0.55
0.49
0.42
0.36
0.32 |
2.84
2.07
1.71
1.47
1.33
1.22
1.12
1.05
0.99
0.94
0.90
0.86
0.83
0.79
0.74
0.69
0.65
0.58
0.52
0.44
0.39
0.35 |
|
