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Foe selecting control valve if i have the min,normal & max
data of flow rate,inlet pressure& outlet pressure,inlet
tem.&density than how can i assure that these data are
correct,pl. let me know if therew there is any method to
check it.& also how we assure that the Cv we get from this
data is suitable for valve for controlling require flow.
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Answer Posted / sam
Calculate the cv of a LIQUID ONLY control valve for a given
flow, dp and sg
Cv = 11.7 q (SG / dp)1/2
where
q = Expected flow rate (m3/h)
Sg = Specific gravity of the liquid (1 for water in this
example)
dp = Pressure drop across the valve at expected flow rate (kPa)
Example 1.
So if q = 5m3/h
Sg for water = 1
Dp over valve during 5m3/h flow = 10kpa
Do manual calculation:
Cv = 11.7 q (SG / dp)1/2
1) 11,7 x 5 = 58,5
2) 1/10 = 0,1
3) ½ = 1/2 = 0.5
4) 0,1 to the power of 0,5 = 0.31622776
5) So 0.31622776 x 58,5 = 18,49
So a valve with a cv of 18,5 is needed to do a flow of
water(sg=1) at 5m3/h and a dp over the valve of 10kpa
Look at manufacturer valve manual for available valves and
select a valve with a next bigger Cv.
----------------------------------------
Example 2.
So if q = 30m3/h
Sg for liquid = 0,97
Dp over valve during 30m3/h flow = 110kpa
Do manual calculation:
Cv = 11.7 q (SG / dp)1/2
1) 11,7 x 30 = 351
2) 0,97/110 = 0,008818
3) ½ = 1/2 = 0.5
4) 0,008818 to the power of 0,5 = 0.0939042
5) So 0.0939042 x 351 = 32,96
So a valve with a cv of 32,96 is needed to do a flow of
liquid (sg=0,97) at 30m3/h and a dp over the valve of 110kpa
Look at manufacturer valve manual for available valves and
select a valve with a next bigger Cv
---------------------------------------
Example 3.
So if q will be 64m3/h
Sg for liquid is 0,87
Dp over valve during 64m3/h flow is 210kpa
Do manual calculation:
Cv = 11.7 q (SG / dp)1/2
1) 11,7 x 64 = 748,8
2) 0,87/210 = 0,00414285
3) ½ = 1/2 = 0.5
4) 0,00414285 to the power of 0,5 = 0.06436497
5) So 0.06436497 x 748,8 = 48,196
So a valve with a cv of 48,196 is needed to do a flow of
liquid (sg=0,87) at 64m3/h and a dp over the valve of 210kpa
Look at manufacturer valve manual for available valves and
select a valve with a next bigger Cv
------------------------------------------
To do the power off calculation on your windows calculator:
Example:
5 to the power of 4 ------- press 5 then x^y then 4 then =
--------------------------------------------
Control Valve selection Flow Coefficient - Cv - for Air and
other Gases
For critical pressure drop the outlet pressure - po - from
the control valve is less than 53% of the inlet pressure -
pi. The flow coefficient can be expressed as:
Cv = q [SG (T + 460)]1/2/ 660 pi
where
q = free gas per hour, standard cubic feet per hour (Cu.ft/h)
SG = Specific gravity of the gas relative to air at 14.7
psia and 60oF
T = flowing air or gas temperature (oF)
pi = inlet gas absolute pressure (psia)
Example:
5000 = Gas flow (Cu. ft./h) (f.a.d)
100 = Inlet Gas Absolute Pressure (psia)
1 = Specific gravity of the
60 = Gas Temperature (oF
Do manual calculation:
Cv = q [SG (T + 460)]1/2/ 660 pi
1) 60 + 460 = 520
2) 520 x 1 = 520
3) ½ = 1/2 = 0.5
4) 520 to the power of 0,5 = 22,8035
5) 22,8035 x 5000 = 114017,5425
6) 660 x 100 = 66000
7) 114017,5425 / 66000 = 1,727
So a valve with a cv of 1,73 is needed to do a air flow
(sg=1) at 5000Cu.f/h, at a air flow temp of 60F and a inlet
gas absolute press of 100PSIA
Look at manufacturer valve manual for available valves and
select a valve with a next bigger Cv
-----------------------------------------------
For non critical pressure drop the outlet pressure - po -
from the control valve is greater than 53% of the inlet
pressure - pi. The flow coefficient can be expressed as:
Cv = q [SG (T + 460)]1/2/ [1360 (dp po)1/2]
where
dp = (pi - po)
po = outlet gas absolute pressure (psia)
Example:
6000 = Gas flow (Cu. ft./h) (f.a.d)
140 = Inlet Gas Pressure (psia
110 = Outlet Gas Pressure (psia
1 = Specific gravity of the gas
60 = Gas Temperature (oF)
Do manual calculation:
Cv = q [SG (T + 460)]1/2/ [1360 (dp po)1/2]
1) 60 + 460 = 520
2) 520 x 1 = 520
3) ½ = 1/2 = 0.5
4) 520 to the power of 0,5 = 22,8035
5) 22,8035 x 6000 = 136821,051
6) 140 – 110 = 30
7) 30 x 110 = 3300
8) ½ = 1/2 = 0.5
9) 3300 to the power of 0,5 = 57,445
10) 57,445 x 1360 = 78126,05199
11) 136821,051 / 78126,05199 = 1,75
So a valve with a cv of 1,75 is needed to do a air flow
(sg=1) at 6000Cu.f/h, at a air flow temp of 60F and a DP of
30PSIA absolute press
Look at manufacturer valve manual for available valves and
select a valve with a next bigger Cv
--------------------------------------------------
To do the power off calculation on your windows calculator:
Example:
5 to the power of 4 ------- press 5 then x^y then 4 then =
--------------------------------------------------
Please note the "1/2" in the formulas is not as it seems, it
is surpose to be a small "1/2" on the top right hand corner
of the bracket, in other words a "to the power of 1/2" and
not " bracket times 1/2 as it is displayed"
Cannot seem to get it displayed like that on this page -
sorry if it is confusing but look at he calculation to see
where the "power of" is used and you will understand what I
mean.
Good Luck
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