Overview of Primary Product
Measuring the ow rate
Connect a differential pressure gauge to the pressure test ports of the venturi of the valve. Read the Δp value on the gauge,
Knowing this differential
pressure value and the flow coefficient of the Venturi orifice (which remains constant as valve knob is adjusted), the flow rate can be determined by
either referencing the flow characteristic chart in the figure below or by using the formula:
G= C
v
×
where G is flow rate in gpm, C
v
is the flow coefficient of the Venturi orifice, ∆p is the pressure differential in psi, and SG is the specific gravity of the
fluid. For example, let’s assume water is the fluid medium and the balancing valve used is item 130500A (3/4”). We see from the Venturi chart the C
v
value is 6.4. The specific gravity of water is 1.0. If the measured pressure differential is 0.3 psi, we can determine the flow rate G by substituting the
values into the above formula: G=6.4√(0.3/1.0) = 3.5 gpm. Alternatively, using the Venturi chart below, if we plot where the 0.3 psi value intersects
the ¾” graph, and drop a line vertically to where it intersects the gpm axis, we can estimate the value to be 3.5 gpm.
Setting the ow rate manually
To manually adjust the flow rate through the valve, reposition the adjustment knob until the differential pressure indicated by the measuring device
corresponds to the desired flow rate value on the Venturi chart for the valve being used. Or, calculate the pressure drop of the venturi using the
following:
G
2
Δp Venturi =
Cv Venturi
2
Next, turn the adjustment knob until the theoretical
Δp value calculated using the formula
above is reached.
Correcting for uids with a different density
This applies to fluids with viscosities of water and glycol mixtures or less. If using fluids with a specific gravity different from water at 70˚ F (20˚ C)
(SG=1.0), correct the value of the pressure drop
Δp measured using the following formula:
Δp' = Δp x (SG')
where: Δp' = reference pressure drop in psid (kPa) and SG' = non-water specific gravity.
Example: 50% propylene glycol has a specific gravity of 1.0366 at 70˚ F (20˚ C). To measure a flow rate of 3.5 gpm across the fixed venturi for a
fluid with SG=1.0366, the pressure to read on the differential pressure metering device is calculated as Δp' = 0.3 x (1.0366) = 0.3108 psid.
Δp
SG
Venturi chart (water)
.03
.06
∆
p
G
(l/s) (
gpm
)
0.1
1
0.2
0.3
0.5
(psi) (bar)
10
2
3
5
20
0.01
0.1
0.02
0.03
0.05
1.0
0.2
0.3
0.5
0.4
4
0.04
0.4
.30
.50
1.4
2.8
5.6
1
0.2
0.5
2
5
10
2
0
5
0
100
2
00
.015
46.00
20.00
10.00
9.00
7.00
5.00
2.00
1.50
1.00
0.70
0.50
0.20
(feet of head)
0.05
3.00
0.30
0.11
0.003
0.005
7.0
12.5
.02
0.3
.10
.20
.70
3.5 gpm
1/2 “ 3/4“ 1“ 1 1/4“ 1 1/2“ 2“
Valve size
Size Cv
Venturi
1/2" 3.25
3/4" 6.40
1" 11.20
1 1/4" 17.65
1 1/2" 23.80
2" 32.78
USING AND SETTING THE BALANCING VALVE