FLOW MEASUREMENT

Flow Sensor

Measurement of fluid flow rate is one of the most important in the process control flow. This measurement aims to find out how much capacity the flow of fluid to get the price is measured (measurement variables). The flow is generally measured by the amount of fluid velocity through a particular cross-sectional area, or
Qv = A x V. … … … … … … … … … … … … … … … … … … … … … … (2.1)
where: qv: the flow rate (m3 / s)
A: cross-sectional area of pipe (m2)
V: fluid velocity (m / s)
Four important factors in the measurement of fluid flow in pipes is:
• The speed of fluid
• Friction / fluid friction with the pipe
• The viscosity / viscosity of the fluid
• density / density of the fluid


Many of the tools that can be used to measure fluid flow. Flowmeter can be divided into 4 general groups, namely positive displacement meters, head meters, velocity meters, and mass meters. Which includes the displacement meters include: piston, oval gear, and nutatic disk. Head meters are frequently used type. Measurement of fluid flow indirectly, by measuring the differential pressure. Which includes the head meters including orifice plates, venturi tubes, flow nozzles, PITOT tubes, target meters, elbow tap meters and rotatometer. The advantage in using the side plate oriface cheap, can be used in a variety of materials and can be used in a wide range of pipe size, good accuracy when the plate is installed correctly. While the weakness of the sensor oriface permanent pressure loss is relatively high and the accuracy depends on the installation. If we use the flow permanent benefits nozzle pressure loss lower than oriface plate shown in Figure 2.1 and Figure 2.2, good for fluid containing solids, can be used on many materials. While the weakness of the use of nozzle flow, among others are more expensive than oriface plate and limited to a specific pipe size, flow nozzle can be seen in Figure 2.3.
Which includes the velocity meters include turbine meters, electromagnetic flow meters, vortex meters and meters ultrasonc. While included in the mass meters include: thermal meters and Coriolis meters.
Oriface plate made of thin stainless steel plate, at its center hole with the size of the amount calculated, and then installed in the flow pipe to provide the pressure differential. Orifice can be used to clean all the fluid and gas, but not generally used for containing solid fuida / dirt. The advantage of this tool is easy to replace, it’s cheap, easy to maintain but this tool has a pressure loss is high. There are three kinds of species, as shown in Figure 2.4.
There are several places to take the pressure difference on orifice system, among others:
• Flange Tap
Location shooting at the pressure Flange, 1 inch 1 inch upstream and downstream, measured from the upstream surface of orifice.
• Corner Tap
Used in pipes smaller than 2 inches. Hole-making pressure on the flanges close to the surface of orifice.
• Full flow pipe Tap
Hole taking away the pressure on the upstream surface of 2.5 D from the upstream and downstream orifice distance D from the orifice 8.
• Radius Tap
Retrieval is the pressure on the upstream and downstream 1 D 0.5 D of the upstream surface of orifice.
• Taps Vena contracta
Upstream is 0.5 to 2 D and downstream depending on the d / D as shown in Table 2.1 below.

Table 2.1 Location of the hole downstream [5]
Orifice to Pipe
Diameter Ratio,
d / D Location of downstream
Pressure Tap (N)
(Pipe-Diameter)
Minimum Maximum Mean
0.2 0.37 0.85 1.30
0.3 0.44 0.80 1.15
0.4 0.47 0.73 1.00
0.5 0.47 0.65 0.84
0.6 0.42 0.57 0.70
0.7 0.35 0.45 0.55
0.8 0.25 0.33 0.41

Flow Coefficient (CV)
Flow Coefficient (CV) is a coefficient which expresses the flow capacity. To find out the price of the control valve CV where the flow velocity in the valve from 60 0 F (15.6 0 F) of pure water as much as 1 U.S. gal / min with a 1 psi pressure differential. To obtain the CV calculation of several media including:
a. CV calculations for liquid
Cv = Q …. … … … … … … … … … … … … … … (2.4)
b. Calculation of CV for the gas:
CV = … … … … … … … … … … … … … … … (2.5)
c. Calculation of CV for Stem:
CV = … … … … … … … … … … … … … … … .. (2.6)
where:
Q or W = flow rate (m3 / h)
G = specific gravity
P1 and P2 = inlet and outlet pressure (psi)
Tf = flowing temperature (0 F + 460)
v = downstream specific volume in cubic ft / lb

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