F LUID F LOW T RANSDUCERS
F LUID F LOW T RANSDUCERS
To measure fluid flow, you must measure one of two conditions in the process line: pressure differential or fluid motion. The two most common devices for measuring the pressure differential in a process line are Venturi tubes and
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S ECTION PLC Process Data Measurements C HAPTER 4 Applications
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orifice plates. One of the most common fluid flow transducers for detecting fluid motion is the turbine flow meter. This transducer transforms flow directly into electrical signals.
Pressure-Based Fluid Flow Meters. Both the Venturi tube and the orifice plate are based on the Bernoulli effect, which relates flow velocity to the pressure differential between two points. These fluid flow meters use pressure transducers, which transform pressure into an electrical signal to determine the pressure differential. The strain gauge and the Bourdon C-tube (see Section 13-5) are the two types of transducers most commonly used in pressure-based flow meters. These transducers use the bridge circuit and LVDT techniques, respectively, to convert measured pressure values into electrical signals. If low pressures are to be measured, a Venturi tube or orifice plate may incorporate a low-pressure transducer, such as a bellows, dia- phragm, or capsule to enhance the pressure reading resolution. Figure 13-28 shows these low-pressure transducers.
Figure 13-28. Low-pressure transducers.
Figure 13-29 illustrates a diagram of a Venturi tube, while Figure 13-30 shows an orifice plate flow transducer. The pressure differential ∆ P in these
devices is equal to the difference in pressures P 1 and P 2 . The value ∆ P also relates to the velocity of the fluid through the Bernoulli effect. The velocity
at point P 2 as a function of ∆ P is: V = k ∆ P
where:
V = the fluid velocity ∆ P =− P 1 P 2 k = a constant
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Pipe
P 1 Pipe
Inlet
Throat Outlet
P 2 Pressure
V 1 elocity V
Figure 13-29. Venturi tube diagram and the pressure and velocity at points P 1 and P 2 .
Differential Pressure Measurement
Flow
Pressure
Pipe P 1 P 2 Outside
Orifice
Orifice Orifice
Plate
Hole Plate Figure 13-30. Orifice plate flow transducer.
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The constant k takes into account the density of the fluid, the ratio of pipe to obstruction at cross-sectional points P 1 and P 2 , temperature, and other factors. The equation to obtain the flow rate measurement is:
Q = VA = Ak ∆ P = K ∆ P
where:
V = the fluid velocity
A = the cross - sectional area of the pipe
K = a new constant composed of times the area k A 2
The flow rate value Q gives us the volume per unit time of the flow (ft/min × ft 2 = ft 3 /min). Note that the velocity times the area at point 1 (V 1 A 1 ) is equal
to the velocity times the area at point 2 (V 2 A 2 ).
E X AM PLE 1 3 -1 0
Illustrate the PLC connections and functions necessary to implement the ratio control computation shown in Figure 13-31.
Differential
ON/OFF
Pressure Flow
Valve
Meter (Orifice)
Product A
–10 to +10 VDC
ON/OFF
ON/OFF
Valve
0–10 VDC
0–10 VDC Valve
Product B
Product C
Differential
Differential
Pressure Flow
Pressure Flow
Meter (Orifice)
Meter (Orifice)
Product B = 40% Product A Product C = 32% Product A
ON/OFF Valve
Figure 13-31. Ratio control computation application.
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S OLU T I ON
To implement the ratio control of products B and C at the specified percentage of product A (wild flow), we must read the differential pressures ( DP ) from the orifice flow meter and control the output of the analog servo valves. Figure 13-32 illustrates this ratio control imple- mentation using flow ratio as the process variable.
Programmable Controller
120 VAC 120 VAC –10 to +10 –10 to +10 –10 to +10
0–10 VDC –10 to +10 VDC 120 VAC
Discrete Discrete
Analog Discrete
Input Output
To 120 VAC Discrete Ouput in PLC
Figure 13-32. Ratio control implementation. The discrete output (120 VAC) is connected to the ON/OFF valve,
which allows each of the products to flow. Each DP instrumentation symbol represents the differential pressure measurement from the orifice flow meter. These pressure measurements are input to the analog input modules (–10 to +10 VDC). The flow rate for each product, A, B, and C, is:
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where K A , K B , and K C are the given constants. The square root value of the analog input ∆ P should be taken after the input count value corresponding to it has been converted to engineering units (through linearization, etc.). As product A flows, the PLC computes the flows of products B and C to maintain the proper ratios between A, B, and C (B = 0.40A and C = 0.32A). The PLC must control the output control valves for products B and C to maintain the proper ratios.
Motion Detection Fluid Flow Meters. The turbine flow meter is one of the most common types of motion detection flow meters. This device is used in applications that measure liquid and gas flows, as well as in applications with very low flow rates. Turbine meters are widely used in petrochemical and pipeline transfers of petroleum flows. Special types of turbine flow meters are also used in liquid oxygen and nitrogen gas-metering applications.
A turbine meter consists of a multibladed rotor, which is suspended in a liquid flow. The fluid flow passing through the blades creates a rotary motion in the turbine. This rotary motion creates a magnetic flux that is sensed by a coil inside the turbine flow meter. The coil changes the flux into a small voltage (as low as 10 to 20 mV) and then amplifies it. This design allows the turbine meter to convert the movement of its blades into output pulses that are proportional to the volume passing through the turbine. The output pulses generally provide information in gallons per minute (gpm). Some turbine meters also provide an analog output proportional to the flow rate being measured. Figure 13-33 illustrates a simple diagram of a turbine flow meter.
Output Pulses/Voltage Signal-Conditioning
Figure 13-33. Turbine flow meter.
E X AM PLE 1 3 -1 1
A programmable controller system receives an analog signal from a turbine flow meter. The flow rate is given as 60 gpm and the area of the pipe is 2 square inches. Find the velocity of the flow to be displayed in feet per second on a four-digit LED display.
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S OLU T I ON
The flow rate of the fluid is described by : Q = VA
where: Q = the flow rate
V = the velocity of the flow
A = the cross - sectional area of the pipe
The velocity of the flow is:
Note that the units given must be converted to obtain the velocity in ft/sec. To convert gallons to cubic feet, we must first convert gallons to cubic meters and then to cubic feet:
1 gal 3 = 3.785 10 m ×
3 1 3 m = . 35 31 ft Therefore: − 3 1 3 gal =
( . 3 785 10 × )( . 35 31 ft )
= 3 . 0 1336 ft
The cross-sectional area of 2 square inches is equal to 0.0139 square feet and 60 gpm is equal to 1 gallon per second. So, the velocity in ft/sec is:
. 0 0139 = . 9 61 ft/sec
Hence, to obtain the velocity of a fluid in a pipe in feet/second when the flow rate is given in gpm and the area is given in square inches, the following equation can be used:
( )( Q gpm . V 0 3208
( ft/sec) =
A (sq in)
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