VENTURI TUBES

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Venturi tubes are used to measure flow in pipes and ducts. They can be applied to gases such as air and steam, and to liquids such as water and oil. Common applications include airflow in large ducts and steam output from boilers. Venturis are widely used, and a great deal of operating experience has been gained in a variety of applications. They are relatively easy to fabricate, consisting of a tube that gently converges to a low diameter throat and diverges to the downstream pipe diameter (Figure S-ll). If the cost of fluid delivery is high, venturis are desirable because they have a low, permanent pressure loss and do not decrease the delivery rate. Venturis have a low sensitivity to wear and are useful in erosive flows such as air with suspended solids. Venturi meters are especially applicable to large ducts , round or rectangular, where other types of flowmeters are expensive to fabricate and calibrate.

1. Operating Principles: Venturi tubes are inferential meters that do not measure flow directly, but cause differential pressure to occur. An inferred flow rate can be calculated by measuring change in pressure and by knowing proportional relationships. Figure 5-11 illustrates in a cutaway view, the venturi and the areas of high and low pressures associated with their respective velocities.

FIGURE 5-11. Venturi Tube

1.1 Venturi Tube/Orifice Plate Comparison: Venturi tubes can measure the same fluids as orifice plates. Like orfice plates, they can be built to match the size and shape of irregular piping. Venturi tubes cause less permanent pressure drop than orifice plates and are used more often where pumping costs are higher. Venturi tubes are less susceptible to wear from erosive fluids than orifice plate meters. However, as the size of venturi tubes increases, their cost increases more than that of orifice plates.

LIMITATIONS: The primary disadvantage of using venturi metering is their limited range. The maximum turndown ratio is limited to 4:1, which is inherent in the design. A marked change in flow conditions requires a new venturi configuration. Changing venturis is more expensive than orifice plates and requires a longer interruption of the system. Other considerations are as follows:

• Temperature limit is 1,000°F.
• Pressure limit is 6,000 psig.

INSTALLATION: The location of a venturi tube in the system is important. Whenever possible, it is preferable to locate the venturi tube in a horizontal line. To ensure accurate flow measurement, the fluid should enter the venturi tube with a fully developed velocity profile, free from swirls or vortices. Such a condition is best achieved by the use of adequate lengths of straight pipe, both preceding and following the primary element. The minimum recommended lengths of piping are shown in Figure 5-4. The diagram in Figure 5-4, that corresponds closest to the actual piping arrangement for the meter location, should be used to determine the required lengths of straight pipe on the inlet and outlet. These lengths are those necessary to limit errors due to piping configurations to less than +%0.5 percent. Minimum recommended distance is dependent upon the ratio of throat-to-inlet diameter. If minimum distances are not observed, the meter may produce inaccurate data.

MAINTENANCE: The following procedures are the minimum required for the most common types of units. When developing maintenance schedules, refer to the manufacturer’s instructions. Perform the following tasks at the periods prescribed.
1. Quarterly Maintenance:

(a) Flush and clean annular chamber throat and inlet.
(b) Purge trapped air from chamber and connecting pipe.
(c) Clean pressure taps.
(d) Blow down sensor lines on a regular schedule.
(e) Check pressure taps for burrs and/or debris.
(f) Resize venturi if necessary, based on flow of previous year.
(g) Test dp transmitter every 6 months with dead weight pressure tester and rescale if necessary.

2. Annual Maintenance:
(a) Clean and paint exterior of meter.
(b) Dismantle and check for corrosion.
(c) Clean and restore smoothness to internal surfaces.
(d) Check flange gasket and replace if it is leading or intruding into flow path.
ACCURACY AND RELIABILITY: Accuracy is up to ±%O.75 percent of full scale. Accuracy is diminished at lowest turndown. Venturis are more reliable than orifice plate meters because they are less susceptible to wear.

DIFFERENTIAL PRESSURE DEVICES: Dp devices are used to provide a quantitative display of the differential pressure; they are also called delta P and ΔP devices. The four most common dp devices are: manometer, diaphragm, bellows and electronic. All new or replacement installations should be electronic.

1. Manometer: The manometer is a rather simple device. One end of the manometer is attached to the high-pressure tap and the other end to the low-pressure tap of the orifice plate installation. As the dp created by the orifice plate is sensed by the manometer , a column of fluid in the manometer allows the dp to be read directly on a scale (Figure 5-7). Refer to the manufacturer’s instructions before installing a manometer.

2. Diaphragm: The diaphragm device includes a hermetically sealed diaphragm that is in an enclosure with one side open to the high-pressure tap and the other side to the low-pressure tap (Figure 5-8). The diaphragm moves as the dp created by the orifice meter is transmitted to the diaphragm chamber. A pointer attached to the diaphragm pivots about a fulcrum in the wall of the chamber and mechanically indicates the dp directly on a scale. Refer to the manufacturer’s instructions before installing a diaphragm.

3. Bellows: A bellows device is similar to the diaphragm device in that the indicator pointer is attached to a component that is subject to movement caused by the dp. In a bellows device, a partition is hermetically sealed between two bellows in a confined compartment with an opening on one side to the high-pressure tap and another to the low-pressure tap (Figure 5-9). The input ends of the bellows are fixed to the compartment walls. As the dp forces the partition to move, compressing and expanding the respective bellows, a lever system causes a pointer to directly indicate the dp on a scale. Refer to the manufacturer’s instructions before installing a bellows.

4. Electronic: Electronic devices are also known as capacitance devices. In an electronic device, the dp is transmitted through an isolating diaphragm to a hermetically sealed sensing diaphragm in the center of the device (Figure 5-10). The sensing diaphragm is surrounded by silicone oil contained between capacitors. As the sensing diaphragm deflects in proportion to the dp, the position of the diaphragm is detected by capacitor plates on each side of the diaphragm. The differential capacitance between the plates and the diaphragm is converted electronically to a 2-wire, 4-20 mA or 0-10 volt data transmission signal. Electronic devices are available in both square root and linear function models. Solid state, plug-in components simplify maintenance/repairs.

5. Calibration: Static calibration should be performed on all dp devices at least every 6 months.

6. Data Transmission: Readings from various dp devices must often be transmitted to remote data collection and recording sites. This is because the dp device may be too remote to warrant onsite reading. Data transmission may also be necessary because there may be many widely dispersed devices to be read and it would be uneconomical to have each one read onsite; or a central data management point has been set up to collect, record, plot, reduce, and analyze all flow data. All the taps for dp will accommodate remote transmission fittings along with the various dp devices.

6.1 Pneumatic: All the dp devices, except electronic, can have the readings transmitted to a remote site by pneumatic lines.

6.2 Electrical: Rather than pneumatically, the recommended means of data transmission is to have the dp electronically converted to an analog signal and transmitted electrically to a data collection center.

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