1.2 Marco teórico
1.2.6 Estrategias de aprendizaje
Positive displacement meters generally measure velocity as a function of how the fluid being measured produces a motion or rotation to a piston or vane. Examples of positive
displacement meters used for measuring gas flow include vane anemometers, turbine meters, and propeller meters. These devices consist of blades, propellers, or cups, mounted on a rotating shaft; velocity is measured as a function of rotational speed of the shaft induced by the gas flow.
4.4.4.1 Measurement Principal5
Positive displacement type flow meters repeatedly entrap a known quantity of fluid as it passes through the flow meter. The number of times the fluid is entrapped is counted, and therefore the quantity of fluid passed through the flow meter is known. Because the
measurements by a positive displacement meter are independent of time (a positive displacement of the meter occurs for each quantity or volume of fluid), positive displacement flow meters measure total flow and can be classified as volume meters.
4.4.4.2 System Components and Operation2,4
Seven types of positive displacement flow meters are commonly used. Four of these can be classified as rotating positive displacement flow meters: lobed-impeller meters, slide-vane rotary flow meters, retracting vane rotary flow meters, and helical gear flow meters. The other three types of positive displacement flow meters are nutating disk meters, oscillating disk meters, and bellows gas meters. Nutating-disk meters and oscillating piston meters generally are used as household water meters and can be used to measure flow rates up to a maximum of about
760 L/min (200 gal/min). The bellows gas meter is widely used in commercial and domestic natural gas service and has a maximum flow capacity range of 68 to 7,950 L/min (18 to
2,100 gal/min). For industrial applications, rotating positive displacement flow meters are the most commonly used type of positive displacement flow meter. The following paragraphs describe these flow measurement devices in greater detail.
4.4.4.2.1 Lobed-impeller meters. Lobed-impeller meters contain two fixed position rotors that revolve inside a cylindrical housing. The measuring chamber is formed by the walls of the cylinder and the surface of one half of one rotor. When the rotor is in the vertical position, a specific volume of fluid is contained in the measuring compartment. As the impeller turns, due to a slight differential pressure between the inlet and outlet ports, the measured volume is
discharged through the bottom of the meter. This action occurs four times for a full revolution, with the impeller rotating the opposite direction at a speed proportional to the volume of the fluid. Figure 4.4-5 depicts a lobed-impeller flow meter.
Lobed-impeller meters can be used to measure fluid flow in pipes with diameters of approximately 3.8 to 61 cm (1.5 to 24 in.) Measurable maximum flow rates range from 30 to 66,000 L/min (8 to 17,500 gal/min). Table 4.4-4 summarizes the advantages and disadvantages of lobed-impeller flow meters.
4.4.4.2.2 Slide-vane rotary flow meters. Slide-vane rotary flow meters contain a cylindrical rotor that revolves on ball bearings around a central shaft and stationary cam. As fluid flows against an extended blade, the resulting rotation of the rotor and action of the cam cause the blades to act as cam followers, creating measuring chambers that measure fluid throughput. Figure 4.4-6 depicts a slide-vane rotary flow meter.
Slide-vane rotary flow meters can be used to measure fluid flow in pipes with diameters of up to 41 cm (16 in.). This type of flow measurement device can be used in temperatures to 204C (400F) and pressures up to 3,450 kPa (500 [psi]). Slide-vane rotary flow meters are characterized by high accuracy, but have the following limitations:
1. High costs;
2. Limited flow rate range--from 5:1 to 10:1; and 3. Moving parts that are subject to wear.
4.4.4.2.3 Retracting vane rotary flow meters. In retracting vane rotary flow meters, the vanes are jointed. As fluid enters the meter, it is deflected downward against the extended blade, causing rotation of the measuring element. Retracting vane rotary flow meters can be used to measure fluid flow in pipes with diameters up to four inches, at fluid temperatures up to 204C (400F) and pressures up to 6,200 kPa (900 psi). As is the case for slide-vane rotary flow meters, retracting vane rotary meters are characterized by high accuracy, but have the following limitations:
Figure 4.4-5. Lobed-impeller flow meter.2
TABLE 4.4-4. ADVANTAGES AND DISADVANTAGES OF LOBED-IMPELLER FLOW METERS
Advantages Disadvantages
Can be used at relatively high temperatures (204C [400F]) and pressure (8,200 kPa [1,200 psi]) Low net pressure loss
Available in numerous materials of construction No upstream or downstream pipe diameter requirements
Applicable for gases and a wide range of light to viscous liquids
Wide range of flow rates
Susceptible to damage from entrained vapors Larger sizes are bulky and heavy
High cost
Moving parts subject to wear
Best used at high flow rates because of possible slippage at low flow rates
Measure chamber
Cam rollers Stationary cam Housing
Rotor Vanes
Figure 4.4-6. Slide-vane rotary flow meter.2
Bridge
Bridge seals Rotor
Fluid out Fluid in
Retracting vane
Resiliant seal
Blade
Measuring chamber
Meter liner
Figure 4.4-7. Retracting vane rotary flow meter.4 1. High costs;
2. Limited flow rate range--from 5:1 to 10:1; and 3. Moving parts that are subject to wear.
Figure 4.4-7 depicts a retracting vane rotary flow meter.
4.4.4.2.4 Helical gear flow meters. Helical gear meters use two radially-pitched helical gears to continually entrap liquid as it passes through the flow meter, causing the rotors to rotate
Figure 4.4-8. Helical gear flow meter.11
in a longitudinal plane. Flow is proportional to the rotational speed of the gears. System
components include the rotor, bearings, and sensing system. Magnetic or optical sensing systems monitor the speed of the gears. In a magnetic sensor, the gear teeth are sensed by a magnetic pickup and amplified. An optical sensor uses a magnetically driven, optically encoded disc.
Rotation of the disc is sensed by an optical pickup that senses a pulse each time a portion of a revolution occurs. Figure 4.4-8 depicts a helical gear flow meter.
Helical gear meters can be used to measure highly viscous liquid flow in pipes with diameters of approximately 3.8 to 25 cm (1.5 to 10 in.). Measurable flow rates range from 19 to 15,100 L/min (5 to 4,000 gal/min). Table 4.4-5 summarizes the advantages and disadvantages of helical gear flow meters.
4.4.4.3 Accuracy4,5
The following accuracies apply to the four rotating positive displacement flow meters:
Lobed-impeller: ±0.2 percent of flow rate Slide-vane rotary: ±0.2 percent of flow rate Retracting vane rotary: ±0.2 percent of flow rate Helical gear: ±0.2 to 0.4 percent of flow rate.
4.4.4.4 Calibration Techniques5
4.4.4.4.1 Sensor. The meter constant (K-factor), which establishes the relationship between the frequency output of the flow meter, the volumetric flow, and the output of the converter, is fixed by design and cannot be calibrated.
TABLE 4.4-5. ADVANTAGES AND DISADVANTAGES OF HELICAL GEAR FLOW METERS
Advantages Disadvantages
Applicable to highly viscous liquids Low net pressure loss
Good accuracy
Moving parts subject to wear Only applicable to liquids
4.4.4.4.2 System. To calibrate positive displacement flow meter systems, a frequency signal that corresponds to the output of the primary flow meter device at a known flow is injected into the converter so as to verify operation of the converter and set zero and span. Another system calibration that can be performed involves energy or mass balance calculation of fluid flow to process operations; if fluid flow energy or mass agrees (balances) within the performance specifications to actual production rates or heat input, the flow rate measurement system can be assumed to be in calibration. Additionally, comparisons of recent energy or mass balances to past data can be made.
4.4.4.5 Recommended QA/QC Procedures5
Positive displacement flow meters are subject to deterioration due to wear, corrosion, exposure to a dirty liquid, and abrasion. Pluggage can occur if the flow meter is exposed to a dirty liquid. Excessive slippage usually results from corrosion or abrasion. Line cleaning before commissioning a new unit is recommended. Additionally, the meter should not be exposed to steam, which is often used to clean pipes. The following recommended spare parts should be maintained: rotor, sensor, bearings, and electronic components.
4.4.4.5.1 Frequency of calibration. Calibration of the positive displacement flow meter converter should follow a consistent pattern to allow for comparison of performance changes over time. If slippage is suspected, appropriate procedures should be undertaken, and the meter should be sent back to the manufacturer for repair. The recommended frequency of calibration depends largely on site-specific conditions and facility standard operating procedures. Moreover, specific regulations may require a specific calibration frequency (e.g., annually). In general, calibration frequency should be within the manufacturer’s recommendations. These calibration intervals should not be relied on indefinitely; they are starting points. At the end of the initial calibration period, the system should be calibrated or examined, as appropriate, and the data obtained should be charted. If the system is near or beyond the limit of accuracy (80 percent of acceptable error) and no process excursions or conditions are suspected of causing the
decalibration, the calibration interval probably is too long. In such a case, the system should be recalibrated to the center of the acceptance band, and the calibration interval should be shortened.
At the end of the second calibration period, calibration should be checked to determine if the system is drifting. If the system is near or beyond the limit of acceptable accuracy, similar steps should be taken, and the calibration period should be further shortened. This process should be repeated until the system is within the acceptable limit of accuracy at the end of the calibration interval. If, at the end of the initial calibration period, the system is determined to be within acceptable tolerance, adjustment is not necessary. The results should be recorded and the same calibration interval should be maintained for another calibration period. A log of all calibration check results should be maintained at the facility. Any corrective actions or adjustments should be recorded. Calibration data should be reviewed annually in order to spot significant deviations from defined procedures or tolerances.
4.4.4.5.2 Quality control. Written procedures should be prepared for instrument calibrations. These procedures should include:
1. The recommended interval for zero and span calibration checks of the converter (Readings before and after adjustment should be recorded.);
2. The reference zero and span values to be applied;
3. Step-by-step written procedures;
4. Blank field calibration forms (Records should include identification of the instrument component calibrated, the date of calibration, and initials of the person who performed the calibration.);
5. Designation of responsibility to perform the calibration (i.e., name of person(s) or position);
6. Designation of person to whom to report any failed calibration; and 7. Place to store calibration results.
4.4.4.5.3 Quality assurance. The calibration logs should be reviewed to confirm that calibrations were completed and performed properly. The person performing this review and the frequency of review should be specified. The written calibration procedures should be reviewed and updated to reflect any changes (e.g., system modifications or instrument changes).