VELOCITY METERS

previous VARIABLE ANNULAR ORIFICE METERS
TURBINE AND FAN METERS

Turbine and fan meters are inferential type meters. Both determine actual flow by relating rotational speed of a moving element, which acts as a turbine or fan, placed in the flow stream.

1. Operating Principles: Turbine and fan flowmeters use kinetic energy of a flowing fluid to drive a turbine or fan which generate frequencies proportional to flow rate. The rotational velocity of the rotating element and the fluid velocity are linearly proportional over the working range of the meter.

METER DESIGNS: All turbine and fan meters consist of a rotating element or rotor. Rotor speed is linearly proportional to fluid velocity.

1. Turbine Meter: Turbine meters consist of a multiblade rotor mounted within a pipe perpendicular to the fluid flow (Figure 6-l). The rotor spins as the liquid passes through the blades. The rotational speed is a direct function of flow rate and can be sensed by magnetic pickup, photoelectric cell, or gears.

FIGURE 6-1. Turbine Meter

FIGURE 6-2. Fan Meter

2. Fan Meter: Fan meters consist of a housing with a multiblade rotor mounted on a spindle at right angles to the direction of flow (Figure 6-2). Flow enters the meter case, strikes the rotor tangentially, causing rotation. The speed of rotation is determined by the velocity of the liquid and the configuration of the chamber and rotor. Single jet type meters are the simplest example. In the single jet meter, fluid enters the meter through a single tangential inlet. A single outlet port is located diametrically opposite the inlet. The multijet type meter has the same general features as the single jet meter. Liquid enters the working chamber of the multijet meter through a number of tangential inlets around the circumference and leaves through another set of orifices placed at a higher level in the chamber.

3. Pressure Losses: Pressure losses in turbine and fan meters are substantially less than orifice plate meters. Expected pressure loss is in the range of 0.4 to 0.8 psi.

TURBINE METER: The turbine meter is used more often than the fan meter. The rotor is helical and is mounted on a shaft parallel to the flow direction. Turbine flowmeter rotors are supported by ball bearings or ball sleeve bearings. Bearings are exposed to the process as long as the flowmeter is in the process pipeline. Bearing life depends on several factors. One factor is the floweter duty cycle, the actual operation expressed as a percentage of total time. Bearing life also is related to corrosion resistance and type of corrosive impurities in the process. Lubricating qualities and cleanliness of the process stream have a positive effect on bearing longevity; abrasiveness of solid particles present has a negative effect. Hard, abrasive particles in the process stream are the major cause of turbine flowmeter bearing wear. To minimize stream contamination by solids, a strainer should be installed upstream of the turbine flowmeter. Recommended strainer mesh sizes are given in specific product literature.

1. Turbine Meter Designs: Turbine meters are applied in two distinct designs, full-bore and insertion types. Full-bore turbine meters have a rotating element equal to pipe diameter; they measure the total flow. Turbine meters are available for pipe sizes greater than one-half inch in diameter. They are designed for liquid or gas measurement. Full-bore turbine meter costs increase exponentially as line size increases, making them uneconomical for large line sizes. Insertion meter costs are the same regardless of line size, making them an attractive alternative. Insertion meters measure a sample of fluid flow at a local velocity. Insertion meters can be used to measure liquid, gas, and steam when pipe sizes are 3 inches or larger (Figure 6-3).

2. Recommended Applications: Full-bore meters are primarily applied either to clean, low particulate fuel oil or water. Corrosive materials can be measured if a stainless steel rotor is used. Turbine meter applications for steam measurement must be so specified to obtain the correct bearing and turbine materials. Insertion meters are excellent for steam applications. The small, low inertia rotor has a rapid response time. In addition, cost of the meter is independent of the pipe size. Whether the full-bore or insertion type meter is used, the turbine meter is often chosen because of its turndown ratio. Turbine meters have a minimum flow turndown ratio of 10:1. Turndown ratio can be up to 50:1 depending on pipe size and fluid velocity. An insertion turbine meter can also be used as an analytical tool. Because of its ability to move across a pipe section and to orient the rotor head at various angles to the flow stream, the meter can determine the nature of unsymmetrical flow patterns and detect any swirl within the pipe. Turbine meters have an advantage over other types of meters because they are capable of measuring forward and reverse flow.

3 Limitations: Several of the limitations that may preclude application of a turbine meter are as follows:

Turbine meters are restricted to clean fluids.
Insertion turbine meters require a clear space of at least 4 feet perpendicular to the pipe for installation.
Temperature operating range for standard units is -73°C (-100°F) to +427°C (+800°F). Special ranges are available.
Pressure limit is 3,000 psig.

FIGURE 6-3. Insertion Turbine Meter

4. Accuracy and Reliability: The turbine meter is highly accurate through most of its range. Guaranteed accuracy of ±%1.O percent is available. Insertion meters are subject to the inaccuracies that result from locally measuring an average velocity. Reliability of properly installed and maintained turbine meters should yield a 4-1/2 year mean-time-between-repair and a total life expectancy of 10 to 25 years.

SHUNT METERS: Shunt flowmeters are a special class of turbine meters which use an orifice plate to control a bypass flow metered by the turbine.

1. Operating Principles: The turbine rotation is at a speed proportional to a bypass flow controlled by an orifice plate, which in turn is proportional to the main flow rate. A reduction train, that gears down the turbine, is coupled to a driving magnet. The magnet influences another magnet that is the counter. The two magnets operating in unison enable totalization of the turbine rotation.

2. Meter Design: There are two types of shunt meters. One is external to the main flow pipe, as shown in Figure 6-4. The other is mounted as an inline meter within the main pipe. Neither type requires power for totalization.

3. Limitations: The limitations of a shunt meter are as follows:

Requires system shutdown to install.
High maintenance and difficulty in calibration.
Costs are moderate to high.
Turndown ratio is 7 to 1.
Not recommended for steam below 30 psig or over 200 psig.
Not recommended for fuel oils.
Not recommended for air or natural gas below 5 psig.
Permanent pressure loss due to orifice plate.

4. Installation: The flow in the line must be turned off during installation of a shunt meter. A diversion orifice plate with flanges must be placed in the main line. Holes must be cut into the main line before and after the orifice plate to accept the bypass piping which contains the turbine and counter box (Figure 6-4).

5. Maintenance: Maintenance should be performed every 6 months. The diversion orifice plate should be checked for wear and the turbine blades for deformation. The counter should be checked for accuracy and calibrated if necessary.