FAN METERS

A second type of rotational inferential meter is the fan meter (Figure 6-2).

1. Recommended Applications: The use of fan type meters is limited to cold and hot water flow measurement. Fan meters have wide ranges and turndown ratios greater than 10:1 are common. Pressure drop associated with fan type meters increases proportionately with flow rate. Typically, permanent pressure drop for this type of meter is less than 1.0 percent at all flow conditions.

2. Limitations: Several limitations that may preclude application of a fan type meter are as follows:

Fan type meters are restricted to clean fluids.
Meter installation should be horizontal and level.
Temperature range is from -267°C (-450°F) to +260°C (+500°F).
Pressure limit is 3,000 psig.

3. Accuracy and Reliability: Fan type meters are typically not as accurate as turbine or positive displacement meters. They have an accuracy of ±%1.5 percent over the flow range. Reliability is comparable to turbine meters. If installation is proper and maintenance is performed as required, the meters can be expected to have a 4-1/2 year mean-time-between-repair and a total life expectancy of 10 to 25 years.

INSTALLATION: Installation of turbine and fan type flowmeters involves placement of the sensing element directly in the flow channel. The location of the turbine or fan meter in the system is important. Whenever possible, it is preferable to locate the primary element in a horizontal line or a vertical line with flow in the upward direction. To ensure accurate flow measurement, the fluid should enter the primary element with a fully developed velocity profile, free from swirls or vortices. Such a condition is achieved by the use of strainers, straightening vanes, and/or adequate lengths of straight pipe preceding and following the primary element. Using a beta ratio of 0.6, typical ASME recommended lengths of such piping are shown in Figure 6-5, but specific requirements should be obtained from the manufacturer. The diagram in Figure 6-5 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 necessary to limit errors due to piping configurations to less than ±%0.5 percent. If minimum distances are not observed, applying flow equations and calculations may result in inaccurate data. Full-bore meters require shutdown of the system during installation, periodic inspection, and maintenance, but insertion type meters can be hot-tapped and do not interrupt the system for installation or removal. Turbine and fan flowmeter installations must also take into consideration the following items:

Reduce turbulence by observing recommended straight length distances.
Care should be taken to place the meter as far from valves and other turbulence-producing fittings as possible. All regulator and control valves should preferably be located downstream of the meter; if upstream, beyond the recommended straight length distance.
If used, place straightening vanes and strainers upstream of the meter.
When metering liquids, a “horizontal” line should gently slope upward for a sufficient distance downstream of the flowmeter to ensure that it will always be filled with liquid.

MAINTENANCE: The following procedures are the minimum required for the most common types of fan and turbine units. When developing maintenance schedules, refer to the manufacturer’s recommendations.

1. Annual Maintenance: Annually, remove the meter head and inspect as follows:
(a) Ensure that the rotor has no obvious signs of deterioration, abrasion wear, or fouled debris.
(b) Ensure that the rotor shaft and bearings are not excessively worn, are free of debris, and the propeller turns freely.
(c) Return insertion turbine rotor to the manufacturer for recalibration.