How to Size Flow Meters

These industrial flow meters must be carefully selected and sized to be accurate.

Flow-meter sizing is complicated by the fact that the flow information coming from meters is used for different purposes. Flow meters are used for instantaneous flow-rate indication, flow control and totalizing measurements used for both mixing and commercial-dispensing applications. A further obstacle is that the different purposes are best served by different types of flow meters. The process of properly sizing a flow meter must really start with a selection of the right type of meter for the application.

Things You'll Need

• Calculator
• Selected flow meter published data

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Instructions

Continuous Indication and Control Applications

1. Define the application. You will need to know what the flowing media is, such as water. Other mandatory information is the pipe size, maximum flow rate, minimum flow rate, normal flow rate and required accuracy. Determine the maximum, minimum and normal temperature, and pressure, viscosity, specific gravity, entrained substances such as oil or sand, or additives such as anti-freeze--all of which can predicate a meter type.

2. 

   This indicating rotameter is used to measure oxygen in a laboratory.

   Calculate the turn-down ratio by dividing the maximum flow rate by the minimum flow rate. For example, a flow-indicating and control application for water requires a maximum flow rate of 60 gallons per minute (gpm) and a minimum flow rate of 10 gallons per minute (gpm) at ambient temperature. Dividing 60 by 10 results in a turndown ratio of 6:1. Knowing the turndown ratio will help you select the best meter type first and then size it correctly.

3. 

   Gas flow meters are carefully sized for each application.

   Select the meter type and size it using the meter application data supplied by the manufacturer. A differential pressure-type meter or a vortex meter can work well in this application. For this example (water-flow) application, pick the vortex-type electronic meter, which works well for flow control with a 6:1 turndown ratio, ambient temperature and no contaminants.

4. Size the meter by selecting a meter-body size and model number from published manufacturer's data. For a 1 1/2-inch pipeline, a typical 1 1/2-inch wafer-type vortex meter will handle minimum and maximum flow rates between about 8 and 160-gpm, which is a 20:1 turndown ratio. Most flow meter manufacturers put basic meter size selection data on their websites.

Commercial Dispensing and Batch Applications

1. 

   Flow meters may supply information for this control panel.

   Define the application, as in the case of plain indication and control applications. Flow meters used in dispensing and batch-mixing in critical applications are better served by turbine or positive-displacement meters. Use the example of distilled water being metered into precise 250-gallon batches in a pharmaceutical mixing tank at a rate of 25 gallons per minute.

2. Select and size a positive-displacement meter or turbine meter specifically set up for batching. Consult the manufacturer's meter data pertaining to the pipe size and flow-rate range. Select a pre-tested (validated) turbine meter for this example. A 7/8-inch turbine pulse-meter in a common 1-inch pipe-size body would work with this flow rate, being designed for about a 3-to-30-gpm range with a basic 1 percent accuracy specification.

3. Complete the meter sizing by configuring the meter electronics to output zero-to-30-gpm based on the pulse rate of this validated turbine meter body.

Tips & Warnings

• Take full advantage of manufacturers' application experience and expertise in selecting and sizing flow meters, as most provide this assistance free to support their customers.

• Use both types of metering if the application requires both continuous control and accurate totalizing.

• The tendency in this process is to oversize meters, which compromises accuracy.

• Particulates and grit can destroy costly meters with moving parts (such as the turbine meter used in Section 2).

References

• Omega Engineering: Flowmeters (Types and Basics)
• MEMFlo Flow Meter Sizing & Field Correction Calculators
• Cole-Parmer Technical Library: How Volumetric Flowmeters Work

Resources

• Sierra Instruments: Vortex Meter Sizing