Considerations in Flow Meter Selection

Flow measurement is a critical aspect in a wide range of plant operations. Users choosing equipment to meter the flow of liquid or gas processes must consider various factors to arrive at an optimal solution. Experience has shown there are significant differences between meter technologies, with each type of device having its own advantages and disadvantages for industrial facilities.

The previous article in this series (Pumps & Systems, October 2017) described common flow applications and types of flow technology. This article will look at important selection criteria.

In a typical industrial facility, fluid characteristics (single or double phase, viscosity, turbidity), flow profile (laminar, transitional, turbulent), flow range and accuracy requirements are all important considerations in determining the right flow meter for a particular measurement task. Additional considerations such as mechanical restrictions and output-connectivity options impact the user’s choice.

Some of the key criteria in flow meter selection include:

1. Process media. Different flow meters are designed to operate best in different fluids and under different operating conditions. That’s why it is important to understand the limitations inherent to each style of instrument. Fluids are conventionally classified as either liquids or gases. The most important difference between these two types of fluid lies in their relative compressibility (i.e., gases can be compressed much more easily than liquids). Any change that involves significant pressure variations is generally accompanied by much larger changes in mass density in the case of a gas than in the case of a liquid.
2. Type of measurement. Industrial flow measurements fall under one of two categories: volumetric or mass. Volumetric flow rate is the volume of fluid passing through a given volume per unit time. Mass flow rate is the movement of mass per time. It can be calculated from the density of the liquid (or gas), its velocity and the cross-sectional area of flow. Volumetric measuring devices, like variable area meters or turbine flow meters, are unable to distinguish temperature or pressure changes. Mass flow measurement would require additional sensors for these parameters and a flow computer to compensate for variations in these process conditions. Thermal mass flow meters are virtually insensitive to variations in temperature or pressure.
3. Flow rate information. A crucial aspect of flow meter selection is determining whether flow rate data should be continuous or totalized. A flow rate has to do with the quantity of a gas or liquid moving through a pipe or channel within a given or standard period of time. A typical continuous flow measurement system consists of a primary flow device, flow sensor, transmitter, flow recorder and totalizer.
4. Desired accuracy. Accurate flow measurement can be the difference between on-spec quality and wasted product. Flow meter accuracy is specified in percentage of actual reading (AR), percentage of calibrated span (CS) or percentage of full-scale (FS) units. It is normally stated at minimum, normal and maximum flow rates. A clear understanding of these requirements is needed for a meter’s performance to be acceptable over its full range.
5. Application environment. Flow meters can be employed under a host of varying conditions in an industrial plant. For example, users must decide whether the low or high flow range is most important for their metering application. This information will help in sizing the correct instrument for the job. Pressure and temperature conditions are equally important process parameters. Users should also consider pressure drop (the decrease in pressure from one point in a pipe to another point downstream) in flow measurement devices, especially with high-viscosity fluids. In addition, viscosity and density may fluctuate due to a physical or temperature change in the process fluid.
6. Fluid characteristics. Users should make certain that the selected flow meter is compatible with the fluid and conditions they are working with. Many plant operations involve abrasive or corrosive fluids, which move under aerated, pulsating, swirling or reverse-flow conditions. Thick and coarse materials can clog or damage internal meter components—hindering accuracy and resulting in frequent downtime and repair.
7. Installation requirements. Planning a flow meter installation starts with knowing line size, pipe direction, material of construction and flange-pressure rating. Complications due to equipment accessibility, valves, regulators and available straight-pipe run lengths should also be identified. Nearly all flow meters must be installed with a run of straight pipe before and after their mounting location. Where this is not possible, a flow conditioner can be used to isolate liquid flow disturbances from the flow meter while minimizing the pressure drop across the conditioner.
8. Power availability. Pneumatic instrumentation was once used in most hazardous area applications, since there was no power source to cause an explosion. Today’s installations normally call for intrinsically safe instruments, which are “current limited” by safety barriers to eliminate a potential spark. Another option is to employ fiber optics. Turbine flow meters offer an advantage in environments where a power source is not available. They do not require external power to provide a local rate/total indicator display for a field application, and instead rely on a battery-powered indicator. Solar-powered systems can also be used in remote areas without power.
9. Necessary approvals. Approvals for the use of flow measurement equipment in hazardous plant locations include FM Class 1 Division 1, Groups A, B, C and D; and FM Class 1, Zone 1 AEx d (ia) ia/IIC/T3-T6. Standards such as the Measuring Instruments Directive (MID) in the European Union (EU) apply to fiscal and custody transfer metering for liquids and gases. In terms of environmental emissions, industrial flow meters must meet the Electromagnetic Compatibility (EMC) Standards EN55011:1992 and EN61326-1:1997.
10. Output/indication. Flow meter users must decide whether measurement data is needed locally or remotely. For remote indication, the transmission can be analog, digital or shared. The choice of a digital communications protocol such as HART, FOUNDATION Fieldbus or Modbus also figures into this decision. In a large industrial facility, flow readings are typically supplied to an industrial automation and control system (IACS) for use in process control and optimization strategies.

Summary

Choosing the right flow measurement solution can have a major impact on operational and business performance. For this reason, companies anticipating a flow meter purchase should consult with a knowledgeable instrumentation supplier in the early stages of a project. The effort spent learning about basic flow measurement techniques, and available meter options, will ensure a successful application once the equipment is installed.