A Beginners’ Guide to Flow Meters

Flow measurement is a critical aspect of operations involving pumps, valves and other industrial systems. Users choosing flow meters to measure the flow of liquids or gases must consider key factors to make the right decision. There are significant differences between meter designs, with each type of device having its own pros and cons.

Evaluating Meter Technologies

Flow meters are excellent tools to measure, monitor and control the distribution of a host of fluids. There is the question of which technology to use because a wide variety of meter configurations are available and each must be properly deployed to achieve optimal performance. Coriolis: Coriolis flow meters directly measure fluid mass over a wide range of temperatures with very high accuracy. Their unobstructed, open flow design is suitable for viscous, non-conductive fluids. With no internal moving parts, Coriolis meters require minimal attention once installed (see Image 1). Differential pressure: Differential pressure (DP) meters employ a proven, well-understood measuring technology that does not require moving parts in the flow stream. DP meters are not greatly affected by viscosity changes. Electromagnetic: Electromagnetic meters measure virtually any conductive fluid or slurry. They provide low pressure drop, high accuracy, high turndown ratio and excellent repeatability. The meters have no moving parts or flow obstructions, and are relatively unaffected by viscosity, temperature and pressure when correctly specified. Positive displacement: Positive displacement meters are highly accurate (especially at low flows) and have large turndown ratios. These devices have only one or two moving parts, making them easy to maintain. There is no need for straight pipe lengths as with other metering approaches. Thermal mass: Thermal mass meters carry a relatively low purchase price. They are designed to work with clean gases of known heat capacity, as well as some low-pressure gases not dense enough for Coriolis meters to measure. Turbine: Turbine meters are known for high accuracy, wide turndown and repeatable measurements. They produce a high-resolution pulse rate output signal proportional to fluid velocity and, hence, to volumetric flow rate. Turbine meters are limited to use with only clean fluids, and they require periodic recalibration and service. Impeller: Impeller meters are frequently used in large diameter water distribution systems. Their attributes include direct volumetric flow measurement (often with visual indication); universal mounting; fast response with good repeatability; and relatively low cost. Their performance suffers in applications with low fluid velocity. Ultrasonic: Ultrasonic flow meters have no moving or wetted parts, suffer no pressure loss, offer a large turndown ratio, and provide maintenance-free operation. Clamp-on ultrasonic meters can be used for troubleshooting a wide range of flow issues (see Image 2). Variable area: Simple, inexpensive and reliable, variable area meters provide practical flow measurement solutions for many applications. Be advised most of these meters must be mounted perfectly vertical. They also need to be calibrated for viscous liquids and compressed gases. Vortex: Vortex meters have no moving parts that are subject to wear, so regular maintenance is not necessary. They can only measure clean liquids. Vortex meters may introduce pressure drop due to obstructions in the flow path. Oval gear: The latest breed of oval gear meters directly measures actual volume. It features a wide flow range, minimal pressure drop and extended viscosity range. This design offers easy installation and high accuracy, and measures high temperature, viscous and caustic liquids with simple calibration. Nutating disc: Nutating disc meters have a reputation for high accuracy and repeatability, but viscosities below their designated threshold adversely affect performance. Meters made with aluminum or bronze discs can meter hot oil and chemicals.

Identifying Selection Criteria

In a typical industrial facility, fluid characteristics, flow profile, flow range and accuracy requirements are important for determining the best flow meter for a particular measurement task. Additional considerations such as mechanical restrictions and output-connectivity options impact the user’s choice. For plant operations, the key factors in meter selection include: Process media: 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). Consequently, any change that involves significant pressure variations is generally accompanied by much larger changes in mass density in the case of a gas than a liquid. Type of measurement: Industrial flow measurements fall under one of two categories: mass or volumetric. 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. 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. Desired accuracy: 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. Application environment: Users must decide whether the low- or high-flow range is most important. This information will help in sizing the correct instrument for the job. Pressure, temperature, density and viscosity conditions are equally important parameters. Fluid characteristics: Users should be cautious that the selected flow meter is compatible with the applicable fluid and conditions. Thick and coarse materials can clog or damage internal meter components, hindering accuracy and resulting in frequent downtime and repair. Installation requirements: Planning a flow meter installation starts with knowing the 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. Power availability: 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. 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. 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. Choosing the right flow measurement solution can have a major impact on operational and business performance. Companies anticipating a flow meter purchase should consult with a knowledgeable instrumentation supplier in the early stages of a project. This will ensure a successful application once the equipment is installed.