Flowmeters – 2003-10-10

Flowmeters are used for determining the amount of product passing through a pipe for purposes of product blending, determining billing or cost, machine lubrication, process heating or cooling, and many other applications. Reliability and accuracy are two of the selection factors used in choosing a flowmeter.

By Joseph L. Foszcz, Senior Editor, PLANT ENGINEERING Magazine October 10, 2003
Key Concepts
  • Types

  • Selection

  • Comparison

    Selection factors

    Flowmeters are used for determining the amount of product passing through a pipe for purposes of product blending, determining billing or cost, machine lubrication, process heating or cooling, and many other applications.

    Reliability and accuracy are two of the selection factors used in choosing a flowmeter. Accuracy is important because it can make a difference between profit and loss when dispensing fluids. Inaccurate measurement could result in serious damage to equipment or product.

    Flowmeter selection is not easy because there are about two dozen designs to choose from. Many are established designs. Thermal mass, Coriolis, and ultrasonic have benefited from new technology and electronics to become popular.

    There are two basic types of flowmeters used with pipes: full-bore inline and insertion. Inline flowmeters allow the entire flow to pass through and derive a flow rate from average velocity. Other designs use positive displacement or mass flow techniques.

    Insertion-type flowmeters protrude onto the pipe. They sample a point in the flow stream that represents average velocity or create a pressure differential dependent on flow.

    Orifice plates are the most popular flowmeters in use today (Fig. 1). They produce the best results when measuring turbulent flow of clean liquids. Major advantages are no moving parts and low cost, regardless of pipe size. Metering accuracy depends on installation, orifice area ratio, and fluid properties. They must be installed in straight pipe runs.

    Venturi tubes can handle large flows with low pressure drop and good accuracy. They can be used with most liquids, including those with high solids content. Venturis are not recommended for highly viscous liquids or those containing large amounts of sticky solids.

    Flow nozzles represent a compromise between an orifice and a venturi. They can handle large solids, high velocities, high turbulence, and very high temperatures. Liquids with suspended solids can be metered.

    Variable area flowmeters maintain a relatively constant pressure differential with varying flow rates by using a moveable restriction in the flow path (Fig. 2). The position of the piston in the housing indicates the flow rate. Because the flow rate can be read directly, secondary reading devices are unnecessary.

    Thermal mass flowmeters operate independently of pressure and viscosity (Fig. 3).

    The flow stream conducts heat from the heated sensing element. The conducted heat is directly proportional to mass flow rate. The amount of heat carried away depends on the fluid’s velocity, density, specific heat, and thermal conductivity. If the probe becomes coated, heat transfer is changed, which negatively affects accuracy and response time.

    Coriolis mass flowmeters accurately measure flow rates independent of temperature, pressure, viscosity, and solids content (Fig. 4).

    In these units, fluid flow causes two, constantly vibrating tubes to twist. The amount of twist depends on the flow rate. The design is noninvasive and is used with many fluids over a wide range of flow rates. Since these meters maintain accuracy, they are used in applications that require tight control, management of high-value fluids, and custody transfers.

    Turbine flowmeters use a rotor with propeller-like blades (Fig. 5).

    Flow rate is proportional to rotational speed and is sensed by a magnetic pickup, infrared beam, or a radio frequency field. This design provides excellent short-term accuracy, repeatability, and rangeability. It is usually used with clean fluids and is not effective with swirling or high viscosity fluids. Meters must be calibrated for each application.

    Magnetic flowmeters are constructed with a coil around the flow stream that creates a magnetic field (Fig. 6).

    An electrically conductive fluid generates a voltage as it moves through the magnetic field. This voltage is proportional to the flow rate. These flowmeters can measure difficult and corrosive liquids and slurries and forward and reverse flow. The fluid must be electrically conductive and nonmagnetic. Most water-based fluids can be measured, petroleum-based fluids cannot.

    Positive displacement flowmeters measure incremental volumes of flow as line pressure fills and displaces each chamber’s volume downstream (Fig. 7).

    Flow rate is determined by counting the number of times this action occurs. Because these meters have many moving parts, they are not suited for dirty or gritty fluids. Leakage around the gears or vanes can cause inaccurate readings, but viscous fluids reduce this effect. Designs include reciprocating single or multiple pistons, nutating disks, oval gears, lobed impellers, and rotary vanes.

    Vortex flowmeters use a bluff body or shedder bar to generate vortices in the flow stream (Fig. 8).

    Flow rate is determined by counting the vortices that form behind the bluff body. Frequency of vortex formation is directly proportional to fluid velocity. These flowmeters are rugged devices with no moving parts. Use with slurries or high-viscosity liquids is not recommended. They are not useful at very low flow rates because vortex formation is poor due to a lack of energy in the fluid.

    Ultrasonic flowmeters are available in two designs: Doppler and transit time. Doppler measures the frequency shift of a sound wave to determine flow rate. Transit time measures the time it takes a sound wave to travel a specified distance through a flow stream. The variation in time is related to flow rate.

    Doppler flowmeters use a constant-frequency sound wave transmitted through the pipe walls and fluid to a receiver. The sound wave is reflected back to the receiver by suspended solids, entrained gases, or flow turbulence in the fluid. Because the liquid causing the reflection is moving, frequency of the returned signal is shifted proportionately to the liquid’s velocity.

    Transit time flowmeters have transducers mounted at a 45-deg angle to flow, either on the same side or opposite sides of a pipe, depending on pipe and liquid characteristics (Fig. 9).

    Speed of the signal or shift of frequency between the transducers increases or decreases with the direction of transmission and velocity of the fluid. The liquid being measured must be relatively free of entrained gases or solids.

    Advantages and disadvantages of flowmeters

    Type Advantages Disadvantages
    Differential pressure Low initial cost Subject to plugging
    Familiar technology Pressure drop
    Easy to use Orifice plate wear
    Thermal mass Low cost Periodic cleaning
    Handles low-density fluids Not highly accurate
    Coriolis mass High accuracy Sensitive to vibration
    True mass flow measurement High initial cost
    Not suitable for large pipes
    Turbine Accurate Wear
    Accepted technology High flow velocity can cause damage
    Magnetic Accurate Requires conductive fluid
    No pressure drop Electrodes subject to coating from fluid
    Adaptable to large pipes
    Positive displacement Accurate Wear
    Wide rangeability Limited use on large pipes
    Requires clean fluids
    Vortex Accurate Sensitive to vibration
    Easy to install Lacks approvals
    Ultrasonic Low maintenance High initial cost
    Nonintrusive May require clean fluid
    Adaptable to large pipes Clamp-on installation

    Selection factors

    At a minimum, specifiers of flowmeters should consider the following:

    Ability to withstand the process environment: fluid, pressure, temperature, etc.

    Ability to provide the accuracy of measurement required

    Serviceability and maintenance requirements

    Long-term stability, durability, and frequency of calibration

    Cost of purchase and installation

    Ease of interfacing with existing equipment

    Pressure loss incurred, level of swirl generated, or pulsation produced

    Adaptable to future needs