4.2 Flow Measurement Instruments

Introduction to Flow Measurement

Flow measurement is categorized into volumetric flow (measuring the volume passing a point, e.g., gallons per minute, cubic meters per hour) and mass flow (measuring the actual weight or mass, e.g., pounds per hour, kilograms per minute). Accurate flow data is essential for blending, batching control, custody transfer accounting, and overall process efficiency. Selecting the right technology requires understanding fluid properties, Reynolds numbers, and piping constraints.

Differential Pressure (DP) Flow Measurement

Differential pressure flow meters are the most widely installed industrial flow measurement devices, boasting decades of proven reliability. They operate on Bernoulli's Equation, which describes the conservation of energy in a moving fluid. When a fluid passes through a physical restriction in a pipe, its velocity increases (kinetic energy goes up). To conserve total energy, its static pressure must decrease (potential energy goes down).

By measuring the pressure difference before and after the restriction, we can calculate the flow rate. A critical characteristic of all DP flow measurement is that the differential pressure is proportional to the square of the flow rate.

To obtain a linear flow signal (e.g., a 4-20 mA signal representing 0-100% flow), a mathematical square root extraction must be performed on the DP measurement. This extraction can be configured within the smart transmitter itself or downstream in the DCS/PLC.

Primary Flow Elements

The restriction that creates the pressure drop is called the primary element. Common types include:

• Orifice Plates: The simplest, cheapest, and most common. It is a metal plate with a precisely machined hole.
  • Concentric: Hole is in the center; used for clean liquids and gases.
  • Eccentric: Hole is off-center (usually positioned at the top for liquids to allow entrained gas to pass, or at the bottom for gases to allow condensate to pass).
  • Segmental: A semi-circular opening, useful for slurries or dirty fluids.
  • Conditioning: Features multiple smaller holes to straighten disturbed flow profiles, requiring less straight pipe run.
• Venturi Tubes: A gradual cone-shaped restriction followed by a gradual expansion. They offer high accuracy and very low permanent pressure loss (pressure recovery is excellent), making them ideal for high-volume, low-pressure applications like water treatment, but they are expensive, heavy, and bulky.
• Flow Nozzles: A bell-shaped restriction with a curved entrance. They handle high-velocity flows, steam, and abrasive fluids much better than orifice plates because they resist erosion.
• Pitot Tubes: Measures fluid velocity at a single point by capturing the difference between dynamic (impact) pressure and static pressure.
• Annubar (Averaging Pitot Tube): A multi-port tube that spans the entire diameter of the pipe and averages multiple impact pressure readings, providing a more accurate measurement than a single-point Pitot tube, especially in large ducts or disturbed flow profiles.

Velocity and Positive Displacement Flow Meters

Electromagnetic Flow Meters (Magmeters)

Magmeters operate on Faraday's Law of Electromagnetic Induction, which states that a conductor moving through a magnetic field induces a voltage. In a magmeter, the fluid itself acts as the moving conductor. Electromagnetic coils inside the meter tube generate a magnetic field across the pipe, and electrodes flush with the liner measure the induced voltage, which is directly proportional to the fluid velocity.

Crucial Requirement: Magmeters only work with electrically conductive fluids (e.g., water, wastewater, acids, mineral slurries). They cannot measure gases or electrically non-conductive liquids such as most hydrocarbons. Very low-conductivity liquids, including highly purified water, require a meter specifically rated for the actual conductivity and installation; do not reject or approve the application from the fluid name alone. They have no obstruction or moving part in the measuring tube and therefore add little permanent pressure loss beyond the selected liner, diameter, and installation effects.

Vortex Shedding Flow Meters

Vortex meters place a blunt, unstreamlined object (called a bluff body) in the center of the flow stream. As fluid flows past, it cannot follow the sharp contours and instead creates alternating low-pressure swirls (vortices) on either side of the body. The frequency of these shedding vortices is directly proportional to the fluid velocity. A piezoelectric sensor detects the pressure pulses. Vortex meters are incredibly versatile and can be used for liquids, gases, and steam.

Ultrasonic Flow Meters

Ultrasonic meters use high-frequency sound waves to measure fluid velocity, usually mounted externally (clamp-on) or internally (wetted).

• Transit-Time: Measures the time it takes an ultrasonic signal to travel diagonally across the pipe between two transducers. A signal traveling downstream moves faster than one traveling upstream. The time difference is proportional to velocity. This requires relatively clean liquids with few bubbles or solids.
• Doppler: Relies on the Doppler effect. The meter bounces sound waves off bubbles or solid particles suspended in the fluid. The frequency shift of the returning wave determines the velocity. This is best for dirty, aerated, or slurry flows where transit-time would fail.

Turbine Flow Meters

A multi-bladed rotor is suspended in the flow stream. The kinetic energy of the fluid pushes the blades, causing the rotor to spin at a speed proportional to the volumetric flow rate. A magnetic pickup coil detects the passing blades. Turbine meters are highly accurate and are frequently used in custody transfer of clean liquids and natural gas.

Positive Displacement (PD) Flow Meters

PD meters mechanically divide the fluid into precise, known volumes and count the number of volumes transferred, much like a revolving door. Examples include Oval Gear, Nutating Disc, and Rotary Vane meters. They are highly accurate, particularly for highly viscous fluids (like syrups or heavy oils), and uniquely do not require straight pipe runs upstream or downstream because they don't rely on a fully developed velocity profile.

Mass Flow Measurement

While volumetric meters measure the physical space a fluid occupies, mass flow meters measure the actual mass. Mass is critical for chemical reactions and combustion control, as these processes rely on molecular mass, not volume, which fluctuates with temperature and pressure.

Coriolis Mass Flow Meter

A Coriolis meter splits the fluid into one or two vibrating tubes (often U-shaped or straight). The fluid's inertia resists the vibration as it moves toward and away from the center of rotation, causing the tubes to twist slightly. Sensors measure the phase shift between the inlet and outlet sides of the vibrating tube. This phase shift is directly proportional to the mass flow rate.

Coriolis meters directly infer mass flow and are not converted from volume using an assumed density, although installation, two-phase flow, coating, pressure, temperature, and device limits still affect performance. They can also infer fluid density (by analyzing the resonant frequency of the tubes) and temperature.

Thermal Mass Flow Meter

Thermal mass meters are primarily used for gases. They insert two temperature sensors into the flow stream. One sensor measures the ambient gas temperature, while the other is heated. The meter measures the amount of power required to maintain a constant temperature difference between the two sensors. As mass flow increases, the cooling effect increases, requiring more power. This cooling effect is directly proportional to the mass flow rate of the gas, leveraging the specific heat capacity of the fluid.