Flow Rate Measurement Techniques:

  • Indirect Measurement:

Many flow measurement instruments rely on indirect methods like measuring the differential pressure across a restriction to determine flow rate.

Principle:
These instruments use a restriction in the fluid path, which causes a pressure drop as the fluid flows through it.
The resulting differential pressure is directly proportional to the flow rate.

Common Restrictions:

Orifice Plate:

  • A thin plate with a hole in the middle, placed perpendicular to the flow direction.
  • The pressure drop across the orifice plate is measured to determine the flow rate.

Venturi Tube:

  • A tube with a gradually narrowing throat and then a gradually expanding section.
  • The pressure drop occurs in the throat and is used to measure the flow.

Flow Nozzle:

  • Similar to the Venturi tube, but with a simpler design and higher-pressure drops.
  • Used for measuring flow in high-velocity fluid streams.

Dall Tube:

  • A modification of the Venturi tube with a shorter length and simpler design.
  • Generates a slightly lower pressure drop than the Venturi tube, making it more efficient.

These devices leverage pressure differentials created by constrictions in the flow path to infer flow rates, providing an indirect but reliable method for flow measurement. Each type has its specific application based on the required accuracy, efficiency, and flow characteristics.

  • Orifice plates

Orifice plates are reliable, simple, and commonly used instruments for measuring fluid flow rates based on differential pressure. Their design varies according to the characteristics of the fluid being measured and the specific application requirements, but the underlying principle of pressure drop remains constant.

Key Features:
Construction:
The orifice plate is typically a flat, thin metal diaphragm with a precise, constricted hole in the centre.
It is clamped between pipe flanges to allow easy removal and maintenance.

  • Differential Pressure Measurement:
    When fluid flows through the orifice, the constriction creates a pressure drop.
    This pressure drop is directly proportional to the flow rate of the fluid.
    Differential pressure gauges measure the pressure difference across the plate and can be calibrated to reflect flow rates.
  • Ports and Location:
    Differential pressure ports are usually located on either side of the orifice plate.
    They can also be positioned at specific locations in the pipe, chosen based on flow patterns (the vena contracta).
  • Design Considerations:
    The leading edge of the orifice hole is often beveled to reduce turbulence.
    The orifice’s central hole design is adapted based on fluid characteristics:
    Centred for general fluids.
    Offset to the bottom to prevent particulate build up when fluids contain particulates.
    Semicircle at the bottom of the pipe for specific applications.
  • Pressure Profile:
    The pressure drops sharply as the fluid passes through the orifice, reaching a minimum at the vena contracta (the narrowest section of the flow stream).
    The pressure then gradually recovers downstream of the plate but does not reach its original level.

 

  • Venturi tube

A Venturi tube is a device used to measure the flow rate of a fluid by utilizing the principles of fluid dynamics. It consists of a specially shaped tube with a narrow throat between two larger sections, leveraging the Bernoulli principle to create a pressure differential that can be measured and used to calculate the flow rate.

Working Principle:

  • Rotor and Blades:
    The core of the turbine flow meter is a rotor equipped with blades.
    When the fluid flows through the meter, it rotates the rotor, with the rotational speed proportional to the flow rate.
  • Pulse Generation:
    The movement of the rotor is detected magnetically or optically.
    As the rotor spins, it generates pulses, the frequency of which correlates directly to the fluid velocity.
    These pulses are used to calculate the flow rate.
  • Fluid Movement:
    Fluid movement transforms energy into rotational energy, which spins the rotor on bearings.
    The rotational speed is measured mechanically or via sensor detection.
  • Flow Range:
    Turbine flow meters are well-suited for clean, low-viscosity liquids and gases.
  • Custody Transfer:
    Commonly used in custody transfer for hydrocarbons and natural gas.
  • Fluid Properties:
    Measures velocity precisely, requiring calibration to account for pressure, temperature, and fluid properties.
  • Limitations:
    Bearings:
    Wear of bearings affects accuracy, especially with non-lubricating fluids.
    Abrupt Transitions:
    Abrupt flow changes can degrade meter accuracy and damage the meter.
  • Flow Range:
    Accuracy diminishes below 5% of maximum flow due to drag from the rotor and bearings.
  • Flow Conditions:
    Avoid high velocity, which causes premature bearing wear.
  • Flow Nozzle

Design:
A flow nozzle has a convergent section followed by a short cylindrical throat, ending with a divergent outlet.
Advantages:
Provides a good balance between the cost and accuracy of the Venturi tube and orifice plate.
Main Use:
Primarily used for measuring steam flow.
Limitations:
Not suitable for fluids with suspended particles due to the narrow throat.

  • Dall Tube

Design:
Similar to the Venturi tube, but has a shorter length and different profile.
Advantages:
Has the lowest insertion loss among differential pressure meters, offering high efficiency.
Limitations:
Not suitable for slurries due to potential clogging.

  • Elbow

Design:
Utilizes the natural elbow of the pipe as a differential pressure flow meter.
Measures the differential pressure between the inside and outside of the elbow due to fluid direction change.
Advantages:
Can be used as a simple flow meter by leveraging existing pipe infrastructure.
These devices are commonly used in industrial applications where accurate flow measurement is essential. Each type has its specific applications and limitations, making them suitable for different scenarios.