Fluid Flow Measurement: Venturi, Orifice & Pitot Tube

Venturi Meter

A Venturi meter has two entrances at each end, along with a nozzle. Fluid passes through the nozzle, typically made from a single casting. Key components include:

• An upstream section with the same diameter as the pipe, fitted with a brass ring containing piezometric openings to measure static pressure.
• A converging tapered section leading to a cylindrical throat, also equipped with a brass piezometer ring.
• A diverging conical section that gradually expands back to the original pipe diameter. Piezometer rings connect to a differential manometer at each end.

Venturi meter size is specified by the pipe diameter and throat diameter (e.g., a 6"x4" Venturi fits a 6" pipe and has a 4" throat).

For accurate measurements, the Venturi meter requires a straight pipe section at least 10 times the pipe diameter upstream.

As fluid flows from the pipe to the throat, its velocity increases and pressure decreases. The pressure difference, measured by the manometer, is a function of the flow rate.

Venturi meters have various applications, including carburetors and propulsion systems.

Other Features:

• Minimizes pressure drop.
• Features a smooth conical taper to minimize exit shock.
• Suitable for dirty or slightly contaminated fluids.

Orifice Plate

An orifice plate is a perforated plate installed in a pipe. The hole, through which fluid flows, has a cylindrical or prismatic opening. The normalized hole's edge ensures the exiting jet doesn't touch the hole wall.

Flow rate can be determined by measuring the differential pressure. Two taps, located upstream and downstream of the plate, capture this pressure difference.

Plate Types:

• Concentric plates (hole in the center) are used for liquids.
• Eccentric plates (hole offset from center) are used for gases to prevent particle buildup. The high-pressure side tap minimizes measurement errors.
• Segmented plates are used for fluids carrying solids or gases to prevent clogging. The plate includes a vent hole. Common hole profiles include "sharp-edged", "quarter-circle", and "beveled".

Bernoulli's Equation

Bernoulli's equation, a fundamental principle in fluid mechanics, is used to measure flow. It states that the total energy of a fluid particle—the sum of its gravitational potential energy, kinetic energy, and pressure energy—remains constant.

Pitot Tube

A Pitot tube is a simple device used to measure fluid velocity. It consists of a small tube with its opening facing the flow. The fluid velocity at the inlet is zero (stagnation point), converting kinetic energy into pressure energy (P_tot).

Pitot tubes are simple, inexpensive, and available in various sizes. They offer moderate accuracy and are commonly used to measure air velocity. They can also be used with an integration technique to measure total flow in large ducts with virtually any fluid.

Features:

• Measures point velocity.
• Low pressure drop and low cost, making them suitable for large diameter pipes carrying gas or clean fluids.
• Small diameter tube that resists the flow, creating a stagnation point at the tip.

Operation:

The Pitot tube opening measures the total pressure and transmits it to a pressure sensor. Static pressure is measured separately and also transmitted to the sensor. The resulting differential pressure (dynamic pressure) is proportional to the velocity and is analyzed and displayed.

Formula:

By measuring the pressure difference and using the Pitot tube formula derived from Bernoulli's equation, it's possible to determine the total flow rate.