Flow elements are primary sensing devices used in fluid measurement and control systems to monitor the flow of liquids, gases, or other fluids. There are various types of flow elements, each designed for specific applications and measurement requirements.
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Types of Flow Elements
Here are some common types of flow elements
- Pitot tube
- Orifice plates
- Venturi flow element
- Averaging Pitot tube
- Target flow element
- Flow nozzle
- V-cone flow sensor
- Segmental wedge flow sensor
1. Pitot tube:
It is a primary element used to measure fluid velocity in the field of aeronautics and aerodynamics. It is named after Henri Pitot, a French engineer who invented the device in the 18th century.
The Pitot tube consists of a hollow tube with two openings, one facing directly into the fluid flow (stagnation pressure Opening) and the other perpendicular to the flow (the static pressure opening).
Stagnation Pressure: The opening of the Pitot tube facing directly into the fluid flow collects the total pressure of the fluid. This pressure includes both the static pressure and the dynamic pressure. When the fluid comes to a stop in this opening, it momentarily stagnates, hence the name stagnation pressure.
By measuring the pressure difference between the total pressure opening and the static pressure opening, it is possible to measure velocity; its substation in the flow equation gives actual flow.
The Pitot tube is used in various industrial applications apart from the aviation industry, where it measures air speed on aircraft. It is used in various industries where precise measurement of fluid velocity there by flow measurement.
2. Orifice plates
The choice of orifice flow element depends on factors such as type of fluid, flow conditions, accuracy requirements, and potential for obstruction or damage.
Orifice plates create differential pressure when they are placed in a flow path, and the amount of this pressure drop is used to calculate the flow rate. There are several types of orifice flow elements: Concentric, Eccentric, Quadrant edge, and Segmental.
Out of the four, the concentric orifice is most widely used. The eccentric orifice is used in applications where suspended solids or entrained gases are in the fluid. This type of orifice reduces clogging or damage to the orifice plate. A segmental orifice plate is not a full circular opening. Quadrant edge orifices have sharp-edged openings with 90-degree edges. These types of orifices are helpful for non-Newtonian fluids.
Read More- Orifice Flow Meter- Its Working, Types of Orifice Plates
3. Venturi Flow Element
It is a primary sensing element used to measure the flow rate of a fluid, such as gas or liquid, through a pipeline.
The Venturi element consists of a tapered tube with a narrow throat in the middle. When fluid flows through the Venturi element, it accelerates as it passes through the narrowest part (the throat) of the tube and then decelerates as it returns to the wider sections of the tube. This change in velocity causes a corresponding change in pressure within the device.
Measuring the difference in pressure across the throat and the converging part of the flow element makes it possible to determine the flow rates accurately.
4. Averaging Pitot tube
Averaging Pitot tube, also called Annubar (brand name), is a specialized primary sensing element used to measure a fluid’s velocity or flow rate within a pipeline or duct. It is designed to provide an average velocity measurement across the entire cross-section of the flow, as opposed to a single-point measurement that a standard Pitot tube would provide.
It consists of multiple pressure-sensing holes arranged along its length. Different pressures are captured at different hole locations across the cross-section of the flow. By averaging these pressure readings, the instrument provides a more accurate representation of the flow rate within the pipeline.
When fluid flows through the tube, it creates a pressure differential between the upstream and downstream sides. This differential pressure is proportional to the liquid or gas flow rate.
Read More-What is a Torbar?- Averaging Pitot Tubes
5. Target flow meter
A target flow meter, also known as a target-disc flow meter, is a disc-type flow meter. It is a type of flow-sensing primary element used to measure the liquid flow in a pipeline. Its operation is based on the principle of differential pressure. A circular disk is placed perpendicular to the direction of the fluid flow inside a pipe. The target disc is mounted inside with a rod’s help in the pipeline’s center.
When the liquid is directed to flow, it encounters the disc. The flow will move sides, bottom, and below it. It creates a differential pressure (DP) across the upstream and downstream of the target. The differential pressure (DP) across the target disc is directly proportional to the flow rate of the liquid.
It is typically used for sensing slurries and sediment-bearing liquid flows. The Target flow element helps to measure high temperatures and pressures.
6. Flow nozzle
It is a type of flow measurement primary device used to measure the flow rate of a gas or liquid in a pipeline or duct. A flow nozzle generally consists of a converging section, a throat, and a diverging section. In the converging section is the entrance of the nozzle; the converging section is designed to reduce the cross-sectional area of the flow path, where the liquid or gas velocity increases.
The cross-sectional area at the throat is narrow and located at the center of the flow nozzle.
As the liquid or gas enters the diverging section (where the cross-sectional areas increase gradually), the fluid velocity decreases, and the pressure increases.
7. V-Cone flow element
A V-cone flow element is a differential pressure flow meter instrument. It works on the principle that blockage in the pipe increases flow velocity and a corresponding pressure drop. The differential pressure is caused by a cone located in the middle of the pipe. The V-cone shape is specially designed to flatten the fluid’s velocity profile in the tube. This, in turn, helps create a highly stable signal across all flow reductions.
V-cone-type primary flow elements are helpful in situations where the fluid conditions can vary significantly. They can maintain accuracy across a wide range of flow rates.
8. Segmental Wedge flow element
It consists of a specially designed wedge-shaped obstruction placed in the low path.
When the fluid flows through the wedge-shaped opening, it accelerates as it passes through the construction, and this acceleration causes a pressure drop across the wedge. By measuring the pressure drop, engineers can calculate the flow rate of the fluid.
The segmental wedge flow elements cause a relatively significant pressure drop compared to other flow measurement primary elements like orifice plates or Venturi tubes. Segmental wedge flow elements are often used in applications with acceptable high-pressure drops.
