Orifice Meter, Venturi Meter, Pitot Tube and Rotameter

Orifice Meter

A pipe-based orifice meter measures the flow rate of moving fluids (liquids or gases) in the pipeline. It is a topic of fluid machinery.

A fluid's average velocity or rate of flow determines the reduction in pressure.

Definition

In Fluid Mechanics and Machinery, orifice meters or plates are used to measure the average flow rate or velocity of fluids. The orifice meter or Plate operates on the basis of Bernoulli's theorem, which states that the sum of all energy at one location equals the sum of all energy at another.



There is total four types of orifice meter

Conical orifice plate

For lower Reynolds numbers, a Conic Edge orifice plate is beneficial. It features a 45-degree bevel that faces upstream into the running stream.

Quadrant orifice plate

This aperture is used for fluids with a high viscosity.

Eccentric orifice plate

It is used to measure fluids that carry a tiny quantity of or gases that include a small amount of liquid and non-abrasive particulates. A spherical hole (bore) is located at the center of the pipe perpendicular to the interior wall.

Segmental orifice plate

In addition to measuring liquids and gases that are abrasive, the segmental plate can also be used to measure objects that are non-abrasive, such as light slurries or particularly dirty gases.

Orifice meter working principle

The orifice meter operates on the concept of Bernoulli's equation.



As shown in the picture, there is a pipe through which fluid flows from one side to the other, from input to exit. The manometer installed here is used to measure pressure changes between two points.

Now we insert a thin orifice plate with a small hole in the centre through which the fluid will travel. When the velocity rises, the pressure decreases and vice versa.

It just affects the discharge rate at the point where the orifice plates are located in the pipe. The discharge may be computed using the formula, which will be presented in the section on derivation.

Advantages

• The following are the benefits of using an orifice meter:
• When compared to other flow meters such as the venturi meter, the orifice meter is fairly inexpensive.
• The direction might be vertical, horizontal, or sloped.
• There is less room necessary for installation.
• It is often narrow enough to fit between existing pipes.
• It has a relatively low pressure drop.

Disadvantages

• There are restrictions on the length of the vena-contracta that can make determining the minimum pressure for reading the flow difficult.
• Venturi meters can be used to measure downstream pressure. However, downstream pressure cannot be recovered in orifice meters.
• It necessitates a single liquid phase.
• The opening The viscosity, density, and pressure of the fluid can all have an impact on accuracy.
• For optimum precision and accuracy, a straight pipe is required.
• The differential pressure's total head loss ranges from 40% to 90%.

Applications

Natural Gas, Oil Filtration Plants, Water Treatment Plants, Petrochemicals, and Refineries are some of the sites where orifice meters are used to monitor flow rates.

Venturi meter

Venturi meters measure the rate of liquid flow by converting pressure energy into kinetic energy. Clemans Herchel, an American engineer, created it, and Giovanni Venturi, an Italian physicist, named it. It builds upon Bernoulli's Equation as its foundation.

Construction

Venturi meter consists of two conical sections separated by a small section of uniform cross-section. A neck has the smallest area of any part of the body. Each conical part has the same base diameter, however one has a smaller cone angle and is shorter, while the other has a greater cone angle.

Applications of venturi meter

• Water, liquids, gases, dirty liquids, etc. can all be measured with a ventilator.
• It is typically found in the water supply industry.

Advantages of venturi meter

• The differential pressure head loss is about 10%.
• The system can be utilized in any position, for example, horizontally or vertically.
• Due to a smaller throat size and greater pressure differential, higher sensitivities can be achieved.

Disadvantages of venturi meter

• The larger size limits the amount of space available.
• Due to its larger size, venturimeters are more expensive.
• Cavitations of fluid are caused by very small throat diameters.
• The downstream tapping (throat) is more likely to develop errors due to burrs or deposits.

Pitot tube

A pitot tube is a mechanical device used to monitor flow velocity at any point in a pipe or channel. It was created by Henri Pitot, a French engineer, and refined by Henry Darcy, a French scientist.

Pitot Tube operates on a fundamental principle of fluid mechanics: when a fluid ceases to flow, all of its kinetic energy is transformed into pressure energy. It aids us in calculating the pressure energy of a stopped fluid.

The square of velocity is directly proportional to the kinetic energy, so flow velocity can be calculated.

There are three types of pitot tubes that includes -

• Static source
• Simple pitot tube
• Pitot-static tube

Applications of pitot tube

• It is used to determine the speed of an aircraft.
• A watercraft's speed is determined by this device.
• It is used in a variety of sectors to measure fluid flows.
• It is utilized when extreme precision is not necessary.
• A flow profile is determined by measuring the flow of a channel or duct.
• Flow velocity is commonly measured with Pitot tubes.

Advantages of pitot tube

• Installation and removal are easy and inexpensive. Because there are no moving parts, frictional losses are reduced.
• It is compact in size, inexpensive, and results in very little pressure loss.

Disadvantages of pitot tube

• Any foreign item in the flow might cause the correct reading to vary.
• Its precision isn't all that great.
• It has a limited range.
• It is insensitive.
• It is more effective in high-velocity flows.
• Its sensitivity is affected by flow direction.

Rotameter

Rotameter construction

One can build a rotameter using a clear tube, a scale, a float, a transmitter, and other components.

• The transparent tube in the structure is conical in form, with a scale that floats within it. It is very useful to be able to see the measurements immediately after taking them.
• Floats are tiny devices that fit inside tubes of rotameters and have precise dimensions. The float indicates the velocity of liquid flow within the tube, which can be made of plastic, glass, or metal.
• This meter's scale displays flow data by displaying a float.
• When compared to physically observing the flow on the scale, transmitters are highly useful in capturing the flow data precisely.
• It is composed of a float and a tapered tube, with the float positioned within the tapered tube. Pipelines are used to route the network, with flanged connectors at both ends. Rotameters are always linked vertically in pipes, and a scale is accessible on the tube to immediately examine flow rate measurements.

Working

In a rotameter, liquid flows from the base of the tapering tube, part of which directly reaches the bottom of the float, while the remainder flows away from the float. As a result, the float in the rotameter experiences two forces in the opposite direction: gravity force on the underside and drag force on the upside.

The float is propelled upward by the flow of liquid. After some time, the flowing zone reaches a point where the force imposed on the floating body precisely matches the weight of the float. Thus, equilibrium will be reached after the float is dragged enough, similar to weight, for its area to provide sufficient drag.

When both gravity and float weight are steady, the distance the float moves upward is proportional to the flow velocity of the liquid flowing through the tapered tube.

Advantages

• Inexpensive
• There is a low pressure drop
• It is suitable for low flow rates.

Disadvantages

• It must be linked vertically.
• It is tough to handle the glass tube.
• It is not suitable for pulsing services.
• It is limited to a few pipe services.
• Only low temperatures are suitable for its use.
• Its accuracy ranges from 1/2 to 10%.
• It employs in-line mounting.

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