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Friday, December 16, 2011

Recovery of Pressure Drop in Orifices, Nozzles and Venturi Meters

After the pressure difference has been generated in the differential pressure flow meter, the fluid pass through the pressure recovery exit section, where the differential pressure generated at the constricted area is partly recovered.

As we can see, the pressure drop in orifice plates are significant higher than in the venturi tubes.

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Flow Nozzles

Flow nozzles are often used as measuring elements for air and gas flow in industrial applications.

The flow nozzle is relative simple and cheap, and available for many applications in many materials.
The TurnDown Rate and accuracy can be compared with the orifice plate.

The Sonic Nozzle - Critical (Choked) Flow Nozzle:

When a gas accelerate through a nozzle, the velocity increase and the pressure and the gas density decrease. The maximum velocity is achieved at the throat, the minimum area, where it breaks Mach 1 or sonic. At this point it's not possible to increase the flow by lowering the downstream pressure. The flow is choked.
This situation is used in many control systems to maintain fixed, accurate, repeatable gas flow rates unaffected by the downstream pressure.

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Venturi Tube

Due to simplicity and dependability, the Venturi tube flowmeter is often used in applications where it's necessary with higher TurnDown Rates, or lower pressure drops, than the orifice plate can provide.

In the Venturi Tube the fluid flowrate is measured by reducing the cross sectional flow area in the flow path, generating a pressure difference. After the constricted area, the fluid is passes through a pressure recovery exit section, where up to 80% of the differential pressure generated at the constricted area, is recovered.

With proper instrumentation and flow calibrating, the Venturi Tube flowrate can be reduced to about 10% of its full scale range with proper accuracy. This provides a TurnDown Rate 10:1.

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Orifice Plate

With an orifice plate, the fluid flow is measured through the difference in pressure from the upstream side to the downstream side of a partially obstructed pipe. The plate obstructing the flow offers a precisely measured obstruction that narrows the pipe and forces the flowing fluid to constrict.

The orifice plates are simple, cheap and can be delivered for almost any application in any material.

The TurnDown Rate for orifice plates are less than 5:1. Their accuracy are poor at low flow rates. A high accuracy depend on an orifice plate in good shape, with a sharp edge to the upstream side. Wear reduces the accuracy.
Orifice plate flowmeter installation
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Differential Pressure Flowmeters

In a differential pressure drop device the flow is calculated by measuring the pressure drop over an obstructions inserted in the flow. The differential pressure flowmeter is based on the Bernoullis Equation, where the pressure drop and the further measured signal is a function of the square flow speed.

The most common types of differential pressure flowmeters are:
•    Orifice Plates
•    Flow Nozzles
•    Venturi Tubes
•    Variable Area - Rotameters

For detailed explanation see the upcoming posts.

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Different flow meters and their principles of working

The most common principals for fluid flow metering are:

•    Differential Pressure Flowmeters
•    Velocity Flowmeters
•    Positive Displacement Flowmeters
•    Mass Flowmeters
•    Open Channel Flowmeters

Differential Pressure Flowmeters : 
In a differential pressure drop device the flow is calculated by measuring the pressure drop over an obstructions inserted in the flow. The differential pressure flowmeter is based on the Bernoullis Equation.

Velocity Flowmeters : 
In a velocity flowmeter the flow is calculated by measuring the speed in one or more points in the flow, and integrating the flow speed over the flow area.

Positive Displacement Flowmeters :
The positive displacement flowmeter measures process fluid flow by precision-fitted rotors as flow measuring elements. Known and fixed volumes are displaced between the rotors. The rotation of the rotors are proportional to the volume of the fluid being displaced.

Mass Flowmeters :
Mass meters measure the mass flow rate directly.

Open Channel Flowmeters :
A common method of measuring flow through an open channel is to measure the height of the liquid as it passes over an obstruction as a flume or weir in the channel.

In the upcoming posts the detailed explanation of different flowmeters that uses the mentioned principles is covered. See those.

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Flow Meters : An Introduction

An introduction to flow meters: 
Flow meters have a wide selection of applications and are utilized in nearly every industry and in some laboratories too. A flow meter is a machine to measure large quantity fluid movement. You will find many methods used to calculate flow. Considered one of the common kinds of flow meters is the positive displacement flow meter. It basically gathers an allocated volume of the fluid then it will count the number of instances the volume is filled to sum up the flow. Others have a constriction and by calibrating the forces produced by the running stream across the constriction, this will compute the flow indirectly. If we need to employ a flow meter for any application, it is always good to purchase a top quality unit since it will give long years of trouble free service.
Much of the applications for flow meters are:

- Production process.
- To compute effluent emission.
- Steam movement
- Oil flows
- Air flows
- Gas movement
- For laboratory use.
- To compute water and gas usage.
- To measure stream of rivers and opportunities.
- To calculate fuel being dispensed.

These are only some of the uses. Flow meters have hundreds of applications and are of the various types. The form of flow meter depends upon its application. Flow meters can measure in minute quantities and in some massive applications they could measure fluid flow in large volumes.

Some of the different types of flow gauge are:

- Piston meter/Rotary piston: They are one of the commonest flow meters and are utilized to measure domestic water utilization.

- Variable area meter: This flow meter is also called a 'rotameter' and consists of a tapered tube with a float inside that is pushed upwards by the fluid flow and downwards by gravity. While the flow rate keeps increasing, the ball moves up in the tapered tube till the upward and down forces are in a state of equilibrium. The reading at that point is the flow. It is used mainly to measure flow of water or air and it is precise up to =/- 1%.

- Turbine flow meter: It obstructs the stream of the liquid only minimally and this design allows for measurement of greater flow rates. They are mostly used by large commercial users.

- Some of the other kinds of meters are Woltmann gauge, Single jet gauge, Paddle wheel meter, multiple jet meter, Pelton wheel, Oval gas meter, Nutating disk gauge, Current meter etc.

These are generally only a few of the many types of flow meters available. Some of the other types are Pressure based meters. They function on Bernoulli's principle by measuring differential pressure. Venturi meters shrink the flow of the fluid and by utilizing pressure sensors to measure the differential pressure, before and after constriction, the flow is calculated. The Pitot tube measures the fluid flow by computing the stagnation pressure and by making use of Bernoulli's equation, the rate of flow is determined.

Flow meters are regular and we are certain to encounter a flow meter practically daily. One of the most common flow meters is the diesel dispenser. The other is our domestic water meter. A flow meter may be manual or electronic, simple or sophisticated, but plays important part in our way of life either directly or indirectly.

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Basics of Instrumentation & Control

To view more posts on BASICS : CLICK HERE


To view more posts on PRESSURE : CLICK HERE


To view more posts on FLOW : CLICK HERE


To view more posts on LEVEL : CLICK HERE


To view more posts on TEMPERATURE : CLICK HERE

Analytical Instrumentation

To view more posts on Analytical Instrumentation : CLICK HERE
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