Gujarat Technological University L.D. College of Engineering

Year: 2016-17 Subject: Fluid Flow operation(2130502)

Topic: Orifice meter & Pitot tubeName of the Students:

Sindhav Jaydrath 160283105011

Vasava Yogesh 160283105012

ORIFICE METER

The venturi meter is a reliable flow measuring device.

Furthermore, it causes little pressure loss.

For these reasons it is widely used, particularly for large-volume liquid and gas flows.

However this meter is relatively complex to construct and hence expensive.

Especially for small pipelines, its cost seems prohibitive, so simpler devices such as orifice meters are used.

PRINCIPAL OF ORIFIC METER

The principle of the orifice meter is identical with that of the venturi meter. The reduction of the cross section of the flowing stream in passing through the orifice increases the velocity head at the expense of the pressure head, and the reduction in pressure between the taps is measured by a manometer. Bernoulli's equation provides a basis for correlating the increase in velocity head with the decrease in pressure head.

One important complication appears in the orifice meter that is not found in the venturi. Because of the sharpness of the orifice, the fluid stream separates from the downstream side of the orifice plate and forms a free-flowing jet in the downstream fluid and vena contracta forms.

Diagram

Continue…

It is also known as Pipe orifice & Orifice plate. It may be installed in pipeline with a minimum of trouble and expense. Pipe orifice is a device used for measuring the rate of flow of a fluid

through a pipe. It consists of a thin, circular plate with a hole in it. The plate is held in the

pipeline between two flanges. Pipe orifice is a cheaper device as compared to venturimeter. The diameter of orifice is generally 0.5 times the diameter of the pipe (D),

although it may vary from 0.4 to 0.8 times the pipe diameter.

Orifice plates can be a surprisingly good way of measuring small gas flows. To be able to use orifice plates of very small orifice diameter in an existing installation in a declining gas field may be much more economical than to replace the metering or shut the field.

From the beginning of the flow measurement to its conclusion a 4″ orifice meter with different orifice plates (including those described in this appendix) might measure a range of around 3000:1 in terms of mass flow rate.

The orifice fitting, which enables the operator to change or remove an orifice plate easily.

Continue…

APPLICATION Orifice plates are most commonly used to measure flow rates in pipes, when

the fluid is single-phase (rather than being a mixture of gases and liquids, or of liquids and solids) and well-mixed, the flow is continuous rather than pulsating, the fluid occupies the entire pipe (precluding silt or trapped gas), the flow profile is even and well-developed and the fluid and flow rate meet certain other conditions.

Under these circumstances and when the orifice plate is constructed and installed according to appropriate standards, the flow rate can easily be determined using published formulae based on substantial research and published in industry, national and international standards.

Plates are commonly made with sharp-edged circular orifices and installed concentric with the pipe and with pressure tapping's at one of three standard pairs of distances upstream and downstream of the plate.

The edges may be rounded or conical, the plate may have an orifice the same size as the pipe except for a segment at top or bottom which is obstructed, the orifice may be installed eccentric to the pipe, and the pressure tapping's may be at other positions. Variations on these possibilities are covered in various standards and handbooks. Each combination gives rise to different coefficients of discharge which can be predicted so long as various conditions are met, conditions which differ from one type to another.

Once the orifice plate is designed and installed, the flow rate can often be indicated with an acceptably low uncertainty simply by taking the square root of the differential pressure across the orifice's pressure tapping's and applying an appropriate constant. Even compressible flows of gases that vary in pressure and temperature may be measured with acceptable uncertainty by merely taking the square roots of the absolute pressure and/or temperature, depending on the purpose of the measurement and the costs of ancillary instrumentation.

Continue…

Pitot tube

A Pitot tube is a pressure measurement instrument used to measure fluid flow velocity. The pitot tube was invented by the French engineer Henri Pitot in the early 18th century and was modified to its modern form in the mid-19th century by French scientist Henry Darcy. 

It is widely used to determine the airspeed of an aircraft, water speed of a boat, and to measure liquid, air and gas velocities in industrial applications. The pitot tube is used to measure the local velocity at a given point in the flow stream and not the average velocity in the pipe or conduit.

Diagram

Types of Pitot tube

Working The basic pitot tube consists of a tube pointing directly into the fluid flow.

As this tube contains fluid, a pressure can be measured; the moving fluid is brought to rest (stagnates) as there is no outlet to allow flow to continue. This pressure is the stagnation pressure of the fluid, also known as the total pressure or (particularly in aviation) the pitot pressure.

The liquid flows up the tube and when equilibrium is attained, the liquid reaches a height above the free surface of the water stream. 

Measurement of static pressure in this case is made at the boundary of the wall (Fig.-1). The axis of the tube measuring the static pressure must be perpendicular to the boundary and free from burrs, so that the boundary is smooth and hence the streamlines adjacent to it are not curved. This is done to sense the static pressure only without any part of the dynamic pressure.

A Pitot tube is also inserted as shown (Fig.-2) to sense the stagnation pressure. The ends of the Pitot tube, measuring the stagnation pressure, and the piezometric tube, measuring the static pressure, may be connected to a suitable differential manometer for the determination of flow velocity and hence the flow rate.

Fig-1 fig-2

Operations

In industry, the velocities being measured are often those flowing in ducts and tubing where measurements by an anemometer would be difficult to obtain. In these kinds of measurements, the most practical instrument to use is the pitot tube.

The pitot tube can be inserted through a small hole in the duct with the pitot connected to a U-tube water gauge or some other differential pressure gauge for determining the velocity inside the ducted wind tunnel. One use of this technique is to determine the volume of air that is being delivered to a conditioned space.