flow measuring instruements

Assignment

Assign by:Sir Usman Saeed

Prepared by:Rashid Saeed

Regd.#2012-CH-425

MNS-UET Multan

FLOW MEASURING INSTRUEMENTS

Contents:

Rotameter Venturi meter Orifice meter Pitot tube Positive displacement meters Metering pumps

Rotameter

working principle:

A rotameter is a type of flow meter based on the variable area principle and is used to indicate flow rates in fluid systems. The rotameter is placed vertically in the fluid system with the diameter end of the tapered flow tube at the bottom. When the fluid is poured into the tube, the float is raised from its position at the inlet allowing the fluid to pass between it and the wall.

Advantages:

Easy to use Direct readings Low cost Easily maintenence

Venturi meter:

The entry of the venture is cylindrical in shape to match the size of the pipe through which fluid flows. This enables the venture to be fitted to the pipe.

After the entry, there is a converging conical section with an included angle of 19º to 23º.

Following the converging section, there is a cylindrical section with minimum area called as the throat.

After the throat, there is a diverging conical section with an included angle of 5’ to 15’.

Basic principle:

When a venturi meter is placed in a pipe carrying the fluid whose flow rate is to be measured, a pressure drop occurs between the entrance and throat of the venture meter. This pressure drop is measured using a pressure sensor and when calibrated this pressure drop becomes a measure of flow rate.

Application:

It is used where high pressure recovery is required.

Can be used for measuring flow rates of water, gases, and dirty liquids.

Can be used to measure high flow rates.

Advantages:

Coefficient of discharge is high. Can be installed vertically, horizontally or

inclined.

Limitations:

They are large in size and hence where space is limited, they cannot be used.

Expensive initial cost, installation and maintenance.

Orifice meter:

An orifice plate is a device used for measuring flow rate, for reducing pressure or for restricting flow.

An orifice plate is a thin plate with a hole in the middle. It is usually placed in a pipe in which fluid flows. When the fluid reaches the orifice plate, the fluid is forced to converge to go through the small hole; the point of maximum convergence actually occurs shortly downstream of the physical orifice, at the so-called vena contracta point . As it does so, the velocity and the pressure change. Beyond the vena contracta, the fluid expands and the velocity and pressure change once again.

Advantages:

Orifice are small plates and easy to install/remove.

Offer very little pressure drop of which 60 to 65 % is recovered.

Easy maintenance.

limitations:

Orifice flow range is max. to 1/4 of max. Only 60 to 65 % of pressure drop is

recovered.

Pitot tube:

A pitot tube is a pressure measuring instrument used to measure fluid flow velocity.

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.

Principle:

The measuring principle of the pitot tube utilizes the differences between the pressure ridge on the upstream side and the static pressure on its downstream side.

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 the pitot pressure.

POSITIVE DISPLACEMENT

METERS:

PD METERS:A positive displacement meter is a type of

flow meter that requires the fluid being measured to mechanically displace

components in the meter in order for any fluid flow to occur.

Advantages & Applications:

Positive displacement flow meters are very accurate.

having high turndown. They can be used in very viscous, dirty and corrosive fluids essentially require no straight runs of

pipe for fluid flow stream conditioning.

Drawbacks:

Relatively expensive Produce the highest differential pressure subsequently greatest pressure drop

head loss of all the flow meter types

Types:

Nutating disk Rotating vane Lobed impeller

Nutating Disk PD Meters:

Principle of Operation: Liquid enters a machined chamber containing

a disc which nutates (wobbles). The position of the disc divides the chamber into compartments containing an exact volume. Liquid pressure drives the disc to wobble and a roller cam causes the nutating disc to make a complete cycle. The compartments are filled and emptied each cycle. The movements of the disc are transmitted by a gear train to a register/totalizer or pulse transmitter.

Working :

The movable element is a circular disk which is attached to a central ball. A shaft is fastened to the ball and held in an inclined position by a cam or roller. The disk is mounted in a chamber which has spherical side walls and conical top and bottom surfaces. The fluid enters an opening in the spherical wall on one side of the partition and leaves through the other side. As the fluid flows through the chamber, the disk wobbles, or executes a nutating motion. Since the volume of fluid required to make the disc complete one revolution is known, the total flow through a nutating disc can be calculated by multiplying the number of disc rotations by the known volume of fluid.

Advantages and Applications:

Low cost Moderate pressure loss Available in several materials of

construction This type of flow meter is normally used

for water service, such as raw water supply and evaporator feed.

ROTATING VANE METERS:

Rotating Vane meters:

There are two or three couples of positive displacement rotary vanes integrated with a rotor to form a rotating unit that is installed inside of the measuring chamber a meter.The positive displacement rotary vanes are fitted in the grooves in the rotor with the quantities Accordance with the quantities of the positive displacement rotary vanes.

Advantages:

Low pressure loss Applicable to wide variety of gas and

liquid fluids including viscous materials High temperature and pressure rating Good accuracy Available in numerous construction

materials

Disadvantages:

High cost Sustainable to damage from entrained

vapors and dirty fluids Low accuracy at low flow rates Bulky and heavy

LOBED IMPELLER METER:

Lobed Impeller Meter:

In the rotating lobe design, two impellers rotate in opposite directions within the void housing. As they rotate, a fixed volume of liquid is entrapped and then transported toward the outlet. Because the lobe gears remain in a fixed relative position, it is only necessary to measure the rotational velocity of one of them.

Advantages:

Increasingly accurate at higher flow rates where leakage and slip is decreased

Good capacity range Can be used for corrosive services

Thank You