Mechanical Engineering Principles

250 MECHANICAL ENGINEERING PRINCIPLES

h

B

A

Direction of

fluid flow

Figure 22.6

difference ofh, may be calibrated to indicate the
velocity of flow directly.

Applications

A Pitot-static tube may be used for both turbulent
and non-turbulent flow. The tubes can be made very
small compared with the size of the pipeline and the
monitoring of flow velocity at particular points in the
cross-section of a duct can be achieved. The device
is generally unsuitable for routine measurements and
in industry is often used for making preliminary
tests of flow rate in order to specify permanent flow
measuring equipment for a pipeline. The main use
of Pitot tubes is to measure the velocity of solid
bodies moving through fluids, such as the velocity
of ships. In these cases, the tube is connected to
a Bourdon pressure gauge that can be calibrated to
read velocity directly. A development of the Pitot
tube, apitometer, tests the flow of water in water
mains and detects leakages.

Advantages of Pitot-static tubes

(i) They are inexpensive devices.

(ii) They are easy to install.

(iii) They produce only a small pressure loss in
the tube.

(iv) They do not interrupt the flow.

Disadvantages of Pitot-static tubes

(i) Due to the small pressure difference, they are

only suitable for high velocity fluids.

(ii) They can measure the flow rate only at

a particular position in the cross-section of

the pipe.

(iii) They easily become blocked when used with

fluids carrying particles.

22.7 Mechanical flowmeters

With mechanical flowmeters, a sensing element sit-

uated in a pipeline is displaced by the fluid flowing

past it.

Examples of mechanical flowmeters commonly

used include:

(a) Deflecting vane flowmeter (see Section 22.8)

(b) Turbine type meters (see Section 22.9)

22.8 Deflecting vane flowmeter

The deflecting vane flowmeter consists basically of

a pivoted vane suspended in the fluid flow stream

as shown in Figure 22.7.

Scale Pointer

Pivoted vane

Direction of

fluid flow

Figure 22.7

When a jet of fluid impinges on the vane it

deflects from its normal position by an amount

proportional to the flow rate. The movement of the

vane is indicated on a scale that may be calibrated in

flow units. This type of meter is normally used for

measuring liquid flow rates in open channels or for

measuring the velocity of air in ventilation ducts.

The main disadvantages of this device are that it

restricts the flow rate and it needs to be recalibrated

for fluids of differing densities.

22.9 Turbine type meters

Turbine type flowmeters are those that use some

form of multi-vane rotor and are driven by the fluid

being investigated. Three such devices are the cup

anemometer, the rotary vane positive displacement

meter and the turbine flowmeter.