Mechanical Engineering Principles

FLUID FLOW 249

Pressure tappings are made at the entry (atA)
and at the throat (atB) and the pressure differenceh
which is measured using a manometer, a differential
pressure cell or similar gauge, is dependent on
the flow rate through the meter. Usually pressure
chambers are fitted around the entrance pipe and
the throat circumference with a series of tapping
holes made in the chamber to which the manometer
is connected. This ensures that an average pressure
is recorded. The loss of energy due to turbulence
that occurs just downstream with an orifice plate
is largely avoided in the venturimeter due to the
gradual divergence beyond the throat.
Venturimeters are usually made a permanent in-
stallation in a pipeline and are manufactured usu-
ally from stainless steel, cast iron, monel metal or
polyester glass fibre.

Advantages of venturimeters

(i) High accuracy results are possible.

(ii) There is a low pressure loss in the tube (typ-

ically only 2% to 3% in a well proportioned

tube).

(iii) Venturimeters are unlikely to trap any matter
from the fluid being metered.

Disadvantages of venturimeters

(i) High manufacturing costs.

(ii) The installation tends to be rather long (typi-
cally 120 mm for a pipe of internal diameter
50 mm).

22.5 Flow nozzle

The flow nozzle lies between an orifice plate and
the venturimeter both in performance and cost. A

Flow nozzle

Direction of

fluid flow

AB

Figure 22.4

typical section through a flow nozzle is shown

in Figure 22.4 where pressure tappings are located

immediately adjacent to the upstream and down-

stream faces of the nozzle (i.e. at pointsAandB).

The fluid flow does not contract any further as it

leaves the nozzle and the pressure loss created is

considerably less than that occurring with orifice

plates. Flow nozzles are suitable for use with high

velocity flows for they do not suffer the wear that

occurs in orifice plate edges during such flows.

22.6 Pitot-static tube

A Pitot-static tube is a device for measuring the

velocity of moving fluids or of the velocity of bodies

moving through fluids. It consists of one tube, called

the Pitot tube, with an open end facing the direction

of the fluid motion, shown as pipeRin Figure 22.5,

and a second tube, called the piezometer tube, with

the opening at 90° to the fluid flow, shown as

T in Figure 22.5. Pressure recorded by a pressure

gauge moving with the flow, i.e. static or stationary

relative to the fluid, is called free stream pressure

and connecting a pressure gauge to a small hole in

the wall of a pipe, such as pointTin Figure 22.5,

is the easiest method of recording this pressure. The

difference in pressure (pR−pT), shown ashin the

manometer of Figure 22.5, is an indication of the

speed of the fluid in the pipe.

h

T

Direction of R

fluid flow

Figure 22.5

Figure 22.6 shows a practical Pitot-static tube con-

sisting of a pair of concentric tubes. The centre tube

is the impact probe that has an open end which

faces ‘head-on’ into the flow. The outer tube has

a series of holes around its circumference located

at right angles to the flow, as shown byAandB

in Figure 22.6. The manometer, showing a pressure