Introduction..... Symbols..... Standards..... Pitot ..... Venturi ..... Nozzle..... Orifice..... Flow condition..... losses.....

Introduction The notes on this page related to the methods of measuring flow
using devices which are based on bernoulli's equation. There are many other devices
which are convenient to use and are very accurate which are based on other principles including
vortex shedding, ultrasonics (doppler), turbines, and variable orifice. To obtain
information on these devices please consult the linked sites at the bottom of this page. Symbols
Relevant Standards
The following standards provide detailed information on measuring fluid flow using venturis, orifice plates and nozzles. Pitot Meter
Consider three glass tubes positioned in a pipe which is carrying flowing fluid
Now the static head of the fluid (p /ρg ), that is the
height that the fluid rises in the tube with the fluid velocity at zero, is
indicated by the tube at position B. At the interface of a flowing liquid with
a solid surface the fluid velocity is zero. The head at position A is a
measure of the stagnation head (p /ρg +u If ρ
The pitot tube meter is used to indicate the Venturi Meter Reference ..Fluid Flow
A venturi meter includes a cylindrical length, a converging length with an included angle
of 20 Applying bernoulli's equation to the two sections. Therefore the ideal discharge is given by
Now in practice there is a slight friction loss between 1 and 2 which would result in
a high Δh reading and a consequent value of Q which is too high. For real fluids
therefore a factor is introduced called the coefficient of discharge factor (C Design and performance parameters of venturi flow meters are provided in BS EN ISO 5167-4:2003 Nozzle Flow Meter The nozzle as shown is practically a venturi with the diverging part removed.
The basic equations used are the same as for the venturi meter. The friction losses are slightly
larger than for the venturi but this is offset by the lower cost of the unit. The fact that the manometer
connections cannot be located in the ideal positions for measuring the required piezometric pressures is allowed for in
selection of the coefficient of discharge factor C Design and performance parameters of nozzles flow meters are provided in BS EN ISO 5167-3:2003 Orifice Flow MeterThe simplest and cheapest method of measuring the flow using the bernoulli equation is the sharp edged orifice as shown below. The fluid flow pattern in the region of and orifice is shown in the diagram below.. Application of Bernoulli's equation to the fluid flowing through the orifice. Now u To arrive at a final equation a overall discharge coefficient C is introduced. Now letting β = d Note: This equation is very similar to the equation provided in BS EN ISO 5167:2 except that an expansion coefficient (ε )is introduced to cater for the measurement of compressible fluids. The equation provided in the standards is . Values of the discharge coefficient C are provided in BS EN ISO 5167:2 for the different meter tapping arrangements,
for different values of β against Reynold number ranges.
Flow Conditioning The accuracy of the flow
measuring devices is very much affected by uniformity of the approaching
fluid flow.
Therefore ideally there should be a straight length of piping before the
flow measuring
device. It is generally accepted that for accurate flow readings there should
be 50 pipe diameters of straight piping before the metering device following any
pipe bend, valve, tee, reducer etc. The relevant standard provides a
range of recommended minimum straight lengths
depending on the nearest upstream fittings varying from 5 to 44 lengths. This length can be reduced if flow
straighteners or flow conditioning devices systems are used upstream of the flow
measuring device. A flow straightener is designed to remove swirl from
the flowing fluid. A flow conditioner is a device which removes swirl
and also redistributes the velocity profile to produce near ideal metering conditions.
Losses resulting from flow metering devicesThe orifice plate and, to a lesser extent the nozzle has significant kinetic energy losses downstream of the metering device as the locally generated kinetic energy is dissipated. The figure below illustrates the extent of these losses. |

- Omega Fowmeters.. A supplier with some useful notes on selecting flowmeters
- Wikipedia - Flow measurement.. Clear and detailed notes
- Engineeringtoolbox flow meterss.. Lots of useful notes and links
- lmnoeng .. Venturi Flowmeter Calculation for Liquid Flow (includes C coefficient values)