Modern wat er flowmeters [611819]

Modern wat er flowmeters
Differential pressure flowmeters

Assoc. Lecturer dr. eng. Gabriel Constantin S ârbu
“Gheorghe Asachi” Technical University of Iasi
Faculty of Hydrotechnics, Geodesy and Environmental Engineering Iasi, Romania
gabrielsarbu26 @yahoo.ro

Abstract —Being a part of the no moving parts instruments,
differential flow meter are mainly used in the industrial envir onment,
not being found in house hold use.
The basic functional principle consists in the appe arance of a
differe ntial pressure between two points of the flow, if between this
comes a flow res istivity object (obstacl e, strangulation, elbow).
The relati onship between pressure and velocity is described by
the Bernoulli equation. Measuring the pressure di fference the
velocity can be calculated and the flow implicitly.
There are a several kinds of differential pressure flow meters :
Venturi, Nozzle, Pitot, and measuring diaphragms .
This paper refers to an experimental method for determining the
constant K, for the constant flow cross -section of the flow meters and
the pressure drop variable

Keywords —differential pressure, flow meter, diaphragm,
Venturi, Nozzle , Pitot.

I. INTRODUCTION
This class of flowmeters are indirect f lowmeters with
measurement section constant and no moving parts .
These meters have the characteristic "flow differential
pressure drop static", introducing a hydraulic resistance by
decreasing the flow path.
Primary transducer of the flow meter is character ized by
its reference element , which may be a diaphragm, a Venturi ,
Pitot tube or N ozzle elements, which reduces the local flow
section corresponding to a measuring sections area, fixed
positioned and having constant area .
The equation characteris tic of these flowmeters is:
12
24ρπαε pdQm ∆ =
Where: (1)
Q m is the mass flow
α – flow coefficient
ε – coefficient of expansion
∆ p = p 1 – p2 – differential pressure
p 1, p 2- static pressure upstream , downstream of the
reference element
ρ1 – density of the fluid upstream reference element
For volume flow Qv relationship is:
m vQ Q=/ρ (2) Venturi meters are one of the most accurate type of flow measuring device that can be used in a water supply system.

Venturi meters are often used in the laboratory to calibrate other closed conduit flow measuring devices.
In effect, the flow N ozzle is a V enturi meter that has been
simplified and shortened by eliminating the gradual downstream expansion. The str eamlined entrance of the
Nozzle causes a straight jet without contraction, so its
effective discharge coefficient is nearly the same as the Venturi meter. Flow N ozzles allow the jet to expand of its
own accord.
The Pitot tube measures a fluid velo city by converting the
kinetic energy of the flow into potential energy. The
conversion takes place at the stagnation point, located at the
Pitot tube entrance. This "static" pressure is measured by comparing it to the flow's dynamic pressure with a differ ential
manometer.
More conveniently, a differential pressure cell that senses
velocity head directly can be used. A recording digital
voltmeter attached to the pressure cells can provide continuous records of velocity.
The flow measurements can be very accurate at moderate
flow velocities.
The most common differential -pressure type flowmeter
used in pipelines is the measuring diaphragms .
Measuring diaphragms are the primary devices used to measure the flow of fluids (liquids and gase s) that was flowing
under pressure through pipes with circular section. With a
simple construction, robust, no moving parts, used under
proper conditions are highly reliable, precise measurement provides premises for extended periods of time, being usable
for a wide range of fluids in different conditions of pressure
and tempe rature (Fig. 1).

Fig. 1
The diaphragms are sensitive elements of the category
section of constant bottleneck. The diaphragm is a steel disc with a central hole of diameter d
1 less than the diameter D of
the pipe.