Intro4u2u

Positive displacement meters measure the volume
flow rate (QV) directly by repeatedly trapping a
sample of the fluid. The total volume of liquid
passing through the meter in a given period of time
is the product of the volume of the sample and the
number of samples. Positive displacement meters
frequently totalize flow directly on an integral
counter, but they can also generate a pulse output
which may be read on a local display counter or by
transmission to a control room. Because each pulse
represents a discrete volume of fluid, they are
ideally suited for automatic batching and
accounting. Positive displacement meters can be
less accurate than other meters because of leakage
past the internal sealing surfaces. Three common
types of displacement meters are the piston, oval
gear, and nutating disc.

Head Meters

Head meters are the most common types of meter
used to measure fluid flow rates. They measure
fluid flow indirectly by creating and measuring a
differential pressure by means of an obstruction to
the fluid flow. Using well-established conversion
coefficients which depend on the type of head meter
used and the diameter of the pipe, a measurement
of the differential pressure may be translated into a
volume rate.
From the Equation of Continuity, assuming
constant density (incompressible fluid) it can be
seen that:

This equation is one of the most important
relationships in fluid mechanics. It demonstrates
that for steady, uniform flow, a decrease in pipe
diameter results in an increase in fluid velocity. In
addition, from Bernoulli’s equation on the
conversation of energy, it is further seen that total
head pressure (H) must remain constant
everywhere along the flow or:

The first term of the equation is called “potential
head” or “potential energy”. The second term is
known as the “velocity head” or “kinetic energy”.
Because potential and kinetic energy together are
constant, it is clear that an increase in velocity as
described by the Equation of Continuity must also
be accompanied by a decrease in potential energy or
line pressure. It is this relationship between velocity
and pressure that provides the basis for the
operation of all head-type meters.
Head meters are generally simple, reliable, and
offer more flexibility than other flow measurement
methods. The head-type flowmeter almost always
consists of two components: the primary device and
the secondary device. The primary device is placed
in the pipe to restrict the flow and develop a
differential pressure. The secondary device
measures the differential pressure and provides a
readout or signal for transmission to a control
system. With head meters, calibration of a primary
measuring device is not required in the field.

The
primary device can be selected for compatibility
with the specific fluid or application and the
secondary device can be selected for the type or
readout of signal transmission desired.

Orifice Plates

A concentric orifice plate is the simplest and least
expensive of the head meters (Figure 2). Acting as a
primary device, the orifice plate constricts the flow
of a fluid to produce a differential pressure across
the plate. The result is a high pressure upstream
and a low pressure downstream that is proportional
to the square of the flow velocity. An orifice plate
usually produces a greater overall pressure loss
than other primary devices. A practical advantage
of this device is that cost does not increase
significantly with pipe size.

Venturi Tubes

Venturi tubes exhibit a very low pressure loss
compared to other differential pressure head
meters, but they are also the largest and most
costly. They operate by gradually narrowing the
diameter of the pipe (Figure 3), and measuring the
resultant drop in pressure. An expanding section of
the meter then returns the flow to very near its
original pressure. As with the orifice plate, the
differential pressure measurement is converted into
a corresponding flow rate. Venturi tube applications
are generally restricted to those requiring a low
pressure drop and a high accuracy reading. They
are widely used in large diameter pipes such as
those found in waste treatment plants because their
gradually sloping shape will allow solids to flow
through.

Flow Nozzle

Flow nozzles may be thought of as a variation on the venturi tube. The nozzle opening is an elliptical
restriction in the flow but with no outlet area for
pressure recovery (Figure 4). Pressure taps are
located approximately 1/2 pipe diameter downstream
and 1 pipe diameter upstream. The flow nozzle is a
high velocity flowmeter used where turbulence is
high (Reynolds numbers above 50,000) such as in
steam flow at high temperatures. The pressure drop
of a flow nozzle falls between that of the venturi
tube and the orifice plate (30 to 95 percent).

Pitot Tubes

In general, a pitot tube for indicating flow consists
of two hollow tubes that sense the pressure at
different places within the pipe. These tubes can be
mounted separately in the pipe or installed together
in one casing as a single device. One tube measures
the stagnation or impact pressure (velocity head
plus potential head) at a point in the flow. The other
tube measures only the static pressure (potential
head), usually at the wall of the pipe. The
differential pressure sensed through the pitot tube
is proportional to the square of the velocity. To
install a pitot tube, you must determine the location
of maximum velocity with pipe traverses. Although
a pitot tube may be calibrated to measure fluid flow
to ±1/2 percent, changing velocity profiles may cause
significant errors. Pitot tubes are primarily used to
measure gases because the change in the flow
velocity from average to center is not as substantial
as in other fluids. Pitot tubes have found limited
applications in industrial markets because they can
easily become plugged with foreign material in the
fluid. Their accuracy is dependent on the velocity
profile which is difficult to measure.

Target Meters

A target meter consists of a disc or a “target” which
is centered in a pipe (Figure 5). The target surface is
positioned at a right angle to the fluid flow. A direct
measurement of the fluid flow rate results from the
force of the fluid acting against the target. Useful
for dirty or corrosive fluids, target meters require no
external connections, seals, or purge systems. Much
data is necessary, however, to determine the
optimum size of the target and calibration is
essential for its proper operation.

Elbow Tap Meters

An elbow tap operates by using a 45 degree pipe
elbow in the fluid flow. A high pressure tap is taken
from the outside of the elbow and a low pressure tap
is taken from the inside of the elbow. This provides a
differential pressure which is proportional to the
flow rate. Measuring the differential pressure
depends on the centrifugal force of the fluid flowing
through the elbow. Hence, gas with its low density is
not a good application for elbow taps. This also
explains why a short curvature in the elbow
develops a much greater differential pressure than
a long curvature. The pressure drop of an elbow tap
is no greater than that of the elbow. Though
repeatable, accuracy of an elbow tap meter is only
within ±5 percent.

Rotameters

Rotameters (also known as variable-area
flowmeters) are typically made from a tapered glass
tube that is positioned vertically in the fluid flow
(Figure 6). A float that is the same size as the base
of the glass tube rides upward in relation to the
amount of flow. Because the tube is larger in
diameter at the top of the glass than at the bottom,
the float resides at the point where the differential
pressure between the upper and lower surfaces
balance the weight of the float. In most rotameter
applications, the flow rate is read directly from a
scale inscribed on the glass; in some cases, an
automatic sensing device is used to sense the level
of the float and transmit a flow signal. These
“transmitting rotameters” are often made from
stainless steel or other materials for various fluid
applications and higher pressures. Rotameters may
range in size from 1/4 inch to greater then 6 inches.
They measure a wider band of flow (10 to 1) than an
orifice plate with an accuracy of ±2 percent, and a
maximum operating pressure of 300 psig when
constructed of glass. Rotameters are commonly used
for purge flows and levels.