How Flowmeters Work
Variable area flowmeters are basically vertical internally
tapered tubes mounted with the large end at the top. A float
or rotor with an outer diameter slightly less than the minimum
diameter of the tube is placed inside the tube. The clearance
space between the float and the tube forms an annular passage
or orifice. As the tube is tapered, the area of this orifice
is larger when the float is near the top than it is when
the float is near the bottom. By connecting the tube into
a fluid flow line so flow direction is from bottom to top,
the float will move upward and be supported at a point where
the orifice is just large enough to pass the fluid flowing
through the system.
Several Forces are involved
- The precise position of
the float within the tube is determined by several forces
acting on it. These forces are:
1. The weight of the float;
2.
The velocity pressure of the flowing fluid multiplied by
the area of the float; 3.
Buoyancy of the float (weight of fluid volume displaced
by the float); 4.
Viscous aerodynamic or hydrodynamic drag of the fluid on
the float. The float's weight acts downward - while velocity
pressure, buoyancy and drag all act upward on it.
Read-out and scales -
By making the tube transparent so
that float position can be seen and by providing a scale
along side, we may "read" the float position in
terms of numbers on the scale. These scale numbers can be
of two types: 1.
Of an arbitrary nature (by themselves they mean nothing,
but become meaningful when compared with a calibration curve)
or: 2.
They can be precalibrated for direct reading; to show the
actual volume flow (in cfm, cc per minute, etc.) of the
fluid for which the flowmeter is calibrated.
Limiting factors - Although
the basic relationship outlined above is linear; certain
fluid properties tend to modify this relationship. These
changes are accentuated where the variable orifice formed
between float and tube becomes either very large or very
small. For example...
Sonic velocity - Where
the orifice is particularly large and is combined with a
heavy float, velocities of gas or other fluid through the
orifice area can approach sonic velocity. In this velocity
range, reflected shock waves cause the float to become unstable,
and it will typically hunt from side to side and from top
to bottom within the bore. At best, this condition makes
it impossible to take a reading, and at worst may even destroy
the flowmeter. The addition of flow guides (built into certain
Dwyer® Rate-Master® flowmeters) can often improve
the stability and performance of the system by channeling
the flow into multiple streams that equalize dynamic effects
and preventing oscillation of the float. This has the new
effect of moving the limit of satisfactory operation upward,
and thereby expanding the range and accuracy we can achieve
with any given flowmeter tube.
Laminar flow - In
flowmeters where the orifice area is extremely small, the
conditions result in smooth flow, or laminar flow. Other
factors which contribute to the transition to laminar flow
are low velocity (often associated with a lightweight float),
low density or specific gravity and high viscosity of the
flowing fluid. When laminar flow conditions prevail, a greatly
expanded scale is usually required. Meters operating in
the laminar flow area are very difficult to manufacture
and calibrate to the degree of accuracy usually expected
of variable area flowmeters.
Turbulent flow - Most
variable area flowmeters operate in the turbulent flow range
which occurs below sonic velocities and above the laminar
flow range. In turbulent flow, the flowing fluid particles
move in random paths within the stream - rather than in
violent shock waves as in sonic flow or very smooth parallel
paths as in laminar flow. In turbulent flow, variables follow
the relationships shown in the curves. These curves are
quite accurate for small changes in pressure and specific
gravity. For large changes or where a change in viscosity
is involved, it is much better to have the flowmeter recalibrated
for the specific conditions under which it will be used.
Reynolds Numbers - Reynolds
Numbers are useful in the study of fluid behavior and are
quite helpful in separating laminar and turbulent flow.
The Reynolds Number of a fluid flow system is described
as a dimensionless index. It is equivalent to the diameter
of the orifice in feet times the average velocity of the
fluid in feet per second times the density of the fluid
in pounds per cubic foot divided by the absolute viscosity
in pounds per second foot. A system operating with a Reynolds
Number of less than 2000 is said to be subject to laminar
flow, whereas Reynolds Numbers above 3000 are clearly in
the turbulent flow area.
How Sight Flow Indicators Work
A sight flow indicator basically consists
of a small housing equipped with a glass window which is
inserted in a run of pipe to observe the flow of the fluid
in the pipe. To enhance the visibility of the flow, a spinner
is often incorporated in the flow stream so that fluid impinging
on the spinner vanes cause it to turn. The spinner also
aids in the detection of low flows as well as providing
visibility of low from a distance. In addition, the speed
of rotation gives a relative indication of flow velocity.
Midwest sight Flow Indicators are also available with hinged
flappers instead of spinners to indicate bi-directional
flow.
Sight flow indicators can be provided
with a single window on the front of the indicator or double
windows, one on the front and one on the back of the indicator.
Double window units are best when observing the clarity
or color of a liquid. Midwest single window units are always
equipped with spinners to provide for observation of clear
fluid flows at the lowest cost.
Vertical tube-type sight flow indicators
consist of a clear glass tube, equal to or greater than
the diameter of the pipe into which it is inserted, and
are utilized to observe high flow rates in vertical pipe
runs. No spinners or other type of detection devices are
incorporated in these units. As a result, they offer no
significant resistance to flow and therefore provide the
lowest pressure drop of any type sight flow indicator.
Midwest Sight Flow Indicators are available
with special materials to meet various applications. Optional
materials are available for the sealing gaskets, spinners,
and housings or flanges.
Typical
Applications Using Flowmeters and Sight Flow Indicators
