Despite the growing popularity
of other flowmeter types, the variable area flowmeter
continues to command a major portion of the industrial
market. Although it has not enjoyed the same degree
of publicity granted more exotic meter forms in
technical literature, continuing innovations and
developments in variable-area metering have greatly
enhanced the capabilities of this popular flow
monitor. Their basic advantages--relatively low
cost, accurate and reliable performance, simplicity,
and inherent versatility--remain valid today.
Available in a wide variety of metals and plastics,
and with current alarm and control options, variable
area meters can be used with a wide variety of
liquid, gas, and steam applications.
Perhaps the best known of the variable area meter
family is the "rotameter," first patented
in the United States in 1868. In its basic form,
the rotameter consists of a vertically oriented
tapered glass tube with the large end at the top,
and a metering float which is free to move within
the tube. Fluid flow causes the float to begin
to rise in the tube as the upward pressure differential
and buoyancy of the fluid overcome the effect
of gravity.
The float will rise until the annular area
between the float and tube increases sufficiently
to allow a state of dynamic equilibrium between
the (upward) pressure differential and buoyancy
factors, and (downward) gravity factors. The
height of the float is an indication of flow
rate, and the tube can be graduated in appropriate
flow units.
These meters typically can have up to a 12
to 1 turndown (ratio of maximum to minimum measures
flow), and industrial accuracies of +/- 2% or
even 1% of the full scale rating.
To meet the demands of a variety of industrial
requirements, a variety of constructions has
become common. Glass is often replaced with
various plastic or metal components, with some
form of magnetic position sensing being used
in the latter case. The use of magnetic float
sensing is also used for alarm and signal transmission
functions, in many cases.
SLOTTED TUBE FLOWMETERS
Another variation of industrial variable area
meters uses a slotted cylinder and piston rather
than a tapered tube (see figure). This piston
portion of the meter float travels within a
precision-honed cylinder, with the piston acting
as the fourth side of the slot. As with tapered
tube meters, fluid flow raises the piston until
sufficient slot area is exposed to bring all
forces into dynamic equilibrium. To assure constant
buoyancy, a "snorkel" device allows
the top of the unit to fill with fluid in liquid
applications.
The metering cylinder and piston are contained
within a T-shaped body, and flow is read by
means of a knife-edged disk supported by an
extension from the piston. This disk moves up
and down in response to flow within a transparent
sight tube containing an externally mounted
scale. As the tube is cylindrical and the disk
always close to its walls, flow can be read
in dirty fluids. If a transparent sight tube
cannot be used, a magnet is added to the float
and an external indicator used. Use of a magnet
on or encapsulated in the float also provides
alarm and control functions with appropriate
accessories.
Advantages of this meter style include more
compact and less costly construction, high rangeabilities
of 25 to 1, and easy access to flow internals
for cleaning or changing meter capacity without
removing the unit from the pipeline. Standard
accuracy is +/-2% full scale,
with +/-1% calibrations as an option, and repeatability
+/-0.3% of rate. Pressure drops are reasonably
low, and similar to tapered tube models.
VANE AND PISTON FLOWMETERS
Yet another style of variable area meter incorporates
a spring-loaded orifice and tapered plug or
a vane within a "bowl." Fluid flow
moves the vane or orifice, exposing a greater
area around the tapered plug or within the bowl
until equilibrium is achieved. Flow is read
by
means of a pointer mechanically positioned by
the vane, or by a magnetic indicator. Although
use of a spring does have drawbacks in terms
of pressure drop and repeatability, the principal
flow element does not rely on gravity and, therefore,
these meters can be mounted in any position.
Accuracies vary from +/-2% to +/-5%, depending
on the model, and average repeatability is +/-1%.
Rangeability is usually 6 or 10 to 1. Because
of the incorporation of the spring, pressure
drops are high (often expressed in pounds rather
than inches W. C. as with other
variable area meters), and generally go up from
2 psig.
