Viscosity considerations and the lack
of capillary sealing
Mechanical meters need to be applied with some concern given to the
fluid characteristics. A
mechanical meter which has a oscillating piston, nutating disc (wobble
plate) or similar
measuring element is known as a positive displacement meter. However,
what makes it
“positive” is a capillary seal between the measuring element
and the side wall or the meshing
components. If you are dealing with a low viscosity fluid, i.e. alcohol,
toluene, MEK, etc. you
lack the viscosity to develop a good capillary seal. This will manifest
itself in leakage around
the measuring element and generally make the meter read low.
Low lubricity and pressure drop
Positive displacement meters have surfaces in the measuring chamber
which touch each
other. In the nutating disc (wobble plate) the center ball rides in
a shaped cup, in oscillating
piston meters the center pin is in contact with the roller and the
piston diaphragm rides on a
hub. This means that non-lubricating fluids allow these parts to drag
on each other which
increases frictional loading. This loading creates higher pressure
drops and increases wear
on the meter.
How to meter these fluids
The best approach for this type of fluid is to use a turbine flow meter.
The turbine rotor is
suspended by a ball or sleeve bearing giving it almost no drag coefficient.
This allows a
turbine meter to be used for low viscosity as well as non-lubricating
fluids.
Another choice is the oval gear meter, a positive displacement device
using dual oval shaped
rotors. The rotors inherently provide better sealing because their
gear design traps a pocket
of fluid right at the capillary sealing point, thus preventing leakage
around the measuring
element.
How to select the proper meter
Selecting a flow meter for chemical service can be tricky. We’ll
need to
know exactly what the fluid is and the operating conditions. Of particular
importance is the temperature and line pressure. The more information
you can provide us, the better job we can do at selecting the right
flow
meter for your application.
Never just ask for a body material and expect the meter to work properly.
The body is only a pressure vessel which contains the working mechanism.
The materials in the mechanism must be specified carefully. Always
review
the list of materials in the specifications. If you get into a chemical
compatibility problem, it will be in the working mechanism, not the
body.
Never assume that a plastic bodied meter will work for you just because
you are using PVC lines. PVC and CPVC can handle chemicals which will
melt a plastic bodied meter in seconds. Plastic bodied meters designed
for
water service are made of polyacetals and polycarbonates, not PVC.
They
cannot handle low pH fluids and in some cases will be attacked by solvents.
For more information on the various meters we offer for this type of
service, please click on one of the links below.
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