We
offer several designs of PD meters:
Oscillating Piston,
Nutating Disk and Oval Gear designs.
Meter accuracy
For a discussion of the differences between accuracy statements
expressed as % of Full Scale and % of Rate, please see our glossary
section.
People think that a flow meter is accurate when they install it. Maybe,
but maybe not! It would be extremely unusual for you to install a meter
and get
exactly the same accuracy results that the factory gave you on the calibration
sheet, unless you are running the same fluid and have the same piping arrangement
as exists on the manufacturer's test stand.
Lets take a typical large industrial grade positive displacement flowmeter.
This meter is calibrated on water, you are running molasses. From experience
the fellow at the factory doing the calibration knows that you are running
a higher viscosity fluid so he "estimates". He gets his calibration
data on water and says "the molasses is going to affect me about 2%, so
I'll change my register reduction gearing ratio to match". This change
offsets the correct reading (on water) by his estimate. His estimate could
be wrong.
The meter is calibrated in a test stand, facing upward, on water or Stoddard
Solvent, typically with a circulating pump putting some pulsation's in the
flow stream. (Which improves low end performance). Your installation may have
a very long pipe between the pump and the flowmeter which dampens the pulsation's
out, the meter may be installed in a vertical line, etc. If you can set up
a volumetric or gravimetric (weighscale) test, you will get different results
from what appears on the calibration card which arrived with the meter.
If you want an accurate meter ... install the meter and then do a volumetric
or gravimeteric test. In other words, run fluid for at least 2 minutes at the
desired flow calibration into a calibrated tank or into a tank on a weighscale.
Correct the meter to match the weighscale reading.
Small meter size designation
You call and say that you want a 3/4" meter for water service. If the
person you are talking to knows meters and wants to give you the best possible
selection, he will say "Do you want a 5/8 x 3/4 or a 3/4 x 3/4?" The
normal reaction is "Why would I want a meter with a 5/8" inlet and
a 3/4" outlet?" Fortunately, that is not what these fractions mean.
At one time, before design improvements which have come about in the
early '90's, we would tell people:
The first fraction is the chamber capacity:
5/8
meant a 25 GPM capacity with a 13 psig pressure drop
3/4 meant a 30 GPM capacity with a 13 psig pressure drop
1 meant a 50 GPM capacity with a 13 psig pressure drop (seldom used)
The
second fraction is the line size the meter couples into, when used
with the provided end couplings.
Note that the second fraction is not the meter end connection, but the line size
the meter will install into when the coupling connections are used. The cases
on meters which comply with AWWA C700 (American Water Works Assoc) have straight
threads, which are 1 line size over the size pipe. Here are the actual threads:
5/8
x 1/2 meter Actual end threads are 3/4 x 14 TPI, NPSM (male)
5/8 x 3/4 meter Actual end threads are 1" x 11 TPI, NPSM (male)
3/4 x 3/4 meter Actual end threads are 1" x 11 TPI, NPSM (male)
1" meter Actual end threads are 1.5 x 11.5 TPI NPSM (male)
Why
this straight thread? The meter is designed to be periodically removed
and tested. If the meter ends had tapered threads, the connecting pipe
coupling rides over the meter thread (interference fit). In order to
remove the meter, the connecting pipe must be pushed backwards to disengage
the NPT threads. But the pipe is buried under tons of dirt and will
not move. By using straight threads and couplings which utilize a gasket
to seal the line, one can loosen the coupling nut, spring the line
slightly and pull the meter out.
If you try to screw a tapered coupling (FIP, NPT) to the meter, it will leak.
Use the couplings offered with the meters.
Metering low viscosity, low lubricity fluids
such as mineral spirits, toluene, isopropyl alcohol, acetone, etc.
Typically you will want to use mechanical meters because these fluids are hazardous
and you would like to avoid the problems of using explosion proof enclosures
on an electronic flow meter.
Viscosity considerations and the lack of capillary sealing:
Mechanical meters are tried and true devices, but they need to be applied
with some concern given to the fluid characteristics. A mechanical meter
which has
an oscillating piston, nutating disc (wobble plate) or similar measuring element
is termed a positive displacement meter. However, what makes it "positive" is
capillary sealing 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. This leakage can be
substantial. In general, the larger the meter, the greater the mass of the measuring
element and the greater the leakage.
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 a non-lubricating fluid
allows these parts to drag on each other which increases the frictional loading.
This loading creates higher pressure drops.
There is a intermediate product ... The oval gear meter is 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, plus the gear ends are large and have a small gap of about 0.001" to
the meter body. Small oval gear meters do not develop enough torque to drive
a mechanical register and require electronic displays, however, larger units
ranged to 40 GPM and higher can be equipped with mechanical registers. Oval gear
meters have lower flow ranges on a per line size basis and higher costs than
a turbine meter.
Controlling pressure drop - derating meters
for high viscosity fluids
There are many ways to measure viscosity and consequently many ways to express
it. Most flowmeters use centiPoise (cP) or centiStokes (cStks) to define pressure
drops. If you need help in converting viscosity numbers provide us with the specific
gravity or density (English).
Crane (the valve people) publishes the ASTM equivalents of Kinematic (cStks),
Saybolt Universal, Saybolt Furol and Absolute (cP) in a nomograph. This, along
with a fine collection of associated data, is published in the Engineering section
of their sales manual which is widely distributed.
PD meters will work over wide viscosity ranges, with minimal accuracy shifts.
It is a rare PD meter which will show greater than a +1.5% accuracy shift with
increasing viscosity. However, the pressure drop rises quickly as viscosity goes
up. To reduce the pressure drop you choose a larger meter. To get the proper
size meter you must derate the maximum recommended flow statement in the literature.
Many derating curves are in the literature, but as a rule of thumb you can:
Take the maximum continuous flow rating and multiply it by:
500
cP ............ multiply by 0.7
1000 cP .......... multiply by 0.55
2000 cP .......... multiply by 0.4
4000 cP .......... multiply by 0.3
10000 cP ........ multiply by 0.2
Volumetric compensation
Let's say that you unload 2,000 gallons of a hydrocarbon base oil
that comes off the tanker at 190 deg F. The meter says it saw 2000
gallons
come in. But
the next day, when the oil has cooled down to 140 deg F, the tank level gauge
says that you only added 1950 gallons -> you've lost 50 Gallons someplace,
the meter must be off.
Both the meter and the tank level gauge are correct. The oil has a expansion
factor of about 5 parts in 10,000 per degree F. When your temperature dropped
the oil contracted.
We can provide a temperature sensor and mass flow display which will correct
for the temperature variations, always providing a reading to some base (reference)
temperature.
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