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The Line Size
In general, it is a bad idea to select a flowmeter on the basis
of line size. From practical experience we know that there are specific
applications where the existing lines are frequently oversized.
For these applications a meter should never be selected with the
primary consideration being the line size. These are:
a.
Waste water lines
b. Any compressed gas, especially compressed air and natural gas.
c. Any gravity drain application
d. Oil burner fuel lines
e. Steam lines
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Flow range ... same size meters can
be different
Meters of the same line size can have widely different flow ranges.
This is frequently seen on small oil meters used for burner service.
An example would be a 800 HP boiler with a 3/4" oil line to the
burner and at high fire is burning about 4 GPM.
If he selects a Neptune positive displacement meter on the basis
of the 3/4" line size, he will get a meter ranged for 3 to 30 GPM.
The morale of the story - don't buy on the basis of line size alone!
A second major mistake is to assume that different meter designs
have the same flow range. For example:
A
1" oval gear meter is typically 4 to 40 GPM
A 1" positive displacement design has a range of 5 to 50 GPM.
A 1" axial turbine has a 2 to 75 gpm range.
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Foreign matter & Cleanliness
Use a magnetic or ultrasonic flowmeter if you have fibers, silt,
bits of matter coming from a cleaning process, biological matter,
anything that can (a) get into bearings, (b) wrap around something,
(c) cause damage by impact (turbine rotor blades) or (d) erosion.
Strainers - remember that what counts is the mesh of the
strainer, not the fact that a customer has a strainer body in place
ahead of the meter.
Guidelines -
For
1" and smaller PD meters .......
For 1/8" & 1/4" oil meters .............
For oval gear meters under 1/2" .....
For oval gear meters over 1/2" .......
Precision turbines (Sponsler) ........
Turbines (GPI, Blancett) ...............
Bulk meters (Kent T3000,Helix) .....
Irrigation .....................................
Magnetic ....................................
Ultrasonic ...................................
Vortex shedder ............................ |
40
mesh
felt
80 mesh
60 mesh
60 mesh
40 mesh
10 mesh
Use sealed bearings (McCrometer)
Use grounding ring
Not req'd
Not req'd |
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Chemical service
It is a mistake to specify a body material for a chemical service
meter and not be equally specific about the internal working parts.
We are commonly requested to quote a 316 SS positive displacement
meter with no mention of the desired internals. Most 316 SS P.D.
meters have Kynar (Polyvinylidene Fluoride) working parts. Kynar
has a wide range of compatibility, but there are some fluids, especially
acids at higher temperatures which will attack it.
We offer a very wide range of materials in our turbine meters. A
turbine meter can machined from any material which will retain dimensional
stability, Typically we offer 304 and 316 st. st., CPVC, Polypropylene,
Kynar, Kel-F, Teflon, Hastelloy B & C, Titanium, Tantalum, Monel
and Inconel.
Never assume that a plastic bodied meter will work for you just
because you are using plastic lines. PVC and CPVC can handle chemicals
which melt a plastic bodied flow 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. The market for domestic water meters is
the largest single meter market in the world. The volumes are huge
and the meters are optimized for water service, they are not general
purpose design meters. If you need a meter to measure chemicals,
buy a chemical meter, not a water meter.
If the viscosity is right consider the Sponsler Corrosive Service
meter. It has the best selection of body and internal materials
of any product we offer.
The magnetic flowmeter is available with a teflon liner and a variety
of electrode materials.
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High Pressure
We have PD meters which are available in 2500 and 5000 psig bodies,
but only with NPT ends.
For meters with high pressure fittings, i.e. 30 deg. flare, Autoclave,
Grayloc, SAE Code 61 and 62 flanges, consider the use of the Sponsler
turbine meter.
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Viscosity
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 (Metric,
i.e. cP or cStk) or density (English).
Crane (the valve people) publish 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.
Flowmeters react differently to viscosity changes, plus remember
that most viscosity's will drop with increasing temperature or velocity
(thixotropic fluids).
Magnetic and ultrasonic flowmeters - are immune to viscosity
considerations. Almost all other meters are affected by viscosity.
Positive displacement meter - If a magnetic or ultrasonic
meter is not suitable, a PD meter is your best choice. 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
Turbine meters - stay below 20 cP viscosity on meters 1" and
under. On larger meters as the viscosity goes up try to size the
meter so that the flow rate is at the upper end of the meter curve
- always avoid low flow rates at the bottom of the meter curve.
If you must use a turbine meter under higher viscosity conditions,
you can because the turbine meter is repeatable in nature, however,
you need to add a "linearizer" to compensate for the drop off in
low end performance. Another option is to have a viscosity calibration
done by the manufacturer, in the field, or by an outside flow lab.
Our policy is to shift to a different meter design which does not
have this problem, however, we do offer linearizers.
Vortex shedding meters - always stay below 20 cP.
Rotameters (Variable area meters) - Rotameters come in a
variety of sizes and materials, but they all have a tapered tube
and a float. The "float" is the thing that rises and falls inside
the tapered tube. As the float rises the area of the annular orifice
between the float OD and the tube ID increases, thus rotameters
are a particular form of variable area meter. For each tube you
are offered a selection of float materials, of different weights.
This gives you as many different flow ranges as there are floats.
The viscosity handling characteristics vary with the float design.
Ball
floats ... no viscosity immunity at all only found on small meters.
Cylindrical floats ............ large "drag" surface, little immunity.
Sharp edged floats, 1" and larger ... maximum immunity to viscosity.
Practical viscosity limits using sharp edged floats:
1/2"
size ....... 10 cP
3/4" size ....... 15 cP
1" size .......... 20 cP
1.5" size ....... 40 cP
2" size .......... 60 cP
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Accuracy & Repeatability requirements
Accuracy is defined as "% FS, i.e. % of full scale range" and "%
rate, i.e. % of actual flow rate".
% FS ... if a meter is rated for 100 GPM @ 1% FS accuracy:
At
100 GPM it is 100 (1 GPM).
At 50 GPM it is 50 (1 GPM).
At 10 GPM it is 10 (1 GPM).
% Rate ... if a meter is rated for 100 GPM @ 1% of rate accuracy:
At
100 GPM it is 100 (1 GPM).
At 50 GPM it is 50 (0.5 GPM).
At 10 GPM it is 10 (0.1 GPM).
With
the exception of oval gear meters, most PD meters are rated as %
FS.
It is not fair to rigorously apply the mathematical interpretation
of % FS. In the above example, at 10 GPM we have an error of 10%
of actual flow. The actual error is never stated ... but you can
apply some intuition and get reasonable results. For example, if
the meter has a 10:1 turndown range (max flow divided by minimum
flow = 10), the bottom end of the meter curve is probably straying
up to 1.5% to 2.0% of actual range accuracy. PD meters with better
low end accuracy's will have greater than 10:1 turndown statements
and in some cases will provide specific statements on low end accuracy's.
Alternatively, some meters will define accuracy's as a function
of turndown or specific flow spans. An alert manufacturer will give
you an interpretation of his accuracy rather than let you assume
10% and larger errors.
For truly precise measurement, accuracy doesn't matter. Repeatability
is what counts. If you look at our precision electronic turbine
literature you will see that accuracy is not even given. In a mechanical
meter, you can change the register gear train ratio and make the
meter give you any reading you want. In a electronic meter with
a pulse output, the display will have a "K-Factor Divider", which
allows you to make the display say anything you want. Accordingly,
what really matters is how well the flow meter repeats itself. Flowmeter
repeatability specifications run from 0.01 up to 0.3% and are always
5 to 10 times better than the accuracy statement.
Certain meter designs are inherently very accurate and the accuracy
becomes a selling point. Magnetic flowmeters are 0.15% or 0.2% accurate
of rate with turndown ranges of up to 1000:1.
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Lubricity (oiliness) & 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 a 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.
A positive displacement meter has moving parts which touch, typically,
the ball in the middle of a nutating disc, a piston sliding across
a partition plate, etc. If the fluid is very dry in nature, like
alcohol, MEK, acetone, etc., it does not provide any lubrication
to the parts which touch. PD meters can develop substantial frictional
loading which increases pressure drop and destroys low flow rate
performance.
It is common for mass produced positive displacement water meters
for municipalities to have carbon impregnated working parts.
If you have a non-lubricating fluid use a turbine, vortex shedder,
magnetic or ultrasonic flow meter. The turbine bearings would typically
be teflon sleeves or ball bearings with teflon inserts.
We have a problem with companies who want to batch these types of
solvents using mechanical meters in order to avoid using electricity
and explosion proof housings. We understand the desire, a explosion
proof housing for a electronic batcher is very expensive. The meters
are typically 1.5" and 2" st. st. positive displacement meters with
batching registers and integral valves. These meters have lots of
mechanical loading, the register, the valve, etc., and when we add
the frictional drop of a nonlubricating fluid, they have huge 10
to 20% errors at low flows. The fluid is of such a low viscosity
that a capillary seal never forms, allowing the fluid to squirt
through all the gaps between parts.
There are ways to get around this and provide good service using
a mechanical oval gear meter (which costs more). See our discussion
on oval gear meter designs in another paper.
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Gas flow measurement
Very important - never size the meter by line size.
A meter responds to what it sees, which is actual flow, not flow
at STP.
SCFM
- Gas flow rate at standard temperature (70 (F) and pressure (14.7
psia)
ACFM - Gas flow rate at the actual temperature and pressure.
Gas and compressed air lines are almost always oversize and demand
carefull sizing based on line size, flow rate, pressure and temperature.
Lets say that you have 400 SCFM at 15 psig. Unless temperature is
specified we assume 70 deg.F
Using the Sponsler meter as a typical choice, look at the Gas Sizing
table. 198 ACFM would work well in a 2" turbine meter.
If you have a large line, let's say a 4" line, the next thought
is that you will be choking your flow by dropping to a 2" line.
This is seldom the case, the 4" line may have been used simply to
allow for future expansion, or because at one time a lower pressure
was used. To evaluate, look at the pressure drop curve in the Sponsler
literature ... @ 200 ACFM a 2" line will provide a 3 psi drop, a
3" line would provide 0.5 psi drop and a 4" a .15 psig drop.
Can you use a larger meter? Yes, if the low fire rate is above the
turbines minimum flow rates. Looking at the Sponsler literature,
if you stay above 30 ACFM a 3" meter is usable. If above 40 ACFM,
you can use a 4" meter.
If you need the low end performance, you might adjust your gas regulator
to deliver a higher pressure, or resize the turbine meter for installation
on the high pressure side of the regulator.
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Gas flow measurement and Pressure/Temperature
Compensation
In the above example, let us assume that the pressure is not regulated
at 15 psig, but varies from 13 to 15 psig depending on the load
(in other words, the pressure regulator can't keep up with the demand
volume and the pressure is varying)
At 15 psig, the ACFM is:
At 13 psig, the ACFM is:
The error, in a system calibrated for 15 psig is:
Bottom line, a pressure compensated system is required.
This is a normal situation and the need for a compensated system
should be evaluated every time a gas application is under consideration.
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