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 Neptunr positive displacement meter on the basis of the
3/4" line size, he will get a meter ranged for 3 to 30 GPM. By contrast
a 3/4" Kent ABB20 meters is ranged min/max/peak of 8/265/400 GPH
(0.13/4.42/6.67 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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