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“Wide Range Flow Metering Using Differential Pressure Technology
Based on the Modified Short Form Venturi Tube Design”
By: Bruce Briggs, President, Primary Flow Signal
Today, more than ever, the requirement for accurate and reliable flow measurement is on the top
of every process design list. One of the difficult issues that designers face is the increasingly
wide minimum-to-maximum flow rates that they are required to plan for. A major reason that
wide range measurement is of greater concern is the significant increase in both the cost and
value of many measured commodities.
For example, in the water processing market, raw water is at historically low supply levels. Due
to this shortage, there is a growing need to document each step in the water processing chain to
be certain that intake water is accurately measured since it may be used for accountability
purposes; effective filtered and backwash water measurements so that proper rate of flow control
(water filtration process control) results in the optimal filter run time before backwash so
production levels can be cost effectively met and filtered water chemistry meets governmental
regulations; finished water flow out of the plant needs to be accurately measured so it can be
compared to distributed and consumed water totals in order to determine if there are water loss
issues within the distribution system. This requires monitoring over a minimum-to-maximum
flow rate range that is much wider than previously required. Another change in thinking, because
of the excessively high costs associated with new plant construction, is that new water plants are
not being built nearly as often as older facilities are being upgraded, expanded, and modernized.
This trend will continue and has already begun to modify the most frequently-used design
philosophy concerning minimum-to-maximum flow rate range.
On the industrial side, chemical refining, oil and gas production, and plant process control
systems are noting the same “stretching” of the range as facility engineers are required to
increase and decrease production rates according to demand. The challenge is to have a process
that is scalable so that when demand is high, opportunities are not lost; and when demand drops,
cost and quality can be controlled.
For the most part, all flow metering technologies claim some degree of range in their
specifications along with an accuracy statement that is, in many cases, affected by the minimum
–to-maximum flow rate range that the system operates under. The process engineer is, today,
challenged more than ever before with:
a) determining what the true range requirements are;
b) determining which measurement technology meets that requirement; and
c) carefully considering all of the error sources ‒ one of which is range ‒ so that an
integrated system accuracy analysis can be developed and used to confirm that the
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equipment selected based on the design flow rate range can meet the system accuracy
requirement.
For years, the range attributed to Venturi meter performance has been incorrectly stated to be 3:1
or 5:1 on flow when, in actuality, the true range of the modified, short form Venturi meter can be
50:1 or 100:1 or greater because the largest error source is developed by the differential pressure
transmitter. As seen in the following examples, using the correct differential pressure transmitter
model, including multiple transmitters in a system, can result in far wider ranges than previously
imagined.
Consideration for using a modified, short form Venturi should be based on a number of
operational advantages of this technology. With a factory bench calibration, modified Venturi
meters have a basic accuracy of +/- 0.5%. When a laboratory flow calibration is applied,
modified Venturi meter accuracy is improved to +/- 0.25%. These accuracies can be maintained
across a wide flow rate range, making it a more versatile choice for many different applications.
Additionally, they:
a) are tolerant of short upstream piping configurations and with optional laboratory flow
calibration of the Venturi meter with upstream piping configuration, they can be installed
with NO upstream straight pipe;
b) don’t require any straight pipe downstream piping;
c) have very low energy consumption (headloss) and minimal impact on overall line
pressures;
d) have flexible meter length and end connection configurations to suit application space
end connection requirements;
e) have no line size or throat size limitations like the ISO 5167 and ASME Classical type
Venturi meters have; and
f) can accurately measure all types of materials, such as sludge, slurries, tar sand, and many
others using sealed diaphragms to isolate the process from the differential pressure
transmitter, thus eliminating any potential for plugging the impulse lines.
Venturi meters have a usable life expectancy of 100 years or more based upon proper material
selection. What’s unique about differential pressure meters, unlike electronic meters, is the stated
accuracy can be virtually guaranteed for the life of the meter, assuming no dramatic changes in
flow condition or with the physical geometry of the internal profile. Additional features such as
the Internal Condition Assessment System (ICAS) offered by Primary Flow Signal – means that
it can be easily determine if there is any change to the internal profile of the Venturi meter that
impacts its accuracy without internal inspection.
When using a differential pressure based, wide range metering system, there are two basic
components and several application conditions to consider. The prime measurement device is the
Venturi meter, which is commonly referred to as the flow “primary”. The differential pressure
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transmitter senses the high and low pressure that the Venturi meter develops and converts the
input “differential pressure” to an electronic output signal which generates rate and total results.
The differential pressure transmitter is therefore called the “secondary” part of the system.
When designing a differential pressure wide range metering system, it’s important to adhere to
the following process:
a. Analyze process conditions that impact on the design of the Venturi meter, such as
available line pressure, low pipe Reynolds number conditions, and differential
development at the minimum flow rate, to be certain the differential pressure is high
enough for the differential pressure transmitter to sense and process but not excessively
high at the maximum flow rate.
b. Next review what the accuracy requirement is that must be achieved for the total
metering system (primary and secondary) so that the accuracy statement becomes an
integrated +/- system value at the indicator/totalizer. Once the target system accuracy is
established, analyze what differential pressure transmitter accuracy is required and how
many transmitters will be used to cover the wide range application. When selecting the
correct differential pressure transmitter model, take into consideration what the maximum
differential produced by the Venturi meter will be at the maximum flow rate so that the
right differential pressure transmitter range code can be selected.
To illustrate the previous point, Figure 1 shows an example taken from a commonly used
Emerson’s Rosemount* 3051CD Differential Pressure Transmitter with a selected range of 0 -
250 inches of water column. However, at start-up the transmitter is calibrated to what the
maximum differential of the application would be (plus a small buffer) of 154.35 inches of water
column = 20.0 mA output signal from the transmitter. Typically, most people would calibrate
the transmitter for figure 1 to 165.0 so that any excursion beyond the assumed maximum flow
rate of 2600 GPM would be accurately reported.
Let’s take a look at how a wide range system performs based on the PFS-Halmi Venturi Tube,
which is a modified, short form Venturi meter and a state-of-the-art differential pressure
transmitter based on a minimum-to-maximum flow rate range of 8:1 on flow.
Once the application conditions and accuracy requirements noted above have been analyzed and
the Venturi meter design that best suits those requirements has been defined, process engineers
can use a System Accuracy Profile (such as the one in Figure 1) to visualize how the metering
system (primary and secondary equipment) will perform. Figure 1 is an example of a standard
range (8:1) Venturi metering system with a single differential pressure differential pressure
transmitter.
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(FIGURE 1)
Columns 1 and 2 give the percent of maximum flow rate and the flow rate in gallons per minute
(gpm).
Column 3 gives what the differential pressure produced by the Venturi meter is for the
minimum-to-maximum flow rate.
Column 4 shows what the differential pressure transmitter uncertainty is as a percent of span (in
this example, using an Emerson’s Rosemount* 3051CD Differential Pressure Transmitter, which
has a basic uncertainty of +/-0.04% of max calibrated span/full scale).
Column 5 is used only with certain differential pressure transmitter models and reflects the error
as expressed in % of reading rather than % of span.
Column 6 provides the differential pressure transmitter uncertainty in “inches of water” form.
Columns 7 and 8 provide the differential pressure transmitters +/- uncertainty as a percent of
flow. Now the standard differential pressure transmitter uncertainty statement has been converted
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from “percent of full scale” to “percent of actual rate of flow”, which is consistent with how the
uncertainty of the Venturi meter is stated. Note that while the differential pressure transmitter
uncertainty is +/-0.020% of flow rate at maximum rate, it changes to +1.27/-1.28% of flow rate
at the minimum rate because, while the accuracy of all differential pressure primary elements is
always stated as a +/- % of actual rate of flow (down to the primary elements minimum pipe Rd
requirement), all differential pressure transmitter accuracies are stated as a +/-% of max
calibrated span or full scale, which means that the error contribution of the differential pressure
transmitter increases as the flow rate/differential pressure drops. The exception to this rule is the
Emerson’s Rosemount* 3051S Ultra for Flow which specifies accuracy as a percent of reading.
Column 9 shows what the uncertainty of the primary element/Venturi meter is across the
minimum-to-maximum flow rate range.
Column 10 gives the pipe Reynolds number over the minimum-to-maximum flow rate range.
Note that there are a number of bias error sources to consider when using a Venturi meter (there
are similar bias error sources for all measurement technologies), such as low pipe Reynolds
number conditions and upstream straight pipe requirements.
Columns 11 and 12 provide the +/- bias and random errors that are the result of the pipe
Reynolds number noted in Column 10 dropping below the minimum accepted for the specific
Venturi meter design being considered. Note that a significant benefit of the modified short form
venturi meter design (PFS model HVT) compared to the ISO5167 and ASME type venturi
meters is that while the ISO and ASME designs have a minimum pipe Reynolds number
limitation of about 200,000 for +/-1.0% basic accuracy, the PFS modified short form HVT
requires only 75,000 pipe Reynolds number for +/-0.5% basic accuracy.
Column 13 shows what the bias error is if there is not adequate straight upstream pipe. Note that
with a modified, short form Venturi meter there is no downstream straight pipe requirement and
the upstream requirement is considerably shorter than the classical Venturi meter design and is
functionally tied to what the first upstream disturber is (elbow, reducer, tee, etc.) and what the
beta ratio of the Venturi meter is (Beta is the ratio of throat size to line size d/D with the lower
beta ratio’s requiring less straight pipe and the higher beta ratios requiring great upstream
straight pipe. Generally, Venturi meter beta ratio’s range from 0.25 to 0.75.
Columns 14 and 15 provide what the integrated system accuracy is, based on the components
noted in Columns 1 to 13.
Figure 2 shows us what the integrated system accuracy would be if the flow rate range were to be
extended to 15:1 (325 to 5000 gpm) using the same Emerson’s Rosemount* 3051CD Differential
Pressure Transmitter.
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(FIGURE 2)
Note the change in differential pressure transmitter performance as noted in Columns 7 and 8.
Rather than the +1.27/-1.28 uncertainty for the transmitter based on 8:1 (Figure 1), the effect on
the uncertainty performance of the differential pressure transmitter becomes apparent as the
range extends to 15:1, which results in a minimum flow rate transmitter uncertainty of +4.67/-
4.85% of flow rate. If our expectation of total system accuracy over a 15:1 flow rate range is +/-
<1.0% of actual rate of flow, clearly that requirement cannot be met with this model differential
pressure transmitter system. Note also that the accuracy of the modified, short form Venturi
remains constant at +/-0.5% of actual rate of flow.
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(FIGURE 3)
In order to meet a total system accuracy uncertainty level of <+/-1.0% of rate, process engineers
can “split the flow rate range” (also known as stacking differential pressure transmitters) into two
discrete elements: high and low flow rate ranges. Then, select the “range code” for the high and
low transmitter’s best suited for and differential pressure range of the application. As an
example, engineers can use the same differential pressure transmitter range code noted in Figure
1 for the high flow portion of the system, but introduce a low range differential pressure
transmitter that has a maximum calibrated span of 25.0” of water column. The next challenge is
to determine what the “crossover” point will be between processing the signal from the high or
low range transmitter.
By simply calibrating the low range transmitter to a higher value, which changes the crossover
point, the system accuracy performance can be modified and improved. Note also that the
crossover point can be adjusted seasonally or based on changes in plant process requirements.
The dual transmitter system can also be used as a diagnostic tool where a comparison of the
output signals from both the high and low range transmitters at the crossover point will indicate
if one of the transmitters is out of calibration. In Figure 3, it has been determined that the optimal
crossover point is 26.0/25.998% of flow or approximately 1300 gpm, which, as noted in Figure 3
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Columns 14 and 15, will result in a total system performance of better than +/-1.0% of actual rate
of flow over the full range.
Another system design tool can be considered is to change the model of the differential pressure
transmitter to one whose uncertainty performance has enhanced capabilities. Figure 4 provides
an example of what happens to the system performance with a change from the Emerson’s
Rosemount* 3051CD Differential Pressure Transmitter (0.04% of span) to the Emerson’s
Rosemount* 3051S1CD Differential Pressure Transmitter with Ultra Performance Class (0.025%
of span).
(FIGURE 4)
In Figure 2, it’s noted that the integrated system accuracy over a 15:1 flow rate range and using
the Emerson’s Rosemount* 3051CD Differential Pressure Transmitter was from +/-0.501% (at
the maximum flow rate) to +7.434 to -8.029 % at the minimum flow rate. Figure 4 shows what
happens to the system when replacing Emerson’s Rosemount* 3051CD Differential Pressure
Transmitter with the Emerson’s Rosemount* 3051S1CD Differential Pressure Transmitter with
Ultra Performance Class. The result improved to +/-0.5% of rate at the maximum flow to
+2.959/-3.045% at the minimum flow rate. For instance, if the application requirement for
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system accuracy was +/-3.0% of rate, by changing the transmitter model, process engineers can
achieve the desired performance with a single differential pressure transmitter.
Figure 5 shows what happens when changing to the Emerson’s Rosemont* 3051S1CD
Differential Pressure Transmitter with Ultra Performance Class (0.025% of span) and utilizing a
split range or dual transmitter system. The crossover point remains best at 26.0% of max flow
with the resulting system accuracy ranging from +/-0.5% at max rate to +0.875% to -0.879% at
the minimum rate.
(FIGURE 5)
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Figure 6 gives a final system design that provides better than +/-0.78% accuracy across the full
15:1 range by upgrading to the Emerson’s Rosemount* 3051S3CD Differential Pressure
Transmitter with Ultra for Flow Performance Class (0.04% of reading). Unlike the other
transmitters that represent their accuracy/uncertainty as % of span, this transmitter has the ability
to provide accuracy/uncertainty in % of reading.
(FIGURE 6)
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While Figures 1-6 utilize a factory bench calibrated Venturi meter with +/-0.5% of rate accuracy,
Figure 7 includes the same basic data (including the use of an Emerson’s Rosemount*
3051S1CD Differential Pressure Transmitter with Ultra Performance Class) as Figure 4, but with
a change to a lab calibrated +/-0.25% Venturi meter uncertainty. Based on the lab calibration of
the Venturi meter, there is significant improvement in the system accuracy from about 12% to
100% of the application flow rates. However, at flows below about 12%, there is less
improvement due to the differential pressure transmitter performance at and below the 12% rate.
(FIGURE 7)
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Figure 8 is the same as Figure 4, but includes a change to laboratory flow calibrated Venturi (+/-
0.25% uncertainty) and uses the Emerson’s Rosemount* 3051S3CD Differential Pressure
Transmitter with Ultra for Flow Performance Class. With these two changes to the system
design, the system accuracy statement is just about +/-0.65% or better across the 15:1 range with
an even greater level of accuracy at all flow rates save the lowest one.
(FIGURE 8)
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Conclusions:
1. With proper analysis of the application requirements, including an understanding of what
the system accuracy goal is, a Venturi metering system can be designed to suit most
requirements and very effectively benefit from the basic advantages of the PFS HVT
modified short form Venturi meter technology.
2. The first step is to size the Venturi meter such that all of the application requirements are
met in terms of range, differential magnitude, energy consumption, etc.
3. Once the sizing process is completed, the selection of the best suited differential pressure
transmitter can be accomplished using an integrated system accuracy program such as is
presented in Figures 1-8. Choosing a high performance transmitter with % of reading
accuracy delivers the best system performance.
4. Adjustments to the overall system accuracy can then be made by including laboratory
flow calibration or changing the differential pressure transmitter to one that has enhanced
performance.
5. If the example used in the figures above were a real application, our recommendation
would be to use the system designed around Figure 8, which provides excellent accuracy
performance with a single properly selected differential pressure transmitter. This proves
the point that a wide range metering system using a Venturi primary element does not
necessarily mean stacked transmitters; it means that a thought process must take place
that leads the process design engineer to the conclusions presented for each option and,
with the help of an Integrated System Accuracy Profile, success can be achieved.
6. One example for the use of a wide range metering system analysis is when product loss
issues arise and there is concern, as there frequently is, that some portion of the flow in
the line is not being captured by the metering system. It may be simply a case of
excessively high errors due to the minimum/maximum flow rate operating outside of the
accurate range of the equipment in use or, outside of the calibrated span of that
equipment. Stated another way, some portion of a product loss number that is higher than
allowed may be easily corrected by a careful analysis of the secondary instrumentation.
*The performance data for the Rosemount brand differential pressure transmitters used in all of the
system accuracy figures was provided courtesy of Rosemount Inc.