“Wide Range Flow Metering Using Differential Pressure ... · “Wide Range Flow Metering Using...

“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

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

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.

(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

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.

(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.

(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

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

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)

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)

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)

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)

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.

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