The Versatile Venturi Design

The Versatile Venturi Page No: 1

The Versatile Venturi

Part 1--History

The Classical (Herschel) Venturi

The Herschel Venturi meter design was first introduced commercially during the 1800's and quicklygained active use.

This device was revolutionary at the time of its introduction in that it was the first flow meter to provide a method to measure liquids with high accuracy and repeatability. It also provided the user with a simpleyet effective way to be sure that it was operating properly.

The basic material of construction was Cast Iron for the body and flanges and Bronze for the throat section.

The design included an averaging pressure sensing ring (called an annulus or annular chamber) at thehigh and low pressure sensing cross sections.

The length of the Classical Venturi was quite long because its convergent and divergent angles weresmall.

The concept of the device as a means of measurement is simple:

1.) High pressure is sensed at the inlet tap cross section.

2.) Low pressure is sensed at the throat tap cross section.

When the two pressures are compared ( in a variety of ways,) the resulting difference is what wecommonly call the meter’s Differential pressure signal.

3.) As the flow passes the high pressure sensing point, its energy content value is noted.

We call it high pressure but in reality it is an indication of the available energy in our flowing stream.

4.) Next the flow is transitioned to a narrower cross section which we call the Throat Section. Here again, the energy content is measured.

5.) The difference between the two sensing point values equates to the amount of energy consumed by the meter as the line velocity increases through the narrower cross section.

6.) Once the flow passes through the throat cross section, it begins a long transition from the smaller throat diameter back to the full downstream pipe diameter. The purpose of this recovery section of the Venturi is to smoothly and efficiently recover as much of the energy consumed by the upstream portion of the meter as possible.

7.) Finally we take the differential value (in inches of water or psi,) input it into our Venturi flow formula with the result being a reliable indication of the rate of flow which is passing through

the device.


Part 2--Nomenclature

Design and performance language for Venturi type devices is consistent for all differential producers. The following are the most important terms necessary for the design and application of such devices.

1.) Line Size of the meter is designated as “D.”

2.) Throat Size of the meter is designated as “d.”

3.) Beta Ratio of the meter is d divided by D (d/D.)

4.) Differential is the difference between the high pressure sensation and the low pressure sensation.

5.) Unrecovered Headloss is the amount of energy which the meter consumes in doing its“work.”

6.) Reynolds Number (Pipe Reynolds Number) is the ratio of all forces driving our flow divided by all forces trying to prevent the material from flowing.

Each type of Venturi meter has its own Reynolds number characteristics which must be stated and substantiated by the meter manufacturer.

Reynolds number is a limiting factor when applying Venturi meter technology. A short definition of Reynolds number is:

... Pipe Reynolds Number is the ratio of all forces driving the flow


through a pipeline divided by all of the forces trying to prevent the flow from moving through the pipeline.

The calculated value which we call Pipe Reynolds number also expresses the relationship between turbulent and laminar flow conditions.

Venturi meters need turbulent flow conditions and the closer the flow approaches full laminar flow conditions, the greater is the Pipe Reynolds Number effect on the ability of the pressure taps of

the meter to accurately sense true cross sectional static pressure.

As an example, the PFS-HVT product has a minimum Pipe Reynolds Number requirement of 75,000. This means that if you calculate the Pipe Reynolds number of your application and the result exceeds 75,000,

the accuracy of the meter is not affected by the potential Turbulent versus Laminar conditions.

However, if your calculation resulted in a value below 75,000, it does not mean you cannot use the Venturi meter. It simply means that you cannot obtain the standard accuracy of +/- 0.50 % of actual rate of flow.

A quick calculation for (approximate) Pipe Reynolds Number is as follows:

For GPM: 2790 x GPM/Line Size in Inches.

For MGD: 1.935 x MGD/Line Size in InchesPlease Note: These formulae will produce results within +/- 10% of the actual value, which is close enough to determine if you have a problem.

Since the Venturi meter manufacturer is responsible for publishing the minimum Pipe Reynoldsnumber requirement for his meter, if you are operating in a questionable range, the

manufacturer should also be able to advise you what the error would be at the lower than standardPipe Reynolds number level.

Ergo: Contact your manufacturer to be sure you are not in a questionable range.

Part 3--Key Design Criteria

1.) The size of the throat cross section determines the magnitude of the differential produced by the Venturi meter.

The smaller the throat (lower Beta ratio,) the higher the differential and unrecovered headloss.

As you make the throat larger (larger Beta ratio,) you lower the differential and headloss.

2.) Typical Headloss percentages for true Venturi meters is a function, therefore, of the Beta ratio:

Standard 0.5 Beta (6.0" throat in a 12.0" line) -- 10% of the differential.

Standard 0.7 Beta (8.4" throat in a 12.0" line) -- 4% of the differential.

3.) Venturi meters are subject to the effects of upstream Non- Straight piping ONLY!

Venturi meters are not subject to any downstream straight pipe requirements.


Please Note: Each manufacturer must provide information as to the exact nature and value of those effects. For a complete explanation of this subject see pages B8, B9, and B10 of the PFS Standard Technical Literature.

4.) Venturi meters can be used to measure most liquids and gasses.

5.) There is no phase requirement for orienting Venturi meters. They can be used for horizontal or vertical (upwards or downwards) flow, or any angle in between.

6.) If a Venturi meter is to be used for liquid flow metering and in a horizontal orientation, the pressure tap orientation should be on the horizontal plane. This will keep air out of the lines (which will cause a maintenance problem with the Differential PressureTransmitter) and prevent the accumulation of dirt or sediment, as well.

It is not a good practice to orient the high andlow pressure taps vertically and should be avoided ifat all possible.

Part 4 -- Modified Short Form Venturi

The Venturi meter in use today has undergone a number of changes from the Classical design but the basic metering concept remains unchanged.

As an example of the Short Form Type Venturi, the PFS model HVT-CI gives you thebasic layout with the following key features:

1.) The overall length of the device is considerably shorter because the

intersecting angles are much greater and the dischargeend of therecovery cone does notend in theFull downstream pipe line

size, but rather isTruncated or

Cut Short whilethe flange is

designed tomate directly to the

downstream pipe flange.

This allows the device tobe shorter and MoreEnergy Efficient since the


frictional loss through the long Classical Venturi meter recovery cone consumed a great amount of energy.

2.) The HVT-CI throat section is about one-half the length of the Classical Venturi meter throat section. The present HVT-CI design makes the length of the throat section ½ the diameter.

3.) Instead of the gradual radius to and out of the throat section of the Classical Venturi meter, theHVT-CI short form device has sharper intersecting angles.

4.) While the Classical Venturi meter had annular chambers at both high and low pressure tap cross sections, the short form utilizes a single tap design whereby the tap SEES directly the material being measured.

Part 5 -- Insert Type Venturi Meters

A useful addition to the world of Venturi metering is theInsert design.

This design approximates the shape of a true venturi, butthe profile is entirely inside the pipeline, save for the thickness

of the center flange. This design is similar in its installationto an orifice plate.

This design utilizes a single tap Static low pressure throat tap but violates one of the key requirements of a “True Venturi Meter” in that its high pressure sensation comes from a corner location on the inlet side of the center flange. Note that the Classical Venturi senses true static pressure at both the high and low pressure tap

cross sections, as does the Short Form Venturi design.

In order to claim that a tap senses “True Static Pressure” the tap must sense pressure perpendicularto the axis of What is Flowing.


The Classical and Short Form Venturis do that very successfully in the flanged design, but without an upstreamsection in which to locate the high pressure tap, the Insert Type Venturi Meters look at the flow Head On at thehigh pressure tap.

While this makes the insert Type Venturi meter more sensitive to the effects of upstream Non-Straight piping, thebasic accuracy of these meters is identical to the flanged design at +/- 0.50% of actual rate of flow.


Part 6 -- The Venturi Flow Metering System

Once we create a very accurate and repeatable pressure signal, we need an efficient and accurate way toconvert that signal into an indicated rate of flow.

We can use a number of methods:

1.) Manometer

2.) Gauge

3.) DP Transmitter - Differential Pressure Transmitter or Flow Transmitter.

The most common secondary device in use today is either the analog or digital differential pressure transmitter (DPT.)

This device takes the high and low pressure signal from the Venturi and converts it into a 4-20 mA (typically) output signal which is used for the final indicated rate of flow signal.

Part 7 -- Wide Range Flow Metering

With the advent of the Smart/Digital DP Transmitter, Venturi flow meters can be very effectively used for wide flow range metering. By “Stacking” the DP Transmitters from the same signal source, you can


achieve accurate flow measurement for ranges of 200:1, 300:1 or more.

In order to do this, we design our Venturi with a throat size which will produce a differential at the lowest desired rate of flow that the low range DP Transmitter can properly sense. From that point and beyond,

the transmitters can be switched simply by using a local electronic device called a Signal Discriminator or it can be done in a PLC or a Central Processor (CPU.)


Part 8 -- Special Designs for Specific Uses Bi-Directional Flow Meters

Today we see Venturi meters used for a wide variety of flow metering applications. As an example, consider the following:

One very good application is for accurate and repeatable metering when one needs to measurein both forward and reverse directions.

Bi-Directional Venturi meters come in Cast (or Ductile) Iron, Fiberglass, Fabricated insert, or Fabricated flanged out of any applicable material.

The accuracy of the meter is the same in either direction (PFS Standard is +/- 0.50% of actual rate of flow)in the symmetrical design. Special Bi-Directional meter designs allow you to measure accurately in one direction while measuring a little less accurately in the reverse direction, but with higher headloss in the opposite direction.

The application for this example would be a water plant pumping into an elevated storage tank then from the tank into a distribution system.

Flowing from the plant into the tank, we may not care as much about accuracy as we do headloss and pumping costs, while as the flow leaves the tank into the distribution system (where it may be billed,) accuracy becomes more critical while we do not care about headloss.


Figure #12 illustrates a Full Bi-Directional metering system including the secondary instrumentation and switch over mechanisms. The system operates as follows:

1.) The Bi-Directional meter has a common low pressure tap and two distinct high pressuretaps, one for each flow direction.

2.) The DP Transmitters are piped as shown and each have a distinct readout and totalization feature.

3.) When the flow changes direction, a number of methods can be employed to isolate the opposite direction transmitter. Typically, a paddle switch (as shown) can selectively power or depower the proper transmitter. Otherwise, this can be accomplished in a PLC or on

command from a pump starting or valve closing event.

Many features can be added to the metering system, including a (2) pen Chart Recorder to record either direction separately.

Part 9 -- Rate of Flow Controllers

Venturi meters can be mated with butterfly valves(or almost any other type of valve) to form a very reliable and efficient Rate of Flow Controller.

Typical applications are Filter Effluent rate control, Backwash Control, Aeration Header rate control todrop leg diffusers, and process flow control.


The typical loop has either a flanged Venturi or an Insert Venturi direct or Close coupled to a butterflyvalve. Please Note: This application is possible for Venturi meters without concern of any adverse

back pressure effects of the valve on the metering performance of the Venturi. This Is Not The Case With Other Types of Flow Meters Like the Magnetic Flow Meters and Ultrasonic Flow Meters.


The output signal from the DP Transmitter is routed to a set point controller (this can also beaccomplished in a CPU.) The input rate of flow signal is compared to a pre-determined set point value and ifthere is a difference, the set point controller sends a signal to the valve actuator causing it to move until thedesired flow rate agreement is reached.

Rate of Flow Controllers are designed specifically for a given hydraulic loop condition and can utilize a “same as line size” or “less than line size” butterfly valve in order to achieve proper overall control.

Part 10 -- Use of Venturi Meters for Flow with Suspended Solids Content.

By using a sealed sensor at the point of high and low pressure sensation, the advantages of the Venturi meter can be enjoyed in applications where fear of plugging the taps and sensing lines to the DP

Transmitter would otherwise not permit.

This approach is particularly useful for raw sewage or sludge applications where long term lifeexpectancy is desired.

The system is also useful when compared to other types of Non-intrusive devices (like magnetic flow meters) in that the secondary DP Transmitter system can be calibrated In-Place by the use of a

manometer which checks the high and low pressure sensation independently of the DP Transmitters. Forfield calibration the DP Transmitters are then re-calibrated to the manometer readings and full reliability of the

metering system is achieved. This cannot be accomplished with Magnetic Flow Meters, as acontrast. The only thing one can check on a Magnetic Flow Meter is the electronic circuitry, which hasnothing to do with the relationship of the output signal to the actual rate of flow!


Part 11 -- Steam and Gas Flow Using a Venturi Metering System

For applications where energy consumption is a concern and high, defensible accuracy is required, the Venturi metering system provides optimum performance.

Note that historically, Orifice plates have been used extensively for gas and steam flow measurement. Since they are very “Headloss Intensive” consuming as much as 70% of the available energy with no “Designed” and little “Natural” energy recovery, the use of Orifice plates can greatly limit the maximum capacity of a system despite the fact that the steam or gas generator may be capable of substantially greater output.

Orifice plates are also subject to the unpredictable change of their discharge coefficient(“C”) over time expressed as a deterioration of accuracy as a function the sharpness of the “Bore Edge.” Worse, as the edge wears and causes a “C” change, the accompanying performance change is difficult to “SEE” and therefore quantify in a meaningful way.The Typical Steam Measurement System (see figure18) consists of:

1.) A True Venturi Meter (either pressure vessel -flanged- or insert type.)

2.) One or More Smart Differential Pressure Transmitters.


There are a number of DP Transmitter options depending upon the accuracyrequirement:

a.) Standard Smart Differential Pressure Transmitter with no temperature/pressure compensation.

The use of this DP Transmitter is dependent on the range of the temperature and pressure change expected. If the temperature is fairly constant, the use separate, stand-alone pressure and temperature transmitters may not be required to achieve the required accuracy

b.) Today, a new range of Differential Pressure Transmitters are availablewhich contain built-in pressure and temperature compensation capabilities. In

addition, they carry an internal micro-processor which provides 4-20 mA output signal fully compensated for the effects of different line pressures and

temperatures.

This type of DP Transmitter system provides the most accurate performance and is less expensive than the use of stand-alone pressure, temperature,

and DP Transmitters coupled with a flow computer to process and provide a 4 - 20 mA output.

3.) One or More Local Indicator/Totalizer(s.)

4.) Single signal discriminator for inter-transmitter switch over (Optional.)

5.) Nema 4X or 7X Enclosure (Optional.)

6.) Chart Recorder (Optional.)

Part 12 -- Integrated System Accuracy with Venturi Meters.

Today, users typically do not concern themselves with individual equipment accuracy statements. Rather, their interest is in “Total System Accuracy” since that is the value used for process control purposes,billing, reporting to a regulatory authority or custody transfer applications.

For Venturi metering systems, the accuracy of the primary element, Pipe Reynolds number effect andflow pattern sensitivity must be integrated with the maximum to minimum accuracy of the Differential Pressure

Transmitter and other devices whose performance is a function of a maximum calibrated value.

Prior to considering the System Accuracy, one must consider the confidence level with which we accept accuracy statements claimed by suppliers of each device that comprises our metering system.

Sigma is the symbol for Standard Deviation. Confidence Level refers to the probability that a given measurement will fall within a stated accuracy band and can be referred to in terms of the number of standard deviation, or Sigma values.

There are four Sigma levels which are of significance to us:

1 Sigma = 68.27% Confidence Level.





Please Note: -The above refers to all types of measurement equipment, not only Venturis.-When working with any metering equipment supplier, ask what their Sigma value is.

If they do not have a Published Sigma, Proceed with Caution!How Does Sigma Affect the System Performance?

Responsible Venturi meter manufacturers state their accuracy based upon a 2 Sigma level (sometimes called “Twice Standard Deviation.”) In the case of the PFS-HVT product line, the accuracy statement is:

+/- 0.50 % of the actual rate of flow, from 75,000 Pipe Reynolds numberAnd beyond, 95.45% of the time.

Differential Pressure Transmitter manufacturers provide a 3 Sigma or 4 Sigma accuracystatement. Rosemount, for instance, states their accuracy as follows:

For the 3051CD Model: +/- 0.075% of the calibrated span, 99.73% of the time.

Johnson-Yokogawa, on the other hand, states the following:

For the EJA series: +/- 0.10% of the calibrated span, 99.99% of the time.

Therefore in developing a proper understanding of your system accuracy, it is essential toconvert all accuracy statements (for each piece of the system) to a common Confidence Level. Sincethe Venturi meter forms the basis of the system, we convert all other equipment, as required, to the2 Sigma Confidence Level.

Therefore, the DP Transmitter accuracy becomes +/- 0.05 % of calibrated span at 2 Sigma.

3 Sigma Accuracy = +/- 0.075 % of calibrated span.

Divide by 3 for 1 Sigma Accuracy = +/- 0.025 % of calibrated span.

Multiply by 2( for 2 Sigma Accuracy) = 0.05 % of calibrated span.

Integration of the DP Transmitter and the Venturi Meter:

a.) As an example, if we assume that we have an application where the DP Transmitter has a maximum rate of flow differential of 100.0" of water, then the accuracy of the DP Transmitter at maximum flow is +/- 0.05% of 100.0" of 0.05" of water.

b.) The Venturi meter provides differentials as follows, over a 10:1 range of flows: Q Differential

100.0% 100.0" 80.0% 64.0" 50.0% 25.0" 10.0% 1.0"

Therefore the accuracy of the differential pressure transmitter as it is related to flow is as follows:


+/- [(100.05/100)0.5 -1] x 100 = +/- 0.025 of flow.

+/- [(64.05/64)0.5 -1] x 100 = +/- 0.039 % of flow.

+/- [(25.05/25)0.5 -1] x 100 = +/- 0.100% of flow.

+/- [(1.05/1)0.5 -1] x 100 = +/- 2.470 % of flow.

The Venturi / DP Transmitter Integrated System Accuracy ( in percent of actual rate offlow) then becomes:

Q Venturi DP Trans. System 100 % +/- 0.5 % +/- 0.025 % +/- 0.501 % 50 % +/- 0.5 % +/- 0.100 % +/- 0.510 % 10 % +/- 0.5 % +/- 2.470 % +/- 2.520 %

Once we have determined the performance of our system per the above, we can furtherdetermine whether it will be beneficial to split the flow range into two or more DP Transmittersystems to optimize the system accuracy.

If, for instance, the application dictates that no point along the entire flow range can tolerate an integrated System Accuracy exceeding +/- 1.00 % of the actual rate of flow, it would be necessary to Split the Range. By adding a second DP Transmitter at approximately 17 % of the maximum rate, it would begin its performance at its full maximum calibrated span accuracy of

+/- 0.025 %.

In addition to splitting the range by adding a second transmitter, we also would need to calculateour Venturi Pipe Reynolds number to determine that it exceeded the minimum necessary to

assure a constant Discharge Coefficient.

To complete our system, if we found that we would experience a Pipe Reynolds number bias effect, we would introduce an electronic component called a Linearizer. This device takes the output signal from the DP Transmitter and applies a correction to it based upon the Low Pipe Reynolds number versus Discharge Coefficient Curve provided by the Venturi manufacturer.

Please Note: If the Venturi manufacturer does not have Pipe Reynolds number data for therange of your application, you can either choose to Flow Calibrate the metering system in a Hydraulic

Laboratory covering the flow range(s) where Pipe Reynolds number data is unavailable,otherwise, you can accept the extrapolation provided by the Venturi manufacturer down to the pointat which your application is operating.

Part 13 -- Conclusions

Venturi meters are useful devices to measure liquids or gasses. They are versatile in that their lengthcan be changed to fit a defined space; they can be modified to provide rate of flow control or measure sewage;they can be used reliably for billing or custody transfer and they can be used for rectangular circular metering.

The Limitations are Few:

1.) Pipe Reynolds number is a consideration though not usually a problem that can be “designed out of.”

2.) Venturi meters do consume some energy but, unlike other devices, the recoverycone renders their energy consumption almost insignificant. Typically, the energy loss

through a Venturi is between 0.10 PSI to 0.25 PSI.


3.) Venturi meters can be oriented in any plane and can measure accurately whether flowing upwards or downwards.

4.) Venturi meters are not subject to downstream piping, only upstream piping, theeffects of which are well known and predictable.

5.) There is no intrinsic design limitation on either temperature or pressure of aproposed application.

6.) There is no intrinsic design limitation as to line size.

The Versatile Venturi Design

Documents

Transcript of The Versatile Venturi Design