Flow measurement is one of the most challenging and sometimes the most difficult of all process measurements. Process temperature, pressure, fluid density, viscosity, velocity, and piping must be considered when choosing the correct flow meter. Unfortunately, a flow meter that will measure all applications does not exist. However, the strain gage type target flow meter is as close to a universal flow meter as any flow meter available today.

OPERATION AND CALIBRATION
The strain gage type target force transducer provides flow measurement by sensing the fluid force acting on the target suspended in the flow stream. The following equation describes the operation of the strain gage target flow meter:

Cd = Overall drag coefficient obtained from empirical data.
A = Target area
P = Fluid density
V = Fluid velocity at the point of measurement
g = Gravitational force of the earth

In a given flow application, the drag coefficient, target area, and gravitational force would be constant. The flow meter is actually measuring the following:

Fluid density x fluid velocity2

Flow is equal to the square root of the force. The transmitter amplifies the output signal, extracts the square root, and produces a linear analog (4-20ma) and digital (0-1000 Hz) output signals.

A typical strain gage target flow meter (figure 1A & 1B) consists of the sensing element, mounting flange or housing, and a terminal strip or transmitter enclosed in a junction box.

The sensing element (Figure 2) is the heart of all strain gage target flow meters. The sensing element consists of a wiring connector, target rod, calibrated target, mounting base, protective case, and the sensing tube where the actual strain gages are attached. The sensing element is constructed of 316 SS with the sensing tube being MP35N alloy. MP35N alloy is a nickel-cobalt resistance, making it the ideal material for the sensing tube. Each component of the sensing element is press fit together then welded, completely isolating the strain gages from the process fluid.

Four strain gages (variable resistors) are attached to the sensing tube, two on the leading side of flow, and two on the trailing side of flow. The strain gages are interconnected, forming a four active arm strain gage bridge circuit. At zero flow (no force on the target), the bridge circuit is balanced, producing zero output. Flow produces a
strain on the sensing tube, compressing the leading side strain gages and tensing the trailing side strain gages, causing their resistance to decrease and increase respectively. The change in resistance of the strain gages offsets the bridge circuit, producing an output (figure 3).

The most difficult process in producing the strain gage type target flow meter is the precise application of the strain gages to the sensing tube. Each of the four strain gages must be applied identically to ensure that they respond equally to changes in temperature and pressure, therefore, not affecting the bridge circuit output. The bridge circuit output must also be proportional to force (flow squared) and return to zero when the flow force is removed.

The calibration and range of the flow meter is determined by the target size. Given the flow parameters for an application and knowing the desired amount of stress to be applied to the sensing tube at full-scale flow, the approximate target size is determined. The flow meter is then tested in a flow test stand and the final target is obtained.

Since the flow meter is a force transducer and cannot determine what is causing the force, all fluid flow application can be mathematically converted to a water flow equivalent. This water flow equivalent represents the same force as the actual fluid application allowing water to be used as the primary calibration medium. The following applications all exert the same force on the target, producing the same bridge output:

UNIQUE FEATURES
As stated earlier, the flow range of the strain gage target flow meter is determined by the target size. By changing targets the flow range can be altered. For example, using water in a 3-inch line, a flow range of 5-75 GPM, 15-225 GPM, and 24-360 GPM can be obtained with each flow range having a unique target. The 3"line example, any full scale water flow equivalent force from 70 to 360 GPM is available, with each maintaining a 15:1 turndown. In other words, every flow meter is calibrated to match each individual flow range. This meter has no moving parts to wear out, has the ability to change target (flow ranges), and the capability to withstand a fifty percent over range without damage, providing tremendous flexibility. This unique combination also makes the flow meter very forgiving in terms of correcting for erroneous process information.

The strain gage target flow meter calibration can be easily verified in the field without the use of a flow prover. Again, taking advantage of the force transducer design, hanging a test weight (0-1000 grams) in the direction of flow simulates flow force. Each flow meter data sheet includes the 1000-gram force output, in mV/V, along with the target type and size. A visual check of the target will verify the target type. A set of calipers will verify the target size (three decimal places). Securing the flow meter as if installed in a vertical pipe with flow down will allow the test weight to be suspended from the target rod simulating flow. By taking the square root of the test weight divided by the full-scale weight, and then multiplying by the full-scale flow, the flow simulated by the test weight is obtained. This procedure allows complete system verification from the primary flow sensor, through the transmitter, to the final readout device. Each transmitter has a calibration circuit, which can be used to check the strain gage bridge circuit, transmitter, and the final readout device. The calibration circuit forces the bridge to have a known output which represents a specific flow rate. This procedure can only be performed under a no-flow condition. Both calibration procedures require zero verification (zero output at zero flow) as the first step. At installation, a functional check can be made by simply applying force to the target (by hand) in the direction of flow. This will also verify all wiring connections.

The strain gage bridge circuit, which measures the force produced by flow, will measure both forward and reverse force. The polarity of the output signal indicates the direction of the flow, making the target meter a true bi-directional flow meter. A special target is used in ensure accuracy in both directions.

SPECIFICATIONS
The sensing element, the heart of the flow meter, can be installed in any line size and in almost any mounting configuration. Inline flow meters, supplied with mounting housing such as wafer, flanged, MNPT, and flare tube, are available for one half to six inch line sizes. Fixed insertion type flow meters are available for line sizes of four to sixty inches. Retractable insertion type flow meters are available for line sizes of four to thirty six inches.

The type of mounting configuration limits the pressure rating of the flow meter. In flow meters that have a flange, the flange determines the maximum operating pressure. The strain gage sensing element is available in three pressure ratings: 1000, 5000 and 10,000 PSIG.

The strain gage target flow meter is available in three temperature ranges, from -65° to +425° F, -65° to +500° F, and -320° to +250° F. Design work is currently underway to develop a force transducer that will operate up to +1200° F.

The accuracy of the flow meter, with flow calibration, is ± 0.5%. For line sizes, 8 inches and larger, a ± 2.0% calibration is also available. All strain gage target flow meters have a repeatability and hysteresis of 0.15%.

STEAM FLOW MEASUREMENT
The strain gage target flow meter has all the features desired in a saturated or super heated steam flow meter. It has an all welded design, which eliminates potential leak paths created by seals, gaskets, or o-rings. It has a low-pressure drop; no moving parts (bearings, springs), and is not damaged by slugs of condensate. The retractable flow version allows the flow meter to be inserted into service without shutting off the steam flow. Seasonal flow ranges, such as large flow rates in the winter and small flow rates in the summer, can be easily obtained by changing targets.

During each cycle, an amount of liquid is displaced equal to the difference in the volumes of the chambers minus the volumes of the pistons.

This process repeats itself in a continuous flow mode at the rates of 1 to 210 complete cycles per second proportionately to the fluid flow through the meter. In the FMTD4 Microflowmeter, each cycle displaces approximately .02 cc. Approximately 200,000 cycles will displace one gallon. The FMTD20 Microflowmeter has a flow rate capacity five times larger than the FMTD4.

Signal detection is accomplished by light interruptions of a photo emitter/detector device. A ferromagnetic wire tracks the magnet in the nutator [through a pressure tight barrier] causing these interruptions. The interruptions are electronically manifested as sine waves which are then conditioned by conventional electronic means to provide a square wave output.

The Doppler flowmeter measures the velocity of particles moving with the flowing fluid . Acoustic signals of known frequency are transmitted, reflected from particles, and are picked up by a receiver. The received signals are analyzed for frequency shifts and the resulting mean value of the frequency shifts can be directly related to the mean velocity of the particles moving with the fluid. Software can be used to reject stray signals and correct for frequency changes caused by the pipe wall or transducer protective material. Doppler flowmeter performance is highly dependent on physical properties such as the liquid's sonic conductivity, particle density, and flow profile. Likewise, nonuniformity of particle distribution in the pipe cross section results in a computed mean velocity that is incorrectly weighted. Therefore, the meter accuracy is sensitive to velocity profile variations and to distribution of acoustic reflectors in the measurement section. Unlike other acoustic flowmeters, Doppler meters are affected by changes in the liquid's sonic velocity. As a result, the meter is sensitive to changes in density and temperature. These problems make Doppler flowmeters unsuitable for highly accurate measurements in some applications. Doppler meters play a very important role where other meters will not work. These might be liquid slurrys, aerated liquids or liquids with some small or large amount on suspended solids

The Lease Automatic Custody Transfer (L.A.C.T.) unit is designed for the automatic transfer of ownership of crude or condensate between the buyer and seller.

This can be on land or offshore, into pipelines, barges, and tanker loading and offloading operations.

On land L.A.C.T. units are usually small, single run systems with portable proving connections designed for loading into trucks.

Because of the geographical area, this paper will focus on the offshore L.A.C.T. units. These are dual run with 100% back-up and with mechanical displacement meter provers at 150 ANSI working pressures.

The basic components and function of a L.A.C.T. unit are:

1. Charge Pump and Motor - Largely overlooked and undersized, special care should be taken into consideration during sizing to ensure correct NPSH is available to prevent cavitation, and discharge pressure is enough to overcome pressure drop through the L.A.C.T. to allow the required flow and pressure to the pipeline pump inlet.

2. Strainer/Air Eliminator - Strains solids larger than the perforations in the removable basket with liner. They should have differential pressure indicator to show pressure drop caused by debris accumulation and be cleaned periodically. Essential to prevent premature meter wear or breakage. The air eliminator is located on top of the strainer at the highest part of the system to allow air to be discharged and not metered. This should be piped with a soft-seated check valve to prevent air from being introduced into the system during shutdown.

3. Sample System - Installed with an upstream static mixer usually flow-proportional, isokinetic, and tubed to a vapor tight storage vessel sized to allow 25 to 30 days storage. The vessel is provided with a recirculation pump, the samples are mixed and then drawn off to be checked for composite API Gravity and BS&W during the delivery period.

4. BS&W Monitor and Probe - Installed downstream of the sampler and upstream of the three-way divert valve, this unit consists of a flanged probe that monitors the flowing stream for basic sediment and water and communicates with the "monitor" unit that is normally installed in the control panel. The "monitor" is usually set at 1.0%, and is wired to the solenoid valves controlling the three-way valves on the bad oil divert line. These will send oil to be treated if a high

BS&W signal is received for a given time. When a good oil signal is received for a set time, then the valves will return to normal flow position.

5. Meter - Installed downstream of the three-way valve and downstream of a properly sized thermal relief valve. The meter measures the product stream and allows totalization either through a local totalizer or electronic pulses to a flow computer. This meter can be a positive displacement or turbine meter.

The meter will provide signals to the flow computer or PLC to allow:

* Sample Pacing
* Totalization
* Meter Proving
* Meter Fail

6. Meter Instrumentation - Downstream of the meter a spool consisting of:

Temperature transmitter with platinum RTD installed in a S.S. thermowell.

Pressure transmitter with a pressure gauge mounted with a three-way gauge valve.

Test S.S. thermowell used to calibrate the temperature transmitter.

The temperature and pressure transmitters are used to send a live reading to the flow computer for compensation.

7. Check Valves - Downstream of the meter to prevent backflow to the meter in case the downstream block and bleed valve is left open and the opposite meter train is running.

8. Block and Bleed Valves - Located downstream of the check valves at the end of each run and as the main line divert valve separating the to and from lines to the prover four-way divert valve. This is to ensure all fluid is being diverted to the prover during proving, or a false meter factor could be obtained during proving of the meter.

9. Prover Instrumentation - On the outlet of the prover four-way divert valve a spool consisting of:

* Temperature transmitter with platinum RTD installed in a S.S. thermowell.
* Pressure transmitter with a pressure gauge mounted with a three-way gauge valve.
* Test S.S. thermowell used to calibrate the temperature transmitter.
* Thermal relief valve properly sized.

10. Back Pressure Valve - On the skid outlet to maintain pressure above the vapor pressure of the fluid being metered and provide a constant pressure and flow on the meter during proving.

11. L.A.C.T. Control Panel - This can be located on the skid with PLC controls and local manual proving connections would then be required, for a prover counter, detector switch plug in, power for the counter, and a portable pulse generator for P.D. meters.

An electronic temperature averager could then be used in lieu of temperature transmitters; however, due to their flexibility and relative cost, flow computers are rapidly replacing them.

If located in the MCC room, the panel could then be equipped with a PLC, flow computer, and printer to allow for automatic proving and batch reports by pushing a button, provided the prover's four-way valve is equipped with a remote actuator, and pressure and temperature transmitters were installed.

The control panel will have the following functions:

* Start and Stop Off High and Low Level Switches
* Hand-Off-Automatic Switch
* BS&W Divert Controls
* Meter Fail
* Monitor Failure
* Internal Battery Back-Up for Power Loss

12. Calibrated Bi-Directional Meter Prover - Because of the versatility of configuring a bi-directional prover in tight offshore spaces and its' cost associated with normal offshore flow rates; the bi-directional prover has become the prover of choice versus the uni-directional and small volume prover.

The continuous flow technique of meter proving is accomplished by repeatable displacement of a known volume of liquid in a calibrated section of pipe between two signaling devices (detector switches).

A slightly oversize prover sphere inflated to normally 2% over the pipe inside diameter is used to displace the fluid.

The fluid is run through the meter and the prover. The metered volume is recorded by the electronic meter proving counter (built into flow computers).

The known volume displaced is checked against the meter's indicated output and a "meter factor" is obtained after correction factors (Ctl)(Cps)(Cts) and (Cpl) are applied.

The use of new work over chemicals now seen offshore suggests internal coatings be exclusively a baked on phenolic versus an air-dried epoxy for longer life.

The prover sphere most seen nowadays has gone from the standard 53 Durometer hardness to a 58 Durometer harder material for longer life and durability.


Design Considerations

*Customer's Specifications

First and foremost, if your L.A.C.T. unit does not meet your buyer's or shipper's standard specifications, you may find yourself offshore with a tank full of oil having to shut in your wells because your L.A.C.T. skid has not been approved, or they just were notified that you want to come on-line with a skid they have never seen. Get hold of the specifications and get them involved early in the fabrication of the unit.

*Space

Offshore space is a premium. Units can be built in any number of configurations and footprints, but also consider the serviceability of the unit. Viscosity

High viscosity can cause several problems.

1. Does the meter (P.D. type) require high viscosity clearances and temperature trim on units above 150° F?

2. If heat traced and insulated, the instrumentation may require new trim.

3. Check prover sphere operating temperature.

4. Is NACE trim required?

Flow Rates

Is the L.A.C.T. unit only going to see one platform's oil, or is there a chance that a new platform's oil will be shipped over next year when it comes on-line?

Do you need to design the skid for an additional meter run?

Conclusion

The recent acceptance and development of new electronic equipment for crude oil measurement in the past few years has made them more reliable and accurate while requiring less maintenance. However, the technician needs to be more highly trained than ever before.

Many factors go into the design of L.A.C.T. units flow rates, space limitations, temperature, viscosity, corrosion, and customer specifications. As long as these are taken into consideration, one can end up with a quality measurement system.

It has long been known that some of the largest reserves of oil and gas lay below the waters of the world's oceans. For the last several decades the relatively shallow coastal waters have been the center of activity to locate and extract these reserves. With the ever increasing demand for energy and the increasing price of energy, oil companies have been forced to look in deeper and deeper water to locate new reserves. Until the last few years, the limitation on how deep underwater a reserve could be tapped was the ability to build a fixed production platform structure tall enough and big enough to reach the ocean floor. This limited production to water depths of no more than a few hundred to just over 1000 feet.

That is no longer the case. The invention of advanced subsea production systems, remotely operated vehicles (ROV's) and floating oil and gas production facilities have now pushed the limitation on water depth out to the 10,000-foot mark and soon beyond. Accurate flow measurement of a variety of fluids used in these subsea production systems located on the ocean floor and on the ROV's used to service them at these extreme depths is critical to their proper and safe operation. Measurement applications include specially formulated hydraulic control fluids, drilling mud and seawater.

Hoffer Flow Controls is a leading supplier of highly customized flowmeters to meet the extreme conditions associated with working at depths to 10,000 ft or more. In addition to high internal operating pressures of up to 10,000 PSIG and more, the meters must withstand high external pressures created by the sea. These pressures can reach as much as 5000 PSIG. The temperature of the seawater to which the meters are exposed is often well below 32° F (0° C). The salt content of the seawater prevents the water from freezing but still reduces the strength of metals which must be taken into account. The need to transmit the flow signal to a remote input control device, either located subsea or on the surface aboard the production facility, requires the use of specialized electrical connectors to permit wiring interfaces.

These connectors are often welded to the top of special risers that enclose the meter coil (See Illustration 1). This coil riser enclosure must also be able to withstand the high external pressure exerted by the sea and remain watertight. Above all else, however, the flowmeters must be exceptionally reliable. Day-in and day-out the flowmeters must function accurately and reliably at these depths without routine servicing or maintenance. When a subsea system has to be serviced, it is a logistically complicated and financially expensive operation. Though scheduled servicing is required on the systems; an unscheduled service call due to a flowmeter failure is simply not acceptable. The extraordinary reliability of Hoffer turbine meters under these conditions has made them the preferred choice for many subsea system manufacturers. Pictured in Illustration #2 is one of three custom-built Hoffer 4" flow- meters recently built for use on a new Illustration #2 4" Subsea flowmeter Illustration #1 5/8" Subsea flow meters with special electrical connectors subsea system being developed for operations at depths to 10,000-feet and beyond. These are perhaps the largest turbine meters ever built for operation at these depths. The meters feature a proprietary high-pressure clamp-type end fitting specified by our customer. They are rated and tested to internal and external operating pressures of 10,000 PSIG. The coil rise atop the meter is built from a smaller version of this same connector. It mates with a blind connector (not shown) to form the riser enclosure. In the upper portion of this riser enclosure, a HIT-1B transmitter is also located to provide a 4-20 mA output signal that is transmitted to a controller remotely located in another subsea enclosure. From concept to final product, these meters were designed, built and tested in 12-weeks, an astonishingly quick turnaround for such a highly customized product. Once again, when the pressure to perform was on, Hoffer delivered. Whether your requirements for flow measurement take you to the depths of the ocean, the outer reaches of the Solar system or on plain Terra Firma, call on Hoffer. We have the experience to deliver the performance you need, when you need it.

Savant Measurement Corporation is committed to providing the user community with a variety of cost effective, engineered solutions for everyday application challenges. As such, we maintain a broad product line, including both Flow Conditioners (GFC® and pFC™) and Acoustic Filters, (SAFE™ and Destroyer™) to fulfill the varied needs of our customers.

The SAFE and Destroyer provide the user with the option of using either or both devices as required to satisfy the need for noise reduction for Multi-path Ultrasonic Meters (MUSM). If the noise is of such a magnitude that line of sight and total absorption is required then both maybe used, if it is not convenient to employ the use of a single blind tee.

If the anticipated noise reduction required is in the order of 20-30 dB a single SAFE would be sufficient for the installation even if the noise is coming directly at the meter. Testing has shown that the SAFE demonstrates gross noise absorption of 40-50 dB with indirect noise and 20-30 dB with direct noise. Thus the user has the flexibility to determine which installation option best suits the application and mechanical limitations of the station design.

Based on research carried out it is anticipated that in the worst-case flow\dP noise scenarios a noise
reduction of 40-50dB satisfies most "noisy" applications. This reduction provides the MUSM with the
opportunity to work within a reasonable signal to noise or gain ratio level.

The Destroyer and SAFE have both been designed for simple retrofit and new installations. They both utilize a single flange connection. In the case of the SAFE it is 3\4" between flanges, with a single pin connection downstream on the element canister of the assembly, thus ensuring mechanical robustness in real world installations.

The SAFE has been thoroughly tested in both wet\dry and dirty\clean scenarios. Wet testing used a combination of both water and hydro-carbon components to emulate the combination of water, pipeline rouge and compressor oil.

The SAFE may also be retrofitted easily with new absorbent elements as required.

Installation of the devices is easily accomplished with either existing or new flow conditioner installations and may be deployed where the noise source is upstream or downstream in either unidirectional or bi-directional installations.

This article is the second in a three-part series on flowmeter technologies and selection. February's installment described new technologies and introduced the paradigm-case selection method. In April, users reveal how they choose flowmeters.

A great deal of attention is being paid today to new-technology flowmeters. Coriolis, magnetic, ultrasonic, and vortex are called new technologies mainly in contrast with differential-pressure flowmeters, which have been around for nearly 100 years.

New-technology flowmeters also contrast with traditional-technology flowmeters. Traditional technologies include turbine, positive displacement, thermal, variable area, and open channel. While some traditional technologies are less complex than new technologies, there are still some very interesting developments among these flowmeters. New products are still being introduced, and the annual sales of some of these types of meters exceed those of some new technology meters.

This article describes the operating principles of each of the traditional-technology flowmeters. It also looks at the paradigm-case applications for each type: the case where conditions are optimal for the operation of that type of flowmeter. Paradigm cases are a subset of the broader class of applications where a given technology will work.
Table 1.
Paradigm Cases

Technology	Paradigm case conditions	Comment
Turbine	Clean liquids or gases Enough flow to spin	Low-viscosity liquids are best
Positive-displacement	Liquid and gases Clean, non-corrosive, non-erosive	Medium to high viscosity is best
Thermal	Clean gases Known heat capacity	Low cost is an advantage
Variable-area	Clean liquids Low viscosity High accuracy not required	Spot checks of flow Most have no output signal
Open-channel: Weir and flumes Area-velocity	Free-flowing streams Hydralic structure is feasible Partially filled pipes	Weir: Stream has substantial slope Flume: Stream has little slope Liquid has impurities Flow velocity is high Diameter 6 in. and larger

Which flowmeter is best for which application?

According to this method, users should select the type of flowmeter whose paradigm cases are closest to their application. The method then advises looking at application, performance, cost, and supplier criteria to narrow the choice down to a particular flowmeter.

Turbine Style
Turbine for Clean, Low-Viscosity Fluids
Turbine meters have a spinning rotor with propeller-like blades mounted on bearings in a housing. The rotor spins as water or other fluid passes over it. Flowrate is proportional to the rotational speed of the rotor. A variety of methods are used to detect the rotor speed, including mechanical shafts and electronic sensors.

Turbine meters differ according to the design of the spinning rotor. Several variations include paddlewheel meters and propeller meters. Paddlewheel meters have the axis of rotation perpendicular to the direction of the flow--many paddlewheel meters are insertion devices. Propeller meters have a rotor that is suspended in the flowstream.

Turbine meters can be used on both liquids and gases. Paradigm-case conditions for turbine flowmeters include clean liquids or gases flowing at sufficient speed to operate the meters. Since turbine meters are sensitive to swirl and to flow profile effects, a straight run prior to the meter is recommended. Dirt or impurities in the liquid or gas can damage the meter.

Turbine meters are also sensitive to viscosity: low-viscosity fluids are best. Gas and liquid meters require different designs due to the different densities.

1. Overview
In this article, use of a Pitot Static tube, in conjunction with a manometer will be explained. Reference will be made to the FlowKineticsTM LLC FKT series manometers, as these instruments greatly simplify velocity acquisition. The Pitot Static tube allows the direct measurement of dynamic pressure allowing calculation of the gas velocity in ducts, pipes wind tunnels etc.

2. Measurement of Velocity

The Pitot Static tube measures the total pressure (or impact pressure) at the nose of the Pitot tube and the static pressure of the gas stream at side ports. The difference of these pressures, i.e. the dynamic or velocity pressure (Pdynamic) varies with the square of the gas velocity.

When selecting a Pitot Static tube to be used in conjunction with the FKT Series (or any manometer for that matter), it is necessary to select a tube with a constant close to unity, if errors in velocity are to be avoided. If data for a particular Pitot tube is not available,

the constant C may be estimated. This constant is dependent on the spacing of the Pitot tubes' static pressure ports (see Fig. 1) from the base of the Pitot tube's tip and the stem's center line. Prandtl type Pitot tubes typically have constants C close to 1. Figure 2 shows the effect and error of the location of the static pressure tappings on the static pressure error.

The lower line gives the static pressure error associated with the distance of the static ports from the base of the tip, expressed in diameters. The upper line presents the static pressure error due to the distance of the static ports (expressed in diameters) from the stem center-line.

Taking Measurements with the FKT Series

To measure velocity with the instrument with the greatest accuracy, it is necessary to measure the target gases absolute pressure and temperature as well as Relative Humidity, to allow the FKT Series to calculate the correct gas density. This is achieved by connecting a length of Silicon or Tygon® tubing from the Pabs port to a wall static pressure tap (or averaging ring) at the measurement point location. Alternatively, the Pabs port may be connected to the static port of a Pitot Static tube, provided C » 1 for the tube. Temperature/RH is measured by partially inserting the temp/RH sensor into the duct/wind tunnel etc.

Measurement starts with attachment of Silicon or Tygon® tubing to the Pitot Static tube and the pressure transducer of choice. The "P+" connection barb of the transducer is connected to the Total pressure port of the Pitot tube, and the Static pressure port of the Pitot tube is connected to the transducers "P-" barb connection, see Fig. 1 and the picture below. The appropriate transducer for the expected velocity range should be used for maximum accuracy. However, if in doubt as to the expected velocities, use the largest pressure range available to avoid overloading. If using the FKT 2DP1A-C Series (which accounts for compressibility and displays accurate velocities up to approximately 250m/s), the ratio of the specific heats, g, must be set.

The Pitot Static tube can then be carefully inserted into the gas flow. It may be necessary to drill holes into the ducting for insertion. The absolute pressure and temperature/RH must be measured simultaneously with the differential pressure measured by the Pitot Static tube for best accuracy. A "T" tubing barb can be used to connect the static port of the Pitot Static tube to the P- port of the differential pressure transducer as well as the Pabs absolute pressure transducer, see the sketch below. A Pitot Static tube with C of approximately unity should be used when this type of connection is employed.

In many applications, the ambient density may be close to the target gas density. This can readily be determined using the FKT Series by recording the ambient density (displayed continuously), followed by the target gases density. The density will be calculated and autonomously presented by the FKT Series through measurement of absolute pressure, temperature and RH. If the density is comparable, then simultaneous measurement of target flow density is unnecessary, i.e. the temp/RH sensor can be left in its housing.

3. Pitot Static tube duct surveys
If average duct velocities, or mass or volumetric flow rates are required, it is necessary to perform a Pitot traverse of the duct. This involves taking measurements at various positions across the duct. Before a traverse is conducted, it is necessary to select a suitable location to perform the survey. If possible, avoid traverses close to fans, dampers pipe bends, expansions etc. Try to survey at least 8 duct diameters downstream of the aforementioned elements and 2 duct diameters upstream of these elements. The survey is performed with the aid of Fig. 3. Either the Centroids of Equal Areas or Log-Tchebycheff point distribution may be used. A survey proceeds as follows:

1.
Decide on the number of survey points and then mark these on the Pitot tube using a marker or adjustable spring clips (present on some Pitot Static tubes).
2.
At the selected survey location, drill two perpendicular holes in the duct (for a round duct) or the desired number of holes for a rectangular duct, ensuring sufficient hole clearance to safely insert the Pitot Static tube.
3.
Partially insert the temperature/RH sensor in an additional hole located close to the previously drilled holes.
4.
Connect Pabs to a static pressure tap/ring close to the survey location, or use a "T" barb to connect to the static Pitot tube port, see sketch above.
5.
Carefully insert the Pitot Static tube into the duct and position at the first traverse location. Ensure that the Pitot Static tube is aligned with the axis of the duct using the alignment guide on the tube as a reference.
6.
Wait for the readout on the display to stabilize. If the readout continues to oscillate increase the damping (DAMP). If the magnitude of the oscillations is greater then 25%, then another measuring point should be considered as the results may not be representative.
7.
When stabilized, record the desired reading(s).
8.
Move the Pitot Static tube to the next traversing point and repeat 5 and 7 until the traverse is complete.
9.
Repeat points 5-8 for the other traverse locations.

Parshall Flumes

The most widely known flume and still the most widely used for permanent installations. Used in monitoring sewage, plant effluent and irrigation water. Available in sizes 1" through 12 foot.

Palmer-Bowlus Flumes

Known in the sanitary field where it is widely used for measuring flows in manholes, temporary installations or pipelines.


Trapezoidal Flumes

Originally an irrigation flume where it has been used to monitor flow in furrows and sloping-sided irrigation ditches. Small size trapezoidal flumes have a v-shaped throat, the bottom of which is at the same elevation as the channel invert - thus able to produce accurate readings at very low flows without the disadvantages of a v-notch or rectangular weir.

H Flumes

The H-flumes have been introduced to measure effluent, sewage, or stormwater having a very wide range of flow that otherwise would be difficult or impossible to measure in other types of flumes. Flume must be installed with a free fall off the end

Cutthroat Flumes

Similar to the Parshall flume except that the bottom is at the same elevation as the channel invert throughout the length of the flume. The cutthroat flume's greatest advantage is in its use where head loss is limited.

A Parshall flume has a special shaped open channel flow section which may be installed in a ditch ,canal, or lateral to measure the flow rate. The Parshall flume is a particular form of venturi flume and is named for its principal developer, the late Mr. Ralph L. Parshall (Water Measurement Manual, U.S. Bureau of Reclamation, 1984)."

Ralph L. Parshall saw problems with stream measurements when he began working for the USDA in 1915, as an irrigation research engineer. In 1922 he invented the flume now known by his name. When this flume is placed in a channel, flow is uniquely related to the water depth. By 1953 Parshall had developed the depth-flow relationships for flumes with throat widths from 3 inches to 50 feet. The Parshall flume has had a major influence on the equitable distribution and proper management of irrigation water


Parshall flumes are apparently the most widely used types of flumes now for fixed flow monitoring installations. They have wide flow ranges, resistance to submersion,and are simple to calibrate..
Parshall flumes are sized by throat width and conform to standardized dimensions published in the U.S. Department of the Interior, Bureau of Reclamation.

Laminar and turbulent flow are most common in flow regimes or in liquid flow measurement operations but there is also transitional flow.

If we want to calculate the Reynolds number , we can use the following equation

R = 3160 x Q x Gt D x µ

where: R = Reynolds number
Q = liquid's flow rate, gpm
Gt = liquid's specific gravity
D = inside pipe diameter, in.
µ = liquid's viscosity, cp

When the Reynolds number is less than 2000, flow will be described as laminar
When the Reynolds number is greater than 4000, flow will be described as turbulent
When the Reynolds number is in the range of 2000 to 4000 the flow is considered transitional.
Viscosity can be a major factor that affects the value of the Reynolds number.

Introduction

Design and test engineers for fuel cells, reformers, and fuel processors all need to be able assert accurate flow control of hydrogen and other gasses into their systems. In addition to requiring that these flow rates are stable, accurate, and repeatable, the commercial realities of the evolving fuel cell marketplace demand that these systems, furthermore, are light, compact and highly reliable. These requirements are essentially identical to the needs spacecraft designers faced in the 1960’s when engineering small rocket engines and thrusters. The sometimes very low flow rates of propellants had to be controlled with accurate, robust, and highly reliable flow regulating equipment - and that solution was very often chokes. Even now, forty years later, critical flow venturies, also known as sonic chokes or Laval nozzles, are still the primary device for regulating a multitude of gas flow rates into the chemical laser at the core of the Airborne Laser being built by Boeing at Edwards AFB. This is an application where reliability, accuracy, and compactness are crucial - and chokes are the chosen solution. In contrast, most fuel cell test labs, fuel processing systems, and even fuel cells themselves have adopted a brute-force, expensive solution to flow regulation - the triple-headed combo of control valve+flowmeter+PLC, all tweaked thirty times per second to maintain fixed flow rates over the tiniest of changes in ∆P. Sonic chokes – an elegant solution considered commonplace amongst aerospace designers for forty years – offer substantial advantages.

A Sonic Choke: Flow Rate Independent of Differential Pressure (∆P)

There is nothing new about Sonic Chokes. Bernoulli understood them, their operational characteristics are described in detail in every fluid mechanics textbook, and they have been commercially available for over fifty years. What do they do? What sonic chokes do is very simple: When provided with a fixed inlet pressure, they maintain stable, constant flow rates that are unaffected by downstream pressure or changes in inlet-to-outlet differential pressure. (This is true as long as the outlet pressure is below about 88 - 90% of the inlet pressure - a value referred to as ‘recovery.’) In simpler terms - this means that if you set the inlet pressure to a sonic choke flowing hydrogen at 100 psia, then the discharge pressure can change from 15 psia to 50 psia to 75 psia to 85 psia with absolutely no change in flow rate. See Fig. 2. The only moving part in the entire system is perhaps the diaphragm in the upstream pressure regulator. The flow control elements of the this system - the sonic choke - has no moving parts at all. The flow rate, which can be calibrated to ±1/4%, is now solely a linear function of inlet pressure. Fuel cells or fuel processing systems already have a pressure regulating system. Coupled with a sonic choke, the existing pressure regulating system suddenly becomes transformed into a flow regulating system - and a very compact one at that. Therefore, with just a few extra psig/kPa on the inlet side to ensure that the minimum recovery level of 85-90% is achieved - flow rate into a fuel cell or reformer is fixed, stable, repeatable and unaffected by pressure changes in the stack or fuel processor.

Sonic chokes, which can be machined from any metal, are in use today with gasses with temperatures ranging from -450° F to +1500° F and with pressures ranging from 5 psia to 10,000 psia.
In the conventional, valve+flowmeter+PLC approach, every wisp of pressure fluctuation in the fuel cell or reformer causes a resultant change in ∆P across the valve, resulting in a change in flow rate, which is sensed by the flowmeter, which sends a signal to the valve, which adjusts the flow, which causes a new ∆P, which must be again compensated for, and so on. In a sonic choke, a shock wave at the venturi throat establishes a barrier that prohibits propagation of any downstream perturbations upstream beyond the throat. The inlet flow pattern into the throat - and hence flow rate - is undisturbed and unaffected by ∆P across the choke.

Adjustable Area Sonic Chokes

So far, we have discussed fixed area sonic chokes, where flow rate through a single venturi throat establishes a single curve - a straight line - relating inlet pressure to flow rate. What if we wish to be able to vary the H2 flow rate into a fuel cell, yet still take advantage of the features of a sonic choke where, once we establish the desired flow, it is unaffected by any changes in ∆P or backpressure.

This requirement is met by adjustable area sonic chokes,which have been used to vary flow rates into rocket engines and high energy lasers since the 1970’s. Precision-machined needles are inserted into a venturi throat, and can be accurately repositioned by manual, electrical, or pneumatic means. A calibration then determines the precise flow area corresponding to every valve position along its stroke. This “effective area” (CdA) can then be used to predict flow rate for any gas, at any pressure, at any temperature.

Elimination of Flowmeters: Regulating Flow Rates that Don’t Need to be Measured

It is important to remember that once sonic chokes are being used in a system, flowmeters should be eliminated from the process. This has sometimes been a difficult concept to understand. Sonic chokes - whether fixed or adjustable - can be calibrated traceable to the NIST to ±1/4% or better. Although you may wish to use them as a flow regulating device in your fuel cell, you can also remove them and use them as a calibration reference standard with which you can calibrate the other flowmeters (turbine, laminar flow, hot wire, etc.) in your facility. Do you have ISO-9000? If so - you don’t need to send your flowmeters out for recalibration if you have a calibrated sonic choke in your building: they can be calibrated against the choke. And certainly, you do not need to install a flowmeter downstream of a sonic choke in a reformer to verify performance, since the choke will probably be regulating gas flow rates with a higher precision than the flowmeter can measure.