Pipe Flow Conditions

The most important-and most difficult to measure-aspects of flow measurement are flow conditions within a pipe upstream of a meter. Flow conditions refer to: the gas velocity profile, irregularities in the profile, varying turbulence levels within the velocity or turbulence intensity profile, swirl and any other fluid flow characteristics which will cause the meter to register flow different than that expected. This will cause the meter to differ from the original Calibration State referred to as reference conditions that are free of installation effects.

Installation effects which cause flow conditions within the pipe to vary from reference conditions are: insufficient straight pipe, exceptional pipe roughness or smoothness, elbows, valves, tees and reducers, just to name a few. Certainly, a common understanding of how these installation effects impact the meter is important since devices which create upstream installation effects are common components of any standard metering design. Flow Conditioning refers to the process of artificially generating a reference, fully-developed flow profile and is essential to enable accurate measurement while maintaining a cost-competitive meter standard design.

Industry-accepted nomenclature and discussions are presented which explain commonly referred to flow conditions.

The most commonly used description of flow conditions within the pipe is the velocity flow profile. For general fluid dynamic background Miller (1996) offers a thorough textbook description of velocity profiles and distortions of the profile due to upstream piping effects. The most common method used to describe velocity flow profiles for natural gas measurement is shown in Figure 1, Velocity Flow Profile.

Figure 1, Velocity Flow Profile

Equation 1 describes the shape of the velocity flow profile. The value of n determines the shape of the velocity flow profile. Karnik (1993) and others use Equation I to determine the flow profile's shape within the pipe by fitting a curve to experimentally measured velocity data. Karnik (1993) was the first to actually measure transverse velocities within the high-pressure natural gas environment using hot wire technology to accomplish the data fit.

A fully developed flow profile is used as the Reference State for meter calibration and determination of Coefficient of Discharge (Cd). For Reynolds Number 10⁵ to 10⁶ n is approximately 7.5; for Re of 10⁶, n is
approximately 10.0 where a fully developed profile in a smooth pipe is assumed.

Since n is a function of Reynolds Number and friction factor, more accurate values of n can be estimated by using

where f is the friction factor. It is not the intent here to provide detailed instructions for determining friction factors. The Colebrook (1939) equation or Moody (1944) diagram can be utilized as illustrated and detailed by Karnik (1993).

A good estimate of a fully developed velocity flow profile can be used for those without adequate equipment to actually measure the velocities within the pipe. White (1986) and Karnik (1993) utilize the following straightpipe-equivalent length to ensure a fully developed flow profile exists.

As one can see, the pipe lengths required by equation (2) are significant, hence the need for devices that condition the flow over a shorter pipe length allowing metering packages to be cost competitive and accurate.

It is important to point out that the velocity flow profile is generally three-dimensional. For simplicity, normally the description requires no axial orientation indication if the profile is asymmetric. If asymmetry exists, then axial orientation with respect to some suitable plane of reference is required. Asymmetry exists downstream of installation effects such as elbows or tees.

Normally, the velocity flow profile is described on two planes 90° apart. With today's inexpensive computer and software technology a full pipe cross sectional description of the velocity profile is possible (if sufficient data points are provided of course).

The second description of the flow-field state within the pipe can be made using turbulence intensity. Karnik, Jungowski and Botros (1994) showed that metering errors may exist even when the velocity flow profile is fully developed and pipe flow conditions seem perfect. Conversely, they found zero metering error at times when the velocity profile was not fully developed. They attributed this behavior to the turbulence intensity of the gas flow that can cause metering bias error. This behavior accounts in part for the less than adequate performance of the conventional tube bundle.

Delving into the mechanisms governing the effects of turbulence intensity is not within the scope of this paper. It is therefore highly recommended that the state of technology be pursued via the references provided, namely (Karnik, Jungowski and Botros).

The third description of the flow field's state is swirl. Swirl is the tangential flow component of the velocity vector (The velocity profile should be referred to as the axial velocity profile. Recall that the velocity vector can be resolved into three mutually orthogonal components, the velocity profile only represents the axial component of velocity).

Figure 2, Swirl Angie illustrates the definition of flow swirl and swirl angle. Note that swirl is usually referenced to full body rotation (that which the full pipeline flow follows one axis of swirl). In real pipeline conditions, such as downstream of elbows two or more mechanisms of swirl may be present. Miller (1996) provides additional details pertaining to the effects of Installation effects such as one and two elbows in and out of plane.

Orifice Meter and Flow Conditioning
Recall the basic orifice mass flow equation as provided by API 14.3 and ISO 5167

in order to use the flow equation as stated (essentially, to be allowed to use the Coefficient of Discharge as provided) the flow field entering the orifice plate must be free of swirl and exhibit a fully developed flow profile. API 14.3 (1990) and ISO standards determined the Coefficient of Discharge by completing numerous calibration tests where the indicated mass flow was compared to the actual mass flow to determine coefficient of discharge. In all testing the common requirement was a fully developed flow profile entering the orifice plate as indicated by Scott, Brennan and Blakeslee (1994).

Accurate standard compliant meter designs must therefore ensure that a swirl free, fully developed flow profile is impinging on the orifice plate. There are numerous methods available to accomplish this. These methods are commonly referred to a flow conditioning.

The first installation option is to revert to no flow conditioning, but adequate pipe lengths must be provided via equation (2). This generally makes the manufacturing costs for a flow measurement facility unrealistic due to excessively long meter tubes; Imagine meter tubes 75 diameters long.

The second and most well-know option is the 19 tube tube-bundle flow conditioner. The majority of flow installations in North America contain the tube bundle,

With the advent of hot wire, pitot tube and laser based computerized measurement systems which allow detailed measurement of velocity profile and turbulence intensity Karnik (1994) it is becoming clear that the tube bundle does not provide fully developed flow. Therefore, this device is causing biased orifice flow measurement. As a result of these recent findings few tube bundles are specified for flow measurement in the United States anymore. Canadians are beginning to move away from using this device as well.

Numerous references are now available providing performance results indicating less than acceptable meter performance when using the conventional 19 tube tube bundle. Most convincing are: Morrow (1995,1997), Kamik (1993, 1994) and others. The individual results should be reviewed to ascertain details such as beta ratio, meter tube lengths, Re and test conditions.

The general indications are that the conventional tube bundle will cause the orifice installation to over register flow values up to 1.5% when the tube bundle is 1 pipe diameter to approximately 11 pipe diameters from the orifice plate. This is caused by a flat velocity profile that creates higher differential pressures than with a fully developed profile.

There is a crossover region from approximately 10 to 15 pipe diameters where the error band is approximately zero.

Then, slight under-registration of flows occurs for distances between approximately 15 to 25 pipe diameters. This is due to a peaked velocity profile that creates lower differential pressures than a fully developed profile.

At distances greater than 25 pipe diameters the error asymptotes to zero. Figure 3, Conventional Tube Bundle Performance illustrates typical characteristic behavior of the popular 19 tube, tube bundle.

Figure 3 Conventional Tube Bundle Performance

An additional draw back of the conventional 19 tube, tube bundle is variation in sizing. For each nominal pipe size specified, nominal sizes of tubes are specified. This gives slightly different tube bundle fits for each pipe size specified. The assumption of geometric similarity across the pipe sizes from NPS-2 to NPS-30 (which is required in order to utilize Cd properly) is certainly difficult to defend when using the tube bundle. This is an additional bias error applied to the measurement process.

In summary, the conventional tube bundle provides errors very much dependent on installation details (two elbows on and out of plane, tees, valves and distances from the last pipe installation to the conditioner and conditioner to the orifice plate). The errors are not insignificant. It is strongly recommended that the latest findings regarding conventional tube bundle performance are reviewed prior to meter station design and installation.

The final installation option for orifice metering are perforated plate flow conditioners. With in the last 20 years, a variety of perforated plates have entered the market place. These devices generally, are designed to rectify the drawbacks of the conventional tube bundle (accuracy and repeatability insufficiency). The reader is cautioned to review the performance of the chosen perforated plate carefully prior to installation. A flow conditioner performance test guideline such as provided by Morrow (1997) should be utilized to determine performance. The key elements of a flow conditioner test as recommended by Morrow (1997) are*:

1. Perform a baseline calibration test with an upstream length of 70 to 100 pipe diameters of straight meter tube. The baseline Coefficient of Discharge values should be with in the 95% confidence interval for the RG orifice equation ( i.e. the coefficient of discharge equation as provided by AGA-3).

2. Select values of upstream meter tube length, and flow conditioner location, to be used for the performance evaluation. Install the flow conditioner at the desired location. First, perform a test for either the two 90° elbows out-of-plane installation, or the high swirl installation for β = 0.40 and for β = 0.67. This test will
show whether the flow conditioner removes swirl from the disturbed flow. If the ΔCd Is within the acceptable region for both β = 0.40 and β = 0.67 tests, and if the Cd results vary as β^3.5, then the conditioner is successful in removing swirl. The tests for the other three installations (good flow conditions, partly closed valve, highly disturbed flow) may be performed for β = 0.67, and the results for other (i ratios predicted from the ΔCd .- β^3.5 correlation. Otherwise, the tests should be performed for a range of p ratios between 0.20 and 0.75.

3. Perform test and determine the flow conditioner performance for the flow conditioner installed in good flow conditions, downstream of a half closed valve, and for either the double 90° elbow out-of-plane or the high swirl installation.

* Taken from "Technical Memorandum GRI Report No. GRI-97/0207 Metering Research Facility Program, Orifice Meter Installation Effects: Development of a Flow Conditioner Performance Test, Prepared by Dr. Tom B. Morrow Southwest Research Institute, San Antonio, Texas,

Also note that for service in Canada, a Measurement Canada Provisional Specification will be required for the flow conditioner when used for custody transfer service. Other considerations may be:

• Acceptable pressure loss coefficient
• Cost
• Installation details

Table 1, Flow Conditioners, illustrates the number of flow conditioners available to the measurement industry (including perforated plates and vane type) (list supplemented by Gallagher, LaNasa, Beaty 1994).

Table 1, Flow Conditioners

• 19 Tube
• Akashi
• AMCA
• Bellinga
• Bosch & Hebrard
• Etoile
• Gallagher
• ISO
• K-Lab
• Kinghorn
• Laws
• NOVA 50E
• PG&E
• Sens & Teule
• Spearman
• Sprenkie
• Stuart C-3
• Zanker

Turbine Meter and Flow Conditioning

The turbine meter is available in various manufacturer's configurations of a common theme; turbine blades and rotor configured devices. These devices are designed such that when a gas stream passes through them they will spin proportionally to the amount of gas passing over the blades in a repeatable fashion. Accuracy is then ensured by completion of a calibration, indicating the relationship between rotational speed and volume, at various Reynolds Numbers.

The fundamental difference between, say, the orifice meter and the turbine meter is the flow equation derivation.

The orifice meter flow calculation is based on fluid flow fundamentals (a 1st Law of Thermodynamics derivation utilizing the pipe diameter and vena contracta diameters for the continuity equation). Deviations from theoretical expectation are assumed under the Coefficient of Discharge. Thus, one can manufacture an orifice meter of known uncertainty with only the measurement standard in hand and access to a machine shop.

The need for flow conditioning, and hence, a fully developed velocity flow profile is driven from the original determination of Cd which utilized fully developed or 'reference profiles' as presented in the previous section.

Conversely, the turbine meter operation is not rooted deeply in fundamentals of thermodynamics. This is not to say that the turbine meter is in any way an inferior device. There are sound engineering principles providing theoretical background. It is essentially an extremely repeatable device that is then assured accuracy via calibration.

The calibration provides the accuracy. It is carried out in good flow conditions (flow conditions free of swirl and a uniform velocity flow profile) this is carried out for every meter manufactured. Deviations from the as-calibrated conditions would be considered installation effects, and the sensitivity of the turbine meter to these installation effects is of interest here. The need for flow conditioning is driven from the sensitivity of the meter to deviations from as calibrated conditions of swirl and velocity profile.

Generally, recent research indicates that turbine meters are sensitive to swirl but not to the shape of the velocity profile. A uniform velocity profile is recommended, but no strict requirements for fully developed flow profiles are indicated. McBrien (1996), Park (1995) and Micklos (1997) indicate that no significant errors are evident when installing single or dual rotor turbine meters downstream of two elbows out-of-plane with out flow conditioning devices. Dijstelbergen (1995) also indicates very good performance of the turbine meter downstream of strong ISO 9951 reference disturbances. The Dijstelbergen (1995) also provides some variations of the standard tube or vane type straightener, which provide adequate swirl control for acceptable turbine meter performance.

if a specific UM design may be affected by the planned upstream piping configuration, and to evaluate any benefits of installing a flow conditioner or altering the piping configuration.

*Taken from, New AGA Report No. 9. Measurement of Gas by Mulbpath Ultrasonic Gas Meters, John W. Sturat, Principle Engineer, Pacific Gas and electric Company, AGA Operating Section Proceedings 1997 AGA Cat X59707.

Due to the relative age of the technology, it may be beneficial to discuss the operation of the muti-path ultrasonic meter to illustrate the effects of flow profile distortion and swirl.

There are various types of flow measurements utilizing high frequency sound. The custody transfer measurement devices available today utilize the time of travel concept. The difference in time of flight with the flow is compared to the time of flight against the flow. This difference is used to infer average flow velocity on the sound path. Figure 5 Ultrasonic Meter sound path no flow, illustrates this concept.

Figure 5 Ultasonic Meter sound path no flow.

Miller (1996) provides the resulting flow equation for the mean velocity experienced by the sound path:

The case of no flow is shown to illustrate the actual path of the sound when there is zero flow (Equation (5) equates to zero, of course). If one invokes a theoretical flow profile, say a uniform velocity flow profile where the no-slip condition on the pipe walls is not applied, Figure 6 Ultrasonic Meter sound path - uniform velocity profile, illustrates the resultant sound path.

Figure 6 Ultrasonic Meter sound path - uniform velocity profile

A theoretical derivation of the Mean velocity equation for this sound path becomes much more complicated and it is recommended the reader undertake the derivation of the equation including the non-linearity of the sound path for mathematical exercise.

Let us now present a perfect fully developed real velocity profile. Figure 7 Ultrasonic Meter sound path - fully developed flow, indicates a possible sound path as a result of an installation in a real flow.

Figure 7 Ultrasonic Meter sound path fully developed flow

Once again, a mathematical derivation is beyond the scope of this paper. Developing a robust flow algorithm to calculate the mean flow velocity for the sound path can be quite complicated.
Now add to this; sound path reflection from the pipe wall, multi-paths to add degrees of freedom, swirl and departure from axisymmetric fully developed flow profile and the problem of integrating the actual velocity flow profile to yield volume flow rate can be an accomplishment. Karnik, Studzinski, Rogi (1996) provide indications of the real performance of ultrasonic meters downstream of perturbations, and the need for calibrations and further research.

Determining sensitivity of multi-path ultrasonic meters to installation effects is presently being undertaken by a number of industry research and development participants. Thus, adherence to the previous recommendations regarding the calibration or verification of ultrasonic meters Stuart (1997) is strongly recommended. Also understanding the implications of flow conditioning prior to using a measurement device is strongly recommended, consult the device manufacturers for additional information.

Conclusions

The process of flow conditioning refers to the modification of the flow characteristics within a pipe to reference conditions as required by a specific meter. Utilization of this measurement aid can reduce the capital costs of an accurate metering facility.

The orifice meter does not require individual calibrations to ensure accuracy due to the availability of a though and complete standard. Flow Conditioning is absolutely essential in order to utilize the supplied coefficient of discharge if flow proofs are not to be carried out.

Turbine and Ultrasonic Meters must be calibrated in order to guarantee accuracy. The need for flow conditioning is determined by sensitivity of the meter to changes from as calibrated conditions to installation conditions. Flow conditioning can help to isolate the installation conditions to an acceptable level resembling the calibrated state. The manufacturers and applicable standards should be consulted for correct installation details - ask the right questions when specifying installations.

References

Miller, W. Richard, "Flow Measurement Engineering Handbook", McGraw-Hill, Third Edition, 1996, ISBN 007-042366-0

Kamk, U., "Measurements of the Turbulence Structure Downstream of a Tubs Bundle at High Reynolds Numbers", ASME Fluids Engineering Meeting, Washington D.C., June 1993

Colebrook, C.F., 'urbulent Flow in Pipes, with Particular reference to the Transition between the Smooth and Rough Pipe Laws", J. Inst Clv. Eng., vol. 11, pp. 133-136, 1938-1939.

Moody, L.F. "Friction Factors for Pipe Flows", Trans. ASME, vol. 66, p 671, 1944.

White M. Frank, "Fluids Mechanics", Second Edition, McGraw-Hill, 1986, ISBN 0-07-069673-x

Kamlk U., Jungowskl W.M., Botros -K., "Effect of Turbulence on Orifice Meter Performance", 11'" International Symposium and Exhibition on Offshore Mechanics and Arctic Engineering, ASME, May 1994, Vol. 116

American Gas Association Report No. 3, American Pertroleum Institute API 14.3, Gas Processors Association GPA 8185-90, "Orifice Metering of Natural Gas and Other Related Hydrocarbon Fluids", Third Edition, October 1990, A.G.A. Catalog No. XQ9017

The Intematonal Oragnlzatlon for Standardization, "ISO 5167, Measurement of Fluid Flow by Means of Orifice Plates, Nozzles and Venturi Tubes Inserted In Circular Cross-Section Conduits Running Full", first Edtton, 1980-02-01, Ref. No. ISO 5167-1980 (E)

Scott L.J., Brennan J. A., Blakeslee, NIST, U.S. Department of Commerce, National Institute of Standards and Technology, "NIST DataBase 45 GRI/KIST Orifice Meter Discharge Ceoffcient", Version 1.0 N1ST Standard Reference Data Program, Gaithersberg, MD (1994).

Kamlk, U., "A compact Orifice Meter/Flow Conditioner Package", 3rd international Symposium of Fluid Flow Measurement, San Antonio Tx., March, 1995

Morrow, T.B., 'Orifice Meter Installation effects in the GRl MRF", 3rd International Symposium of Fluid Flow Measurement, San Antonio Tx., March, 1995

Morrow T. B., 'Orifice Meter Installation Effects: Research Update for X = 29 D Meter Tubes", AGA Operations Meetings, Nashville, TN., May, 1997.

Morrow T. B., Metering Research Facility Program, " Orifice Meter Installations Effects, Development of a Flow Conditioner Performance Test', GRI-9710207. Dec. 1997.

Gallagher J.E., LaNasa P.J., Beaty R.E., 'he Gallagher Flow Conditioner", 1994 North Sea Flow Measurement Workshop.

Mcarien R.K., "Performance of a Single and Dual Rotor Turbine Meter in Short and Close Coupled Installations", AGA Operating Section Proceedings, Montreal, 1996, p. 586.

Park J.T., "Reynolds Number and Installation Effects on Turbine Meters", Fluid Flow Measurement 3r6 International Symposium, Mar, 1995.

Micklos J.P., "Fundamentals of Gas Turbine Meters", American School of Gas Measurement Technology 1997 Proceedings p. 35.

The international Oragn€zation for Standardization, "ISO 9951, Measurement of Gas Flow in Closed Conduits - Turbine Meters€", first Edtion, 1993,

Stuart J.S., "New A,G.A. Report No. 9, Measurement of Gas by Multipath Ultrasonic Gas Meters", 1997 Operating Section Proceedings, Nashville, TN., May, 1997.

Kamik U., Studzinski W., Geerligs J,, Rogi M., "Effect of Flow Conditioning and Pulsation on the 8" Multipath Ultrasonic Meter", International Pipeline Conference, June 1998 Calgary (to be published)

Kamik U., Studzinskl W., Geerligs J., Rogi M., "Performance Evaluation of 8 Inch Mutipath Ultrasonic Meters", A.G.A. operating Section Operations Confernce, May, 1997, Nashville TN.