INTRODUCTION TO GAS METERING
ROBERT BENNETT SOLUTIONS ARCHITECT HONEYWELL SMART ENERGY
INTRODUCTION NATURAL GAS Science interprets nature in terms of matter and Natural gas is a mixture of different types of energy. Energy is defined as the capacity to do gases whose final composition is dependent on work. There are many types of energy such as the area where the gas originated and how it has heat energy, electrical energy, chemical energy, been scrubbed and mixed with other sources. kinetic energy (energy of motion), and potential energy (intrinsic energy of an object due to the BOYLE’S LAW position of the object). Matter is the material of which the universe is composed and is defined as anything that occupies space and has mass. There are three normal states of matter - solid, liquid, and gas. Robert Boyle first gave the quantitative Under certain conditions, most substances can be relationship between the volume and pressure of made to exist in any of the three states, i.e. water a gas in 1662. Boyle's Law states: as steam, liquid, or ice. If the constant temperature remains Solid matter is rigid, generally crystalline, constant, the volume of a given mass of gas is and will exhibit a definite shape. Liquids will inversely proportional to the pressure. flow, assume the shape of the container they are This means that doubling the absolute stored, and considered to maintain a constant pressure on a given amount of gas will decrease volume and density. Gaseous matter is much its volume by half. Mathematically the law is more difficult to qualify since it consists of written: widely separated molecules in rapid motion. The comparatively large distances between the V is proportional to 1/Pressure molecules make it possible for one gas to accommodate molecules of another gas or be Therefore compressed to force the individual molecules closer together. Since the molecules are in V = k/P constant motion, they will expand to fill any container and strike the walls of the vessel. P x V = k = Constant These myriad impacts result in a pressure. which means: P1 x V1 = Constant = P2 x V2 where: V1/T1 = k = Constant = V2/T2
V1 = Original Volume where:
P1 = Original Pressure (psia) T1 = Original Temperature
V2 = New Volume V1 = Original Volume
P2 = New Pressure (psia) T2 = New Temperature
CHARLES’ LAW V2 = New Volume
IDEAL GAS LAW By combining Boyle's and Charles' laws, it is possible to solve problems in which all three variables - volume, pressure, and temperature - change.
Mathematically expressed: Boyle's Law describes the behavior of gases under conditions of constant temperature. V is directly proportional to the absolute Studies to determine the effect of temperature on temperature and inversely proportional to the the volume and pressure of a gas sample were absolute pressure. first undertaken by Jacques Charles (1787) and this work was considerably extended by Joseph V = kT/P Gay-Lussac (1802). Therefore: Charles' Law states: P1V1/T1 = k = Constant = P2V2/T2 If the pressure remains constant, the volume of a given mass or weight of a gas varies directly with Where P1, V1, T1, P2, V2, and T2 have the same the absolute temperature. significance as before. This means that if the absolute temperature is doubled, the volume is also KINETIC THEORY doubled with the pressure remaining constant. The Ideal Gas Law correlates observed facts concerning the behavior of "ideal" gases. Mathematically expressed, the law is: Real gases obey these laws reasonably well over certain temperatures and pressures depending on V is proportional to T the molecular properties of the specific gas being considered. or V = k x T The theory that offers a model to explain Therefore: these laws and observations is known as the kinetic theory of gases. This theory postulates up between the individual molecules that comes the following for an ideal gas: into bearing as they become closer together.
1. Gases consist of molecules widely 2. There are some attractive forces separated in space. The total volume of the between the molecules that tend to draw them molecules is negligible in comparison to the closer together than the Ideal Gas Law would volume of the gas as a whole. indicate. At high temperatures, the molecules move so fast that the attractive forces between 2. Gas molecules are in constant, rapid, the molecules is effectively overcome. But at straight-line motion colliding with each other low temperatures, the forces of attraction pull the and their container. The collisions are perfectly molecules together so that the volume observed elastic with energy being transferred from is less than that indicated by the ideal gas law. molecule to molecule but with no net decrease in The gas becomes more compressible. kinetic energy. Note that these two forces act in 3. Gas molecules translate heat opposition to each other and the net result is energy into kinetic energy. Although different dependent on the pressure, temperature and molecules have different kinetic energies, the composition of the gas. average kinetic energy of the molecules depends To compensate for this variance, another upon the temperature and increases as factor must be added to the Ideal Gas Laws temperature increases. The molecules of any gas have the same average kinetic energy at a given (P x V)/T = Z x k temperature. where Z is a Compressibility Factor. 4. No attractive forces exist between the molecules of an ideal gas. The Gas Law with Compressibility Factors becomes: IDEAL GAS VERSUS REAL GAS At higher pressures and lower (P1xV1)/(T1xZ1) = (P2xV2)/(T2xZ2) temperatures, real gases do not quite follow the Ideal Gas Law and another factor must be Solving for V2: considered. The reason for the variance from ideal behavior comes from some inaccuracies in V2=V1 x (P1/P2) x (T2/T1) x (Z2/Z1) the assumptions of the Kinetic Theory. To simplify this equation, Z2/Z1 is 1. As the pressure is increased, the defined as the supercompressibility ratio (s). molecules are forced closer together and the Natural gas is assumed to have a Z factor equal volume of the individual molecules becomes to 1 at 14.73 psia and 600F. therefore the significant when compared to the total volume. supercompressibility ratio (s) = 1/Z1 where Z1 is Therefore at high pressures, the volume of a real the compressibility factor for the gas at flowing gas is materially larger that that predicted for an conditions. ideal gas. There is in effect a repulsive force set The equation now becomes:
V2 = V1 x (P1/P2) x (T2/T1) x s Rate Structure – How will the gas used be billed and under what conditions; i.e. cubic feet, The more common term is Supercompressibility base pressure, and base temperature. This is 2 Factor which is the Square Root of s, so s = Fpv . where Pb and Tb are set. In the gas industry the gas law equation is normally written as follows Metering – Determining the volume used by some means of metering device; i.e. 2 Vb= Vm x (Pm/Pb) x (Tb/Tm) x (Fpv) diaphragm meters, rotary meters, turbine meters, ultrasonic meters, orifice meters, etc. These where: provide the Vm.
Vb = Volume of gas at base temperature Regulation – What is the pressure being and base pressure delivered to the customer and how accurate is that pressure being maintained over the whole Vm = Volume registered by metering range of flows going through the meter? This device provides the Pm.
Pb = Absolute Base Pressure Instrumentation – What is the nature of (Usually 14.73 PSIA) the gas being delivered; i.e. its heating value, its specific gravity, odorization, etc? Pm = Absolute Metering Pressure (Gauge + Atmospheric Pressures) HISTORY The first gas company in the U.S., The Gas Tb = Absolute Base Temperature, Light Company of Baltimore, Maryland, = (600F) + 460 = 520 R) founded in 1816, struggled for years with financial and technical problems while operating Tm = Absolute Gas Temperature on a "flat rate" basis. Its growth was slow = Gas Temperature (0F) + 460 because the charge for gas service was beyond the pocketbook of the majority of the population. Fpv= Supercompressibility Factor By comparison, the New York Gas Light Company, founded in 1823, prospered and = s1/2 expanded. They built their system on "the use of MEASUREMENT COMPONENTS gas meters to measure the supply of gas to Natural gas measurement centers around customers, and a large meter to register the solving the Basic Gas Law Equation(s) correctly quantity made at the station before it was conveyed to the gasometers. 2 Vb= Vm x (Pm/Pb) x (Tb/Tm) x (Fpv) The pattern of operation used by this New York company was quickly copied by other companies throughout the East Coast, including the Baltimore company. Due to this success, In order to accomplish this, there are 4 separate New York businessmen formed new gas components that need to be considered: companies in Albany, Boston, Philadelphia, New
York, etc. The new U.S. gas distribution industry began to flourish.
METERING Since this early beginning, meters have been an important, integral element in every phase of gas industry operations. Various types of meters are used; diaphragm, rotary, turbine, ultrasonic, and orifice; each serving a definite purpose and meeting specific requirements.
These common types of meters can be broken down into two distinct categories: positive displacement and inferential. Diaphragm and rotary meters fall into the positive displacement category because they have well-defined measurement compartments that alternately fill and empty as the meter rotates. By knowing the Diaphragm Meter volume displaced in each meter revolution and by applying the proper gear ratio, the meter will A diaphragm meter can be compared to a two- read directly in cubic feet or cubic meters. piston double-action engine in which the Turbine, ultrasonic, and orifice meters, on diaphragms correspond to pistons and the meter the other hand, have no measurement body to the cylinders. Each stroke of the compartments to trap and then release the gas. diaphragm displaces a fixed volume of gas and These meters are inferential meters in that the the diaphragms operate 900 out of phase so that volume passing through them is "inferred" by when one is fully stroked, the other is at mid- observing or measuring some physical stroke characteristic. This provides a smooth flow of gas to the
DIAPHRAGM METERS meter outlet and insures the meter will always A diaphragm meter is physically composed of: start regardless of its static position. When a demand for gas is made on the downstream side 1.) A body to contain the gas pressure and of the meter, a pressure drop is created across the form part of the compartments that measure the meter and its diaphragms. This differential, gas which amounts to 0.1" W.C., provides the force 2.) Diaphragms that move as gas pressure to drive the meter. fluctuates on either side 3.) Valve covers and seats that control the flow of gas into each side of the diaphragm 4.) Linkage to connect the diaphragm with the valves and index, and finally 5.) The index which registers the number of revolutions of the entire mechanism. length increases the diaphragm’s stroke which increases the meter proof and vice versa. The crank makes a certain number of turns per cubic foot and transmits this motion to the front counter (index) by means of an axle shaft driven by a worm and wheel. The crank also drives the sliding valves, which are timed to the motion of the diaphragm. Diaphragm meters meeting the requirements of both B109.1 and B109.2 will measure the gas flow to a pilot light starting at .25 ft3/hour for a Class 250 meter and smaller one up to 7 ft3/hour for a class 3500. The allowable accuracy at this flow is +10%. A typical accuracy curve for a diaphragm meter is shown below.
Diaphragm Meter Accuracy
In addition to the capacity of the meter, its body Movement of a Diaphragm Meter must be physically constructed to withstand the Above each diaphragm is a "D" shaped valve internal pressure of the gas. Modern diaphragm (See Figure 2.). Under the valve are three port meters have sufficient structural integrity to openings that direct the flow of gas in and out of withstand a minimum shell test of 10 PSI. This the case and diaphragm compartments. As the is necessary to meet the federal rules contained diaphragm expands, it forces the gas in the case in Part 192 of Title 49, Department of compartment up through the case port. The Transportation, Office of Pipeline Safety valve directs the flow of gas into the center port regulations. that leads to the meter outlet. A similar process occurs when the diaphragm contracts. The The end connections must be of sufficient size to stroke of the diaphragm is controlled by linkage allow easy passage of gas through the meter and in the upper port of the meter and a rod (flag can be some type of insert, NPT screw rod) that extends down into the diaphragm connections, or flanges. (See Table For compartment. The tangent link, as it is called, is capacities and connections.) attached to the top of the meter crank and is adjustable in length. Increasing the tangent
between one impeller and the housing. As the meter continues to rotate, this volume of gas is discharged at the meter outlet. The other impeller is rotating in the opposite direction during this cycle and as the discharge commences from the first impeller, the second is beginning to confine another and equal volume of gas. Continue rotation causes this volume to discharge. During each full revolution of a single impeller, two volumes of gas are discharged and the meter displaces four volumes in a completer cycle. By knowing the volume displaced per revolution, appropriate gearing allows a readout of actual metered volume to be displayed on a counter.
Diaphragm Meter Capacities
ROTARY METERS A rotary meter relies on the rotation of one or more parts to carry out its measurement operation. The design of these meters allows them to operate at higher speeds than a diaphragm meter, consequently higher volumes of gas can be metered. A lobed impeller rotary meter consists of two (2) figure-eight impellers mounted in a housing. Very close tolerances and timing gears ensure that an effectively capillary seal is achieved at the same time ensuring that the impellers and housing do not come in contact starts. When a demand for gas occurs, a As with diaphragm meters, the model pressure differential is set up across the meter numbering will usually reflect the capacity of the and the impellers start to rotate. As gas enters the meter at an operating pressure of 0.25 psig. The meter, a quantity of gas becomes contained rangeability of a rotary meter will vary from 25:1 up to 160:1 as improved production methods and determined by counting the number of meter bearings have resulted in better performance. rotations. As defined in A.G.A. Report #7, the turbine meter consists of three basic components (See Figure 1 and 1A)
1. The body which houses all of the parts and physically contains the gas pressure.
2. The measuring mechanism consisting of the rotor, rotor shaft, bearings, and necessary supporting structure.
3. The output and readout device which may ACCURACY be either a mechanical drive to transmit the A typical accuracy curve will look as: indicated meter revolutions outside the body for uncorrected volume registrations or for Rotary Meter Accuracy Curve electrical pulse meters, it would be the pulse 100.6 detector system and all electrical connections 100.2 needed to transmit the pulses outside. 99.8 20 40 60 80 100
% Accuracy 99.4
99.0 % of Capacity Flow Rate
TURBINE METERS Turbine, ultrasonic, and orifice meters have no measurement compartments to trap and then release the gas. These meters are categorized as inferential meters in that the volume passed Gas entering the meter increases in through them is "inferred" by something else velocity as it flows through the annular passage observed or measured. In the orifice meter the formed by the nose cone or upstream stator and volumes are determined only by knowing the the interior of the body. The movement of the inlet pressure, differential pressure, plate size, gas over the angled rotor blades exerts a force to and piping characteristics, all of which "infer" the rotor causing it to rotate. The ideal rotational the flow rates that in turn can be integrated over speed of the rotor is directly proportional to the time to provide the volume. flow rate of the gas. The actual rotational speed is a function of the annular passageway size and Turbine meters, also called velocity shape, and rotor design. It is also dependent on meters, "infer" the volume of gas passing through the load that is imposed due to internal them by measuring the velocity of the gas stream. mechanical friction, fluid drag, external loading, Gas moving through the meter impinges on a and gas density. bladed rotor resulting in a rotational speed that is proportional to the flow rate. The volume is CAPACITIES such as the AGA Standard No.3, the volumetric flowrate can be determined. In very basic terms; Pressure 3”- 45º 4” – 45º 6” – 45º 8” - 45º 0.25 10,000 18,000 35,000 60,000 1/2. 50 44,000 80,000 154,000 265,000 Flowrate = C x (hw x P1) 100 79,000 142,000 276,000 474,000 200 150,000 271,000 526,000 902,000 Where hw is the differential pressure across the 500 379,000 695,000 1,328,000 2,276,000 plate, P1 is the upstream static pressure and C is Based on Patm = 14.37 psia & Pbase = 14.73 psia a multiplier incorporating several factors. These factors include information on the pipe size, ACCURACY orifice size, Reynolds number, position of the pressure taps, and several others. Turbine Meter Accuracy Curve 100.2 The main use for orifice plate metering in 100.0 on large pipe sizes with high pressure flows, 20 40 60 80 100
99.8 where other suitable metering devices are % Accuracy % available. 99.6 % of Capacity Flow Rate
RANGEABILITY ULTRASONIC METERS
Ultrasonic gas flow meters, employing the Rotor Qmax Qmin Meter Rangeabiltiy absolute digital transit time measurement Angle mscfh mscfh 3” 45 10 0.83 12 principal, have gained acceptance for fiscal 45 18 1.2 15 accounting of gas transfer. Among this No. 4” 30 23 1.9 12 technology's advantages are: 45 35 1.9 18 6”p 30 56 4.7 12 45 60 3.0 20 • Wide measuring range, 60:1 turndown 8” 30 88 5.9 15 • High accuracy 12” 45 150 6.0 25 • High repeatability
• Negligible offset ORIFICE METERS • Negligible pressure drop The primary element is an orifice plate metering system is the orifice plate itself. The • Generally unaffected by dust and liquid plate is usually a flat disk with a round hole deposits (orifice) in it. The hole can be eccentric, but it is • Insensitive to fluctuations of the gas usually concentric with the plate, hence the pipe. composition • Little maintenance As the fluid flows through the orifice, a jetting effect occurs which creates a low pressure Extremely accurate and reliable multipath zone immediately downstream of the plate. ultrasonic metering systems have emerged as There is a pressure drop across the plate which the measurement technology of choice for large increases as the flow rate increases, and by volume gas transfer. Pushed by continuously applying the formulae in a recognized standard, improving signal processing technology and pulled by increasing customer requirements, multipath instruments provide state of the art • Pulsating flow is present gas flow measurement. Its accuracy and reliability was initially certified by NMI, and is Using state of the art digital signal now continuously certified during routine flow processing electronics, modern ultrasonic calibrations by users in N. America at CEESI, instruments employ the method of absolute Southwest Research Institute and TC digital travel-time measurement, which is Calibrations. An operating guideline has been discussed in detail below.
established by AGA ("Report 9"), and B contractual requirements have been successfully defined by parties for its use in custody transfer φ v applications. D L
Operating Principal Acoustic techniques for the measurement of A flow have been applied for almost four decades. In ultrasonic flow measurement, acoustic Depending upon the magnitude of the Mach pulses are transmitted and received by a pair of number, v / c, and due to the rapid change in piezoelectric transducers. This figure shows the electronics, different measurement methods simple geometry of two transducers, A and B, at have been applied in meters which appeared on an angle with respect to the axis of a straight the market over this time interval. For instance: cylindrical pipe. D denotes the diameter, L the path length and V the velocity vector. 1. The sing-around method (1950's) which Some instruments employ reflection essentially is phase-shift measurement paths like that shown below, where the acoustic 2. Continuous wave frequency shift pulses reflect one or more times against the pipe method (1960's) wall. 3. Analog differential time-delay measurement (1970's) 4. Digital absolute travel time measurement (1980's)
Although some of the older methods v occasionally reappear in new instruments D (mainly because of their inexpensive L/ L/ manufacturing cost), they cannot compare with modern methods that employ high speed digital signal processing techniques and advanced piezoceramic transducers, in terms of accuracy Other instruments employ chordal, or and repeatability. The first three methods are point-to-point, pulse transmission paths. problematic when: Regardless of the path geometry, the operating principal is the same, and basic transit • Gas composition, temperature or time equations applicable. At zero flow, the pressure are fluctuating travel time is equal in both directions and the measured time of flight difference between • The gas is not completely clean them (tu-td) is zero. However if there is flow, the one notes that the cancellation of c from the travel time of the sound pulse transmitted in the average velocity equation (gas properties such same direction as the flow decreases due to its as density affect both tup and tdown equally) being accelerated by the moving gas. means that absolute velocity measurement is not Conversely, the pulse traveling upstream suffers dependent on gas density. That is, pressure, an increased transit time due to the retarding temperature and gas composition have no effect effect of the gas flow. Transit times in the up on the velocity calculation from pulse transit and down stream directions may be calculated times. as: Pulse Generation Specially designed transducers are used L for generation and reception of ultrasonic 1) tup = c - v cos pulses. These transducers are designed to and transmit and receive. The main component within a transducer performing these functions L is a piezoceramic element. In the transmitting 2) tdown = mode these piezoceramic elements are excited c + vcos with a characteristic voltage that results in the emission of a well-characterized sound pulse. where L is the length of the path, φ is its angle When used as a receiver, the incoming sound with respect to the axis of the pipe, and c is the pulse generates a small voltage, which is speed of sound in the gas (about 300-450 m/s or processed after amplification. The frequency 1000-1500 ft/s at typical transmission and directivity pattern of a transducer depends pressures). If the speed of sound is constant for the most part on the dimensions and during both measurements, the two equations characteristics of the piezoceramic element. can be combined The transducers have been designed for the generation of short, powerful pulses in order L 1 1 3) v = ( - ) to exploit the advantages of single and double 2 cos t down tup reflection paths at high repetition rates at operating pressures ranging from atmospheric where v denotes the flow velocity, positive in up to 5000 psi. Because they are fabricated downstream direction. The history of this within tight specifications under strict quality method goes back to Rütten (1928), who filed control, and with detailed characterization, they the first patent on the application of ultrasound can be exchanged without parameter adjustment in flow measurement. From the above equations or meter recalibration. the speed of sound can be calculated as follows: Pulse Detection L 1 1 Before pulse detection and recognition 4) c = ( + ) take place, the received acoustic pulse is pre- 2 t down tup processed using an Automatic Gain Control and filtering circuitry to ensure pulse discrimination. Since the speed of sound is related to the After the pre-processing stage (detection) takes density of the medium in the transport system, it place, the signal is digitized and compared with can be used to calculate mass flow. Further, a 'fingerprint' of a reference pulse. This method provides the unique ability to check the quality Path Weighting Factors of every single pulse against preset standards The velocity v calculated by equation (3) before processing for velocity measurement represents an average along the acoustic path. purposes. The pulses are either: The velocity of interest, however, is the mean, or bulk, value V over the pipe cross section. * accepted when the signal completely meets This variable is computed by the preset quality standards or: 5) V = k v * rejected when a deviation from these quality standards is detected. where the meter factor k expresses the influence of the flow velocity profile. The value of the Only when both pulses are accepted, are path weighting factor depends on the velocity their travel times used to calculate flow velocity profile in the duct as sensed by the acoustic and speed of sound. This method results in the path. highest precision yet achieved by flow measurement devices. During signal detection Acoustic path configuration and processing, built-in diagnostics supply real- To create optimal acoustic path time information about the system's configurations for multipath flowmeters, performance to the user, and may be used to set knowledge is required about the actual flow alarm limits on meter performance. These patterns in transport systems. parameters will be discussed in more detail For smooth straight circular ducts the later. velocity profile is determined as a function of the Reynolds number (“Re”) of the flow. This Timing characteristics dimensionless number is calculated using the flow speed, the duct diameter, the density and The accuracy required in the travel time the dynamic viscosity of the flowing medium. measurement can be found from the equations. For low Reynolds numbers the flow is laminar For example, when a velocity of 3 ft/s is with a measured with 0.5 % accuracy along a 3 ft path length in a gas with sound velocity of 1300 ft/s, both travel times are of the order of 2.5 ms, but their difference is about 6 μs, which must be measured with an error no greater than 30 ns! This small travel time difference requires high speed, high accuracy digital electronics. The travel times of only a few milliseconds enable individual ultrasonic flow velocity measurements at high repetition rates. Typical rates are 20 - 50 Hz, depending on pipe diameter. The need for high repetition rates is parabolic (Hagen-Poiseuille) profile, while for evident in cases such as surge control high Reynolds numbers the flow becomes applications, where the flow may drop from its turbulent and with a plug-like (logarithmic) set point to its minimum in less than 0.05 s. profile.
Since the transition from laminar to turbulent takes place (Schlichting, 1968) Accuracy somewhere between Reynolds number 2000 and The average velocity flux measured by a 4000, while in practice the Reynolds numbers single path ultrasonic flowmeter is calculated usually encountered are in the range from 104 to using the following equation: 108, the turbulent profile is that most commonly encountered in high pressure gas transmission L 1 1 systems. 6) Q = k A ( - ) 2 cos t down tup Due to the presence of one or more, possibly out of plane, bends in the transport system, the flow profile will always be distorted where A denotes the cross-sectional area of the with respect to the ideal logarithmic one. A pipe. From this equation, the total accuracy single elbow induces a dual-eddy pattern, which depends upon the individual accuracies of all has two counter rotating vortices on either side factors involved and can be split into three of the center plane of the elbow. The resulting parts: transverse flow is directed outward, with axial velocities much lower than in corresponding • the accuracy of the geometry ideal velocity profiles. This double-eddy pattern • the accuracy in the travel time decays faster than the single-eddy one induced measurement by double bends out of plane. This important • the accuracy of the velocity profile form of distortion is called swirl. Although the presence of swirl does not contribute to the total Geometry flow, it causes a distortion of the velocity The geometry is determined by the profile, which results in an effect on travel time acoustic path length L, angle φ, and the cross- that may introduce an error in flow velocity sectional area, A of the pipe. Tight mechanical measurement. Mathematical modeling has been tolerances, and application of acoustic path used to characterize and account for swirl's length determination assure that high resolution influence on average velocity measurement. of path length is achievable. Therefore errors caused by geometrical factors can be minimized to the point where they no longer have significant impact on measurement accuracy. Travel Time • Will a building enclose the meter runs, The accuracy of the time measurement is and clearance between meter and limited only by the signal to noise ratio and the building wall be an issue if transducers digital clock frequency. The pulse's travel time are retracted? measurement is based on high resolution quartz controlled electronics. Since samples of travel These are several of the potential questions times are available at a rate of about 20 to 50 hz, designers should consider when laying out a the resulting mean error can be reduced to just a station: there may be others peculiar to a given few ns. installation.
Meter Station Design Considerations Gas Quality Space limitations, and the environment In wet gas environments (hydrocarbons in which the station will operate, often dictate or water vapor), designers need consider equipment selection and configuration. If whether the meter run needs to be angled, or shorter meter runs are required, headers and include siphon drains to assure liquids don't several tees may be involved which generate collect in the pipe or can be drained if they do. significant flow disturbances. High performance If Sulfur content is a consideration, it flow conditioners may be desirable to insure needs to be specified to insure corrosion consistent velocity profiles in short coupled resistant transducers are provided. meter tube installations. Carbon Dioxide in concentrations exceeding 15% (this level varies somewhat Sizing depending on operating pressure) can attenuate Ultrasonic meters are typically sized on ultrasonic signals such that transmissiblity of the basis of actual velocity. Therefore, when pulses fails, and measurement does so also. selecting the meter, one must consider the Good, representative, samples of gas pressure, temperature and flow range stated in quality are necessary to facilitate calculation of SCF per unit time. Basic calculation programs reference speed of sound values needed to to size ultrasonic meters using these parameters evaluate meter operating condition. Depending are available from most manufacturers. on the nature and importance of a particular In addition to meter size, designers need meter station, designers need consider whether a consider the nature of the operation and the gas chromatograph is necessary, or whether a maintenance requirements for the particular spot sample will suffice. If it is determined a station: GC is needed, gas quality may dictate whether the instrument required to characterize SOS • Are multiple runs needed to provide need be C9+ or C6+. Likewise, if sampling is redundancy, or flexibility should a meter determined to be an acceptable mechanism, require out-of-pipe service or judicious selection the sampling point is needed: recalibration? only a spot sample (composite will not provide • Are multiple runs needed in stepped line desired results) will suffice since the meter size to enhance station rangeability? measured SOS must be chronologically • Are there pressure or control valves that correlated with the spot sample draw to provide might require installation of additional a valid comparison of meter measured SOS to attenuating elements such as blind tees? that calculated from the analysis. Pressure and temperature data are also required as part of the Flow calibration not only certifies meter data collected at the time of sample draw. performance traceable to a recognized standard, it also alleviates many measurement disputes. Meter Installation These tests generally consist of flowing gas Several steps occur prior to physical through the meter under test ("MUT") at various installation, the judicious monitoring of which flow rates across its capacity range, and can assure a successful start-up as well as comparing the MUT's output to a reference, or providing benchmark performance criteria upon transfer, standard. AGA Report 9 does not which to evaluate meter's operating condition require flow calibration, but does specify that over the life of the station. manufacturers meet uncalibrated ("out of the box") performance criteria of +/- 0.7 % for Dry Calibration meters 12 inch and larger, or 1.0% for meters of This terminology is somewhat of a lesser diameter. It is obvious that these misnomer, since this process is intended to established criteria are not sufficient for characterize electronic performance, and in the acceptable fiscal measurement, particularly in case of Instromet, tighten up path length data, light of recently high natural gas prices. rather than generate a meter factor as the result Therefore, it is prudent practice to certify a of an actual calibration. meter at a traceable facility. Pure Nitrogen is used to assess meter Key factors to assess during calibration functionality at high pressure prior to flow are the repeatability and linearity of the meter's calibration of these devices. Electronics (SPU proof curve. Proof curves may be linearized and transducers) are given their final QC check (usually to characterize low flow performance), by running a static test on the meter at stable but an optimal proof curve is one composed of conditions (known gas, steady temperature and tightly clustered data points that form a flat, pressure). straight line. Criteria for acceptable linearity In addition to insuring electronic and repeatability are published in AGA Report functionality, some manufacturers utilize this 9. opportunity to compare meter measured speeds of sound to calculated, certified, values. Path Physical Installation length is adjusted to provide agreement with the Report 9 also describes criteria for ID calculated, certified, values so that the meter match of spool pieces that comprise the meter exits the assembly process with tight per path run. When bolting up for final installation, it is performance tolerances that may reliably be essential to assure proper spool alignment and used as baseline meter performance criteria. insure joint gaskets do not protrude into the flowing stream. One can make that assurance Flow Calibration by either assembling the meter run at the site Once a custody meter is successfully dry and installing it as a unit, or by making a visual calibrated, it is generally sent to an independent inspection of the assembled run as each spool is testing facility to certify its meter factor. Flow installed in the station piping. tests are recommended for any meter that is used for custody service, and particularly Start-up whenever a flow conditioner is proposed for use Once the meter is physically installed, it as part of the meter run. is important to generate base-line documentation of its performance. Such meters. While measurement accuracy is not information, generated when the meter is new compromised, pulse detection may become and in pristine condition, may be used during impossible causing a loss of measurement. subsequent routine inspections to assure meter condition has not changed. Key elements to Performance Monitoring capture data for baseline characterization are Meter diagnostics, made available by average SOS, per path SOS, per path gain levels virtue of signal processing routines, may be and per path gain limits. Interpretation of these applied to determine if sediments or ultrasonic parameters is addressed in the maintenance noise compromise meter function. section that follows. Transducer Gain Levels: The "sound volume" of the pulse is usually controlled Field Applications & Routine Maintenance automatically with electronic gain controls. Monitoring gain levels over time provides an Dirty Gas indication of whether sediments may be In real-world gas pipeline systems, attenuating pulse transmission (gains will be actual conditions may differ considerably from found to increase). the ideal encountered in flow measurement labs. Signal Rejection: Pulse signals are Major disturbing factors are pollution (i.e., dirt rejected when they fail to match the fingerprint and liquids) and ultrasonic noise. Many gas of an electronically stored reference pulse. flow meters are sensitive to dust and liquid Signal rejection indicates potential transducer residue in the flowing stream. Through the use failure, but is usually indicative of noise of digital pulse recognition techniques, the interference from devices such as control acoustic flow meter can be made relatively valves. immune to these deposits. If the signal is Speed of Sound: Ultrasonic meters attenuated too much by deposits on transducers, measure the speed of sound in the flowing measurement is no longer possible. Due media (reference equation 4). Using AGA however to digital signal processing of time of Report 8 equations of state, the speed of sound flight measurements, dust and liquid residue may be accurately calculated using flowing does not affect the accuracy of the meter. temperature, pressure and gas composition as inputs. Comparisons of meter measured SOS 5.2 Ultrasonic Noise may be made against this calculation as a "health check". Direct correlation between Although many new control valve designs are meter accuracy and SOS has yet to be promoted as 'low noise', they are the main established, but it is known that correct meter source of interference encountered in the field. function is doubtful if the SOS calculation is in During tests at various installations these 'low error. Per equation (4), poor SOS comparisons noise' valves, when nearly closed, appeared to suggest clock or transducer problems. create much non-audible ultrasonic noise that Using the sophisticated capabilities of may interfere with the transmitted sound. This is flow computers, or an on-board electronics problematic for ultrasonic meters since the archive, these parameters may be trended and reduction of audible control valve noise has alarm limits established for these important been accomplished by shifting it to ultrasonic, operating characteristics, thus signalling users or non-audible, frequencies used by these of maintenance requirements or failure onset. Ultrasonic Meter Maintenance inspected and/or experimented with to Ensuring proper function of custody determine if accommodation can be made (i.e., measuring devices is a measurement change in valve position or trim). technician's major responsibility. Field SOS Comparisons: Discrepancies operating experience indicates ultrasonic between measured and calculated SOS indicate meters, while nearly trouble free, may require a fundamental meter problem (clock or special maintenance in addition to routine transducers). However, one must recognize the inspection. A typical routine inspection might sensitivity of the equation of state calculations consist of the following: to gas composition and temperature, prior to assuming meter malfunction. Seemingly 1. Pressure transmitter calibration. insignificant concentrations of heavier 2. Temperature transmitter calibration. hydrocarbons greatly influence the accuracy of 3. Verification of pulse output (if used) the calculation (comprehensive sensitivity accuracy (i.e. validation of D/A converter analysis of this effect is lacking, but it is performance). advisable to obtain an extended analysis for 4. Collection and review of meter data logs SOS calculations if aggregate C6+ is greater which typically include SOS, signal acceptance than 0.5 mol %). Likewise, accurate rate, gain and gain limit data. temperature measurement is necessary: Calculated SOS can differ from that measured, Performance parameters from collected by as much as 3-5 fps, if measured temperature logs should be compared to a baseline log or is in error by 1 degree Fahrenheit at typical trended against previously recorded logs. A pipeline operating pressures (800-1000 psig). "baseline" log is one collected when the meter's SOS comparison is an extremely useful tool, but condition was known to be satisfactory; usually be sure inputs to the calculation are correct taken at the time of initial meter start-up, or (good gas analysis and assured temperature after re-certification. transmitter calibration) before spending time Special maintenance is required when and money to review meter characterization. performance monitoring dictates, or complete meter failure occurs. The signals identified for Performance monitoring may be interpreted as follows: At present, nearly all custody grade Increasing Gain Levels: If performance ultrasonic meters are .flow calibrated to assure monitoring reveals gain levels have increased that even minor deviations of these relatively over time, it is an indication of potential high capacity devices are accounted for in transducer fouling. In this event, transducers actual operations. Approximately 100 separate should be carefully removed, inspected, and calibrations were run on 8”, 3 path meters in the cleaned if necessary. If the meter is blown- past year. A typical proof curve is depicted in down to accomplish this, it is advisable to clean figure 6 below. Flow is stated in ACFH, but the the nozzles (transducer receptacles in the meter low rate point represents approximately 4% of body) as best as possible. meter capacity (about 5 fps) and the high rate Signal Rejection: Should performance point approximately 90 fps. Note the maximum monitoring reveal excessive signal rejection deviation of this proof curve, corrected with a rates, suggesting ultrasonic noise is a problem, single meter factor to correct the meter to 0.0% control valves or throttled valves should be proof, is less than 0.1%.
1.00 Deviation Measured 0.80 0.60 0.40 0.20 0.00
-0.20 Deviation [%] -0.40 -0.60 -0.80 -1.00 0 20000 40000 60000 80000 100000 120000 Flow ACFH
Figure 6
This performance curve is typical of path meters irrespective of line size. The slight “roll off” usually begins at less than 7 fps, and meters are linear at flow rates above that roll-off point. Many manufacturers have introduced curve fit routines to eliminate non-linearity such as that witnessed at very low flow rates. It is recommended these curve fit routines be applied only if operation is contemplated at very low meter capacities.