Keywords

flow; meter; fluid; oil and gas industry; reference standard; fluid measurement; custody transfer

Chapter Overview

The vast majority of this book relates to “conventional” flowmeters; for example, the admonition about single phase flow. Obviously, this comment does not apply to multiphase meters. Other exceptions are noted as they appear.

The book’s general approach is to look first at basic principles, particularly with respect to differential and linear meters and the types used to measure fluid flow in the oil and gas industry. After a review of basic reference standards, “theory” is turned into “practice,” followed by an overview of fluids and the fluid characteristics. “Flow” itself is examined next, followed by operating and maintenance concerns. Next, comments are offered on individual meters and associated equipment with a detailed review of the two classes of meters: differential and linear readout systems. Meter proving systems are covered in detail, followed by measurement data analysis and “lost and unaccounted for” procedures. The book concludes with a discussion of conversion to volumes, conversion of the volumes to billing numbers, and the audit procedures required to allow both parties to agree to the final measurement and money exchange.

Emphasis is not so much on individual meter details as on general measurement requirements and the types of meters available to solve particular problems.

Specifically, this first chapter presents some background information, overviews the requisites for “flow” and defines the major terms used throughout the book. Chapter 2 introduces various relevant subjects, starting with basic principles and fundamental equations. Chapter 3 details the types of fluid measurement: custody transfer and non-custody transfer. Chapter 4 is devoted entirely to listing the basic reference standards. Chapter 5 applies theory to the real world, and describes how various practical considerations make effective meter accuracy dependent on much more than simply the original manufacturer’s specifications and meter calibration. Chapter 6 covers the limitations that fluid characteristics place on accurate flow measurements. Chapter 7 looks at flow in terms of the characteristics required, measurement units involved, and installation requirements for proper meter operation.

Chapter 8 reviews the necessary concerns in operating the meters properly, with examples of real problems found in the field. Chapter 9 covers the maintenance required for real metering systems to allow proper performance over time. Chapter 10 reviews meter characteristics, with comments on all the major meters used in the industry. Chapters 11 and 12 detail head and linear meters. Chapter 13 deals with related readout equipment. Chapter 14 discusses proving systems. Chapter 16 covers measurement data analysis and Chapter 15 covers material balance calculations and studies (i.e., lost and unaccounted for). Chapter 17 introduces the auditing required in oil and gas measurement.

Requisites of Flow Measurement

In this book, fluids are the common fluids (liquids, gases, steam, etc.) that are handled in the oil and gas industry, both in the pure state and in mixtures. However, each fluid of interest must be individually examined to determine whether it:

The flow should be examined to see if it:

Certain meters may have special characteristics that can handle some of these problems, but they must be carefully evaluated to be sure of their usefulness for the fluid conditions actually encountered (Figure 1-1).

Measurement can usually be undertaken with any one of several meter systems, but certain meters have earned acceptance for specific applications based on their service record. This is an important factor in choosing a meter. Reference to industry standards and existing users within an industry are important points to review when choosing the best meter for a given application.

Background of Flow Measurement

The terms below form the background for fluid flow measurement, and should be understood before embarking on the task of choosing a flow measurement system. “Fluid,” “flow” and “measurement” are defined in generally accepted terms (in Webster’s New Collegiate Dictionary) as:

Further limitations require that the fluids have a relatively steady state mass flow, are clean, homogeneous, Newtonian, and stable with a single phase non-swirling profile with some limit of Reynolds number (depending on the meter). If any of these criteria are not met, then the measurement tolerances can be affected, and in some cases measurement should not be attempted until the exceptions are rectified. These problems cannot be ignored, and expected accuracy will not be achieved until the fluid is properly prepared for measurement. On the other hand, the cost of preparing the fluid and/or the flow may sometimes outweigh the value of the flow measurement, and lower accuracy should be accepted.

History of Flow Measurement

Flow measurement has evolved over the years in response to demands to measure new products, measure old products under new conditions of flow, and for tightened accuracy requirements as the value of the fluid has increased (Figure 1-2).

Over 4,000 years ago, the Romans measured water flow from their aqueducts to each household to control allocation. The early Chinese measured the flow of salt water to brine pots used for producing the salt used as a seasoning. In each case, control over the process was the prime reason for the measurement.

Flow measurement for the purpose of determining billings for total flow developed later.

Well known names among the developers of the differential meter are Castelli and Tonicelli who, in the early 1600s, determined that the rate of flow was equal to the flow velocity times the area, and that discharge through an orifice varies with the square root of the head (pressure drop or differential).

In the early 1700s, Professor Poleni provided additional work on understanding the discharge of an orifice, and at about the same time Bernoulli developed the theorem upon which the hydraulic equations of head meters have been based ever since (Figure 1-3).

In the 1730s, Pitot published a paper on a meter he had developed. Venturi did the same in the late 1790s, as did Herschel in 1887. In London in the mid-1800s, positive displacement meters began to take form for commercial use. Then, in the early 1900s, the fuel-gas industry started to develop in the United States (Baltimore Gas Light Company).

An early practice in the United States was to charge for gas on a per-light basis; this certainly did not reduce waste, as customers would leave lights on day and night. It is interesting to note that the first positive displacement meters were classified as “5-light” and “10-light” meters, referencing the number of lights previously counted in a house that could be measured by the meter.

The first of these meters installed outdoors were water-sealed; in the winter, ethanol had to be added to the water to prevent freezing. One of the immediate problems was that not all the ethanol made it into the water baths—and some service personnel found it hard to make it home! In the 1800s a “dry” type meter was developed that replaced the “wet” meters (the prohibitionists cheered).

Rotary meters did not become available until the 1900s. About this time, Professor Robinson at Ohio State University used the pitot to measure gas flows at gas wells. Weymouth calibrated a series of square edged, thin plate orifices with flange taps. His work was reported in a 1912 paper to the American Society of Mechanical Engineers entitled “Measurement of Natural Gas.” Similar tests were run on an orifice by Pugh and Cooper. Crude oil in this time period was measured by tank gauging. Tank gauging was the method used for storage tank batches from production to the final measurement of the refined products.

Around the same time, Professor Judd at Ohio State conducted tests on concentric, eccentric, and segmental orifice plates, and the forerunners of the present-day meter companies also conducted their own research, including Metric Metal Works (later American Meter), the Foxboro Company, and Pittsburgh Equitable (later Rockwell and Equimeter). To study the data and coordinate results, a committee of the American Gas Association (AGA) (1925) began additional testing. This work culminated in AGA Report No. 1, published in 1930 and reporting results to date for the test programs in progress. Work began immediately on AGA Report No. 2, which was published in 1935. The first AGA Report No. 3 was published in 1955.

Since that time, the quantity of additional data available is reflected in the reports published most recently. The AGA Report No. 3, published in 1992, reflects a new discharge coefficient; a revision published in 2000 outlines new installation requirements. Current studies are evaluating the need for further revisions.

Paralleling these gas measurement efforts is the development of liquid meters for use in other areas of flow measurement, meters such as positive displacement, vortex shedding, ultrasonic, magnetic, turbine, and laser.

Flow measurement continues to change as the needs of the industry change. No end to such change and improvement is likely as long as mankind uses gas and liquid energy sources which require the measurement of flow.

Definition of Terms

Absolute Viscosity (mu) The absolute viscosity (mu) is the measure of a fluid’s intermolecular cohesive force’s resistance to shear per unit of time.

Accuracy The ability of a flow measuring system to indicate values closely, approximating the true value of the quantity measured.

Acoustical Twining The “organ pipe effect” (reaction of a piping length to a flow-pressure variation to alter the signal). Effects are evaluated based on acoustics.

Algorithm A step-by-step procedure for solving a problem, usually mathematical.

Ambient Conditions The conditions (pressure, temperature, humidity, etc.) externally surrounding a meter, instrument, transducer, etc.

Ambient Pressure/Temperature The pressure/temperature of the medium surrounding a flow meter and its transducing or recording equipment.

Analysis A test to define the components of the flowing fluid sample.

Base Conditions The conditions of temperature and pressure to which measured volumes are to be corrected (alternatively known as reference or standard conditions). The base conditions for the flow measurement of fluids, such as crude petroleum and its liquid products, having a vapor pressure equal to or less than atmospheric pressure at base temperature are:

The base conditions for the flow measurement of natural gases are (in the USA):

For both liquid and gas applications, these base conditions can change from one country to the next, from one state to the next, or from one industry to the next. Therefore, it is necessary that the base conditions be identified for “standard” volumetric flow measurement.

Beta Ratio The ratio of the measuring device diameter to the meter run diameter (i.e., orifice bore divided by inlet pipe bore).

Calibration of an Instrument or Meter The process or procedure of adjusting an instrument or a meter so that its indication or registration is in close agreement with a referenced standard.

Calorimeter An apparatus for measuring the heat content of a flowing fluid.

Certified Equipment Equipment with test and evaluations with a written certificate attesting to the device’s accuracy.

Chart Auditing A visual review of field charts to find questionable dates.

Check Meter A meter added in series to check the billing meter.

Chilled Meter Test A test used to determine dew points (water and/or hydrocarbon) by passing the natural gas over a mirror while gradually reducing the temperature of the mirror until condensation forms.

Clock Rotation The time (in hours) needed to make a 360° chart rotation.

Coefficient of Discharge Empirically determined ratio from experimental data comparing measured and theoretical flow rates.

Compressibility The change in volume per unit of volume of a fluid caused by a change in pressure at constant temperature.

Condensing Reduction to a denser form of fluid (such as steam changing to water); a change in state from gas to liquid.

Condensing Point A point, measured in terms of pressure and temperature, at which condensation takes place.

Contaminants Undesirable materials in a flowing fluid that are defined by the quality requirements in a contract.

Control Signal (Flow) Information about flow rate that can be transmitted and used to control the flow.

Critical Flow Prover A test nozzle that is used to test the throughput of a gas meter where the linear velocity in the throat reaches the sonic velocity of the gas.

Critical Point That state at which the densities of the gas and liquid phases and all other properties become identical. This is an important correlating parameter for predicting fluid behavior.

Critical Pressure The pressure at which the critical point occurs.

Critical Temperature The temperature above which the fluid cannot exist as a liquid.

Custody Transfer A flow measurement whose purpose is to arrive at a volume for which payment is made/received as ownership is exchanged.

Dampening A procedure through which the magnitude of a fluctuating flow or pressure is reduced.

Density The density of a quantity of homogeneous fluid is the ratio of its mass to its volume. The density varies with temperature and pressure, and is therefore generally expressed as mass per unit volume at a specified temperature and pressure.

Density, Base The mass per unit volume of the fluid being measured at base conditions (Tb, Pb).

Density, Relative (Gas) The ratio of the specific weight of gas to the specific weight of air at the same conditions of pressure and temperature. (This term replaces the term “specific gravity” for a gas.)

Density, Relative (Liquid) The ratio of a liquid’s density at a given temperature to the density of pure water at a specific base temperature. (This term replaces the term “specific gravity” for a liquid.)

Diameter Ratio (Beta) The calculated orifice plate bore diameter (d) divided by the calculated meter tube internal diameter (D).

Differential Pressure The drop in pressure across a head device at specified pressure tap locations. It is normally measured in inches or millimeters of water.

Discharge Coefficients The ratio of the true flow to the theoretical flow. It corrects the theoretical equation for the influence of velocity profile, tap location, and the assumption of no energy loss with a flow area between 0.023 to 0.56 percent of the geometric area of the inlet pipe.

Electronic Flow Meter (EFM) An electronic flow meter readout system that calculates flow from transducers measuring the variables of the flow equation.

Element, Primary That part of a flow meter which is directly in contact with the flow stream.

Element, Secondary Indicating, recording, and transducing elements that measure related variables needed to calculate or correct the flow for variables in the flow equation.

Empirical Tests Tests based on data observed in experiments.

Energy The capacity for doing work.

Energy, External Energy existing in the surroundings of a meter installation (normally heat or work energy).

Energy, Flow Work Energy necessary to make the upstream pressure higher than that downstream, so that flow will occur.

Energy, Heat Energy of the temperature of a substance.

Energy, Internal Energy of a fluid due to its temperature and chemical makeup.

Energy, Kinetic Energy of motion due to fluid velocity.

Energy, Potential Energy due to the position or pressure of a fluid.

Equation of State The properties of a fluid are represented by equations that relate pressure, temperature, and volume. The usefulness of these equations depends on the database from which they were developed and the transport properties of the fluid to which they are applied.

Extension Tube (Pigtail) A piece of tubing placed on the end of a sample container used to move the point of pressure drop (point of cooling) away from the sample being acquired. See GPA 2166.

Flange Taps A pair of tap holes positioned as shown in Figure 1-4. The upstream tap center is located 1 inch (25.4 mm) upstream of the nearest plate face. The downstream tap center is located 1 inch (25.4 mm) downstream of the nearest plate face.

Flashing Liquids with a sudden increase in temperature and/or a drop in pressure vaporize to a gas flow at the point of change.

Floating Piston Cylinder A sample container that has a moving piston whose forces are balanced by a pre-charge pressure.

Flow

Flow, Fluctuating A variation in flow rate that has a frequency that is lower than the meter-station frequency response.

Flow, Ideal Flow that follows theoretical assumptions.

Flow, Layered Flow that has sufficient liquid present to permit gas flow at a velocity above that for liquid flow at the bottom of a line. This flow is not accurately measured with currently available flow meters.

Flow, Non-Fluctuating Flow that varies gradually in rate over long periods of time.

Flow, Non-Swirling Flow with velocity components which move in straight lines with a swirl angle of less than 2° across the pipe.

Flow, Pulsating Variations in flow rate that have a frequency that is higher than the meter station frequency response.

Flow, Slug Flow that has sufficient liquid present to cause the liquid to collect in low spots and then “kick over” as a solid slug. This flow is not accurately measured by currently available flow meters.

Flow, Totalized The total flow over a stated period of time, such as per hour, per day, per month.

Flow Conditioning Preparing a flowing fluid so that it has no flow profile distortion or swirl.

Flow Nozzle A differential measuring device with a short cylinder that has a fluted approach section, as defined by the ASME standards.

Flow Profile A relationship of velocities in planes upstream of a meter that defines the condition of the flow into the meter.

Flow Proportional Composite Sampling The process of collecting gas over a period of time at a rate that is proportional to the pipeline flow rate.

Flow Rate The volume or mass of flow through a meter per unit time.

Flow Regime The characteristic flow behavior of a flow process.

Flow Temperature The average temperature of a flowing stream measured at a specified location in a metering system.

Fluid Flow Measurement The measurement of smoothly moving particles that fill and conform to the piping in an uninterrupted stream to determine the amount flowing.

Fluid Dynamics The mechanics of the flow forces and their relationship with the fluid motion and equilibrium.

Fluids, Dehydrated Fluids that normally have been separated into gas and liquid with the gas dried to the contract limit by a dehydration unit. (Normally the liquid is not dried, but it may be.)

Fluids, Separated Fluids that have been separated into gas and liquids at the temperature and pressure of the separating equipment.

Force Majeure Forces beyond the control of humans, normally from natural sources.

Frequency Response The ability of a measuring device to respond to the signal frequency applied to it within a specified limit.

Gas Laws These relate the volume, temperature and pressure of a gas; they are used to convert the volume at one pressure and temperature to that found under another set of conditions, such as flowing conditions to base conditions.

Gas Law Boyle’s Law states that the volume occupied by a given mass of gas varies inversely with the absolute pressure if the temperature remains constant.

Gas Law Charles’ Law states that the volume occupied by a given mass of gas varies directly with the absolute temperature if the pressure remains constant.

Gas Lift Injection of gas into a reservoir containing liquid to remove the liquid in the resulting production.

Gas Quality Refers to the physical characteristics determined by the composition (including non-hydrocarbon components, specific gravity, heating value, and dew points) of the natural gas.

Gas Sample Distortion Any effect that results in a sample that is not representative of the flowing gas stream.

Gas Sampling System The system intended to deliver a representative sample of natural gas from the pipeline to the analytical device.

Gaseous Phase The phase of a substance that occurs at or above the saturated vapor line of a phase diagram. It fills its container and has no level.

Gasoline Stripping Plant A separation plant designed to remove the heavier hydrocarbons from a gas stream.

Grade, Commercial Less-than-pure substance that must meet a composition limit. Although it is normally called by the name of its major component, it is actually a mixture.

Grade, Reagent Very pure substance that can be considered pure for calculation purposes.

Head Devices Meters that use the difference in elevation or pressure between two points in a fluid to calculate a flow rate.

Homogeneous Mix A mixture that is uniform throughout a flow stream mix, particularly important when sampling a flowing stream for analysis and the calculation of fluid characteristics.

Hydrates Ice-like compounds, formed by water and some hydrocarbons at temperatures that can be above freezing (32°F). They can collect and block a meter system’s flow.

Hydrocarbon Dew Point The temperature at a specific pressure at which hydrocarbon vapor condensation begins.

Ideal Gas Law Relationship of pressure, temperature, and volume with no corrections for compressibility.

Integration To calculate the recorded lines on a chart for the period of chart rotation.

Internal Controls A company’s rules of operation and the methods used to control these rules.

Lag Time In a sample system, the time required for a molecule to migrate from the inlet of the sample probe to the inlet of an analyzer.

Laminar Flow Flow at 2000 Reynolds number and lower; it has a parabolic profile.

Level Measurement Determination of a liquid level in a vessel.

Manometer A device that measures the height (head) of liquid in a tube at the point of measurement.

Mass The property of a body that measures the amount of material it contains and causes it to have weight in a gravitational field.

Mass Meter Meter that measures mass of a fluid based on a direct or indirect determination of the fluid’s weight rate of flow.

Master Meter A meter whose accuracy has been determined, used in series with an operating meter to determine the operating meter’s accuracy.

Material Balance A comparison of the amount of material measured into a process or pipeline compared with the amount of material measured out.

Measurement The act or process of determining the dimensions, capacity, or amount of something.

Meter, Dynamic Meters that continuously measure a flowing stream.

Meter Factor (MF) A number obtained by dividing the quantity of fluid measured by the primary mass flow system by the quantity indicated by the meter during calibration. For meters, it expresses the ratio of readout units to volume or mass units.

Meter Inspection This may be as simple as an external visual check, or be as complex as a complete internal inspection and calibration of the individual parts against standards and a throughput test.

Meter Proving The procedure required to determine the relationship between the “true” volume of fluid measured by prover and the volume indicated by the meter.

Meter, Static Meters that measure by batch from a flowing stream by fill and empty procedures.

Meter System All the elements needed to make up a flow meter, including the primary, secondary, and related measurements.

Meter Tube The upstream and downstream piping of a flow meter installation required to meet minimum requirements of diameter, length, configuration, and condition necessary to create a proper flow pattern through the meter.

Meter Tube Internal Diameter (D, Dm, Dr) The calculated internal diameter of a meter tube (D) is the inside diameter of the upstream section of the meter tube computed at the flow temperature (Tf); the calculated meter tube internal diameter (D) is used in the diameter ratio and Reynolds number equations. The measured meter tube internal diameter (Dm) is the inside diameter of the upstream section of the meter tube at the temperature of the meter tube at the time of internal diameter measurements determined as specified in API Chapter 14.3, Part 2. The reference meter tube internal diameter (Dr) is the inside diameter of the upstream section of the meter tube at the reference temperature (Tr) calculated as specified in API Chapter 14.3, Part 2. The reference meter tube internal diameter is the nominal, certified, or stamped meter tube diameter within the tolerance of API Chapter 14.3 Part 2, Section 5.1.3, and stated at the reference temperature Tr.

Mixture Laws A fluid’s characteristics can be predicted from a knowledge of the individual components’ characteristics. These mixture laws have limits of accuracy that must be evaluated before applying them.

Mobile Sampling System The system associated with a portable gas chromatograph.

Multiphase Flow Two or more phases (solid, liquid, gas, vapor) in the stream.

Newtonian Liquids Liquids that follow Newton’s second law, which relates force, mass, length, and time. The flow meters covered in this book measure Newtonian fluids.

Non-pulsating (see Pulsation) Variations in flow and/or pressure that are below the frequency response of the meter.

Normal Condensation Caused by an increase in pressure or a decrease in temperature.

Normal Vaporization Caused by a decrease in pressure or an increase in temperature.

Nozzle A flow device with an inlet profile that is elliptical along its centerline and made to a specified standard; they are usually used for high-velocity flows. They are resistant to erosion because of their shape.

Orifice Plate A thin plate in which a circular concentric aperture (bore) has been machined. The orifice plate is described as a “thin plate” and “with a sharp edge,” because the thickness of the plate material is small compared with the internal diameter of the measuring aperture (bore), and because the upstream edge of the measuring aperture is sharp and square.

Orifice Plate Bore Diameter (D, Dm, Dr) The calculated orifice plate bore diameter (D) is the internal diameter of the orifice plate measuring aperture (bore) computed at flowing temperature (Tf). The calculated orifice plate bore diameter (D) is used in the flow equation for the determination of flow rate. The measured orifice plate bore diameter (Dm) is the measured internal diameter of the orifice plate measuring aperture (bore) at the temperature of the orifice plate at the time of bore diameter measurements determined as specified in API Chapter 14.3, Part 2. The reference orifice plate bore diameter (Dr) is the internal diameter of the orifice plate measuring aperture at reference temperature (Tr), calculated as specified in API Chapter 14.3, Part 2. The reference orifice plate bore diameter is the nominal, certified, or stamped orifice plate bore diameter within the practical orifice plate bore diameter tolerance of API Chapter 14.3, Part 2, Table 2-1, and stated at the reference temperature Tr.

Orifice Plate Holder A pressure-containing piping element, such as a set of orifice flanges or orifice fitting, used to contain and position the orifice plate in the piping system.

Phase A state of matter such as solid, liquid, gas, or vapor.

Phase Change A change from one phase to another (such as liquid to gas). Most flow meters cannot measure at this condition.

Physical Constants The fundamental units adopted as primary measure values for time, mass (quantity of matter), distance, energy, and temperature.

Pipeline Quality Fluids that meet the quality requirements for contaminants as specified in the exchange contract; such as clean, non-corrosive, single phase, component limits, etc.

Pitot Probe An impact device with an inlet and return port that provides flow to a “hot loop” by converting velocity into a differential pressure.

Pressure The following terms pertain to different categories of pressure.

Pressure, Ambient The pressure of the surrounding atmosphere.

Pressure, Atmospheric The atmospheric pressure or pressure of one atmosphere. The normal atmosphere (atm) is 101.325 kPa (14.696 psia); the technical atmosphere (at) is 98,066.5 Pa (14.222 psia).

Pressure, Absolute The static pressure plus atmospheric pressure. (Note: calculations use absolute pressure values to determine flow.)

Pressure, Back, Turbine Meter The pressure measured at specified pipe diameters downstream from the turbine flow meter under operating conditions.

Pressure, Differential (dP) The static pressure difference measured between the upstream and the downstream flange taps.

Pressure, Gauge The pressure measured relative to atmospheric pressure (atmospheric pressure is taken as zero).

Pressure, Impact The pressure exerted by a moving fluid on a plane perpendicular to its direction of flow. It is measured along the flow axis.

Pressure Liquid, High-Vapor A liquid that, at the measurement or proving temperature of the meter, has a vapor pressure equal to or higher than atmospheric pressure (see low-vapor pressure liquid).

Pressure Liquid, Low-Vapor A liquid that, at the measurement or proving temperature of the meter, has a vapor pressure less than atmospheric pressure (see high-vapor pressure liquid).

Pressure Loss (Drop) The differential pressure in a flowing stream (which will vary with flow rate) between the inlet and outlet of a meter, flow straightener, valve, strainer, lengths of pipe, etc.

Pressure, Partial The pressure exerted by a single gaseous component of a mixture of gases.

Pressure, Static (Pf) Pressure in a fluid or system that is exerted normal to the surface on which it acts. In a moving fluid, the static pressure is measured at right angles to the direction of flow.

Pressure, Reid Vapor (RVP) The vapor pressure of a liquid at 100°F (37.78°C) as determined by ASTM D 323-58, Standard Method of Test for Vapor Pressure of Petroleum Products (Reid Method).

Pressure, Vapor (True) The term applied to the true pressure of a substance to distinguish it from partial pressure, gauge pressure, etc. The pressure measured relative to zero absolute pressure (vacuum).

Pressure, Velocity The component of the moving fluid pressure that is due to its velocity; commonly equal to the difference between the impact pressure and the static pressure (see pressure, impact and static).

Primary Element The primary element in orifice metering is defined as the orifice plate, orifice plate holder with its associated differential pressure sensing taps, and the meter tube.

Provers Devices of known volume used to prove a meter.

Proving Throughput Testing meter volume against a defined volume of a prover.

Pseudocritical A gas mixture’s compressibility may be estimated by combining the characteristic critical pressures and temperatures of individual components based on their percentages and calculating an estimated critical condition for the mixture.

Pulsation A rapid, periodic, alternate increase and decrease of pressure and/or flow. The effect of this on a meter depends on the frequency of the pulsation and the frequency response of the meter.

Quality Requirements Limits of non-contract material contaminants in the fluid.

Real Gas Law Ideal gas law corrected for the effect of compressibility.

Recirculation Region (“Eddy”) An area within a piping system out of the main flow where gas is not continually being replaced even though gas is flowing through the system.

Refined Products Products that have been processed from raw materials to remove impurities.

Representative Gas Sample Compositionally identical, or as near to identical as possible, to the gas sample source stream.

Residual Impurities Any substances, such as air or natural gas components, that are left in a sample cylinder.

Retrograde Condensation Caused by a decrease in pressure or increase in temperature.

Retrograde Vaporization Caused by an increase in pressure or decrease in temperature.

Reynolds number A dimensionless number defined as (ρ d v)/μ where ρ is density, d is the diameter of the pipe or device, v is the velocity of the fluids and μ is the viscosity—all in consistent units. Its usefulness is in correlating meter performance from one fluid to another.

Sample/Sampling

Sample Container Any container used to hold a natural gas sample. Typical sample containers are constant volume cylinders or floating piston cylinders.

Sample Loop The part of the sampling system that conveys the sample from the probe to the container or analytical device. It is typically external to the analysis device. This should not be confused with the sample loop that exists inside an analytical device such as a gas chromatograph.

Sample Probe A device which extends through the meter tube or piping, into the stream to be sampled.

Sample Source Refers to the stream being sampled.

Sampling A defined procedure for removing a part of the flowing stream that is intended to be representative of the total flowing stream composition.

Saturated Natural Gas Gas that will condense if the pressure is raised or the temperature is lowered. Water content saturated with water. Hydrocarbon content saturated with hydrocarbons.

Saturation A state of maximum concentration of a component in a fluid mixture at a given pressure and temperature.

Seal Pot A reservoir installed on each gauge line to maintain a constant leg on a pressure differential device or to isolate corrosive fluids from the differential device.

Secondary Equipment Equipment used to read the variables at the primary meter.

Shrinkage The amount of loss in apparent volume when two fluids are mixed; caused by the interaction of variably sized molecules.

Single Phase One phase (such as liquid without solids or gases present).

Single Phase Flow Natural gas flowing at a temperature above the hydrocarbon dew point and free of compressor oil, water, or other liquid or solid contaminants in the flow stream.

Slip Stream (“Hot Loop” or “Speed Loop”) Provides a continuous flow of sample.

Slippage Fluid that leaks between the clearance of the meter rotors and the meter body.

Smart Transducers Transducers with the built-in ability to correct for variations in measured or ambient conditions; an important requirement for most flow meters and measuring devices.

Sour A fluid that contains corrosive compounds (often sulfur based).

Specific Gravity (see Density, Relative, Gas and Liquid)

Specific Weight The force (weight/unit area) with which a body under specified conditions is attracted by gravity.

Stacked Transducers The installation of two or more differential pressure transducers of different maximum ranges to measure differential pressure on an orifice meter intending to extend the flow range of the meter.

Standard The following terms pertain to categories of measurement standards.

Standard A measuring instrument intended to define, to represent physically or to reproduce the unit of measurement of a quantity (or a multiple or sub-multiple of that unit), in order to transmit it to other measuring instruments by comparison.

Standard, International A standard recognized by an international agreement to serve internationally as the basis for fixing the value of all other standards of the given quantity.

Standard, National A standard recognized by an official national decision as the basis for fixing the value, in a country, of all other standards of the given quantity. In general, the national standard in a country is also the primary standard.

Standard, Primary A standard of a particular measure that has the highest metrological qualities in a given field. Note: (1) The concept of a primary standard is equally valid for base units and for derived units. (2) The primary standard is never used directly for measurement other than for comparison with duplicate standards or reference standards.

Standard, Secondary A standard, the value of which is fixed by direct or indirect comparison with a primary standard or by means of a reference-value standard.

Standard, Working A standard which, when calibrated against a reference standard, is intended to verify working measuring instruments of lower accuracy.

Standards Organizations Industry/government committees that write standards (see the discussion of testing in Chapter 4).

Steam

Steam, Saturated The end point of the boiling process. It is the condition in which all liquid water has evaporated and the fluid is a gas. Being the end point of the boiling process, its pressure automatically defines its temperature, and conversely its temperature defines its pressure. Saturated steam is unstable; heat loss starts condensation; heat addition superheats; pressure loss superheats; pressure gain starts condensation.

Steam, Superheated Pressure decrease or heat added to saturated steam will produce superheated steam, which acts as a gas and follows general gas laws with increased sensitivity to temperature and pressure measurements.

Steam, Wet (Quality Steam) A two-phase fluid containing gaseous and liquid water. The quality number defines what proportion of the mixture is gaseous; for example, “95% quality steam” indicates that 95% by weight of the mixture is a gas; 5% is liquid water.

Sunburst Chart A recorded chart with a wide and variable differential recording with a pattern associated with a sun symbol.

Sweet Fluids containing no corrosive compounds.

Swirling Flow Flow in which the entire stream has a corkscrew motion as it passes through a pipeline or meter. Most flow meters require swirl to be removed before attempting measurement, although some ultrasonic and Coriolis type meters claim to handle some swirl without flow conditioning.

System Balances (see Material Balance) In a pipeline system this information is reflected in a “loss or unaccounted for” report.

Tank Gauging A defined procedure of measurement of fluids in tanks by level determination.

Tap Hole A hole drilled radially in the wall of a meter tube or orifice plate holder, the inside edge of which is flush and without any burrs.

Temperature Measurement (Tf) Flowing fluid temperature measured at the designated upstream or downstream location as specified in API MPMA Chapter 14.3, Part 2. In flow measurement applications in which the fluid velocity is well below sonic, it is common practice to inset a temperature-sensing device in the middle of the flowing stream to obtain the flowing temperature. For practical applications, the sensed temperature is assumed to be the static temperature of the flowing fluid. The use of flowing temperature in this part of the standard requires the temperature to be measured in degrees Fahrenheit, °F, or degrees Centigrade, °C. However, if the flowing temperature is used in an equation of state to determine the density of the flowing fluid, it may require that the °F or °C values be converted to absolute temperature values of degrees Rankine (°R), or kelvin (K) (Table 1-1).

Temperature Stratification At low flow rates, proper mixing does not take place; layers of flow have different temperatures, densities, and speed of sound. Proper mixing must be achieved to measure the fluid temperature.

Throughput Tests The passage at the flowing fluid through the operating meter compared to volume standard at the operating flow rate.

Transition Flow Flow, with a variable velocity profile, at Reynolds number between 2,000 and 4,000.

Turbulent Flow Flow above 4,000 Reynolds number with a relatively flat velocity profile.

Uncertainty A statistical statement of measurement accuracy based on statistically valid information that defines 95% of the data points (twice the standard deviation).

Vapor Phase This term, used interchangeably with “gas,” has various shades of meaning. A vapor is normally a liquid at normal temperature and pressure, but becomes a gas at elevated temperatures. The term “vapor” is also sometimes used to indicate that liquid droplets may be present. In a strict technical sense, however, the terms are interchangeable.

Velocity Time rate of linear motion in a given direction.

Venturi A defined head metering device that has a tapered inlet and outlet with a constricted straight middle section.

Viscosity A fluid’s property that measures the shearing stress that depends on flow velocity, density, area, and temperature—which in turn affects the flow pattern to a meter and hence measurement results.

Water Dew Point The temperature at a specific pressure at which water vapor condensation begins.

Weight The force with which a body is attracted by gravity.

Wetted Part The parts of a meter which are exposed to the flowing fluid.