Fluid Flow Measurement (eBook)

A Practical Guide to Accurate Flow Measurement

Paul J. LaNasa, E. Loy Upp (Autoren)

eBook Download: PDF | EPUB

2014 | 3. Auflage
296 Seiten
Elsevier Science (Verlag)
978-0-12-409532-8 (ISBN)

Infos

There is a tendency to make flow measurement a highly theoretical and technical subject but what most influences quality measurement is the practical application of meters, metering principles, and metering equipment and the use of quality equipment that can continue to function through the years with proper maintenance have the most influence in obtaining quality measurement. This guide provides a review of basic laws and principles, an overview of physical characteristics and behavior of gases and liquids, and a look at the dynamics of flow. The authors examine applications of specific meters, readout and related devices, and proving systems. Practical guidelines for the meter in use, condition of the fluid, details of the entire metering system, installation and operation, and the timing and quality of maintenance are also included. This book is dedicated to condensing and sharing the authors' extensive experience in solving flow measurement problems with design engineers, operating personnel (from top supervisors to the newest testers), academically-based engineers, engineers of the manufacturers of flow meter equipment, worldwide practitioners, theorists, and people just getting into the business. - The authors' many years of experience are brought to bear in a thorough review of fluid flow measurement methods and applications - Avoids theory and focuses on presentation of practical data for the novice and veteran engineer - Useful for a wide range of engineers and technicians (as well as students) in a wide range of industries and applications

Paul LaNasa currently functions as a Consultant serving industry and manufacturers in instrumentation and energy resource management. Prior to this he was President of CPL & Associates, Senior Vice President, Director of Marketing at Nutech Industries and served on the Board of Directors. He also worked for 14 years in the oil and gas industry with Daniel Industries (now part of Emerson) and has worked with the Boeing Co. doing technical analysis in thermodynamics and life support and environmental control on the Apollo program.Paul is the recipient of the American Gas Association's (AGA) Silver Award of Merit, the International School of Hydrocarbons' Laurence Reid Award for outstanding contributions to hydrocarbon-fluid measurement, and the American Petroleum Institute (API) Certificate of Appreciation and 25 Year Citation of Service. During his career he has been active in the American Society of Mechanical Engineers (ASME), the Instrument Society of America (ISA), the American Gas Association (AGA), the American Petroleum Institute (API), the International School of Hydrocarbon Measurement (ISHM), the National Bureau of Standards (NBS), the Southern Gas Association (SGA), and other schools and societies.

Textauszug

2

Basic Flow Measurement Laws

The chapter goes through the laws which should be recognized and obeyed before flow measurement is attempted. Certain physical laws explain what happens in the “real” world. Some of these laws explain what happens when fluid flows in a pipeline, and these in turn explain what happens to a flowing stream as it goes through a meter. The laws discussed in this chapter include the law of “conservation of mass”, also called the “Law of Continuity,” the law of “conservation of energy”, the Fluid Friction Law and the fundamental flow equation. The Reynolds number is explained, as are the gas laws and ways to correct for the effects of temperature and pressure.

Keywords

physical law; fluid flow; conservation of mass; conservation of energy; fluid friction law; fundamental flow equation; Reynolds number

All of the following laws should be recognized and obeyed before flow measurement is attempted. Certain physical laws explain what happens in the “real” world. Some of these laws explain what happens when fluid flows in a pipeline, and these in turn explain what happens to a flowing stream as it goes through a meter. All variables in the equations must be in consistent units of measurement.

The law of “conservation of mass” states that the mass rate is constant. In other words, the amount of fluid moving through a meter is neither added to nor taken from as it progresses from point 1 to point 2 (Figure 2-1). This is also called the “Law of Continuity,” and it can be written mathematically as follows:

1=M2 (2.1)

(2.1)

where:

M1=mass rate upstream;

M2=mass rate downstream.

Figure 2-1 The amount of fluid flowing is constant at points 1 and 2.

Since mass rate equals fluid density multiplied by pipe area multiplied by fluid velocity, Equation 2.1 can be rewritten as:

1A1V1=ρ2A2V2 (2.2)

(2.2)

where:

ρ=fluid density at a designated point in the pipe;

A=pipe area at the designated point;

V=average velocity in the pipe at the designated point;

1, 2=upstream and downstream positions.

In terms of volume rate this can be restated as:

=AV (2.3)

(2.3)

where:

Q=volume per unit time at flowing conditions;

A, V=as previously defined.

The law of “conservation of energy” states that all energy entering a system at point 1 is also in the system at point 2, even though one form of energy may be exchanged for another. (Note: the Bernoulli theorem relates the same physics in fluid mechanics.) The total energy in a system is made up of several types:

1. Potential Energy due to the fluid position or pressure.

2. Flow Work Energy required for the fluid to flow. The fluid immediately preceding that between point 1 and point 2 must be at a slightly higher pressure in order to exert a force on the volume between 1 and 2, so that this will flow.

3. Kinetic Energy (energy of motion) due to fluid velocity.

4. Internal Energy due to fluid temperature and chemical composition.

5. External Energy is energy exchanged with the fluid existing between point 1 and point 2 and its surroundings. These are normally heat and work energies.

The Fluid Friction Law states that energy is required to overcome friction and move fluid from point 1 to point 2. For the purpose of calculating flows, certain assumptions are made about the stability of the system energy under steady flow. The main energy concerns are the potential and kinetic energies (definitions 1 and 3); the others are either of no importance, do not change between position 1 and position 2, do not occur, or are taken care of by calibration procedures. A generalized statement of this energy balance is given below:

E1+PE1=KE2+PE2 (2.4)

(2.4)

Kinetic energy (KE) is energy of motion (velocity). Potential energy (PE) is energy of position (pressure).

In simple terms, this equation can be rewritten:

1+VE1=PE2+VE2 (2.5)

(2.5)

where:

PE=pressure energy;

VE=velocity energy.

Equation 2.5 is the “ideal flow equation” for a restriction in a pipe. In real applications, however, certain corrections are necessary. The major equation correction is an efficiency factor called the “coefficient of discharge.” This factor takes into account the difference between the ideal and the real world. The ideal equation states that 100% of the flow will pass an orifice with a given differential, when in fact empirical tests indicate that a lower fraction of the flow actually passes for a given differential—for example, about 60% with differential between flange taps on an orifice meter, 95% across a nozzle, and 98% across a Venturi (Figure 2-2). This is caused by the device’s inefficiency or the loss from inefficiency caused by turbulence at the device where energy of pressure is not all converted to energy of motion. This factor has been determined by industry studies over the years and is reported as a “discharge coefficient.”

Figure 2-2 Orifice, flow nozzle, and Venturi meters all have permanent pressure losses somewhat less than 100% of full measurement differential; an ultrasonic meter is like an open section of pipe (non-intrusive) with no additional permanent pressure drop.

Equations 2.4 and 2.5 assume no energy, such as heat, is added or removed from the stream between upstream and the meter itself. This is normally of small concern unless there is significant difference between the flowing and ambient temperatures (i.e., steam measurement), or when measuring a fluid whose volume is sensitive to very small temperature changes, such as near its critical temperature. (Three common examples are ethylene, carbon dioxide gas, and hot water near its boiling point.)

It is also assumed that no temperature change is caused by fluid expansion (because of the lower pressure in the meter) from the upstream pressure to the meter. The small pressure difference between the two locations normally makes this theoretical consideration insignificant. If there is a change in state (i.e., from liquid to gas or gas to liquid) then this “insignificant” temperature change is no longer insignificant. Furthermore, the volume occupied (assuming no mass holdup) is much greater in the gaseous phase than the liquid phase; volume ratios of gas to liquids are as much as several hundred for some common fluids. Because of these problems, flow measurement of flashing liquids or condensing gases should not be attempted.

Reynolds Number

The Reynolds number is a useful tool for relating how a meter will react to a variation in fluids from gases to liquids. Since an impossible amount of research would be required to test every meter on every fluid we wish to measure, it is desirable that a relationship between fluid factors be known. Reynolds’ work in 1883 defines these relationships through his Reynolds number, which is defined by the equation:

=ρDvµ (2.6)

(2.6)

where:

Re=Reynolds number, a dimensionless number;

ρ=density of the fluid;

D=diameter of the passage way;

v=velocity of the fluid;

μ=viscosity of the fluid.

Note: All parameters are given in the same units, so that when multiplied together they all cancel out, and the Reynolds number has no units. Units in the pound, foot, second system are shown below:

Re=no units;

ρ=#/cubic feet;

D=feet;

v=feet/sec;

μ=#/foot-sec.

Based on Reynolds’ work, the flow profile (which affects all velocity-sensitive meters and some linear meters) has several important values. At values of 2,000 and below, the flow profile is bullet-shaped (parabolic). Between 2,000 and 4,000 the flow is in the transition region. At 4,000 and above the flow is in the turbulent flow area and the profiles are fairly flat. Thus, calculation of the Reynolds number will define the flow velocity pattern and approximate limits of the meter’s application. To completely define the meter’s application there must be no deformed profiles, such as after an elbow or where upstream piping has imparted swirl to the stream.

These effects will be further discussed in the sections covering the description and application of different meters, in Chapters 8, 9, and 10, and the equations will be covered more thoroughly later in this book.

These equations can be combined and rewritten in simplified forms. However, it is important to recognize the assumptions which have been made, so that if a metering situation deviates from what has been assumed, a “flag will go up” to indicate that the effect of Reynolds number must be evaluated and treated.

Gas Laws

Gas properties are almost always measured at conditions other than...

Produkt-Details

• Erscheint lt. Verlag: 12.4.2014
• Sprache: englisch
• Themenwelt: Mathematik / Informatik ► Mathematik ► Angewandte Mathematik
• Themenwelt: Naturwissenschaften ► Chemie
• Themenwelt: Naturwissenschaften ► Physik / Astronomie ► Strömungsmechanik
• Themenwelt: Technik ► Bauwesen
• Themenwelt: Technik ► Maschinenbau
• ISBN-10: 0-12-409532-1 / 0124095321
• ISBN-13: 978-0-12-409532-8 / 9780124095328