INTRODUCTION

This chapter does not attempt more than a cursory review of modern control systems, as it is outside my main area of expertise. Those knowledgeable in electronics and control will pass quickly over this chapter! Others who wish to delve more deeply may, for instance, turn to Sanderson (1988), Brignell and White (1994), or other experts, or for a good, recent introduction to the control of machinery Foster (1998) is recommended. I am pleased to acknowledge valuable insights obtained from Cranfield lectures given by Bill Black, David Clitherow, Doug de Sa, and others.

This book is about flowmeters that depend, in the main, on mechanical or thermo/fluid mechanical effects. In only one case of mainstream meters, the electromagnetic meter is the signal electrical throughout. The aim of the book has been to bring together information on the performance of these meters based on physical, experimental, and industrial experience. I have made the assumption that the electrical signal interpretation, which is based in the meter, is capable of doing the job set by the designer of interpreting the meter output into a standard electrical form. This assumption is not always adequate (e.g., where electrical signals from the same instrument do not appear to agree with each other). Nevertheless, the developments in design of electrical systems are assumed to be such that the reader will not benefit from a detailed description or that the circuit will be changing so fast that any statement will be out of date.

INTRODUCTION

The material in this chapter is based on the paper by Baker and Morris (1985) on positive displacement (PD) meters, which in turn has been updated with more recent industrial and published material and extended to gases. P.D. Baker's (1983) paper provided some additional useful information on these meters. The main types of liquid meter were given by Barnes (1982), Hendrix (1982), Henke (1955), and Gerrard (1979) (cf. Mankin 1955). The reader should refer to API (1992) and similar documents.

At least four of the meter designs to be discussed have been around for over 100 years. The nutating disk flowmeter for liquids was developed in 1850. The rotary piston meter appeared in the late nineteenth century (Baker 1998).

The measurement of gas has depended, from an early date, on two types of positive displacement meter: the wet gas meter of high accuracy and credited to Samuel Clegg (1815), and the diaphragm meter of lower performance but greater range for which William Richards (1843) should take the credit.

BACKGROUND

The concept of carrying known volumes of fluid through a flowmeter is a short step from the use of a discrete measure such as a bucket or measuring flask. Thus in each of the designs described later, the flow enters a compartment that is as tightly sealed as is compatible with relative movement of adjacent components.

INTRODUCTION

The first proposal for the use of ultrasound for flow measurement, according to Thompson (1978), seems to have been in a German patent of 1928. It was not until after 1945 that the idea became more widely proposed. But not until the development of piezoelectric transducers in the past 40 years or so have ultrasonic applications become really attractive. Fischbacker (1959) provided an early review of ultrasonic flowmeters in which, essentially, the transit-time, sing-around, and beam deflection methods were mentioned. He also referred to phase-difference measurement, means of obtaining sound speed from time measurement and impedance, and how to obtain density. He saw the advantages of off-axis paths.

Sanderson and Hemp's (1981) review is still a useful source of information on the subject. The ultrasonic flowmeter's attraction as a flow measurement device is its linearity, lack of obstruction to flow, and, in contrast to the magnetic flowmeter, its ability to measure the flow of gases.

In this chapter, we shall consider three main types of ultrasonic flowmeter, and it is important to understand the strengths and weaknesses of each and that they are very different in performance and application.

The transit-time flowmeter, or time-of-flight flowmeter, is the most accurate of the family and is available as a spool piece meter for liquids and gases or as a clamp-on design for liquids only. It can also be retrofitted into a pipe. Measurement uncertainty will be from a fraction of a percent to about 5%.

INTRODUCTION

In a recent book (Baker 1996), I provided an introduction to fluid mechanics and thermodynamics, particularly aimed at instrumentation. I do not, therefore, propose to repeat what is written there but rather to confine myself to essentials. In addition, Noltingk (1988) has provided a very valuable handbook on general instrumentation. In this book, I shall use the term fluid to mean liquid or gas and will refer to either liquid or gas only when the more general term does not apply.

ESSENTIAL PROPERTY VALUES

Flowmeters generally operate in a range of fluid temperature from −200°C (–330°F) to 500°C (930°F), with line pressures up to flange rating for certain designs. Typical values of density and viscosity are given in Table 2.1.

It should also be noted that liquid viscosity decreases with temperature, whereas gas viscosity increases with temperature at moderate pressures. In common fluids, such as air and water, the value of viscosity is not dependent on the shear taking place in the flow. These fluids are referred to as Newtonian in their behavior as compared with others where the viscosity is a function of the shear taking place. The behavior of such fluids, known as non-Newtonian, is very different from normal fluids like water and air. Newtonian fluid behavior is a good representation for the behavior of the bulk of fluids.

INTRODUCTION

The orifice plate flowmeter, the most common of the differential pressure (DP) flowmeter family, is also the most common industrial flowmeter. It is apparently simple to construct, being made of a metal plate with an orifice that is inserted between flanges with pressure tappings formed in the wall of the pipe. It has a great weight of experience to confirm its operation. However, it is far more difficult to construct than appears at first sight, and the flow through the instrument is complex.

Some key features of the geometry of the orifice and of the flow through it are shown in Figure 5.1. The behavior of the orifice plate may be predicted, but the predictions derive from experimental observation and data. The inlet flow will usually be turbulent and will approach the orifice plate where an upstream pressure tapping (one diameter before the orifice plate) will measure the pressure at the wall, that is in this case the static pressure. (Flange and corner tappings will be discussed later). The pressure across the pipe will have a constant time-mean value because the only velocity across the pipe is due to turbulent eddies. The flow close to the orifice plate will converge toward the orifice hole, possibly causing a recirculation vortex around the outside corner of the wall and orifice plate.

INTRODUCTION

In this chapter, we are concerned with devices defined in international standard documents. Among these, the venturi meter is one of the oldest industrial methods of measuring flow, although other methods may be more common. Chapter 8 deals with other devices such as variable area meters, target meters, averaging pitot tubes, Dall meters, and other venturi, nozzle, and orifice-like devices.

As in the case of the orifice plate, the reader should have access to a copy of the latest version of the ISO standard (5167-1). I have refrained from repeating information that should be obtained from the standard, and include only the minimum information to provide an understanding of the discharge coefficients, the likely uncertainty of the various devices, and the type of material from which they may be constructed. However, although the amount of material published recently is not great, there is a growing interest in the use of some of these devices for difficult metering tasks and a reevaluation of their behavior, which may well lead to several reports in the next few years.

INTRODUCTION

There are broadly two concepts of thermal flowmeter now available for gas mass flow measurement, one of which is also applicable to liquids. I shall follow the useful terminology in ISO committee draft (ISO/CD 14511:1998) for the two types. The first is the capillary thermal mass flowmeter (CTMF), which has broad applications in the control of low flows of clean gases, but which can also be used with a bypass containing a laminar element to allow higher flow rates to be measured. The arrangement of heaters and coils between the various manufacturers differs, but the basic approach is the same, with heat added to the flowing stream and a temperature imbalance being used to obtain the flow rate.

The second is the full-bore thermal mass flowmeter (ITMF), which is available as both insertion probe and in-line type. It has a widely used counterpart in the hot-wire anemometer for measurement of local flow velocity, but, as the need for a gas mass flowmeter has become evident, it has been developed as a robust insertion probe for industrial usage and then as the sensing element in a spool piece flowmeter. It has been produced by an increasing number of manufacturers in recent years as a solution to the need for such a mass flowmeter.

CAPILLARY THERMAL MASS FLOWMETER – GASES

In various examples of the CTMF, the gas flows through a very small diameter tube that has heating and temperature-measuring sensors.

MARKET DEVELOPMENTS

Lawton Smith (1994, cf. Kinghorn 1988) gave some overall figures (cf. Chapter 20) for the flow measurement industry worldwide, which, with likely growth, suggest that by the year 2000 it could be in excess of US $1 billion.

With this growing market in mind, we look at the new challenges that face the flowmeter engineer, the current and future devices that may provide solutions, the implications of information technology, and new production methods. The chapter concludes with some suggestions for the way ahead, after a brief review of comments that I made on future developments in an earlier book (Baker 1988/9).

EXISTING AND NEW FLOW MEASUREMENT CHALLENGES

Oil Exploration and Processing

The oil industry, dealing as it does with high value products, is likely to continue to ask for more accurate meters capable of operating in adverse conditions, in multiphase flow, and in subsea installations.

Corneliussen (1991) of the Amoco Norway Oil Company described field experience with Hod metering, which was the first small unmanned production platform in the Norwegian sector of the North Sea and started production in September 1990. Gas and liquid were transported in a three-phase pipeline 13 km to another platform from which Hod was operated. He described problems that appeared to have stemmed from trapped air and gas in the pipeline and from a highly acidic well that eventually caused the destruction of some of the meters. This is one example of the increasingly difficult environment in which North Sea instrumentation must work.

INTRODUCTION

This is one of the most important recent meter developments, and its application in the next few years is likely to impinge on many areas of industry. Its importance is recognized by the main national and international advisory bodies (cf. Mandrup-Jensen 1990 who described initial work in Denmark on pattern approval and ISO developments).

BACKGROUND

Plache (1977) makes the interesting point that mass cannot be measured without applying a force on the system and then measuring the resulting acceleration. This is a point that I have long considered a possible requirement, and it is certainly supported in the Coriolis meter.

Possibly the first application of the Coriolis effect for mass flow measurement was proposed by Li and Lee (1953). The meter is shown in Figure 17.1. The T-piece flow tube rotated with the outer casing and was linked to it by a torque tube. As the flow increased, so the T-piece experienced a displacing torque due to the Coriolis acceleration, and this was measured from the displacement of the T-piece relative to the main body.

In the Li and Lee meter, the liquid was forced to move radially, and therefore a force was applied to it through the tube. This force, in turn, was balanced by an equal and opposite one applied by the liquid to the tube. The force caused the tube to twist, and the small rotation was sensed to obtain the mass flow rate.

INTRODUCTION

The possibility of inducing voltages in liquids moving through magnetic fields was known by Faraday in 1832, but the first flowmeter-like device was reported by Williams in 1930. The first real advance in the subject came from the medical field where Kolin (1936, 1941) introduced many ideas that are now standard practice.

The industrial interest in electromagnetic flowmeters (sometimes referred to as EM or magnetic flowmeters) grew in the 1950s with

• the Tobiflux meter (Tobi 1953) in Holland for rayon viscose, sand and water, and acid slurries;

• Foxboro, to whom the patent was assigned in 1952;

• the first commercial instruments in 1954 (Balls and Brown 1959).

• nuclear reactor applications;

• the work that resulted in an essential book by J. A. Shercliff (1962).

In this chapter, we shall concentrate on the application of the flowmeter to fluids that are of low conductivity, such as water-based liquids (Baker 1982). The flowmeter has also been used with liquid metals (Baker 1969, 1970b, 1977), and a few designs have been built for use with nonconducting dielectric liquids (Al-Rabeh et al. 1978). The reader is referred to the original papers because space prevents their inclusion here. Three papers by Wyatt (1961, 1977, 1982), a pioneer in blood flow measurement research, are referenced for those interested.

OPERATING PRINCIPLE

We start with the simple induction, which occurs when a conductor moves through a magnetic field. Figure 12.1 shows a copper wire cutting the flux of a permanent magnet.

INTRODUCTION

The measurement of mass flow, a fundamental requirement for any fluid, has been an elusive goal due to the problems of developing a suitable flowmeter. Despite this, the availability of mass flowmeters has increased greatly over the last 15 years. This is partly due to the increasing value of products (Hall 1990), but it is also due to an increasing realization that volumetric flow measurement is often inappropriate. In addition, the advent of the Coriolis flowmeter has stimulated engineers to find other mass flowmeters. General reviews of mass flowmeters were given by Sproston et al. (1987), Betts (1990), and Medlock and Furness (1990).

Mass flow measurement is commonly categorized as direct (true) or indirect (inferential). However, it may be useful to allow a few more than two categories.

1. a. True (direct) mass flow measurement by a single instrument is rare. It appears that to achieve it we need to use one of the fundamental acceleration laws. We can do this by creating:

• the force (or torque) resulting in a linear (or angular) acceleration (Chapter 16 describes an example) or

• the force that produces Coriolis acceleration (Chapter 17).

1. b. Fluid-dependent thermal mass flow measurement uses the temperature rise resulting from heat addition but is affected by other parameters such as the specific heat of the fluid (Chapter 15).

2. c. Multiple differential pressure flowmeters used in a dedicated system (Section 14.2) depend on the nonlinearity of the flowmeter equation.

INTRODUCTION

This meter uses a fascinating effect that occurs when gas flows at very high velocity through a nozzle. As the gas is sucked through the nozzle, the velocity increases as the cross-section of the nozzle passage decreases toward the throat. At the throat, the maximum achievable velocity is sonic – the speed of sound. Downstream of the throat, the velocity will either fall again returning to subsonic or will rise and become supersonic. In normal operation of the nozzle, the supersonic region is likely to be small and to be followed by a shock wave that stands across the divergent portion of the nozzle and causes the gas to drop, very suddenly, from supersonic to subsonic. The existence of a shock wave does not mean that one will hear a “supersonic bang”! Such bangs are usually caused by moving shock waves carried forward by high speed aircraft.

The fascinating effect of sonic conditions at the throat is that changes in the flow downstream of the throat have no effect on conditions upstream. The sonic or critical condition, as it is called, appears to block any information that is trying to penetrate upstream. A simple picture of this is that the messengers carrying such information travel at the speed of sound and so are unable to make any headway over the fast flowing gas stream.