This is a book about flow measurement and flowmeters written for all in the industry who specify and apply, design and manufacture, research and develop, maintain and calibrate flowmeters. It provides a source of information on the published research, design, and performance of flowmeters as well as on the claims of flowmeter manufacturers. It will be of use to engineers, particularly mechanical and process engineers, and also to instrument companies' marketing, manufacturing, and management personnel as they seek to identify future products.

I have concentrated on the process, mechanical, and fluid engineering aspects and have given only as much of the electrical engineering details as are necessary for a proper understanding of how and why the meters work. I am not an electrical engineer and so have not attempted detailed explanations of modern electrical signal processing. I am also aware of the speed with which developments in signal processing would render any descriptions which I might give out of date.

In the bibliography, other books dealing with flow measurement are listed, and my intention is not to retread ground covered by them, more than is necessary and unavoidable, but to bring together complementary information. I also make the assumption that the flowmeter engineer will automatically turn to the appropriate standard; therefore, I have tried to avoid reproducing information that should be obtained from those excellent documents. I include a brief list that categorizes a few of the standards according to meter or application.

INTRODUCTION

BACKGROUND

Spirals, screws, and windmills have a long history of use for speed measurement. Robert Hook proposed a small windmill in 1681 for measuring air velocity and later one for use as a ship's log (distance meter). A Captain Phipps, in 1773, created a ship's log, using the principle that a spiral, in turning, moves through the length of one turn of the spiral. Many centuries earlier, it appears that a Roman architect, Vitruvius, suggested a more basic form of this device.

In 1870 Reinhard Woltmann developed a multibladed fan to measure river flows (Medlock 1986). The device was a forerunner of the long helical screw-type meter still called after him and used widely for pipe flows in the water industry. The first modern meters, of the type with which we are mainly concerned, were developed in the United States in 1938 (Watson and Furness 1977; cf. Furness 1982). These were attractive for fuel flow measurement in airborne applications. They consisted of a helically bladed rotor and simple bearings. Improved sleeve bearings were developed for longer life with hardened thrust balls or endstones to withstand the axial load. An alternative, developed over several years and patented by Potter (1961), was to profile the hub of the rotor so that the pressure balance across the rotor, rather than the thrust on the bearings, held it against the axial drag forces. This allowed the rotor to run on a single journal bearing.

INTRODUCTION

The object of this chapter is to assist the reader in specifying a proposed flowmeter's operational requirements as fully as possible and hence to enable the reader to select the right flowmeter. The reason for wishing to select a flowmeter may be obvious: a flow rate to be measured. However, the reason in some cases may be less obvious. If you are a flowmeter manufacturer, or prospective manufacturer, you may wish to identify a type and design that would meet a particular market niche. If you are a member of an R&D team, you may be exploring measurement areas that are inadequately covered at present.

In preparing this chapter, I have developed earlier ideas (Baker 1988/9, Baker and Smith 1990), but I have also benefited from the ideas of others such as Endress et al. (1989). Baker (1988/9) was the best distillation I could offer in the late 1980s. There is much there that I would still endorse. In Baker and Smith (1990), we took a different line. We provided the means to specify the user's needs in as much detail as possible. We then provided a form for communication with the manufacturer.

Many people have attempted to provide a means of selecting a flowmeter. A glance at only a few of the many books on flow measurement in the bibliography will indicate this. I used to feel that the way forward was to write an expert system to do the job.

INTRODUCTION

I am indebted to Medlock (1989) for notes on three early devices that attempt to use change of angular momentum to obtain mass flow.

Katys (1964) used an electric synchronous motor with a special rotor supported internally in the pipe and a stator outside the pipe. Fluid passed through vanes in the rotor and acquired angular momentum. From the power used or the torque needed to drive the rotor, it was claimed that the mass flow could be deduced.

According to Medlock, the Bendix meter “measures the torque required to impart angular momentum to the liquid. One end of a calibrated spring is driven at a constant speed and the other end is connected to a freely rotatable turbine. The torsion developed in the spring is a measure of mass flow rate.”

The twin rotor turbine meter (Potter 1959) attempted to measure mass flow by means of two in-line turbine rotors of different blade angles, which are joined by a torsion spring. Despite discussions with Medlock about this meter, I am not convinced that this is a true mass flow meter and suggest that the valid derivative of the Bendix meter is the fuel flow transmitter described later.

An early design with many similarities to the current commercial designs was a device due to Orlando and Jennings (1954) shown in Figure 16.1. A constant speed motor drives an impeller imparting swirl to the liquid.

INTRODUCTION

The object of calibration is to benchmark a flowmeter to an absolute datum. Just as a benchmark tells us how a particular geographical point compares in height with sea level datum, so a calibration of a flowmeter tells us how the signal from the flowmeter compares with the absolute standard of a national laboratory. The analogy is not perfect at this stage. The national laboratory standard must also be compared with other more fundamental measures of time and mass, and it will be essential to check and compare different national standards in different countries from time to time.

There is a desire to reference back to fundamental measurements such as mass, length, and time. Thus if we can measure mass flow on a calibration facility by using fundamental measurements of mass and time, this will bring us nearer to the absolute values than, say, obtaining the volume of a calibration vessel by using weighed volumes of water and deducing the volume from the density. The first is more correctly termed primary calibration, whereas the second fails strictly to achieve this. It is likely that the final accuracy will reflect this. Liquid calibration facilities can achieve a rather higher accuracy than is possible for most of the gas calibration facilities. In part, this difference will result from the lower density and the increased difficulties of handling a gas.

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.