Baker R.C. Flow Measurement Handbook: Industrial Designs, Operating Principles, Performance, and Applications

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14.5 CHAPTER CONCLUSIONS 367

The Kongsberg/Shell meter uses capacitance to obtain the gas-liquid interface in

the channel, the oil-water fraction, and, by cross correlation, the slug velocity and

to estimate the individual flow rates.

The Multi-Fluid International (MFI) meter is suitable for flows with continuous

oil component. The composition is obtained with microwave and gamma radiation,

and velocity is measured using cross correlation between two dielectric sensing sec-

tions.

Okland and Berentsen (1994) described tests of the MFI, which suggested that

after optimization the MFI could achieve uncertainty on oil flow rate within 5%.

Using a combination of venturi, capacitor, and single-beam gamma densitome-

ter, Fischer (1994) showed that, provided the mixture was homogeneous, the mass

flow rates of the components of oil-water-air could be obtained within about ±25%.

Another method that has been looked at is pulsed neutron techniques (Anon.

1993).

Ekrann et al. (1988) included flow measurement as part of a capacitive system

presumably by using a correlation method.

The trend appears to be toward a meter that allows full-bore measurement of

all components, although removal of a small sample may allow a higher precision

while the technology develops.

Whitaker and Millington (1993) concluded that, from available evidence, the

then state of the art should have allowed phase flow rates to be measured to ap-

proximately 10% over reasonably wide operating envelopes. Whitaker (1993) also

appeared to conclude that 10% or so bulk measurement uncertainty was unlikely to

be bettered without new technologies and more recently (Whitaker 1996) confirmed

the ±10% of reading for four meters: Agar, Fluenta, Framo, and MFI.

See also North Sea Flow Measurement Workshops 1992, 1993, and 1994 for fur-

ther papers on multiphase flow meters and mixers.

14.5 CHAPTER CONCLUSIONS

This chapter has been concerned with a wide range of mass flow measurement

that falls outside the meters discussed in Chapters 15-17. Apart from the hydraulic

Wheatstone bridge method that has had a particular market niche, the other multi-

meter methods are likely to be in competition with the increasingly versatile Coriolis

meter. The most promising multimeter development is likely to include ultrasonics.

It is in the area of multiphase flows that the major developments are likely to oc-

cur, backed by the considerable cost advantages that result from subsea multiphase

flow measurement. These are likely to have improved component ratio measure-

ment and clever data handling to obtain the most information. We shall, therefore,

concentrate on these, always remembering that other areas of industry beyond the

oil industry will benefit from such developments.

14.5.1 WHAT TO MEASURE IF THE FLOW IS MIXED

It would be satisfactory to have a flowmeter that could interpret all the variables that

occur in a multicomponent flow. Such a device might identify the position on a flow

pattern chart for well-documented fluids. It could also deduce the mass flow of each

component. The state of the art has not reached that point yet, and it is likely to be

368 MASS FLOW MEASUREMENT USING MULTIPLE SENSORS

several years before it does. It is therefore important to consider what variable we

are seeking in the measurements that are being made now. The most useful measure

is clearly mass flow of the separate components. But the sophistication needed to

achieve this for gases and liquids, without errors due to water, sand, and wax, is

high, and most methods will need to compromise.

Beg and Toral (1993) reported on the use of pattern recognition techniques to

obtain superficial gas-liquid flow rates. They reckoned that the techniques could be

extended to oil-water-gas flows by using pressure sensing and an orifice plate with

capacitance and gamma ray. They introduced a new hydrodynamic system for scal-

ing pipe diameter and fluid properties and used neural network pattern recognition

techniques (cf. Toral et al.'s 1990 computer package).

The use of mixing to create an almost homogeneous flow also has drawbacks.

The flow downstream of a mixer is extremely disturbed, and such a position would

certainly not be recommended for precise conventional flow measurement. In many

cases,

the flow also starts to separate as soon as it is mixed. The flowmeter would

need to be calibrated in these flow conditions, and such a calibration would need to

take account of the effect of varying component ratios. The possibility of using the

pressure loss across the mixer was noted earlier.

14.5.2 USABLE PHYSICAL EFFECTS FOR DENSITY MEASUREMENT

Because it is unlikely that direct mass flow measurement will be suitable for multi-

phase flows, we need to consider the indirect measurements that could be combined

with a flow rate measurement. These are

• weighing elements;

• vibrating elements;

• acoustic methods (sound speed, acoustic impedance);

• electromagnetic field methods (magnetic resonance, electrical capacitance, static

charge); and

• nuclear radiations (X-ray, microwave, gamma).

It becomes apparent that, for two-component flows, some of these are unsuitable

because, for instance, bubbles and particles do not follow vibrating movements. Op-

tical (transmission problems) and thermal (heat transfer across boundaries) methods

have not been listed because they, also, are unlikely to be suitable (cf. King 1990).

However, Dykesteen et al. (1985) suggested that the measurement of resistance

and capacitance could achieve multicomponent ratio measurement. One manufac-

turer has offered a full-bore device for measuring water in oil from 0 to 80% using

capacitance methods for pipe diameters up to 300 mm and with the possibility of

a ceramic-lined device for pressures above 16 bar. Microwaves are used by another

manufacturer for the measurement of water in oil by absorption. Nucleonic mea-

surement has also been offered commercially as part of a three-phase unit for oil,

water, and gas. Frantzen and Dykesteen (1990) report a multiphase fraction meter

combining capacitance and gamma radiation, which looks very promising. Gains-

ford (1990) described meters to obtain water fraction in oil, as well as oil, water,

and gas fractions. It appears that microwave or gamma radiation were used for the

former and a combination for the latter.

14.5 CHAPTER CONCLUSIONS 369

Work in Shell laboratories (van Santen et al. 1995) also suggested that dual-

energy absorption techniques with gamma- or X-rays may offer an on-line moni-

toring method for individual fractions of oil, water, and gas in pipelines but may

be subject to errors from change in water salinity and also possibly from changes in

distribution in space and time.

14.5.3 SEPARATION

MULTICOMPONENT METERING

The problems relating to the accurate measurement of multicomponent flow are

so great that separation has been the only viable route in most cases. Whether

separation of the components continues to be used will depend on the following

conditions.

a. The components are separable. This may be difficult in truly two-phase fluids

and in fluids where the finished product contains several components such as

liquid foods.

The environment is conducive to separation. Subsea separation may be more

difficult than multicomponent flow metering.

c. The ultimate level of accuracy required can be achieved without separation.

d. A complete system for multicomponent flow, pumps, valves, etc., is available.

Separation of the components and subsequent measurement of each component

may offer the quickest route to an accuracy that approaches fiscal requirements, but

in the longer term competitive methods that do not separate may result from elegant

applications of more advanced physics and clever computational interpretation of

signals.

14.5.4 CALIBRATION

National Engineering Laboratory, Scotland, now has the facilities to test multiphase

flowmeters for hydrocarbon flows (cf. King 1990).

It is tempting to suggest, as for single-component calibration rigs, that the test

section of such a rig should be preceded by a long settling length to obtain fully

developed flow. However, there are good reasons why such an approach is less ap-

propriate for a multicomponent rig.

i. The settling length may be impracticably long.

ii.

The orientation of the pipe will affect the flow, and a long settling length will

prohibit other than horizontal orientations.

iii.

It is virtually certain that actual installation positions will not have fully devel-

oped flow conditions.

iv. The enormous range of flow conditions requires that the flowmeter accuracy is

not dependent on particular flow conditions.

more appropriate design for such a rig is likely to be a test section preceded by a

short upstream section and a manifold for component injection. By selection of the

injection manifold, flow regimes could be constructed upstream of the flowmeter.

The whole unit including manifold, upstream section, and test section would be

short enough to rotate to any orientation so that both horizontal and vertical flows

could be simulated. The outlet flow through a flexible pipe would enter a separator

370 MASS FLOW MEASUREMENT USING MULTIPLE SENSORS

before recirculation. Such a rig may also have a small enough total volume to allow

live crude to be used as one component for oil-related flows.

Other combinations of flow components for other industries may lend them-

selves to other rigs using more conventional calibration methods such as volume

and mass measurements of the total flow passed.

See also Mazzoni et al. (1994) for a description of a multiphase test facility.

14.5.5 ACCURACY

The errors in the papers reviewed on multicomponent/multiphase flows range from

±1 % for an ultrasonic correlation flowmeter in paper pulp flow, for example, to as

much as 25% for one of the offshore oil flow measurement systems. This range of

error is symptomatic of the problem for the flowmeter designer. For control and dis-

tribution, a measurement uncertainty of order ±10% may be acceptable. However,

high accuracy is required for fiscal and custody transfer applications, and here the

level that can be achieved by the designer is far from clear. Is even ±5 % achievable

in multicomponent oil flow measurement?

CHAPTER

Thermal Flowmeters

15.1 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 use-

ful 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 by-

pass 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.

15.2 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. For larger flows, this is used

with a bypass laminar flow element in the main gas stream. In many cases, the

mass flowmeter is in one unit with the control electronics and the gas control valve.

However, in this chapter our interest is in the meter

itself.

15.2.1 DESCRIPTION OF OPERATION

In this design of thermal flowmeter, the gas flows through a very small diameter

tube,

with a sufficient length-to-diameter ratio to ensure laminar flow, and on the

outside of which there are heating and temperature-sensing windings. The heater

winding transfers heat through the wall of the tube to the gas. If there is a gas flow,

the heated gas is carried downstream. The downstream temperature sensor will then

sense a higher temperature than the upstream sensor. This differential can then be

used to deduce the mass flow. A diagram is shown in Figure 15.1

(a).

The equation

371

372

THERMAL FLOWMETERS

HEATER

(a)

(b)

TEMPERATURE^-

SENSORS

HEATERS

(c)

(d)

LAMINAR FLOW

BYPASS ELEMENT

Figure 15.1. Diagram to show the various configura-

tions of CTMFs: (a) Central heater with upstream and

downstream temperature sensors; (b) Three heaters

with sensors to ensure that they remain at constant

temperature; (c) Two combined heaters and resis-

tance thermometers; (d) One downstream heater with

two sensors; (e) Bypass with laminar flow element.

for this is given in the steady state by

= Qh/(Kc

AT) (15.1)

where q

is the mass flow rate to be mea-

sured in kilograms per second, Qh is the

heat input in Joules per second, K is a con-

stant, c

is the specific heat at constant pres-

sure in Joules per kilogram Kelvin, and AT

is the measured temperature difference in

Kelvins. A knowledge of K, Qh, and c

al-

lows q

to be deduced from AT.

In an alternative mode of operation [Fig-

ure 15.1(b)], the sensed temperature is used

to adjust the heat into three heaters to keep

them at a constant temperature. The energy

required in this mode to restore equilibrium

to the windings can be used to obtain the

instantaneous rate of flow. This results in a

sensor with a short response time.

Figure 15.1(c) shows a design that uses

the temperature difference between two

coils positioned along the tube axis, both

of which are heated. The coils also act as re-

sistance thermometers. When flow occurs,

the downstream coil is cooled less than the

upstream coil, and the differential temper-

ature is used as a measure of the flow.

Figure 15.1(d) shows a design with two

temperature-sensing coils and one heater

coil.

In order to accommodate a greater flow

rate than is possible through the capillary, a

laminar flow bypass is used [Figure 15.1(e)].

The total flow rate through capillary and

bypass is then proportional to the flow through the capillary. In some designs, a

control valve is incorporated. In others, an air-actuated valve may also be offered by

the manufacturer.

Figure 15.2 shows an industrial CTMF with bypass through n identical elements,

where n can be selected for the appropriate flow range. It also shows the valve,

operated from a second amplifier, which controls the flow.

In another design [Figure 15.3(a)], the means of heating the tube is through a

current passed along the tube wall, which, in turn, heats the fluid. Two heat sinks,

one at each end, conduct away the heat that has passed up the tube. The fluid cools

the tube, and the resultant temperature difference is measured in two places with

the thermocouples TCi and TC

; the flow rate can be deduced from the difference.

As with the other designs of thermal flowmeter, this one can be used with a by-

pass.

The distributions of temperature for zero flow and for a small flow are shown

in Figure 15.3(b). It can be seen that the temperature difference between the two

15.2 CAPILLARY THERMAL MASS FLOWMETER - GASES

373

SENSOR TUBE

(CAPILLARY}

wwwwwwwwwww

^\\\\\\\\3(\\\\\\\\\\\\\M\v

OUTPUT

EXTERNAL

CONTROL

BYPASS DIVERTS SMALL PERCENTAGE OF GAS FLOW

THROUGH SENSOR (CAPILLARY) TUBE.

Figure 15.2. Industrial capillary thermal mass flowmeter showing schematic of flow paths and

electrical diagram (reproduced with permission of

BOC

Edwards).

thermocouples will be zero at zero flow but will increase with flow. The equation

derived for this flowmeter is reproduced by Hemp (1995a) (cf. Appendix 15.A.4).

All these varieties are dependent on a knowledge of c

, which depends on the

gas and varies with pressure as shown in Table 15.1.

HEAT

SINK

FLOW

TC,

- ELECTRIC •

CURRENT

•e-

(a)

HEAT

SINK

TC,

•\

Figure 15.3. Heated tube thermal flow-

meter (after Hastings): (a) Schematic

diagram; (b) Temperature distribution

under static and flowing conditions.

NO FLOW

TEMPERATURE

HEAT

SINK

(b)

FLOW

DISTANCE

^ ALONG

HEAT

SINK

Table 15.1. Variation of c

with pressure

for four gases

Gas

Air

Approximate

Variation

0-10 bar

+0.1

40-100 bar

+1.6

+16

+18

+31

374 THERMAL FLOWMETERS

It is important for the calibration of the

meter to be close to, or referenced to, oper-

ating conditions; otherwise, it will be nec-

essary to look up the data for the gas and

pressure at operation. Temperature varia-

tion has only a slight effect on the value of

. For example, for air it is 0.01%/°C, and

for methane it is 0.11%/°C. Viscosity vari-

ation has a negligible effect unless the gas

is near liquefaction. Calibration may be ref-

erenced to primary volumetric standards at

atmospheric conditions and corrected normal temperature and pressure or standard

temperature and pressure.

15.2.2 OPERATING RANGES

AND

MATERIALS

FOR

INDUSTRIAL DESIGNS

Rangeability is up to 50:1. Flow ranges are from 0 to 3 ml/min or lower to 0-100

1/min or even up to 1,000 m

/h for special bypass designs.

100-mm diameter meter

would typically give a range up to about 250 m

/h and a 200-mm diameter meter up

to about 1,000 m

/h. Note that, for air at ambient conditions, 1,000 m

has a mass

of about 1.2 kg.

These meters may be able to operate for some designs up to about 200 bar or

even, in some cases, 300 bar pressure. Temperature operating range may be 0-65°C

with a coefficient of less than 0.1%/°C. Pressure drop on the line is up to 0.02 bar

for the maximum flow rate.

Materials in contact with the gas include 316, 316L, and high alloy ferritic

stainless steel; anodized aluminum, brass, and Viton; Buna-N; Kalrez; and Teflon

or silicone O-rings.

Power supply requirements usually relate to 15 and 24 V DC versions; when

combined with a controller, these may draw up to about 180 mA resulting in power

consumption of order 3.5 W.

Flowmeters are likely to be factory-set to a preselected gas, pressure, and flow

range. Correction factors range from about 0.55 for ethane to about 1.40 for argon

against air. These may also be referred to nitrogen.

15.2.3 ACCURACY

Uncertainty is typically claimed as ±1% full-scale deflection or FSD (or for a recent

design ±0.75% rate ±0.25% FSD) but may be ±1.5% for the top end of the flow

range and ±4% FSD in some other cases, depending on design and the temperature

and pressure range. Repeatability is typically 0.2-0.5%

FSD,

but in some cases it may

be quoted as being of order ±0.25% of rate and in at least one case as 0.05%

FSD.

addition, there may be pressure and temperature coefficients.

15.2.4 RESPONSE TIME

An exponential curve is usually used to indicate the response of a flowmeter. The

exponential curve is given by

5 = 5

(l -

t/T

)

15.2 CAPILLARY THERMAL MASS FLOWMETER - GASES 375

To Approach Within

0.5%

Response Time

4.6r

5.3r

The time constant x gives the time for £

'^ " ' "

i:i

' ^ -^ >,/ ,^ ,, ,^,^,

«.*

the flow signal 5 to reach about 60% of its I Table 15.2. Time to approach the final

final value S

. Table 15.2 gives the times to i

value after a flow

change

approach the final value.

is of order 250 ms to over 1 s. A stan-

dard instrument may take 1-5

to settle

to within 2% of final value for a 0-100%

command change. However, manufacturers

may claim a response of about

s for some . .

-<-.<, ,

designs.

Thermal drift may be reduced against ambient temperature changes by insu-

lating the unit, but with the consequence of a longer time constant (Sullivan

no date). A solution is to enclose the sensor in a temperature-controlled envi-

ronment. Sensors may also suffer from radiofrequency interference and may be

incompatible with corrosive gases. Typically, a 30-min warm-up time may be

needed.

15.2.5 INSTALLATION

There is unlikely to be any effect from upstream pipework for the basic device with-

out bypass because the bore of the tube is so small that the profile will be readjusted

before reaching the elements. For bypass designs, there may be an effect, and it

may be necessary to include 20D upstream straight pipe (Gray et al. 1991). Designs

should ensure that flow is truly laminar in the bypass, by suitable matching of size

and pressure drop.

There may be significant effects on sensitivity that can seriously impair accu-

racy under certain conditions of installation (e.g., mounting angle), gas density, and

pressure when outside normal operating envelopes. Zero offset appears to be affected

by meter orientation and may result from free convection when the meter is near

vertical. Rarefied gas flows may also create problems.

Temperature effects on meter zero are up to 0.15% FSD/°C and on span are less

than ±1% FSD over a 10-50°C range. Pressure sensitivity is of order ±0.4% per bar

up to about 14 bar. Power supply sensitivity is of order

±0.1%

FSD

per percent power

supply voltage variation.

Typical weight is of order 0.5-2.5 kg. For bypass designs, the weight may be

up to 20 kg. Meters should have been subjected to burn-in and cycling prior to

shipment.

Gray et al. (1991) discussed environmental factors that affect mass flow con-

trollers (MFC). The MFC is tested with a vacuum diagnostic system to which the

MFC is connected. The test chamber is evacuated to below 50 mtorr, and the valve

to the vacuum pump is then shut. The initial flow rate is set, and the pressure is mea-

sured as it

rises.

From this, the time to achieve controlled flow, actual flow rate, error,

and deviations from flow are obtained. They tested various effects and concluded

that, to minimize effects, temperature extremes must be avoided and surroundings

should not be allowed to raise the temperature of the gas boxes, pressure should

be carefully regulated, and vibration should be avoided. Where possible, tubing dis-

tances should be minimized, and MFCs should be mounted in a normal upright

orientation.

376 THERMAL FLOWMETERS

15.2.6 APPLICATIONS

CTMFs are most commonly applied to low flows of clean dry gases above their dew-

points (e.g., gas blending, semiconductor industry). Gases quoted by manufacturers

include air, acetylene, ammonia, argon, arsine, nitrogen, butane, carbon dioxide,

carbon monoxide, chlorine, ethane, ethylene, fluorine, Freon 11, Freon 12, Freon

13,

helium, hydrogen, hydrogen sulfide, krypton, methane, neon, nitrous oxide,

oxygen, propane, propylene, silane, and xenon. Examples of problem gases that

have been used with this type of meter include

• BCI3 (boron trichloride) corrosive gas;

• SiCU (silicon tetrachloride) corrosive and low vapor pressure gas;

• CCI4 (carbon tetrachloride) low vapor pressure gas;

• SiH

(silane) typical of gases that react and create deposits, resulting in clogging

of flow controllers; and

• WF

(tungsten hexafluoride) similar to the handling problems of silane.

The meter may be used with a wide range of gases for processes involving crystal

growth, thermal oxide, diffusion, chemical vapor deposition, sputtering, ion im-

plantation, and plasma etching, and the gases involved will each require a correc-

tion factor against the calibration gas, possibly nitrogen, which may vary between

as low as 0.17 for Freon-C318 (C

) or perfluoropropane, and as high as 1.543 for

krypton. The manufacturer should be asked for the compatibility of any particular

gas and the correction factor (Sullivan no date). In selecting a flowmeter, particular

care should be taken where the inlet pressure is less than atmospheric or has a vapor

pressure less than atmospheric.

15.3 CALIBRATION OF VERY LOW FLOW RATES

One method of calibration of these flowmeters for low flow rates is by means of a

special piston proven A piston moves within a precision glass tube. The piston is

connected to a counterweight by means of a tape that turns an encoder wheel. The

temperature and pressure of the gas are measured while flow is taking place.

Positive displacement devices or piston meters may achieve calibration to an

accuracy of about 0.2% of reading over a range from 0.02 to 30 l/min (Sullivan et al.

no date). The volume flow at standard temperature and pressure (STP), subscript s,

may be obtained from the actual, subscript a, using

Zpa_

15.4 THERMAL MASS FLOWMETER - LIQUIDS

15.4.1 OPERATION

The diagram in Figure 15.4 shows the layout of a meter with parallel tubes running

between a heating block and a heat-sink block. At the midpoint of each tube is the