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

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3.A.1 SPECIFICATION QUESTIONNAIRE 57

Response

C4 Other features if known

C4.1 Abrasiveness

C4.2 Opacity

C4.3 Compressibility

C4.4 Flammability and other safety considerations

D Site details (a site plan should be attached if possible)

Dl Flow range: maximum

minimum

normal flow

likelihood and size of over-range flows

D2 Pipework (a diagram should be attached if possible

extending to at least 50D upstream, particularly in cases

where there are fittings that create swirl)

D2.1 Outside diameter

D2.2 Inside diameter

D2.3 Material of pipe and pipe lining if present

D2.4 Flange size and details or other connection method

D2.5 Nearest upstream pipe fitting

D2.6 Distance to upstream fitting

D2.7 Distance to nearest downstream fitting

D2.8 Orientation of pipe run

D3 Environment, etc.

D3.1 Hazards including required safety and explosion-proof

classification

D3.2 Access, size, and weight limitations including

vehicle access, machinery transport, and shelter

D3.3 Flow source (gravity, pumped, etc.)

D3.4 Flow stability

D3.5 Mounting requirements (including maximum weight)

D3.6 Ambient working pressure: maximum

minimum

normal

D3.7 Fluid working pressure: maximum

minimum

normal

D3.8 Ambient working temperature: maximum

minimum

normal

D3.9 Fluid working temperature: maximum

minimum

normal

D3.10 Maximum acceptable pressure drop

D3.ll Electromagnetic and radio frequency interference at site

E Electrical and control details

El Overall control system in use with communication protocol

details

E2 Power supply availability and variability

58 SPECIFICATION, SELECTION, AND AUDIT

Response

E3 Preferred signal transmission if not defined previously:

0-20 mA

4-20 mA

0-1 kHz

0-10 kHz

Pulse output: frequency range

pulse size

Digital

Other

E4 Failure system:

fail soft one operating and one stand-by channel

fail passive two operating channels

fail operational three operating channels and one

decision element

fail safe output goes to safe condition

E5 Interelement protection (IP) given by IP XY

X has range 0-6 for protection against access of foreign

bodies (Endress et al. 1989)

Y has range 0-8 for protection against ingress of water

(Endress et al. 1989)

F Other user information

Fl Accuracy: uncertainty

repeatability

F2 Response time

F3 Preferred construction materials

F4 Existing instrument maintenance schedules

F5 Line cleaning arrangements

G Manufacturer's proposed meter and requirements

Gl Manufacturer's proposed meter specification

G2 Special requirements resulting from this proposal (e.g.,

filtration, power supply, environment)

G3 Calibration details, rig conditions, and results

G4 Warranty period and conditions

G5 Delivery

G6 Price

H Any other considerations and conclusions

HI Considerations and constraints by the manufacturer not

covered elsewhere

H2 Agreement of the user to the proposed meter

H3 Conclusion and decision

3.A.2 SUPPLEMENTARY AUDIT QUESTIONNAIRE

This questionnaire should be read in conjunction with the specification question-

naire in Appendix 3.A.I.

3.A.2 SUPPLEMENTARY AUDIT QUESTIONNAIRE 59

Answer

A Information about the manufacturer

Not relevant

B User's business and specific application

If the auditor is not part of the user's company, then

this section may usefully be completed.

C Fluid details

Is the meter of suitable design, construction, and

materials for this fluid?

D Site details (Is the site plan correct?)

On the plan, mark power and signal cables and the position of

the main meter electronics.

What is the meter power consumption?

What is the flow range?

Are these within the meter specification?

Pipework

Is the site of the meter satisfactory? If not, should

meter be moved?

Environment, etc.

Is the meter suitable for this environment?

Is meter design suitable for these hazards?

Is the meter capable of operating within these

parameters?

Is access sufficient for proper maintenance?

E Electrical and control details

Do power, control, and signal systems match requirements

for operating and protection?

Is there power and system access for audit and

maintenance work?

What is the signal level?

Is the meter permanently energized and if not what is

the warm-up procedure?

What is the control system for the meter?

Are there special: alarms?

low flow cutoffs?

F Other user information

Has the meter accuracy been checked? If so, what

is the uncertainty? repeatability?

What are the materials of the meter? Do they match preferred

construction materials?

Have instrument maintenance schedules been adhered to?

If so, are there any apparent operational changes or

peculiarities that may have caused problems etc.?

Has the meter reading changed unexpectedly?

What are the line-cleaning arrangements?

60 SPECIFICATION, SELECTION, AND AUDIT

Answer

G Manufacturer's information and requirements

Meter specification:

Manufacturer?

Type?

Model?

Serial number?

Specification?

Where specification is unavailable, obtain key details

as far as possible.

Expected accuracy?

Construction date?

Installation date?

Subsequent checking/calibration/maintenance?

Is documentation available? If so, list.

Are all special requirements met (e.g., filtration, power

supply, environment)?

Has the meter met the expected accuracy? If not, was it

unrealistic/unnecessarily stringent?

H Any other considerations and conclusions

Actions resulting from this audit?

CHAPTER

Calibration

4.1 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.

The result of a calibration will be both a comparison with the national standard

and also a range within which the reading is likely to lie.

4.1.1 CALIBRATION CONSIDERATIONS

Figure 4.1 shows three different approaches to obtaining the data. Although the

selection of calibration points and their optimum distribution over the range is not

obvious, most of the reference books seem to omit any comment on this. Figure

4.1 (a) records data at one flow rate. This might be appropriate if the flowmeter were

expected to operate at one rate. It shows the fact that there is variation with time to

which the repeatability refers. Such a plot might be used to compare the calibration

facility itself with other facilities or to test the consistency of diverter operation.

Figure 4.1(b) is probably the most usual plot. Here a number of points have been

taken over the full range of the flowmeter. Figure 4.1 (c) shows

characteristic that has

been investigated in four positions, and three readings have been obtained in each.

The number of points will depend on the confidence of the user in the calibration

facility, the quality and linearity of the flowmeter, and the statistical requirements for

62 CALIBRATION

(a)

ERROR IN

MEASURED

QUANTITY

SET

•FLOW

RATE

(b)

TIME AFTER

-H

1 1 1 1 1 1 1 1 1 1 1 1 1 START OF

START CALIBRATION

TIME

ERROR IN

MEASURED

QUANTITY

x x , FLOW

~~~ RATE

10%

ERROR IN

MEASURED

QUANTITY

10%

50%

100%

FLOW

RATE

(c)

Figure

4.1.

Diagram to show different calibration plots, (a) Measurements at a constant flow rate

taken over a short period of time, (b) Measurements of the full characteristic of the flowmeter.

the required confidence level. Thus a manufacturer may have sufficient confidence

in the quality of manufacture of its products to use, say, just two calibration points,

and three readings might be taken for each to guard against one spurious reading. It

might then be justified in claiming a ±1% envelope for its product. This is unlikely

to be a nationally accredited calibration. On the other hand, 20 points or more along

a complete characteristic of adequate linearity may be needed to ensure, for a high

4.1 INTRODUCTION 63

quality meter, an acceptable value of uncertainty. At any point on the characteristic,

there will need to be sufficient neighboring points to achieve the required confidence

level. (See Section 1.A.4 where there is such a calculation.)

Ideally the flowmeter would be calibrated under conditions identical with service

ones.

In practice, in situ calibration is not very accurate, and a calibration facility

will provide the reference conditions for an instrument given that installation and

other influence conditions will increase the uncertainty. We shall therefore wish to

consider the following factors.

Fluid: If the service fluid is water, then we shall have few problems, although

it will still be necessary at least to check for gas in solution, for purity, and for

temperature. However, if the fluid is anything else, there will be a range of decisions.

If we cannot match the fluid perfectly, we shall add to the final uncertainty of the

calibration because the effect of, say, calibrating a meter for oxygen service on air will

require a calibration adjustment. This adjustment will,

itself,

have an uncertainty

in it.

Flow Range: It is obviously preferable if this is identical to that for the service.

However, if the fluid conditions are not identical, then there may be a compromise

between velocity through meter, mass/volumetric flow through meter, and Reynolds

number. If the fluid for calibration differs from the fluid for service, the differing

viscosity may cause the Reynolds numbers not to match. The importance of this is

that the Reynolds number defines the shape of the flow profile and the turbulence

spectrum (if the pipe is smooth). The profile shape (and the turbulence), as we noted

in Chapter 2 and as we shall see for many of the flowmeters in this book, can

affect performance. The viscosity may also change the nature of the flow over the

internals of flowmeters, which in turn may alter their performance. Even if the

fluid is the same, the temperature may cause changes in viscosity, which may be

significant. There are also, of course, flow rate limitations due to the calibration

facility.

Pipe Size and Configuration: The service pipe may not be of the same size as

that on the calibration facility. Many flowmeters are sensitive to step changes in

pipe diameter upstream of the inlet. It may therefore be necessary to calibrate the

flowmeter with the upstream service pipework in place. It may be wise to spigot the

flowmeter to the pipe, so that if they are separated, they can be reunited in precisely

the same way as when they were calibrated as a unit. In cases where there are major

changes in the diameter, or fittings causing disturbance, and it is impractical to

install enough upstream pipe in the actual facility, it may be necessary to use a flow

conditioner package upstream, and the calibration, if possible, should be done with

the complete conditioner package and flowmeter bolted together in the calibration

facility. These requirements will be discussed in relation to the various flowmeter

designs.

In addition, the upstream straight length of pipe should be sufficient to create a

fully developed flow profile. This length is usually taken as of order 60D, although

some suggestions have been made that, even after 70D, the flow has not settled, and

it is known that swirl is very persistent.

Pipe Material and

Finish:

One aspect of

this,

which may be important (although

possibly of secondary effect in many applications), is the roughness of the inside

surface - the pipe should be smooth to ensure that the flow profile is predictable.

64 CALIBRATION

Roughness may be defined as the arithmetic mean deviation of surface contour from

a mean line based on the minimum sum of the square of the deviation. The maxi-

mum permitted roughness will probably be in the range 10~

-10~

D. In some cases,

there may be questions of compatibility with the service conditions. For instance,

the electromagnetic flowmeter can be sensitive to surrounding pipework if it has

special electrical or magnetic properties. Also if the flowmeter is transferred with a

length of pipe, this may result in incompatibility with the flow calibration facility.

Steady Flow: Most flowmeters are affected by unsteady conditions. There are

several reasons for this; they are set out in Section 2.6. However, the calibration

facility should be designed to ensure that flow is steady.

Environmental Conditions: Even though ideally the flowmeter should be un-

affected by its environment, in practice this is unlikely to be the case. It is important

that any effects are controlled and recorded. Where the service is very likely to be

subject to external effects, temperature and pressure variation, humidity, immersion

in water, vibration, or hazardous environment, the selection of the meter prior to

calibration will need to allow for these. If the meter is in a vulnerable position, there

is always the possibility of accidental damage. And, of

course,

Joe Bloggs, who always

taps his spanner on any convenient piece of pipe as he walks through the plant, may

find a tender part of the flowmeter!

Experience of instrument performance evaluation (Cornish 1994-5) (reference

condition tests and environmental tests) yielded a table of instruments failing to

meet performance specification. Out of 162 evaluated, 51% failed manufacturers'

specification in 1989-93. Manufacturers' instructions may, in some cases, also be

wanting. On the other hand, I have heard it suggested that a high proportion of

flowmeters (possibly up to 70% in the water industry) may fail to meet manufactur-

ers'

recommended installation requirements.

It should be remembered that, having taken great trouble to calibrate the flowme-

ter in the correct flow conditions, the actual service installation will probably have

many of the shortcomings that we have been at pains to eliminate from the calibra-

tion facility and will, therefore, raise questions about the value of the calibration. In

subsequent chapters, we shall review much of the available advice in these circum-

stances.

4.1.2 TYPICAL CALIBRATION LABORATORY FACILITIES

have found that, among the many papers to which reference is made in this chapter,

Mattingly (1982) and Pursley (1986), although a bit dated, still provide excellent

reviews.

At the top of the range of facilities are national rigs such as those at the National

Engineering Laboratory in Scotland or those listed by Mattingly (1982) at the

National Bureau of Standards (now NIST, the U.S. National Institute of Standards

and Technology):

High water flow 625 kg/s max

±0.13%

Static-weighing

Low water flow 2.5 kg/s max

±0.13%

Dynamic-weighing

High liquid 100 kg/s max

±0.13%

Static-weighing

hydrocarbon flow

4.1 INTRODUCTION 65

Low liquid 10 kg/s max

±0.13%

Dynamic-weighing

hydrocarbon flow

High air flow 4980 m

/h ±0.25% Constant volume tank

and critical nozzles

Low air flow 90 m

/h ±0.25% Bell provers, mercury-sealed

piston devices and critical

nozzles

In a recent draft report by Mattingly, the NIST appears now to have uncertainties of

±0.12%

for the liquid flow facilities and ±0.22% for the dry air flow facility.

The reader is also referred to King (1992 cf. King 1991), who described calibration

facilities at the National Engineering Laboratory, Scotland, in addition to the main

NEL gas and water facilities. These further facilities were for variable viscosity oils,

user-defined oils, and two-component and multiphase fluids. These are in addition

to the standard-type facilities for air, water, and oil. The laboratory includes:

• three gravimetric primary standards for

1-1001/s

for kerosene, gas oil, and 15-cSt

oil to 0.05% of volume at 95% confidence level;

• one high flow secondary for 1-200 1/s for any of the three oils;

• two small gravimetric primary standards for user-defined oils 0.1-10 1/s and

0.01-2.5 1/s to 0.05%;

• a water-in-oil facility of 1-50 1/s for flow studies; and

• a multiphase secondary standard for oil and water with air or nitrogen in hori-

zontal or vertical lines.

In connection with the measurement of multiphase flow, King suggested that un-

certainties of 5% of volumetric flow are likely.

From these examples, the reader can get a good idea of the scope of facilities

available in leading laboratories that specialize in flow calibration. It is also likely

that the uncertainty claims will have improved with time and experience for these

facilities.

4.1.3 CALIBRATION FROM THE MANUFACTURER'S VIEWPOINT

Weager (1993/4), who approached the subject from the standpoint of a manufac-

turer, described aspects of development of a NAMAS (National Measurement Ac-

creditation Service)-approved flow facility. He commented that "the most used and

abused term in any brochure for a measuring instrument is 'accuracy', and even the

phrase 'typical accuracy' has been employed/' Standards committees continue to

address the specification of flowmeters (cf. list of standards). Western European Cali-

bration Cooperation (WECC) document 19 also deals with uncertainty. Figure 4.2

shows the typical quality of a NAMAS-approved laboratory, with uncertainty in the

range ±0.1-0.2%. The quantity passed was derived as in Figure 4.3. The national

authority should ensure that round-robin tests of a transfer standard package are

regularly organized between commercial laboratories and national and international

standard laboratories to ensure that no undiagnosed changes have occurred. It is es-

sential that the national laboratories are as aware of their own unexpected changes

as are other laboratories and are open about the way they address these problems.

CALIBRATION

SUMMARY OF NAMAS ACCREDITATION

Test

Rig

1OOkg

1200kg

Stonnes

46tonnes

Accredited flow range

l/s

0.5

2.5

5-25

6-200

200-1200

Measurement Uncertainty

for Flowrate

+/- 0.1%

+/- 0.2%

+/- 0.1%

+/-0.1%

Measurement Uncertainty

for Quantity passed

+/- 0.1%

Note:-

Measurement Uncertainty for Flowrate above

derived from inierlaboratory comparisons over the whole flowrange,

with the National Engineering Laboratory and

not

the

value

calculated in the table headed

"Calculation of Uncertainty Budgets"

Figure

4.2.

Summary of NAMAS accreditation (reproduced with permission of Danfoss Flowme-

tering Ltd.).

Clay et al. (1981) described just such intercomparison tests between three U.K. lab-

oratories using orifice and turbine meters and disclosed a small,

0.1%,

bias in one of

the laboratories.

Figure 4.3 is an example of the derivation of uncertainty in an actual commercial

laboratory. The traceability for a particular laboratory is shown in Figure 4.4 for a

water facility. In addition, flow rate uncertainty is assessed by laboratory intercom-

parison (with the National Engineering Laboratory) using a transfer standard and

hence providing a further traceability.

The reality is that most flowmeters are sold with a production calibration and

without a fully traceable calibration certificate. The precision claimed for some

flowmeters is in conflict with the reality of the precision of the calibration facil-

ity. A flow rig with ±0.2% uncertainty cannot be used to credit a flowmeter with a

lower uncertainty of, say, ±0.15%. It would be useful if manufacturers justified, on

the basis of a calibration facility accuracy, their uncertainty values.

4.2 APPROACHES TO CALIBRATION

There are several approaches to calibration, and in the remainder of this chapter

they are reviewed in turn.

Dedicated Flow Calibration Facility: This is at present the surest way to achieve

a high accuracy. The facility will usually be traceable back to fundamental measure-

ments of time and mass.