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

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9.3 CALIBRATION, ENVIRONMENTAL COMPENSATION, AND OTHER FACTORS 197

e. The low flow rate and rotation may markedly alter the liquid flows on sliding

and stationary members. The bearing and clearance drag may be changed.

Back Slip Due to System Effects

f. The quantity of liquid in the pipework may not be precisely the same before

and after the slip test. For instance, 20 liters of liquid in a slip test fill 2.6 m of a

100-mm diameter pipe.

g. In tests using manual and automatic valves, one may be closed at the start of

the test and both closed at the end, slightly altering the pipe volume.

h. Upstream valve control may cause pressure changes leading to volume changes,

i. Gas may come out of solution during the test.

Back Slip Due to Calibration Vessel

The measurement uncertainty oi the vessel is not better than about ±0.2% for

so short a run.

It appears that various effects occasionally may come together to cause the observa-

tion of back slip.

The observations of back slip appear likely to result from uncertainties in opera-

tion of the meter at low rotation. Variation during rotation can become important if

flows of about 15 liters are involved because this may result, for a 100-mm meter, in

less than two rotations. Thus the nonuniform calibrator transmission or variation in

vane clearance may occasionally result in an increased recorded rotation for a given

volume passed.

9.3.5 THE EFFECTS OF TEMPERATURE AND PRESSURE CHANGES

The effects of temperature and pressure variation are given in Equations (9.A.9) to

(9.A.18), using values in Figure 9.13. These values are, therefore, for a moving vane

type meter but will give some indication of the effect on others. Making use of values

of a

of 0.23 x 10~

for aluminum, 0.11 x 10~

for mild steel and cast iron, and of

Git

of 9 x 10~

for oil, Table 9.1 gives values for temperature changes of 10°C and

pressure changes of 1 bar taking E for mild steel as 207 GN/m

. One manufacturer

gives the effect of changes in pressure and temperature on the meter excluding the

effect of liquid changes, and this is reproduced in Figure 9.15.

9.3.6 THE EFFECTS OF GAS IN SOLUTION

Another effect not considered so far will be the presence of dissolved gas in solution.

The dissolved gas may cause no change in liquid volume, although it may have a

large volume in gaseous form. The mass of gas involved will often be small enough

to neglect.

However, under sudden pressure reduction, the gas may come out of solution.

This might occur during a slippage test, if the test is controlled on a valve upstream

of the meter, and might cause the rotor to turn due to the presence of the gas alone.

The problem can also arise when metering liquified petroleum gas (LPG) if the

back pressure on the meter is insufficient.

198

POSITIVE DISPLACEMENT FLOWMETERS

Table 9.1. The effects of temperature and

pressure changes

Description

Metal/

Liquid

For temperature change

Flowmeter

expansion

Differential

expansion

Liquid-density

change

M.S.

Al/M.S.

Oil

For pressure change of 1

Flowmeter

expansion

Leakage gap

radial

Leakage gap

axial

M.S.

(°/o)

of IOC

0.07

0.03

—

0.9

bar

0.003

—

At/t(%) '

— '*

18 ?

19 I

— :

1.2 f

0.4 ?

9.4 ACCURACY AND CALIBRATION

percentage

change in

indication

0.02

0.01 -

Accuracy achievable from these meters will be highly dependent on the particular

design. Values have been suggested earlier. At the top end, the sliding vane meters

can achieve an accuracy that may be limited by the means of calibration. It is highly

likely that precision devices such as the oval gear meter will approach the level of

performance of the sliding vane meters.

The calculations of the effect of the pressure and temperature in Table 9.1 indi-

cate that close tolerance control is necessary to achieve the required performance.

However, it should be remembered that, although liquid density may change the

reading of mass flow measurement, the requirement in many cases is for volume

measurement.

The effect of changes in Ap and \i are

indicated by Equation (9.4), and this clearly

illustrates the errors due to change in clear-

ances,

pressure drop, or viscosity. Wear in a

meter will cause t and Ap changes, whereas

temperature variation will cause changes

in \i.

Apart from the published values already

mentioned, Hayward (1977b) developed a

technique for measuring the repeatabil-

ity of flowmeters, which involved compar-

ing readings from two identical flowmeters

in series. Tests at National Engineering

temperature

pressure

°C

bar

Figure 9.15. Sensitivity of a sliding vane meter to

pressure and temperature change excluding the ef-

fect of liquid changes (based on Avery Hardoll data).

9.5 PRINCIPAL DESIGNS OF GAS METERS 199

Laboratory, Scotland, suggested that repeatabilities better than 10 ppm (0.001%)

could be obtained with vane-type flowmeters provided that the drift in readings,

which seemed to arise in the shaft encoder, could be eliminated.

Hayward (1979) commented that the rangeability was about

20:1,

linearity was

±0.05%,

and accuracy when newly calibrated was ±0.2% of volume over the range.

He suggested that these meters perform best in the maximum flow rate range of

0.002-0.05 m

/s.

Barnes (1982) underlined the superior accuracy, repeatability (within 0.05%),

and reliability of positive displacement flowmeters but drew attention to inaccuracies

that arose if the liquid contained free or entrained

gases.

Data from fuel oil, gasoline,

and propane were presented to show that the accuracy was highest when the meters

were used to measure flow at about 25% of the rated flow capacity. At both higher

and lower flows, increased slippage occurs and causes the meter to underregister.

The meters were also relatively insensitive to changes in viscosity. Barnes suggested

that if a meter initially calibrated for a liquid of

cP was used with a liquid of 100 cP,

the resulting shift in accuracy was of order 1.2%.

Reitz (1979) quoted the following figures: repeatability within 0.02%, accuracy

±0.25%

over a

20:1

flow range, and pressure loss 3 psi (0.2 bar) for liquids of viscosity

1 cP. He suggested that fluids with viscosities in the range 0.1 cP (gas) to 10

(liquid) could be metered in this way. He noted that errors arise from excessive wear,

which increases slippage and causes underregistration, and entrained gas bubbles,

which occupy part of the measuring chamber and cause the meter to give too high

a reading.

Kent Meters (Scanes 1974) quoted an accuracy of

±0.5%

for a domestic oil meter

designed for rates as low as 0.1-101/h. The temperature range was quoted as -10°C

to 35°C, although temperatures as high as 90°C had apparently been recorded at

meter boxes sited in direct sunlight.

One manufacturer gives meter calibration curves for the company's range of

meters (Figure 9.16) and claims linearity in the range 0.1-0.3% with repeatability

for all but one of their product sizes in the 0.01-0.02% range.

9.5 PRINCIPAL DESIGNS OF GAS METERS

indicated in Section

9.1,

the wet gas meter and the diaphragm meter have been in

use for over 100

years,

and both still provide service for many applications. However,

the predominant use of the wet gas meter has changed to that of a secondary calibra-

tion standard, whereas mechanical meters offer an important option for commercial

gas metering.

9.5.1 WET GAS METER

Figure 9.17 shows the wet gas meter, which uses a water (or other suitable liquid)

bath as the gas seal to create closed compartments for the transfer of the gas. It is

a high precision meter capable of an operating range of about 10:1, but it is rather

bulky. It was originally developed for the measurement of gas usage in industrial

premises but recently has found a niche as a transfer standard for gas measurement.

200

POSITIVE DISPLACEMENT FLOWMETERS

100%

URV

transmitter only horizontal

Figure 9.16. Typical calibration curves for a sliding vane PD meter (based on

Avery Hardoll data): (a) for various models; (b) for various registration loads

on the meter shaft output.

It needs careful setting up to achieve the highest accuracy. Dijstelbergen (1982) made

the following points.

• With careful use, the calibration curve is stable within

0.1%.

• The use of water has been replaced by Caltex Almag to avoid the problem of gas

take-up of the water.

• The meter can be used as a secondary standard at atmospheric conditions.

Gas out

Emptying

Gas inlet

About to empty

Sightglass

(meter

must be

levelled)

Rotation

\-\— /— —/~

Figure 9.17. Wet gas meter.

Filling

9.5.2 DIAPHRAGM METER

Figures 9.18(a, b) show a diagram of the di-

aphragm meter. It is essentially a piston me-

ter in which the compartments

B, C, and

D form the cylinders and the bellows pro-

vide a common piston for

and

and sim-

ilarly for C and D. The flow into and out of

the compartments is controlled by the slid-

ing valve. In Figure 9.18(a), A has emptied

and

has filled, and the valve is moving to

allow A to fill and

to empty. At the same

time C is filling from the inlet chamber due

to the valve's position, and D is emptying

into the outlet manifold. In Figure 9.18(b),

gas is entering the inlet chamber above the

valves and is routed into compartment A,

while gas leaves compartment B. At the

9.5 PRINCIPAL DESIGNS OF GAS METERS

201

Sliding

* valve

(b)

Error

100% URV

(c)

Ap 2p

mbar

(d)

100% URV

Figure 9.18. Diaphragm meter, (a) A, empty; B, full; C, filling; D, emptying.

(b) A, filling;

emptying; C, full;

empty, (c) Typical error curve (reproduced

with permission of GMT and IMAC Systems Ltd.). (d) Typical pressure loss

curve (reproduced with permission of GMT and IMAC Systems Ltd.).

same time compartments C and D have reached the change-over position so that C

has filled and D has emptied, and the valve is changing over to allow C to empty

and D to fill. Dijstelbergen (1982) made the following points.

• If the bellows are made either of a special type of leather or of fiber-reinforced

rubber, they may dry out with low water content gases, causing overregistration.

• Most meters are manufactured in accordance with OIML recommendation (Or-

ganisation Internationale de Metrologie Legale) and should operate within 2%

at high flows and 3% at low flows.

• Rangeability may be up to

150:1.

202

POSITIVE DISPLACEMENT FLOWMETERS

The typical measurement uncertainty is ±1% [Figure 19.18(c)]. The maximum

allowable inlet pressure on some designs is about 0.1 bar above atmospheric, but

it may be up to 1 bar on other special versions. The coupling from the mechanical

meter to the output may consist of a magnetic coupling to the display. Operating

temperature range is typically -20 to +60°C. The meters operate to DIN 3374 and

4161.

The pressure loss through the meter is about 1.5 mbar [Figure 19.18(d)].

9.5.3 ROTARY POSITIVE DISPLACEMENT GAS METER

The rotary positive displacement gas meter is, sometimes, referred to as a Roots

meter, but this name, in a particular style, is the registered trademark of a particular

manufacturer's design (Dresser). Figure 9.19(a) shows a diagram of another design of

this meter. Two rotors mesh and rotate within an oval body contour so that leakage

is at a minimum between the rotors, but the gas is transferred at the outer point of

the oval. In recent years they have performed to an increasingly high specification

and offer one of the best options for high precision gas flow measurement. There

is a possible problem due to pulsating flow resulting from the use of these meters

(Dijstelbergen 1982), and rapidly changing flow rates may damage them or down-

grade their accuracy. The two rotors have a very small gap between (described by

one manufacturer as less than a human hair), and the manufacture is to tight toler-

ances on impellers and meter body. It is, therefore, necessary for the gas to be free

of particulate matter and for the meter to be mounted so that the tight tolerances

are not distorted. The gas flow can be distributed to give an even distribution over

the full length of the rotor entry.

Maximum flows are from 28 m

/h at 0.5 in. (13 mm) water gauge to 360 m

/h,

with a pressure loss of 0.5 to

in. (13-25 mm) of water gauge. Figure 9.19(b) shows a

typical performance characteristic with a performance well within 0.5% over a 10:1

turndown. In fact, the claims are that 80:1 may be achievable. Temperature range is

from -40 to 65°C.

Transmission can be through timing gears on the end of the shafts, with direct

counter driven or with magnetic couplings.

The rotors may be hollow extruded aluminum with self-cleaning tips and a body

of cast aluminum. The surfaces may be anodized.

(a)

% Error

-2

-3

(b)

20 30 40 50 60 70 80 90 100 110 120%

URV

Figure 9.19. Rotary positive displacement gas meter (reproduced with permission of Romet Ltd.

and IMAC Systems

Ltd.):

(a) Diagram of meter; (b) Plot of the performance characteristic for the

meter.

9.6 POSITIVE DISPLACEMENT METERS FOR MULTIPHASE FLOWS 203

The CVM meter

of similar design to Figure 9.3(b) with a main rotor that consists

of four vanes or moving walls that form metering compartments. The second rotor

is a sealing rotor that returns the vanes to the inlet side of the meter. The second

rotor may be two- or three-lobed, and its rotation will be precisely linked to that

of the main rotor. According to Dijstelbergen (1982), it is suitable for high pressure

metering, does not cause pulsations in the flow, handles flows up to 1,200 m

/h, and

a repeatability of 0.05% may be achievable.

9.6 POSITIVE DISPLACEMENT METERS

FOR MULTIPHASE FLOWS

In a patent (1981), Arnold and Pitts proposed temperature and pressure sensing

and a gamma-ray densitometer for sensing density inside a positive displacement

flowmeter (Figure 9.20), but they gave no experimental data. However, the patent

presents an attractive device, which has the attribute of forcing the flow into a

closed volumetric space regardless of the flow components.

major disadvantage is

the possibility of solids causing it to seize up and block the line, requiring designs

to have large clearances and consequent loss of precision.

An alternative approach has been taken by Gold et al. (1991) who used a he-

lical rotor meter (Figure 9.21). In this design, two rotor sections are used in se-

ries,

and between them is a section where the condition of the multiphase flow is

measured.

Gold et al. (1991) described their solution, which controls the flow being mea-

sured and so avoids the problem of slip between phases. The helical rotors are also

claimed to be more resilient, with suitable bearings, to the rigors of multicomponent

flow, reducing the possibility of damage due to transients. Between the two rotors,

Temperature

and pressure Radiation source and

sensing sensor region

Radiation

transmission

region

Sliding

vanes

Figure 9.20. Diagram based on Arnold and Pitts' patent (1981) (reproduced from

Baker 1991a; with permission of the Council of the Institution of Mechanical

Enigneers).

204 POSITIVE DISPLACEMENT FLOWMETERS

PRESSURE TAPPINGS

HOSE SHOE BAFFLE PLATE

TWIN HELICAL

SCREWS

GAMMA BEAM WINDOW

GAMMA RADIATION

DETECTOR

Figure 9.21. Helical multiphase meter (Gold et al.

1991,

Priddy 1994; reproduced with permis-

sion of ISA Controls Ltd.; design is the property of

International).

9.8 APPLICATIONS, ADVANTAGES, AND DISADVANTAGES 205

t I

Figure 9.22. Meter using liquid plugs to measure low flows.

the density is measured. The preferred orientation is with flow vertically downward.

The bearings are simple low friction journals but with thrust bearings that give long

service.

The meter was seen as suitable for both surface and subsea service and aimed at a

specification of ±5% rate for total flow and ±5% rate for oil flow rate with ±10% for

both gas and water. The field trial meters were designed for up to 20,000 barrels per

day (130 m

/h) with at least 10:1 turndown and ±5% uncertainty. A nominal 4-in.

(100-mm) bore meter was specified. The body had about 0.3-m outer diameter. The

meter length was 1.2 m, and with nucleonic densitometer it weighed about 450 kg.

The scatter on test appears to be a maximum of about 20% in mid values of void

fraction. Priddy (1994) reported tests at Prudoe Bay, Alaska, which are extremely

promising in terms of the specific aims.

9.7 METER USING LIQUID PLUGS TO MEASURE LOW FLOWS

Figure 9.22 shows a diagrammatic representation of a meter that uses liquid plugs

of a sealing fluid such as distilled water to measure the flow of gas. The plugs are

carried through the tube as shown, and the passage and velocity are sensed optically.

Accuracy claims are ±1% at the optimum rate, and the range is 8-3,000 ml/h. In

this device, dry gases pick up moisture. It is not clear how the size of plug, which

presumably varies, is allowed for. It may be that the optics provide a compensation.

9.8 APPLICATIONS, ADVANTAGES, AND DISADVANTAGES

Applications

The applications will vary to some extent for the various designs; in particular, the

accuracy and cost of the meters will make them more suitable for particular applica-

tions.

In general, they operate on clean or well-filtered fluids. The clearances are so

fine that dirty or aggressive fluids may prevent operation and cause total blockage in

the line. For the same reason, the instruments must be correctly mounted to avoid

mechanical distortion, which could affect the clearances.

In particular, for the sliding vane meter, one manufacturer claimed: blending,

batching, dispensing, inventory control, custody transfer of

oils,

solvents, chemicals,

paints, fats, and fertilizers.

206 POSITIVE DISPLACEMENT FLOWMETERS

For high viscosity fluids, the flow passage may create too great a pressure drop.

For some

oils,

the temperature of the liquid may need to be sensed and compensation

used.

Presumably, the design developed for multiphase flows will eliminate the need

for well-filtered fluids.

Advantages

The advantages are seen in high precision instruments, and their accuracy after cali-

bration is possibly only limited by the calibration facility

used.

Most are little affected

by viscosity. The most accurate ones among them, therefore, provide a device that

is highly suitable for custody transfer and fiscal transfer applications.

Disadvantages

The meters may create pulsations and may

subject to damage from rapidly varying

flows.

Damage to the meters causing them to stop rotating will also cause total line

blockage. They may have a high initial price and may require careful maintenance

with high associated costs. The meters may create a substantial pressure loss.

9.9 CHAPTER CONCLUSIONS

What is likely to be the future of such meter designs? No doubt other clever designs

will be introduced such as that selected as a utility gas meter (Kim et al. 1993b,

O'Rourke 1993,1996), which uses an undulating membrane (Figure 9.23). The sensor

consists of such a membrane, which traps discrete volumes of gas as they move

through the flow chamber in pulses. Movement is sensed by piezoelectric elements

bonded to the membrane. The design is patented. Tests suggest that it is capable of

operating over a range of 0.007-7 m

/h with uncertainty of ±1-2% for -40 to 140°F

(-40 to 60°C), and with a life of 20 years (10-year battery life).

More generally, these essential workhorses of the industry will continue to benefit

from improvement in materials, bearings, and manufacture, so that they can be used

with more difficult fluids.

Electronic control will also ensure that their standard applications are possible

with greater automation and precision.

Figure 9.23. Undulating membrane meter (after Kim et al. 1993 and O'Rourke

1993,

1996).