Product Loss

7/30/2019 Product Loss

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Product Loss DuringRetail Motor Fuel Dispenser Inspection

By: Christian Lachance, P. Eng.Senior Engineer - Liquid MeasurementEngineering and Labo atory ServicesrMeasurement Canada

Date: April 10, 2007


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Measurement CanadaProduct Loss During Retail Motor Fuel Dispenser Inspection page 2 of 18April 10, 2007

Introduction

Retail motor fuel dispensers have traditionally been calibrated and inspected with open-neck 20 Ltest measures. The introduction of new testing methods and equipment has resulted in reports ofdiscrepancies between these new methods and the traditional testing method. The new

methods include closed-loop proving equipment and testing on low-volatility substitute liquids.Previous investigations of the accuracy of the traditional test measure indicated that fuelevaporation during testing may be a significant contributing factor in the uncertainty surroundingthis test method when used with gasoline.

This investigation was launched to determine the effect of fuel evaporation under a wide range ofproving conditions that are typically encountered during field testing. The investigation alsoprovides an analysis of different proving equipment items and method bias.

Test Method

A small bidirectional 20 L pipe prover was used to deliver a known amount of gasoline through adispenser hose and nozzle into a traditional 20 L test measure. The test was repeated with a

vapour retention prover and a calibration cart prover as a comparative study and investigationinto alternative methods of proving. The difference between the liquid volume measured with thetest equipment and the liquid volume delivered by the pipe prover was then used to estimateliquid product loss/evaporation during testing.

The diagram below illustrates the test setup.Figure 1: Evaporative Loss Study Test Setup.


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Five test runs were conducted at both low flow rate and high flow rate for each item of equipmentunder test. Flow rates of approximately 15 Lpm and 30 Lpm were typical of the low and highrates obtained. The average flowing temperature at the prover, prover pressure, producttemperature in the test equipment, and the test equipment readings were recorded.

Testing was conducted on six different occasions over a period of seven months in order to

assess the effects of varying ambient conditions and fuel properties. Samples of the fuel weretaken before and after each series of tests and sent to a laboratory for Reid vapour pressure(RVP) measurement.

In order to understand and analyse the results, it is necessary to become familiar with the varioussources of error when using the test measure and the pipe prover system. The following is alisting of the significant sources of error for the two systems.

Test Measure Pipe Prover

Calibration uncertainty

Liquid temperature change between meter

and test measure

Liquid pressure change between meterand test measure

Test measure thermal expansion

Loss product during transfer

Reading error

Wetting variance due to different productcharacteristics

Drift caused by damage to the body of thetest measure

Calibration uncertainty

Liquid temperature change between meter

and pipe prover

Liquid pressure change between meterand pipe prover

Pressure and temperature effects on theprover

Seal failure

Variations in connecting volume

Repeatability of the piston travel distance

Since the aim of this study was to assess the effects of evaporation, some of the above factorswere eliminated or minimized through corrections.

All test equipment was calibrated directly against the pipe prover hose and nozzle assemblyusing water. This step was conducted to minimize calibration bias between the pipe prover andtest equipment to less than 5 mL.

Corrections were applied for liquid pressure and thermal expansion effects. Corrections werealso applied for the effects of pressure and temperature on the proving equipment. The accuracyof these corrections is estimated to be approximately 5 mL.

A few factors are random in nature, so they do not contribute significantly since the average ofmultiple runs was taken. These factors are reading errors by the test equipment operator(including resolution errors) and repeatability of the pipe prover.


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The pipe prover seal integrity was verified by a leak test. Variations in connecting volume areaddressed through proper procedure and minimizing the connecting volume.

The test equipment used in the study is very stable, and stability can be ensured by visualinspection.

The other factors that can contribute to the difference are product loss during transfer and, to alesser extent, the wetting effect due to the use of a liquid other than the calibration liquid.

Equipment

Pipe Prover

The pipe prover is a bidirectional design of 20 L. There is no pre- or post-run travel length, sincethe volume is defined between the end-of-travel positions. Forward and reverse flow direction isaccomplished through the operation of the 4-way valve. The integrity of the piston seal and 4-way valve seal was verified by a leak test before each series of tests.

Flowing product temperature and pressure is taken at the inlet and outlet. A proving run is

conducted by first circulating product until temperature is stabilized. Stability is assumed whenthe inlet and outlet are within 0.3 C and when the outlet temperature is stable.

The volume of the prover is corrected for the temperature effect of the steel. Product temperatureand pressure at the prover are measured so as to allow for liquid compressibility and thermalexpansion corrections. The temperature of the liquid inside the prover is taken as the averageflowing product temperature at the outlet during a run. The product pressure inside the prover istaken from the product pressure at the inlet before the run is initiated.

The prover was fitted with a standard dispenser hose and nozzle assembly. The nozzle was notfitted with a splashguard.

Test Measure

A traditional 4 inch diameter neck stainless steel test measure was used for the testing. In orderto increase the resolution of this equipment, it was fitted with a removable plunger/displacer forthe neck.

For all tests, the test measure product temperature was measured with an immersion probe afterthe run. This temperature measurement was used for the test measure steel expansioncorrection and the liquid thermal expansion correction.

No precautions were taken to minimize splashing or vapour loss during the test.

Vapour Retention Prover

The vapour retention prover (VRP) design consists of a 20 L, 2 inch neck prover with the drainpiped to a reservoir. The neck is capped and has a vapour line running to the reservoir. Abellows is added to the reservoir. When the prover is drained, air is taken from the reservoir andbellows. When the prover is filled, displaced vapours fill the bellows. This equipment isessentially sealed and provides an environment where the air is saturated with fuel vapour. Thesaturated environment is considered to significantly reduce product evaporation.

Product temperature is measured from a permanently mounted thermometer with the probedirectly immersed in the liquid.


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Calibration Cart

This is essentially the same design as above but with a vent valve instead of bellows, so fresh airin introduced via the vent valve when the reservoir is emptied.

Calculations

The volume of liquid delivered by the pipe prover is obtained from the following:

proverproverbaseprover ctsVV

Where:Vprover = volume of liquid delivered by pipe prover at prover temperature (Tprover) and

prover pressure,Vbase prover = volume of pipe prover at reference temperature (15 C),ctsprover = correction for prover steel expansion due to prover temperature.

The volume of liquid measured by the test measure, taking into account:

the temperature effect of the steel of the test equipment, the liquid thermal expansion from the prover to the test equipment, and the liquid expansion due to the pressure drop from the pipe prover to the test equipment.

was obtained from the following:

over

over

TMTMTM

TMcpl

ctl

ctlctsadingVbase

VPr

Pr

Re

Where:

VTM = test equipment volume measurement of the liquid delivered by the pipe provercorrected to prover temperature and pressure,

VbaseTM = to deliver volume (water) of test equipment at reference temperature correctedfor bias with pipe prover,

ctlTM / ctlProver = correction for liquid expansion due to temperature difference from the pipeprover to the test equipment,

ctsTM = correction for test equipment steel expansion due to thermal effects,cplProver = correction for liquid expansion due to pressure drop from the pipe prover to the

test equipment.

The difference between the test equipment liquid volume measurement, with all correctionsapplied, and the calculated liquid volume delivered by the pipe prover is obtained with:

Difference compared with pipe prover after all corrections = VProver - VTM

This value represents the combined effect of vapour loss and test equipment wetting error.

The difference between the test equipment liquid volume measurement, before any corrections,and the calculated liquid volume delivered by the pipe prover is obtained with:


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Difference compared with pipe prover before corrections = VProver - (VbaseTM+ Reading)

This value represents the test equipment error when no corrections are applied to the testequipment reading.

For the purposes of the analysis, the values for the individual corrections were calculated as:Test equipment steel correction = (VbaseTM+ Reading) x ctsTMLiquid thermal expansion correction = (VbaseTM+ Reading) x ctlTM/ ctlProverLiquid expansion due to pressure drop = (VbaseTM+ Reading) / cplProver

Since the correction values are small relative to the measurement value, an alternate andapproximate method of calculating the test equipment volume is as follows:

VTM (VbaseTM+ Reading) + test equipment steel correction + liquid thermal expansioncorrection + liquid expansion due to pressure drop

Vapour Pressure

The fuel vapour pressure at proving conditions was calculated from the model provided in APIManual Petroleum Measurement Standards, chapter 19.4, Appendix B using a value of s = 3 forthe slope of the ASTM distillation curve at 10% evaporated, in degrees F per percentage point.

The following graph provides the results of this model for the range of gasoline encounteredduring the study. The product vapour pressure was lowest in the summer at 50 RVP and highestin the winter at 110 RVP.

Figure 2. True Vapour Pressure of Refined Petroleum Stocks.

True Vapour Pressure of Refined Petroleum Stocks

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

-30 -20 -10 0 10 20 30 40

Temperature (C)

VapourPressure(kPa)

RVP = 50

RVP = 80

RVP = 110


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Results

The Volume Difference before Corrections values represent the test equipment error when nocorrections are applied to the test equipment reading.

The Volume Difference after all Corrections values represent the combined effect of vapour lossand test equipment wetting error.

The Equipment Steel Temp. Correction is the correction for test equipment steel expansion dueto thermal effects.

The Liquid Temperature Correction is the correction for liquid expansion due to the temperaturedifference between the pipe prover and the test equipment.

The Liquid Pressure Correction is the correction for liquid expansion due to the pressure dropfrom the pipe prover to the test equipment.

All values are the average of 5 runs of 20L test quantity.


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Table 1. Results for the August 23, 2005 test.

TestEquipment

Flow PipeProverTemp.(C)

PipeProverPressure(kPa)

VolumeDifferencebeforeCorrections

(mL)

EquipmentSteel Temp.Correction(mL)

LiquidTemp.Correction(mL)

LiquidPressureCorrection(mL)

TestMeasure

high 29.2 257.8 -62.1 13.1 13.5 -6.6

CalibrationCart

high 29.3 261.3 -26.1 15.2 -8.7 -6.7

VRP high 30.7 256.5 -30.0 16.6 -6.7 -6.7

TestMeasure

low 27.3 95.8 -63.0 11.4 10.1 -2.4

CalibrationCart

low 25.8 94.4 -44.4 11.0 3.9 -2.4

VRP low 25.3 114.4 -29.6 10.7 -0.3 -2.8


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Table 2. Results for the December 7, 2005 test.

TestEquipment




(mL)




TestMeasure

high -5.0 259.2 -7.6 -19.6 12.3 -5.0

CalibrationCart

high -4.8 250.9 5.8 -20.7 4.1 -4.9

VRP high -2.4 262.0 -9.8 -18.2 2.7 -5.2

TestMeasure

low -6.0 99.3 -8.1 -20.7 16.9 -1.9

CalibrationCart

low -3.8 121.3 14.4 -19.4 -4.1 -2.4

VRP low -3.0 126.8 4.8 -18.7 1.4 -2.5


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Table 3. Results for the February 15, 2006 test.

TestEquipment




(mL)




TestMeasure

high 3.6 300.6 -35.0 -11.3 10.7 -6.3

CalibrationCart

high 4.0 303.3 -5.3 -11.6 5.0 -6.4

VRP high 6.2 296.4 -7.2 -9.1 0.8 -6.3

TestMeasure

low 2.6 137.9 -21.5 -12.1 6.3 -2.9

CalibrationCart

low 4.0 131.0 -12.9 -11.4 1.1 -2.7

VRP low 5.2 137.9 -4.7 -10.1 -1.9 -2.9


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Table 4. Results for the January 11, 2006 test.

TestEquipment




(mL)




TestMeasure

high 8.2 289.5 -40.1 -6.8 10.5 -6.3

CalibrationCart

high 8.4 292.3 -7.2 -7.2 7.5 -6.3

VRP high 8.7 289.5 -10.4 -6.6 2.8 -6.3

TestMeasure

low 5.7 124.1 -30.0 -9.0 3.6 -2.6

CalibrationCart

low 3.5 128.2 4.5 -11.8 -2.8 -2.7

VRP low 3.9 136.5 4.7 -11.3 -4.7 -2.9


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Table 5. Results for the January 25, 2006 test.

TestEquipment




(mL)




TestMeasure

high 1.2 295.1 -38.4 -13.6 12.4 -6.0

CalibrationCart

high -0.1 308.9 -6.5 -16.0 8.2 -6.3

VRP high 0.1 297.8 -5.1 -15.6 4.4 -6.0

TestMeasure

low 0.9 142.0 -44.8 -14.0 13.5 -2.9

CalibrationCart

low -0.3 140.6 -7.0 -16.3 11.0 -2.8

VRP low -0.8 159.9 -6.8 -16.5 4.4 -3.2


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Table 6. Results for the May 25, 2005 test.

TestEquipment




(mL)




TestMeasure

high 30.8 383.8 -62.9 16.2 3.3 -10.0

CalibrationCart

high 31.6 358.5 -46.6 15.7 5.3 -9.4

VRP high 31.5 255.1 -25.8 17.1 -0.5 -6.7

TestMeasure

low 23.1 146.5 -39.1 8.5 -1.7 -3.6

TestMeasure

low 20.9 146.5 -22.0 6.7 -12.6 -3.5

CalibrationCart

low 24.7 159.9 -14.8 9.6 -7.3 -4.0

CalibrationCart

low 26.6 125.8 -31.5 11.1 -0.7 -3.2

VRP low 26.3 164.6 -21.6 11.8 -2.8 -4.1

VRP low 23.9 126.8 0.8 9.8 -14.6 -3.1


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Discussion

Product Temperature Range

The results show a very wide range of liquid temperatures, -5 to 30C, which was due to the useof an above-ground fuel storage tank. The temperature range is therefore more representative

of extreme field conditions as opposed to typical field conditions.

Product Vapour Pressure

The range of product RVP varied from 50 to 110 RVP and the product vapour pressure duringtesting was between 25 and 45 kPa. Since fuel is normally formulated with low RVP in summerand high RVP in winter, it is expected that low product vapour pressure will be encountered whenthe fuel storage temperature will be low relative to ambient temperature. We also expect to seethe reverse high vapour pressure when the product storage temperature is high relative toambient temperature.

Steel Thermal Expansion Correction

The most significant correction in this study was the correction for equipment steel expansion.

This effect is approximately 1 mL per C when 304 stainless steel is used. It ranged from -20 mLto 17 mL, and was due to the extreme product temperature range experienced in this study. Wewould expect smaller variations in typical field conditions.

Liquid Temperature Correction

The correction for liquid temperature change between the pipe prover and the test equipment isapproximately 2.5 mL per 0.1C difference for gasoline. The two factors believed to influence thetemperature differential are ambient/product temperature differences and evaporative cooling.

It is estimated that the evaporation of 40 mL of fuel is equivalent to a temperature drop of 0.25Con 20 L of fuel. In practice, however, the temperature drop will be less because not all coolingheat is transferred to the liquid. For the majority of tests, the fuel temperature was very close to

the ambient temperature. As a result, the effects of ambient/fuel temperature differences couldnot be analyzed.

The test measure liquid temperature correction averaged about 10 mL. The calibration cart andVRP showed correction values averaging just above 0. This supports the assumption that higherevaporation rates in the test measure will result in greater evaporative cooling of the product.However, there was no significant correlation between the amount of temperature correction andproduct VP. It should be noted that the accuracy of the temperature measurement isapproximately 0.2 C, which is equivalent to a 5 mL correction.

Liquid Pressure Effect Correction

The pressure inside the pipe prover was approximately 250 kPa (35 psi) at high flow and 125 kPa(18 psi) at low flow, resulting in a small expansion of the liquid as it was transferred to theambient pressure in the test equipment. The expansion for 20 L was approximately 6 mL at highflow and 2 mL at low flow. Similar expansion would occur during dispenser testing, depending onthe metering pressure.


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Difference in Volume after all Corrections

This graph shows the results of the study in terms of volume measurement difference betweenthe pipe prover and the equipment used. Each point is the average of five runs.

Figure 3. Difference in Measured Volume After All Corrections.Difference in Measured Volume After All Corrections

-60.0

-50.0

-40.0

-30.0

-20.0

-10.0

0.0

20.0 25.0 30.0 35.0 40.0 45.0Product Vapour Pressure (kPa)

Difference(mL)

VRP high flow Test Measure high flow Calibration Cart high flow

VRP low flow Test Measure low flow Calibration Cart low flow

Vapour Pressure Effect, TM

The graph shows a close correlation between the measured volume difference for the testmeasure and the product vapour pressure during testing. As expected, the difference increaseswith product vapour pressure. This is consistent with the assumption that vapour loss is themain contributor with a regular test measure, as higher product vapour pressure will inducegreater evaporation rates.

Vapour Pressure Effect, VRP and Calibration Cart

Both the calibration cart and the VRP showed a relatively consistent difference of about 20 mL involume measurement. As seen in the next graph, the average bias for the VRP based on the lasttwo runs is approximately 15 mL. This would seem to indicate that this prover is sensitive to anyair entrained when the reservoir is drained and perhaps some conditioning of the air in the prover.

Other than the liquid pressure expansion from the pipe prover to the test equipment (6 mL at highflow), the cause of this bias is not known but is expected to be due to a combination of:

a small amount of evaporation and perhaps some atomization during transfer, wetting effects, bias errors in the study.


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Wetting effect is caused by the variance in the amount of residue left in the to deliver testequipment when a product other than water is used.

Figure 4. Difference in Measured Volume vs Product VP (VRP values based on last tworuns only).

Difference in Measured Volume vs Product VP(VRP values based on last two runs only)

-60.0

-50.0

-40.0

-30.0

-20.0

-10.0

0.0

20.0 25.0 30.0 35.0 40.0 45.0

Liquid Vapour Pressure (kPa)

Diff

erence(mL)

VRP high flow TM high flow Calibration Cart high flow

VRP low fLow TM low flow Calibration Cart low flow


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Volume Difference before CorrectionsThe following graph shows the results of the measured volume difference before corrections.This is somewhat representative of the expected bias between closed-loop proving and provingusing the test equipment under evaluation.

Figure 5. Difference in Measured Volume Before Corrections.

Difference in Measured Volume Before Corrections

-70.0

-60.0

-50.0

-40.0-30.0

-20.0

-10.0

0.0

10.0

20.0

20.0 25.0 30.0 35.0 40.0 45.0

Product Vapour Pressure (kPa)

Difference(mL)


VRP low flow TM low flow Calibration Cart low flow

As expected, the spread of results is wider, about 15 mL to -65 mL, compared with the range of -10 mL to -50 mL for the corrected results. A smaller range of results would be expected in typicalfield conditions, since the product temperature range and resultant steel expansion effects wouldbe lesser. This is demonstrated in the following graph, with only the steel thermal correctionadded.


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Figure 6. Difference in Measured Volume vs. Product VP (Test Equipment Only Correctedfor Steel Thermla Expansion).

Difference in Measured Volume vs. Product VP

(Test Equipment Only Corrected for Steel Thermal

Expansion)

-70.0

-60.0

-50.0

-40.0

-30.0

-20.0

-10.0

0.0

10.0

20.0

20.0 25.0 30.0 35.0 40.0 45.0

Product Vapour Pressure (kPa)

Difference(mL)


VRP low flow TM low flow Calibration Cart low flow

Conclusion

This study provides an estimate of product loss combined with wetting effects during testing withtest measures. When care is not taken to minimize splashing during testing, the results indicatethat the combined vapour loss and wetting effect error correlates closely with product vapour

pressure. In this study, the error was found to increase from 10 mL to about 50 mL for acorresponding increase in product vapour pressure from 25 kPa to 45 kPa. When other sourcesof error are included, the differences between the volume measured by the test measure and thevolume delivered by the pipe prover ranged from 15 mL to -65 mL. But with the producttemperatures of -5 to 30C observed in these tests, correcting for expansion of the test measuresteel reduced these differences to the 0 to -60 mL range.

The desired accuracy ratio of test equipment to device under test is 1:3. Unless vapour loss andwetting effect are addressed during the use of a test measure, this accuracy target will not bemet.

The performance of the VRP and calibration cart indicates that vapour loss can be significantlyreduced with these designs. When test equipment steel corrections were applied, the measuredvolumes were in agreement with the delivered volumes, assuming a tolerance equal to theretail dispenser tolerance.

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