Product Loss During Retail Motor Fuel Dispenser Inspection

Transcription

1 Product Loss During Retail Motor Fuel Dispenser Inspection By: Christian Lachance, P. Eng. Senior Engineer - ment Engineering and Laboratory Services ment Canada Date:

2 Product Loss During Retail Motor Fuel Dispenser Inspection page 2 of 18 Introduction Retail motor fuel dispensers have traditionally been calibrated and inspected with open-neck 20 L test measures. The introduction of new testing methods and equipment has resulted in reports of discrepancies 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 fuel evaporation during testing may be a significant contributing factor in the uncertainty surrounding this test method when used with gasoline. This investigation was launched to determine the effect of fuel evaporation under a wide range of proving conditions that are typically encountered during field testing. The investigation also provides an analysis of different proving equipment items and method bias. Method A small bidirectional 20 L pipe prover was used to deliver a known amount of gasoline through a dispenser 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 investigation into alternative methods of proving. The difference between the liquid volume measured with the test equipment and the liquid volume delivered by the pipe prover was then used to estimate liquid product loss/evaporation during testing. The diagram below illustrates the test setup. Figure 1: Evaporative Loss Study Setup.

3 Product Loss During Retail Motor Fuel Dispenser Inspection page 3 of 18 Five test runs were conducted at both low flow rate and high flow rate for each item of equipment under test. Flow rates of approximately 15 Lpm and 30 Lpm were typical of the low and high rates obtained. The average flowing temperature at the prover, prover pressure, product temperature in the test equipment, and the test equipment readings were recorded. ing 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 were taken 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 various sources of error when using the test measure and the pipe prover system. The following is a listing of the significant sources of error for the two systems. uncertainty uncertainty temperature change between meter and test measure temperature change between meter and pipe prover pressure change between meter and test measure pressure change between meter and pipe prover measure thermal expansion Loss product during transfer Reading error Wetting variance due to different product characteristics and temperature effects on the prover Seal failure Variations in connecting volume Repeatability of the piston travel distance Drift caused by damage to the body of the test measure Since the aim of this study was to assess the effects of evaporation, some of the above factors were eliminated or minimized through corrections. All test equipment was calibrated directly against the pipe prover hose and nozzle assembly using water. This step was conducted to minimize calibration bias between the pipe prover and test equipment to less than ±5 ml. s were applied for liquid pressure and thermal expansion effects. s were also applied for the effects of pressure and temperature on the proving equipment. The accuracy of 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 of multiple runs was taken. These factors are reading errors by the test equipment operator (including resolution errors) and repeatability of the pipe prover.

4 Product Loss During Retail Motor Fuel Dispenser Inspection page 4 of 18 The pipe prover seal integrity was verified by a leak test. Variations in connecting volume are addressed through proper procedure and minimizing the connecting volume. The test equipment used in the study is very stable, and stability can be ensured by visual inspection. The other factors that can contribute to the difference are product loss during transfer and, to a lesser extent, the wetting effect due to the use of a liquid other than the calibration liquid. The pipe prover is a bidirectional design of 20 L. There is no pre- or post-run travel length, since the volume is defined between the end-of-travel positions. Forward and reverse flow direction is accomplished 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 when the 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 temperature and pressure at the prover are measured so as to allow for liquid compressibility and thermal expansion corrections. The temperature of the liquid inside the prover is taken as the average flowing product temperature at the outlet during a run. The product pressure inside the prover is taken 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 not fitted with a splashguard. A traditional 4 inch diameter neck stainless steel test measure was used for the testing. In order to increase the resolution of this equipment, it was fitted with a removable plunger/displacer for the neck. For all tests, the test measure product temperature was measured with an immersion probe after the run. This temperature measurement was used for the test measure steel expansion correction and the liquid thermal expansion correction. No precautions were taken to minimize splashing or vapour loss during the test. Vapour Retention The vapour retention prover (VRP) design consists of a 20 L, 2 inch neck prover with the drain piped to a reservoir. The neck is capped and has a vapour line running to the reservoir. A bellows is added to the reservoir. When the prover is drained, air is taken from the reservoir and bellows. When the prover is filled, displaced vapours fill the bellows. This equipment is essentially sealed and provides an environment where the air is saturated with fuel vapour. The saturated environment is considered to significantly reduce product evaporation. Product temperature is measured from a permanently mounted thermometer with the probe directly immersed in the liquid.

5 Product Loss During Retail Motor Fuel Dispenser Inspection page 5 of 18 This is essentially the same design as above but with a vent valve instead of bellows, so fresh air in 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: V prover V base prover cts prover Where: V prover = volume of liquid delivered by pipe prover at prover temperature (Tprover) and prover pressure, V base prover = volume of pipe prover at reference temperature (15 C), cts prover = 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: V TM Vbase TM Reading cpl cts TM ctl ctl TM Where: V TM Vbase TM ctl TM / ctl cts TM cpl = test equipment volume measurement of the liquid delivered by the pipe prover corrected to prover temperature and pressure, = to deliver volume (water) of test equipment at reference temperature corrected for bias with pipe prover, = correction for liquid expansion due to temperature difference from the pipe prover to the test equipment, = correction for test equipment steel expansion due to thermal effects, = 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 corrections applied, and the calculated liquid volume delivered by the pipe prover is obtained with: compared with pipe prover after all corrections = V - V TM 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:

6 Product Loss During Retail Motor Fuel Dispenser Inspection page 6 of 18 compared with pipe prover before corrections = V - (Vbase TM + Reading) This value represents the test equipment error when no corrections are applied to the test equipment reading. For the purposes of the analysis, the values for the individual corrections were calculated as: equipment steel correction = (Vbase TM + Reading) x cts TM thermal expansion correction = (Vbase TM + Reading) x ctl TM / ctl expansion due to pressure drop = (Vbase TM + Reading) / cpl Since the correction values are small relative to the measurement value, an alternate and approximate method of calculating the test equipment volume is as follows: V TM (Vbase TM + Reading) + test equipment steel correction + liquid thermal expansion correction + liquid expansion due to pressure drop Vapour The fuel vapour pressure at proving conditions was calculated from the model provided in API Manual Petroleum ment Standards, chapter 19.4, Appendix B using a value of s = 3 for the 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 encountered during the study. The product vapour pressure was lowest in the summer at 50 RVP and highest in the winter at 110 RVP. Figure 2. True Vapour of Refined Petroleum Stocks. True Vapour of Refined Petroleum Stocks 70.0 Vapour Temperature ( C) RVP = 50 RVP = 80 RVP = 110

7 Product Loss During Retail Motor Fuel Dispenser Inspection page 7 of 18 Results The before s values represent the test equipment error when no corrections are applied to the test equipment reading. The after all s values represent the combined effect of vapour loss and test equipment wetting error. The Steel is the correction for test equipment steel expansion due to thermal effects. The Temperature is the correction for liquid expansion due to the temperature difference between the pipe prover and the test equipment. The is the correction for liquid expansion due to the pressure drop from the pipe prover to the test equipment. All values are the average of 5 runs of 20L test quantity.

8 Product Loss During Retail Motor Fuel Dispenser Inspection page 8 of 18 Table 1. Results for the August 23, 2005 test. Flow ( C) before s Steel after all s Vapour high high VRP high low low VRP low

9 Product Loss During Retail Motor Fuel Dispenser Inspection page 9 of 18 Table 2. Results for the December 7, 2005 test. Flow ( C) before s Steel after all s Vapour high high VRP high low low VRP low

10 Product Loss During Retail Motor Fuel Dispenser Inspection page 10 of 18 Table 3. Results for the February 15, 2006 test. Flow ( C) before s Steel after all s Vapour high high VRP high low low VRP low

11 Product Loss During Retail Motor Fuel Dispenser Inspection page 11 of 18 Table 4. Results for the January 11, 2006 test. Flow ( C) before s Steel after all s Vapour high high VRP high low low VRP low

12 Product Loss During Retail Motor Fuel Dispenser Inspection page 12 of 18 Table 5. Results for the January 25, 2006 test. Flow ( C) before s Steel after all s Vapour high high VRP high low low VRP low

13 Product Loss During Retail Motor Fuel Dispenser Inspection page 13 of 18 Table 6. Results for the May 25, 2005 test. Flow ( C) before s Steel after all s Vapour high high VRP high low low low low VRP low VRP low

14 Product Loss During Retail Motor Fuel Dispenser Inspection page 14 of 18 Discussion Product Temperature Range The results show a very wide range of liquid temperatures, -5 to 30 C, which was due to the use of 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 The range of product RVP varied from 50 to 110 RVP and the product vapour pressure during testing was between 25 and 45 kpa. Since fuel is normally formulated with low RVP in summer and high RVP in winter, it is expected that low product vapour pressure will be encountered when the fuel storage temperature will be low relative to ambient temperature. We also expect to see the reverse high vapour pressure when the product storage temperature is high relative to ambient temperature. Steel Thermal Expansion 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 ml to 17 ml, and was due to the extreme product temperature range experienced in this study. We would expect smaller variations in typical field conditions. Temperature The correction for liquid temperature change between the pipe prover and the test equipment is approximately 2.5 ml per 0.1 C difference for gasoline. The two factors believed to influence the temperature 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 0.25 C on 20 L of fuel. In practice, however, the temperature drop will be less because not all cooling heat 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 could not be analyzed. The test measure liquid temperature correction averaged about 10 ml. The calibration cart and VRP showed correction values averaging just above 0. This supports the assumption that higher evaporation 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 and product VP. It should be noted that the accuracy of the temperature measurement is approximately ±0.2 C, which is equivalent to a ±5 ml correction. Effect 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 the ambient pressure in the test equipment. The expansion for 20 L was approximately 6 ml at high flow and 2 ml at low flow. Similar expansion would occur during dispenser testing, depending on the metering pressure.

15 Product Loss During Retail Motor Fuel Dispenser Inspection page 15 of 18 in after all s This graph shows the results of the study in terms of volume measurement difference between the pipe prover and the equipment used. Each point is the average of five runs. Figure 3. in d After All s. in d After All s Product Vapour VRP high flow high flow high flow VRP low flow low flow low flow Vapour Effect, TM The graph shows a close correlation between the measured volume difference for the test measure and the product vapour pressure during testing. As expected, the difference increases with product vapour pressure. This is consistent with the assumption that vapour loss is the main contributor with a regular test measure, as higher product vapour pressure will induce greater evaporation rates. Vapour Effect, VRP and Both the calibration cart and the VRP showed a relatively consistent difference of about 20 ml in volume measurement. As seen in the next graph, the average bias for the VRP based on the last two runs is approximately 15 ml. This would seem to indicate that this prover is sensitive to any air 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 high flow), 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.

16 Product Loss During Retail Motor Fuel Dispenser Inspection page 16 of 18 Wetting effect is caused by the variance in the amount of residue left in the to deliver test equipment when a product other than water is used. Figure 4. in d vs Product VP (VRP values based on last two runs only). in d vs Product VP (VRP values based on last two runs only) Vapour VRP high flow TM high flow high flow VRP low flow TM low flow low flow

17 Product Loss During Retail Motor Fuel Dispenser Inspection page 17 of 18 before s The 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 proving using the test equipment under evaluation. Figure 5. in d Before s. in d Before s Product Vapour VRP high flow TM high flow high flow VRP low flow TM low flow 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 typical field conditions, since the product temperature range and resultant steel expansion effects would be lesser. This is demonstrated in the following graph, with only the steel thermal correction added.

18 Product Loss During Retail Motor Fuel Dispenser Inspection page 18 of 18 Figure 6. in d vs. Product VP ( Only Corrected for Steel Thermla Expansion). in d vs. Product VP ( Only Corrected for Steel Thermal Expansion) Product Vapour VRP high flow TM high flow high flow VRP low flow TM low flow low flow Conclusion This study provides an estimate of product loss combined with wetting effects during testing with test measures. When care is not taken to minimize splashing during testing, the results indicate that 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 a corresponding increase in product vapour pressure from 25 kpa to 45 kpa. When other sources of error are included, the differences between the volume measured by the test measure and the volume delivered by the pipe prover ranged from 15 ml to -65 ml. But with the product temperatures of -5 to 30 C observed in these tests, correcting for expansion of the test measure steel 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 and wetting effect are addressed during the use of a test measure, this accuracy target will not be met. The performance of the VRP and calibration cart indicates that vapour loss can be significantly reduced with these designs. When test equipment steel corrections were applied, the measured volumes were in agreement with the delivered volumes, assuming a tolerance equal to ⅓ the retail dispenser tolerance.