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Metrological performance of diaphragm gas meters in distribution networks
Flow Measurement and Instrumentation 37 (2014) 65–72
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Metrological performance of diaphragm gas meters in distribution networks G. Ficco n DICEM, Department of Civil and Mechanical Engineering of UNICLAM, University of Cassino and Southern Lazio, via Di Biasio 43, 03043 Cassino, Italy
art ic l e i nf o
a b s t r a c t
Article history: Received 21 November 2013 Received in revised form 31 January 2014 Accepted 2 March 2014 Available online 18 March 2014
In natural gas distribution networks a great number of domestic diaphragm gas meters are currently installed (about 15 million in Italy) and most of them are very old meters. Furthermore, some National Authorities nowadays impose strict roll-out procedures of the existing meters with new smart meters. To this aim, diaphragm meters in “hybrid” configuration equipped with electronic and transmission devices are supposed to be widely installed by natural gas distribution companies in the first roll-out campaigns, since they present a fully developed technology despite of the new ultrasonic and thermal mass meters. In this paper the author presents the results of a wide experimental study aimed to assess how metrological performance of diaphragm gas meters worsen over time as function of age, installation conditions, manufacturing technology, etc. The results of the experimental analysis show that a significant number of the meters investigated failed subsequent verification and presented a not excellent stability. On the other hand, the average percentage errors and the weighted mean error of the meters were measured basically close to zero, also for very old meters. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Diaphragm gas meter Smart metering Legal verification Maximum permissible error
1. Introduction EU Directives 2012/27 [1] on energy efficiency and 2004/22 [2] on measuring instruments (known as MID Directive) impose very strict constraints in terms of essential requirements of gas meters both for measuring and billing. Therefore, some National Authorities recently issued mandatory resolutions [3] to gradually substitute domestic gas meters in distribution networks with new smart meters provided with remote data transmission devices useful for management purposes. Furthermore, from a legal metrology point of view, gas meters were not subjected to subsequent or in-service verifications by the competent authorities so far, and a legal duration of 15 years was fixed just in 2009 [4]. In Italy, the roll-out program involves a very large number of domestic gas meters (about 15 million). New gas smart meters are expected to guarantee more strict metrological performance, also allowing the correction of the volumes through the real gas temperature measured by a built-in sensor. In the design of future smart meters, manufacturers are now developing new metering principles with no moving parts (commonly known as “static meters”), like thermal mass [5] and ultrasonic [6]. Even if these new principles in gas metering have very significant potential (such as digital output, the absence of
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moving parts, direct mass measurement), their reliability in some critical conditions typical of natural gas distribution networks (i.e. zero flow, minimum flow-rate, gas quality changes, presence of dust and contaminants, etc.) still has to be demonstrated. In such scenario, diaphragm meters are still the most widely used in natural gas distribution networks and gas operators predict that in the next years they will prevail in the first rollout campaigns, despite of the “static” ones. In fact, because of their fully developed technology and their cheap costs, many manufacturers decided to meet actual roll-out requirements adapting diaphragm gas meters with electronic and transmission devices. Thus, mechanical diaphragm meters in “hybrid” configuration will operate in Italian distribution networks for many other years, since the 15 years legal duration currently in force. Therefore, both manufacturers and distribution companies are deeply interested in improving and controlling the metrological effectiveness of these types of meters. In fact, manufacturers continuously deal with constructive issues strongly influencing the metrological performance and stability of the meters, such as: (i) material and components of the metering unit; (ii) diaphragm stability and seal; (iii) internal and external leakages; and (iv) mechanical wear of the moving parts. On the other hand, gas distribution companies are strongly interested in defining effective roll-out plans based on homogeneous lots as function of drift, age, manufacturer and measuring technology, class of consumptions of the meters and so on.
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Nomenclature AEEG CMM E EN EU ki
Italian Authority for electricity and natural gas Coordinate measuring machine Error of indication, % European norm European Union Weighting factor, adimensional
From a technical point of view, the estimation of the metrological performance of the meters installed in natural gas distribution networks is a very difficult issue because of the wide variability of many influential parameters as: (i) manufacturer, (ii) year of construction, (iii) average annual consumptions, (iv) installation (e.g. indoor/outdoor), (v) user's appliances (e.g. heating, kitchen, boiler), and (vi) size of the meter. Furthermore, the existing meters installed before 1990 are commonly made up of natural diaphragms, which are widely recognized as a source of significant errors because of the performance decay of the diaphragm itself. In recent years (after 1990) natural diaphragms were replaced by synthetic ones, which are expected to be more resistant and reliable and to guarantee more stable metrological performance. Finally, natural gas measurement issues as well as metrological performance of gas meters were deeply investigated in the scientific literature [6–9]. In this paper, the author presents the results of a detailed analysis of the metrological performance of domestic diaphragm gas meters installed in the natural gas distribution network of Genoa (Italy). The distribution network of Genoa represents a very interesting situation. In fact, both natural and synthetic diaphragm meters are currently installed (as in many other networks in Italy) and both wet and dry natural gas is distributed. A wide sample made up of 402 diaphragm gas meters was analyzed; in particular, the author conducted the following metrological tests: (i) accuracy and pressure drop, (ii) leakage, and (iii) visual check and dimensional measurements.
2. Smart gas meters for domestic applications In this section, a brief description of the “static” thermal mass and ultrasonic gas meter for domestic applications is presented, together with a comparison with the traditional diaphragm gas meter. Ultrasonic meters are currently the innovative “static” meters at the most advanced stage of development and experimentation, also for in service application. They surely represent a very good solution in the near future for natural gas domestic measurements. Ultrasonic gas meters for domestic applications are based on the time of flight measurement of an acoustic wave traveling in a gas flowing in a closed section. As regards fiscal metering of domestic gas volumes, normally a single-path configuration with two transducers (a transmitter and a receiver) and a measuring section where the acoustic wave is eventually reflected one or more times is used [6]. Ultrasonic gas meters in domestic applications measure the gas velocity in a given section of a restricted pipe. Therefore, the gas flow-rate is calculated by multiplying the measured velocity by the known section of the measuring pipe itself. Finally, the volume is calculated integrating the flow-rate over time. On the metrological hand, ultrasonic gas meters present good performance and costs comparable to the hybrid diaphragm meter ones. On the contrary, they still have to demonstrate a full reliability in domestic
MID MPE OIML Qmax Qmin Qt UAG WME
Measuring Instrument Directive Maximum Permissible Error, % International Organization for Legal Metrology maximum flow-rate of the meter, m3/h minimum flow-rate of the meter, m3/h transition flow-rate of the meter, m3/h unaccounted for gas weighted mean error, %
applications where installation effects and potential contaminants could be significant. A very promising technology for gas mass measurements also in domestic field is represented by the thermal mass principle. Thermal mass gas meters are based on the relationship between the output voltage of a sensor (constituted in the last releases by a microelectro-mechanical systems, MEMS) and the rate of heat transfer between the sensor itself and the flowing gas. Unfortunately, the amount of heat transfer depends on the gas mass flowrate, as the main effect, but also on the gas composition, which influences its thermo-physical properties, like thermal conductivity and diffusivity. Thus, integrating over time and using thermophysical properties of the gas flowing the volume is obtained for fiscal measurement and billing. The technology of these sensors gives an output voltage directly proportional to the gas mass-flow rate and the small size of these devices leads to a high sensitivity and fast response time [5]. Unlike conventional diaphragm and new ultrasonic meters, for which both consolidated national and international technical standards can be applied [10,11], till today no standard references for thermal mass gas meters are available although many national and international technical groups are working on this task. Unfortunately, even if different laboratory analyses were carried out and some pattern approval tests have been already passed, field analysis about metrological performance of these kinds of meters (in particular in countries in which the gas composition widely varies in an unpredictable way, like in Italy) are still not available in scientific literature. Despite the above mentioned “static” technologies are strongly emerging also in gas metering domestic applications, nowadays, mechanical diaphragm represents the near totality of gas meters installed in distribution networks. A diaphragm meter is a volumetric gas meter that measures the gas counting the number of times a cyclic nominal volume is emptied and filled. The errors occurring in a diaphragm gas meter are mainly due to leakages, geometrical changes and mechanical failures as well as to the unavoidable decay of materials over time. As regards the movement of the distributing valve, manufacturers currently adopt a rotary or alternative technology, as depicted in Fig. 1. The advantage of these meters is the simplicity of their construction and the consequent low costs, whereas their limits are: (i) the presence of moving parts subject to wear, (ii) the high pressure drop, (iii) the mechanical output; and (iv) the inability to indicate an instantaneous flow-rate value [12]. To meet the actual roll-out requirements, hybrid domestic gas meters combine new smart electronic features on a traditional diaphragm mechanical body. In such meters the mechanical register is replaced by an electronic unit, equipped with a data transmission device and a remote firmware upgrade function. Moreover, all data are recorded on a non-volatile memory of the electronic device. According to the recent legal metrology regulations [3], an hybrid smart meter has also to be provided with a temperature sensor and with a remotely controlled mechanical valve that allows or forbids the gas flow.
G. Ficco / Flow Measurement and Instrumentation 37 (2014) 65–72
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Fig. 1. Typical measuring unit technology adopted by diaphragm gas meter manufacturers: (a) alternative distributing valve (courtesy of Sacofgas 1927 Spa) and (b) rotary distributing valve (courtesy of Itron).
From a metrological point of view, hybrid smart meters are unavoidably affected by the mechanical constraints of the metering unit, but they allow the error correction directly via software. It is very important to point out that electronic device allows hybrid diaphragm meters to apply effective correction factors at each test flow-rate, despite of the mechanical zero correction of the traditional ones. This feature could be very important not only in initial verification when the requirements of MID annex D [2] are met by the manufacturer, but also in subsequent ones, where specific corrections could be applied as function also of the drift.
3. Test methods and experimental apparatus A strict procedure for the design of the tests [13] was carried out in order to estimate the metrological performance of old domestic diaphragm gas meters and to better understand the causes of the expected drift. Seven lots, of about 60 diaphragm gas meters each, were collected from the Genoa natural gas distribution network: three lots with natural diaphragms (installed before 1990) and four lots with synthetic diaphragms (installed after 1990). Among the latter, one lot was made up of 52 synthetic diaphragm meters installed in a part of the network where only dry gas is distributed. After removal all the meters were immediately filled with natural gas, sealed and stored in a conditioned room before their transportation to the laboratory. Accuracy and pressure absorption tests were carried out at the university measurement laboratory LAMI of the UNICLAM (accredited calibration laboratory 105 by Accredia, the Italian accreditation body). The other tests were carried out at the Scientific and Technological Park of Southern Lazio (PALMER, Ferentino, Italy, accredited testing laboratory 273 and accredited calibration laboratory 85). A brief description of the tests is hereinafter reported. 3.1. Accuracy and pressure drop The tests were performed through a 600 L bell prover test bench (Fig. 2) placed at the LAMI where both temperature and humidity are continuously controlled at (2071) 1C and (50710)%UR, respectively.
Fig. 2. Bell prover test bench for accuracy and pressure absorption tests.
Traceability of the bench starts from the 50 L volume standard calibrated with an expanded uncertainty of 3.3 ml at about 95% confidence level by INRIM, the Italian Primary Institute of Metrology in Turin. Auxiliary calibrated devices were used during the tests for temperature, humidity and pressure measurements both in the test environment and in the bench. At the end of the metrological chain,
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G. Ficco / Flow Measurement and Instrumentation 37 (2014) 65–72
the typical overall expanded uncertainty is 0.40% at about 95% confidence level. Thus, the calibration method adopted is fully consistent with the legal metrology rule which allows an expanded uncertainty of the test bench in verification not exceeding 1/3 of the maximum permissible errors (MPE) of the device under test [14].
is the tolerance currently adopted by manufacturers in their processes). Planarity uncertainty for measurements can be assumed to be about 0.008 mm with a coverage factor k¼2 (corresponding to a 95% confidence level), as the calibration uncertainty of the CMM is 0.005 mm and type A, repeatability and resolution uncertainty contribution were considered [15].
3.2. Leakage The external leak tightness test was carried out at normal laboratory temperature by immersing the meter, without its index, in water and observing for leakage for about 30 s after any external trapped air has dispersed. The meter under test is pressurized by means of a specific test bench equipped with a calibrated air flowmeter and a digital manometer. During the resistance to internal pressure test the case of the meter under test is pressurized progressively with air to 1.5 times the maximum working pressure of the meter [10]. Test pressure is then maintained for 30 min and then released, also ensuring that the rate of pressurization or depressurization does not exceed 350 mbar/s. 3.3. Visual check and dimensional measurements After the external leak tightness test, the meters were disassembled and checked in order to detect: (i) the integrity of couplings and of the exit pipe; (ii) the presence of evident faults on the body of the meter; and (iii) the possible leakages from grid-distributing valve and their wear conditions. The author also investigated the possible correlation between leakages and the error of indication by means of planarity measurements on grid and distributing valve. To this aim, a calibrated coordinate measuring machine (CMM) in a controlled environment (20 70.5 1C and 50 710%UR) was used. The planarity was measured as the maximum height difference between 12 points in the coupling area (Fig. 3). A tolerance of 0.020 mm was considered both for grid and distributing valve (that
4. Results and discussion In this section, the results and the discussion of the accuracy test were emphasized as they are essential both for consumer protection and integrity of distribution as well as for the correct estimation of unaccounted for gas (UAG) in city distribution networks [16,17]. The error of indication of the meters was measured at four flow-rates values: Qmin, 0.2 Qmax, Qmax in compliance to the Italian legal metrology law in force before 2007 [18,19] and 0.5 Qmax. The weighted mean error of indication (WME), which is a function of the percentage errors and of the flow-rates at which the errors occurred, was calculated for each investigated meter through the following equation given in [14]: n
∑ ki E i
WME ¼
i¼1 n
∑ ki
;
8 < ki ¼ QQ i ; max
for
Q i r 0:7Q max
: ki ¼ 1:4 QQ i ; max
for
0:7Q max oQ i r Q max
i¼1
ð1Þ where ki is the wheighting factor and Ei is the percentage error of indication at the flow-rate Qi. As regards the metrological limits in verification, the EU MID Directive [3] regulates only the initial conformity assessment. Thus, each member state has the faculty to define its own specific approach in subsequent verifications guaranteeing the continuity with the existing national rules.
Fig. 3. Measuring points for planarity on the distributing valve (a) and on the grid (b).
Table 1 Maximum Permissible Error (MPE) for domestic gas meters (MID class 1,5) in initial conformity assessment and in subsequent verification/service in Italy. Flow-rate range
Qmin o Qo Qtn Qtn o Qo Qmax WME
MPE in initial conformity assessment
MPE in subsequent/in-service verification
OIML R137 [14] and MID [2] in force after 2007
Italian law in force before 2007 [19]
OIML R137 [14] and Italian decree n.75/2012 [20]
Italian law in force before 2012 [19]
73.0% 71.5% 70.6%
73.0% 72.0% –
7 6.0% 7 3.0% –
n.a. n.a. –
n The transition flow-rate Qt for diaphragm gas meters is equal to 0,1 Qmax [10] and it is the flow-rate at which the flow-rate range is divided into two zones, the ‘upper zone’ and the ‘lower zone’, each one with a different characteristic MPE.
G. Ficco / Flow Measurement and Instrumentation 37 (2014) 65–72
As an example, in Italy, according to the Decree n.75 dated 2012-04-16 [20] domestic gas meters with Qmax up to 10 m3/h are not subjected to subsequent verifications and only random in-service ones are to be applied. Furthermore, such gas meters are also checked on demand of the users. The maximum permissible error (MPE) in subsequent/in-service verifications is double of the corresponding error in the initial conformity assessment as stated in [14]. Table 1 shows the actual MPEs for domestic gas meters (MID class 1,5) in initial and subsequent verification, together with the ones in force before 2007. As regards the WME, only a MPE equal to 70.6% in initial conformity assessment is allowed, whereas no limit for subsequent and in-service verifications is indicated. In Table 2 the average errors of indication of the investigated meters (grouped by year of construction) are presented, together with the percentage of the meters failing subsequent verification. In Fig. 4 the error curves of synthetic and natural diaphragm meters are presented separately, whereas Fig. 5 shows the percentage of the meters failing subsequent verification is undoubtedly increasing with the age of the meter. It can be pointed out that the percentage of the meters failing subsequent verification is significant, also for the more recent meters. In fact, according to the legal metrology regulations, a meter is considered to fail when the error of indication exceeds MPE even at only one flow-rate. The test demonstrated that the 22% of the investigated meters failed only at one flow-rate, especially at Qmin. Basically, synthetic diaphragm meters present negative WMEs (i.e. in consumer advantage), whereas the natural ones present positive WMEs (i.e. in consumer disadvantage). Furthermore, the percentage of the meters presenting a systematic drift (i.e. negative
69
100% 90% 80% 70% 60% 50% 40% 30% 20%
synthetic natural
10% 0% 0
5
10
15
20
25
30
35
40
45
50
55
Age of the meter (years) Fig. 5. Percentage of the meters failing subsequent verification in respect to the age of the meter.
10.0% 5.0% 0.0%
E%
-5.0% Qmin
-10.0%
0.2 Qmax 0.5 Qmax
-15.0%
Qmax
-20.0% 8
13 18 23 28 33 38 43 50
Age of the meter (years) Fig. 6. Trend of the average error of indication at different flow-rates as function of the age of the meters.
Table 2 Error of indication of the meters grouped by year of construction. Period of manufacturing of the meter
2001–2009 1996–2000 1990–1995 1986–1990 1981–1985 1976–1980 1971–1975 1966–1970 Up to 1965 Investigated meters (all) Synthetic meters (dry gas) Synthetic meters (wet gas) Synthetic meters (all) Natural meters (all) n
Age (years)
8 13 18 23 28 33 38 43 50
Average E%n
Meters
WME
Investigated
Failing sub. ver.
%
Qmin
0.2 Qmax
0.5 Qmax
Qmax
61 89 73 29 28 22 33 34 33 402 29 194 223 179
23 29 33 12 14 14 22 18 25 190 7 78 85 105
37.7 32.6 45.2 41.4 50.0 63.6 66.7 52.9 75.8 47.3 24.1 40.2 38.1 58.7
1.4 1.7 2.3 0.7 3.5 0.4 5.1 6.1 11.6 2.9 2.1 0.8 1.8 4.4
0.2 0.9 1.0 1.6 1.9 3.0 0.7 0.4 0.8 0.6 0.8 0.1 0.7 0.6
0.4 0.1 0.3 1.3 2.3 3.3 1.7 0.7 0.7 0.6 0.0 0.7 0.1 1.5
1.1 0.7 0.3 0.6 1.5 3.1 0.5 0.2 1.6 0.1 0.8 0.2 0.7 1.1
Average errors exceeding MPE were reported in bold.
10.0%
10.0% 5.0% 0.0%
up to 1965 (33) 1966-1970 (34) 1971-1975 (33)
5.0% 0.0%
1976-1980 (22)
-5.0%
1981-1985 (28)
-5.0%
-10.0%
1986-1990 (29)
-10.0%
average
-15.0%
-15.0%
-20.0%
-20.0%
1990-1995 (73) 1996-2000 (89) 2001-2009 (61) average
Fig. 4. Average error of indication of the natural (a) and synthetic (b) diaphragm meters, grouped by year of construction.
0.6 0.1 0.2 1.1 1.9 3.2 0.8 0.3 0.7 0.4 0.1 0.4 0.2 1.2
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G. Ficco / Flow Measurement and Instrumentation 37 (2014) 65–72
or positive average errors at all the test flow-rates) is not negligible (about 28.9% of the whole sample, with 16.2% presenting all positive errors and 12.7% negative ones). Among these meters, natural diaphragm are prevailing. Fig. 6 shows that a systematic negative average error in consumer advantage occurs at Qmin. This average error increases as the age of the meter increases. This is probably due to the loss of flexibility of the diaphragms and particularly for the natural ones. On the other hand, no significant behavior occurred at higher flow-rates and the average errors of the meters always lie within the legal limits in subsequent verification. Fig. 7 depicts the trend of WME against the age of the meter, for synthetic (a) and natural (b) diaphragms. It is very interesting to observe that an increasing trend of WME was observed for the more recent synthetic diaphragm meters, whereas a slight reverse trend is evident for the natural ones. This different behavior can be ascribed to the change in diaphragm materials and components of the metering unit. Furthermore, a positive effect on the old natural
diaphragms could be represented by the fact that the most part of them were operated with wet gas. Nevertheless, this effect is not the same at any flow-rate. Therefore, the meters could exceed the limits in subsequent verifications at only one flow-rate (especially at Qmin) and the percentage of meters failing subsequent verification could be high at the same time. In Table 3 the average errors of indication of the meters are grouped by manufacturer. Even if the investigation involved 7 manufacturers whose meters are more common in the distribution network of Genoa, only for three of them the sample was found statistically significant (i.e. more than 15 investigated meters). Data in Table 3 clearly highlight the different gas meter performance as function of the manufacturer, especially at Qmin. In particular, manufacturer C shows significant errors in consumer advantage at Qmin and a negative WME, whereas manufacturer A presents always positive WMEs. The more reliable meters seems to be the ones of manufacturer B, for which an average behavior is basically within the limits of subsequent verification and a low
% WME
% WME
10.0%
10.0%
8.0%
8.0%
6.0%
6.0%
4.0%
4.0%
2.0%
2.0%
0.0%
0.0%
-2.0%
-2.0% -4.0%
-4.0% 0
0
5 10 15 20 25 30 35 40 45 50 55
5 10 15 20 25 30 35 40 45 50 55
Age of the meter (years)
Age of the meter (years)
Fig. 7. Trend of the WME as function of the age for synthetic (a) and natural (b) diaphragms gas meters.
Table 3 Average error of indication of the meters grouped by manufacturer. Manufacturer
A B C Others (4)
n
a
a
Diaphragm
Natural Synthetic Natural Synthetic Natural Synthetic Natural Synthetic
Average E%n
Meters Invstigated
Failing sub. ver.
%
Qmin
0.2 Qmax
0.5 Qmax
Qmax
WME
62 41 83 114 26 17 15 44
44 12 31 41 20 7 13 27
71.0 29.3 37.3 36.0 76.9 41.2 86.7 61.4
2.6 0.5 3.3 0.5 17.7 5.9 4.8 4.6
1.7 1.3 0.5 0.8 3.6 0.1 2.4 0.1
2.5 1.0 1.6 0.1 0.7 1.1 0.6 0.4
2.2 0.8 0.5 1.3 0.1 1.3 2.0 0.1
2.2 1.0 1.0 0.4 1.0 0.9 1.4 0.2
Average errors exceeding MPE were reported in bold. Manufacturers A and C use rotary distributing valve, whereas manufacturers B uses alternative distributing valve.
10.0%
10.0%
5.0%
5.0%
0.0% -5.0%
<100 m3/year (natural diaph.)
0.0% -5.0%
<100 m3/year synthetic diaph.)
-10.0%
100
-10.0%
100
-15.0%
>500 m3/year (natural diaph.)
-15.0%
>500 m3/year (synthetic diaph.)
-20.0%
-20.0%
Fig. 8. Average error by annual consumptions for natural (a) and synthetic (b) diaphragm gas meters.
G. Ficco / Flow Measurement and Instrumentation 37 (2014) 65–72
average WME is measured. On the other hand, manufacturer B presents the higher number of not functioning meters. It is important to point out that manufacturers A and C adopt a rotary distributing valve technology whereas manufacturer B adopt an alternative one. No significant trends of the errors were found as function of the typical installation (indoor/outdoor), of the size of the meter and of the user's appliances. On the other hand, the average annual consumptions of the meter seems to influence only the error of indication of the older ones (however generally within the MPEs in subsequent/in-service verification). Fig. 8, in fact, shows that in this case average errors increase as the average annual consumption decreases. Finally, the possible variations of the metrological performance of the synthetic diaphragm meters operated with wet and dry gas was investigated. Fig. 9 shows the relative performance in terms of average errors. No particular differences are evident, even if a flattening of the error curve of the dry gas is evident with a positive drift at Qmin. Furthermore, a not-negligible percentage of the meters operated with dry gas were found not functioning. From the data in Table 4 it can be pointed out that these meters were represented by: (i) 5.0% of natural diaphragm meters; (ii) 3.6% of synthetic diaphragm meters operated with wet gas; and (iii) 13.8% of synthetic diaphragm meters operated with dry gas. It is important to highlight that the above mentioned data only considered the meters presenting errors of indication within 50%. In fact, errors higher than 50% were considered as outliers and the corresponding meters contribute to the statistics presented only as
E% 10.0% 5.0% 0.0% -5.0% -10.0%
wet gas -15.0%
dry gas
-20.0%
Fig. 9. Average errors of synthetic meters in dry/wet gas.
71
meters failing subsequent verification. These latter constitute the 15.9% of the whole sample. Most of these meters showed significant negative errors at Qmin (i.e. in consumer advantage), and only very few meters presented a very poor metrological behavior (see Table 4). Some meters were found not functioning (about 4.7% of the whole sample), and this can be reasonably ascribed also to the potentially long time elapsed between removal and testing, since many meters did not work for 2–3 months before tests. It is important to point out that not functioning meters do not allow gas flow. On one hand, the percentages of not functioning meters is predictably increasing up to 40 years age of the meter. On the other hand an unpredictable peak has been found for recent synthetic meters operated with dry gas, and this could be adequately focused by manufacturers since this condition is now the most common in modern distribution networks. The author also investigated the possible correlation between significant errors of indication and other mechanical faults of the meters. All the meters tested for pressure drop (that is difference between the pressure measured at the inlet and outlet connections of the meter whilst the meter is operating) were found largely within the limit of 2 mbar predicted in [10]. Nevertheless, synthetic diaphragm meters showed average values higher than the older natural diaphragms ones. This is probably due to the lower cyclic volume of the synthetic diaphragms compared with the natural diaphragm ones. During disassembly and visual inspection no marks of tampering were found. Moreover, only few natural diaphragm meters failed the external leak tightness and the resistance to internal pressure tests. In particular: (i) 10 meters (i.e. about 2.5% of the sample) failed the external leak tightness test; and (ii) 39 meters (i.e. about 10% of the sample) failed the resistance to internal pressure test. Basically, these were natural diaphragm meters manufactured before 1990. On the contrary, the better performance of recent meters are probably due to the improved technologies applied by manufacturers. As an example, external cases are now welded whereas in older meters mechanical joints were used as well as the internal leakages of the meters were also reduced. As concerns planarity test of grid and distributing valve, the measurements showed that many older meters (about 50%) exceed the typical tolerance in production (0.020 mm). Normally, when a planarity failure is found, it occurs on both the grid and the distributing valve. Almost all the natural diaphragms meters, which failed the internal and the external leakage tests, present a significant planarity error on grid-distributing valve. Finally, the meters with a significant planarity error on grid-distributing valve
Table 4 Analysis of the main faults occurred on the outliers meters. Period of manufacturing
Investigated meters
Meters presenting significant errors at all the testing flow-rates (%)
Meters presenting negative error higher than 50% at Qmin (%)
Meters not functioning (%)
Total (%)
2001–2006 1996–2000 1990–1995 1986–1990 1981–1985 1976–1980 1971–1975 1966–1970 Up to 1965 Investigated meters (all) Synthetic meters (dry gas) Synthetic meters (wet gas) Synthetic meters (all) Natural meters (all)
61 89 73 29 28 22 33 34 33 402 29 194 223 179
1.6 4.5 5.5 0.0 3.6 0.0 3.0 8.8 15.2 4.7 0.0 4.6 4.0 5.6
3.3 1.1 2.7 3.4 0.0 4.5 21.2 11.8 24.2 6.5 3.4 1.5 1.8 11.7
3.3 3.4 6.8 6.9 10.7 13.6 3.0 0.0 0.0 4.7 13.8 3.6 4.9 5.0
8.2 9.0 15.1 10.3 14.3 18.2 27.3 20.6 39.4 15.9 17.2 9.8 10.8 22.3
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G. Ficco / Flow Measurement and Instrumentation 37 (2014) 65–72
and also failing the internal leakage test, present a significant error of indication, especially at low flow-rates.
5. Conclusions This paper presents the results of a wide experimental study carried out on 402 diaphragm meters installed in the natural gas distribution network of the city of Genoa (Italy). The study's aim was to evaluate the metrological performance of domestic diaphragms gas meters as function of different influence parameters such as age, manufacturer, and type of diaphragm. Despite in distribution networks very old meters are commonly installed (since the legal duration of such devices was only recently introduced), the results of the tests were found good in terms of consumers protection. In fact, the average error of indication of the meters is generally close to zero and the average weighted mean error (WME) of the whole population investigated is almost within the limit in initial verification. Nevertheless, it was clearly demonstrated that at minimum flow-rate (Qmin) the error of the meters is almost always negative (i.e. in consumer advantage) and it spreads as the age of the meter increases. On the other hand, in terms of metrological performance of the single meter, tests showed a not excellent stability. This condition may be critical since a mandatory 15-year legal duration of domestic gas meters is allowed in Italy. As regards the reliability of diaphragm meters, the test showed that 4.7% of the investigated meters were found not functioning and particularly synthetic diaphragm meters operated with dry gas seem to show a not good reliability. Finally, on the basis of the experimental campaign performed, the following considerations can be summarized: – the percentage of meters failing subsequent verification is significant (about 48%), also for more recent meters installed by less than 15 years (about 35%); – an increasing trend of WME as function of the age of the meter was observed for synthetic diaphragms, whereas a slight reverse trend came out for natural ones; – natural diaphragm meters operated with wet gas typically present lower WMEs; – a significant number of meters showed a systematic drift of the error, both in consumer advantage (about 12.7%) and disadvantage (about 16.2%); – the number of failures (e.g. meters not functioning or errors greater than 50%) was 15.9%; – the most significant errors occurred at minimum flow-rate Qmin, where a negative trend (i.e. in consumer advantage)
increasing with the age of the meter was observed, especially for certain manufacturers; and – the meters with alternative distributing valve presented slight better performance than the rotary ones in terms of error of indication. Future developments of this research will be focused on the comparison of the new “static” meters with diaphragm ones and the estimation of the short term drift, also to validate the actual legal duration. Acknowledgments This work was sponsored by Genova Reti Gas, the natural gas distribution company of Genoa. The author gratefully acknowledges President Eng. Paolo Del Gaudio for worthy technical discussions and for providing data and used meters for testing purposes. Furthermore, special acknowledgments to Prof. Paolo Vigo, President of the Scientific and Technological Park of Southern Lazio (PALMER), for providing facilities and laboratories to perform some of the mechanical tests. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
[18] [19] [20]
Directive 2012/27/EU on Energy Efficiency; 2012. Directive 2004/22/EU on Measuring Instruments (MID Directive); 2004. AEEG Resolution 155/08; 2008. Italian law n.99/2009, 2009-07-23; 2009. de Matos M, da Silva Ferreira V. Gas mass-flow meters: principles and applications. Flow Meas Instrum 2010;21:143–9. Buonanno G. On field characterization of static domestic gas flowmeters. Measurement 2000;27:277–85. Buonanno G, Carotenuto A, Dell'Isola M. The influence of reference condition correction on natural gas flow measurement. Measurement 1998;23:77–91. Vigo P, Cascetta F. The future domestic gas meter review of current developments. Measurement 1993;13:129–45. Dell'Isola M, Cannizzo M, Diritti M. Measurement of high-pressure natural gas flow using ultrasonic flowmeters. Measurement 1997;20(2):75–89. EN 1359, Gas meters. Diaphragm meters; 2006. EN 14236, Ultrasonic domestic gas meters; 2007. Nilsson U, Delsing J. in situ detection of inaccurate gas flow meters using a fingerprint technique. Flow Meas Instrum 1998;9:143–52. Betta G, Dell'Isola M. Optimum choice of measurement points for sensor calibration. Measurement 1996;17(2):115–25. OIML R137-1, Gas meters, 2012. ISO/IEC Guide 98-3:2008 Uncertainty of measurement – Part 3: Guide to the expression of uncertainty in measurement (GUM:1995), Geneve. 〈www.autorita.energia.it〉. Arpino F, Dell'Isola M, Ficco G, Vigo P. Unaccounted for gas in natural gas transmission networks: prediction model and analysis of the solutions. J. Nat. Gas Sci. Eng. 2014;17:58–70, http://dx.doi.org/10.1016/j.jngse.2014.01.003. Directive 71/318/EU; 1971. UNI 7988, Contatori di gas. Prescrizioni di sicurezza e metrologiche; 1986. Italian Decree n. 75 2012-04-16; 2012.
Contents lists available at ScienceDirect
Flow Measurement and Instrumentation journal homepage: www.elsevier.com/locate/flowmeasinst
Metrological performance of diaphragm gas meters in distribution networks G. Ficco n DICEM, Department of Civil and Mechanical Engineering of UNICLAM, University of Cassino and Southern Lazio, via Di Biasio 43, 03043 Cassino, Italy
art ic l e i nf o
a b s t r a c t
Article history: Received 21 November 2013 Received in revised form 31 January 2014 Accepted 2 March 2014 Available online 18 March 2014
In natural gas distribution networks a great number of domestic diaphragm gas meters are currently installed (about 15 million in Italy) and most of them are very old meters. Furthermore, some National Authorities nowadays impose strict roll-out procedures of the existing meters with new smart meters. To this aim, diaphragm meters in “hybrid” configuration equipped with electronic and transmission devices are supposed to be widely installed by natural gas distribution companies in the first roll-out campaigns, since they present a fully developed technology despite of the new ultrasonic and thermal mass meters. In this paper the author presents the results of a wide experimental study aimed to assess how metrological performance of diaphragm gas meters worsen over time as function of age, installation conditions, manufacturing technology, etc. The results of the experimental analysis show that a significant number of the meters investigated failed subsequent verification and presented a not excellent stability. On the other hand, the average percentage errors and the weighted mean error of the meters were measured basically close to zero, also for very old meters. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Diaphragm gas meter Smart metering Legal verification Maximum permissible error
1. Introduction EU Directives 2012/27 [1] on energy efficiency and 2004/22 [2] on measuring instruments (known as MID Directive) impose very strict constraints in terms of essential requirements of gas meters both for measuring and billing. Therefore, some National Authorities recently issued mandatory resolutions [3] to gradually substitute domestic gas meters in distribution networks with new smart meters provided with remote data transmission devices useful for management purposes. Furthermore, from a legal metrology point of view, gas meters were not subjected to subsequent or in-service verifications by the competent authorities so far, and a legal duration of 15 years was fixed just in 2009 [4]. In Italy, the roll-out program involves a very large number of domestic gas meters (about 15 million). New gas smart meters are expected to guarantee more strict metrological performance, also allowing the correction of the volumes through the real gas temperature measured by a built-in sensor. In the design of future smart meters, manufacturers are now developing new metering principles with no moving parts (commonly known as “static meters”), like thermal mass [5] and ultrasonic [6]. Even if these new principles in gas metering have very significant potential (such as digital output, the absence of
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moving parts, direct mass measurement), their reliability in some critical conditions typical of natural gas distribution networks (i.e. zero flow, minimum flow-rate, gas quality changes, presence of dust and contaminants, etc.) still has to be demonstrated. In such scenario, diaphragm meters are still the most widely used in natural gas distribution networks and gas operators predict that in the next years they will prevail in the first rollout campaigns, despite of the “static” ones. In fact, because of their fully developed technology and their cheap costs, many manufacturers decided to meet actual roll-out requirements adapting diaphragm gas meters with electronic and transmission devices. Thus, mechanical diaphragm meters in “hybrid” configuration will operate in Italian distribution networks for many other years, since the 15 years legal duration currently in force. Therefore, both manufacturers and distribution companies are deeply interested in improving and controlling the metrological effectiveness of these types of meters. In fact, manufacturers continuously deal with constructive issues strongly influencing the metrological performance and stability of the meters, such as: (i) material and components of the metering unit; (ii) diaphragm stability and seal; (iii) internal and external leakages; and (iv) mechanical wear of the moving parts. On the other hand, gas distribution companies are strongly interested in defining effective roll-out plans based on homogeneous lots as function of drift, age, manufacturer and measuring technology, class of consumptions of the meters and so on.
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Nomenclature AEEG CMM E EN EU ki
Italian Authority for electricity and natural gas Coordinate measuring machine Error of indication, % European norm European Union Weighting factor, adimensional
From a technical point of view, the estimation of the metrological performance of the meters installed in natural gas distribution networks is a very difficult issue because of the wide variability of many influential parameters as: (i) manufacturer, (ii) year of construction, (iii) average annual consumptions, (iv) installation (e.g. indoor/outdoor), (v) user's appliances (e.g. heating, kitchen, boiler), and (vi) size of the meter. Furthermore, the existing meters installed before 1990 are commonly made up of natural diaphragms, which are widely recognized as a source of significant errors because of the performance decay of the diaphragm itself. In recent years (after 1990) natural diaphragms were replaced by synthetic ones, which are expected to be more resistant and reliable and to guarantee more stable metrological performance. Finally, natural gas measurement issues as well as metrological performance of gas meters were deeply investigated in the scientific literature [6–9]. In this paper, the author presents the results of a detailed analysis of the metrological performance of domestic diaphragm gas meters installed in the natural gas distribution network of Genoa (Italy). The distribution network of Genoa represents a very interesting situation. In fact, both natural and synthetic diaphragm meters are currently installed (as in many other networks in Italy) and both wet and dry natural gas is distributed. A wide sample made up of 402 diaphragm gas meters was analyzed; in particular, the author conducted the following metrological tests: (i) accuracy and pressure drop, (ii) leakage, and (iii) visual check and dimensional measurements.
2. Smart gas meters for domestic applications In this section, a brief description of the “static” thermal mass and ultrasonic gas meter for domestic applications is presented, together with a comparison with the traditional diaphragm gas meter. Ultrasonic meters are currently the innovative “static” meters at the most advanced stage of development and experimentation, also for in service application. They surely represent a very good solution in the near future for natural gas domestic measurements. Ultrasonic gas meters for domestic applications are based on the time of flight measurement of an acoustic wave traveling in a gas flowing in a closed section. As regards fiscal metering of domestic gas volumes, normally a single-path configuration with two transducers (a transmitter and a receiver) and a measuring section where the acoustic wave is eventually reflected one or more times is used [6]. Ultrasonic gas meters in domestic applications measure the gas velocity in a given section of a restricted pipe. Therefore, the gas flow-rate is calculated by multiplying the measured velocity by the known section of the measuring pipe itself. Finally, the volume is calculated integrating the flow-rate over time. On the metrological hand, ultrasonic gas meters present good performance and costs comparable to the hybrid diaphragm meter ones. On the contrary, they still have to demonstrate a full reliability in domestic
MID MPE OIML Qmax Qmin Qt UAG WME
Measuring Instrument Directive Maximum Permissible Error, % International Organization for Legal Metrology maximum flow-rate of the meter, m3/h minimum flow-rate of the meter, m3/h transition flow-rate of the meter, m3/h unaccounted for gas weighted mean error, %
applications where installation effects and potential contaminants could be significant. A very promising technology for gas mass measurements also in domestic field is represented by the thermal mass principle. Thermal mass gas meters are based on the relationship between the output voltage of a sensor (constituted in the last releases by a microelectro-mechanical systems, MEMS) and the rate of heat transfer between the sensor itself and the flowing gas. Unfortunately, the amount of heat transfer depends on the gas mass flowrate, as the main effect, but also on the gas composition, which influences its thermo-physical properties, like thermal conductivity and diffusivity. Thus, integrating over time and using thermophysical properties of the gas flowing the volume is obtained for fiscal measurement and billing. The technology of these sensors gives an output voltage directly proportional to the gas mass-flow rate and the small size of these devices leads to a high sensitivity and fast response time [5]. Unlike conventional diaphragm and new ultrasonic meters, for which both consolidated national and international technical standards can be applied [10,11], till today no standard references for thermal mass gas meters are available although many national and international technical groups are working on this task. Unfortunately, even if different laboratory analyses were carried out and some pattern approval tests have been already passed, field analysis about metrological performance of these kinds of meters (in particular in countries in which the gas composition widely varies in an unpredictable way, like in Italy) are still not available in scientific literature. Despite the above mentioned “static” technologies are strongly emerging also in gas metering domestic applications, nowadays, mechanical diaphragm represents the near totality of gas meters installed in distribution networks. A diaphragm meter is a volumetric gas meter that measures the gas counting the number of times a cyclic nominal volume is emptied and filled. The errors occurring in a diaphragm gas meter are mainly due to leakages, geometrical changes and mechanical failures as well as to the unavoidable decay of materials over time. As regards the movement of the distributing valve, manufacturers currently adopt a rotary or alternative technology, as depicted in Fig. 1. The advantage of these meters is the simplicity of their construction and the consequent low costs, whereas their limits are: (i) the presence of moving parts subject to wear, (ii) the high pressure drop, (iii) the mechanical output; and (iv) the inability to indicate an instantaneous flow-rate value [12]. To meet the actual roll-out requirements, hybrid domestic gas meters combine new smart electronic features on a traditional diaphragm mechanical body. In such meters the mechanical register is replaced by an electronic unit, equipped with a data transmission device and a remote firmware upgrade function. Moreover, all data are recorded on a non-volatile memory of the electronic device. According to the recent legal metrology regulations [3], an hybrid smart meter has also to be provided with a temperature sensor and with a remotely controlled mechanical valve that allows or forbids the gas flow.
G. Ficco / Flow Measurement and Instrumentation 37 (2014) 65–72
67
Fig. 1. Typical measuring unit technology adopted by diaphragm gas meter manufacturers: (a) alternative distributing valve (courtesy of Sacofgas 1927 Spa) and (b) rotary distributing valve (courtesy of Itron).
From a metrological point of view, hybrid smart meters are unavoidably affected by the mechanical constraints of the metering unit, but they allow the error correction directly via software. It is very important to point out that electronic device allows hybrid diaphragm meters to apply effective correction factors at each test flow-rate, despite of the mechanical zero correction of the traditional ones. This feature could be very important not only in initial verification when the requirements of MID annex D [2] are met by the manufacturer, but also in subsequent ones, where specific corrections could be applied as function also of the drift.
3. Test methods and experimental apparatus A strict procedure for the design of the tests [13] was carried out in order to estimate the metrological performance of old domestic diaphragm gas meters and to better understand the causes of the expected drift. Seven lots, of about 60 diaphragm gas meters each, were collected from the Genoa natural gas distribution network: three lots with natural diaphragms (installed before 1990) and four lots with synthetic diaphragms (installed after 1990). Among the latter, one lot was made up of 52 synthetic diaphragm meters installed in a part of the network where only dry gas is distributed. After removal all the meters were immediately filled with natural gas, sealed and stored in a conditioned room before their transportation to the laboratory. Accuracy and pressure absorption tests were carried out at the university measurement laboratory LAMI of the UNICLAM (accredited calibration laboratory 105 by Accredia, the Italian accreditation body). The other tests were carried out at the Scientific and Technological Park of Southern Lazio (PALMER, Ferentino, Italy, accredited testing laboratory 273 and accredited calibration laboratory 85). A brief description of the tests is hereinafter reported. 3.1. Accuracy and pressure drop The tests were performed through a 600 L bell prover test bench (Fig. 2) placed at the LAMI where both temperature and humidity are continuously controlled at (2071) 1C and (50710)%UR, respectively.
Fig. 2. Bell prover test bench for accuracy and pressure absorption tests.
Traceability of the bench starts from the 50 L volume standard calibrated with an expanded uncertainty of 3.3 ml at about 95% confidence level by INRIM, the Italian Primary Institute of Metrology in Turin. Auxiliary calibrated devices were used during the tests for temperature, humidity and pressure measurements both in the test environment and in the bench. At the end of the metrological chain,
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the typical overall expanded uncertainty is 0.40% at about 95% confidence level. Thus, the calibration method adopted is fully consistent with the legal metrology rule which allows an expanded uncertainty of the test bench in verification not exceeding 1/3 of the maximum permissible errors (MPE) of the device under test [14].
is the tolerance currently adopted by manufacturers in their processes). Planarity uncertainty for measurements can be assumed to be about 0.008 mm with a coverage factor k¼2 (corresponding to a 95% confidence level), as the calibration uncertainty of the CMM is 0.005 mm and type A, repeatability and resolution uncertainty contribution were considered [15].
3.2. Leakage The external leak tightness test was carried out at normal laboratory temperature by immersing the meter, without its index, in water and observing for leakage for about 30 s after any external trapped air has dispersed. The meter under test is pressurized by means of a specific test bench equipped with a calibrated air flowmeter and a digital manometer. During the resistance to internal pressure test the case of the meter under test is pressurized progressively with air to 1.5 times the maximum working pressure of the meter [10]. Test pressure is then maintained for 30 min and then released, also ensuring that the rate of pressurization or depressurization does not exceed 350 mbar/s. 3.3. Visual check and dimensional measurements After the external leak tightness test, the meters were disassembled and checked in order to detect: (i) the integrity of couplings and of the exit pipe; (ii) the presence of evident faults on the body of the meter; and (iii) the possible leakages from grid-distributing valve and their wear conditions. The author also investigated the possible correlation between leakages and the error of indication by means of planarity measurements on grid and distributing valve. To this aim, a calibrated coordinate measuring machine (CMM) in a controlled environment (20 70.5 1C and 50 710%UR) was used. The planarity was measured as the maximum height difference between 12 points in the coupling area (Fig. 3). A tolerance of 0.020 mm was considered both for grid and distributing valve (that
4. Results and discussion In this section, the results and the discussion of the accuracy test were emphasized as they are essential both for consumer protection and integrity of distribution as well as for the correct estimation of unaccounted for gas (UAG) in city distribution networks [16,17]. The error of indication of the meters was measured at four flow-rates values: Qmin, 0.2 Qmax, Qmax in compliance to the Italian legal metrology law in force before 2007 [18,19] and 0.5 Qmax. The weighted mean error of indication (WME), which is a function of the percentage errors and of the flow-rates at which the errors occurred, was calculated for each investigated meter through the following equation given in [14]: n
∑ ki E i
WME ¼
i¼1 n
∑ ki
;
8 < ki ¼ QQ i ; max
for
Q i r 0:7Q max
: ki ¼ 1:4 QQ i ; max
for
0:7Q max oQ i r Q max
i¼1
ð1Þ where ki is the wheighting factor and Ei is the percentage error of indication at the flow-rate Qi. As regards the metrological limits in verification, the EU MID Directive [3] regulates only the initial conformity assessment. Thus, each member state has the faculty to define its own specific approach in subsequent verifications guaranteeing the continuity with the existing national rules.
Fig. 3. Measuring points for planarity on the distributing valve (a) and on the grid (b).
Table 1 Maximum Permissible Error (MPE) for domestic gas meters (MID class 1,5) in initial conformity assessment and in subsequent verification/service in Italy. Flow-rate range
Qmin o Qo Qtn Qtn o Qo Qmax WME
MPE in initial conformity assessment
MPE in subsequent/in-service verification
OIML R137 [14] and MID [2] in force after 2007
Italian law in force before 2007 [19]
OIML R137 [14] and Italian decree n.75/2012 [20]
Italian law in force before 2012 [19]
73.0% 71.5% 70.6%
73.0% 72.0% –
7 6.0% 7 3.0% –
n.a. n.a. –
n The transition flow-rate Qt for diaphragm gas meters is equal to 0,1 Qmax [10] and it is the flow-rate at which the flow-rate range is divided into two zones, the ‘upper zone’ and the ‘lower zone’, each one with a different characteristic MPE.
G. Ficco / Flow Measurement and Instrumentation 37 (2014) 65–72
As an example, in Italy, according to the Decree n.75 dated 2012-04-16 [20] domestic gas meters with Qmax up to 10 m3/h are not subjected to subsequent verifications and only random in-service ones are to be applied. Furthermore, such gas meters are also checked on demand of the users. The maximum permissible error (MPE) in subsequent/in-service verifications is double of the corresponding error in the initial conformity assessment as stated in [14]. Table 1 shows the actual MPEs for domestic gas meters (MID class 1,5) in initial and subsequent verification, together with the ones in force before 2007. As regards the WME, only a MPE equal to 70.6% in initial conformity assessment is allowed, whereas no limit for subsequent and in-service verifications is indicated. In Table 2 the average errors of indication of the investigated meters (grouped by year of construction) are presented, together with the percentage of the meters failing subsequent verification. In Fig. 4 the error curves of synthetic and natural diaphragm meters are presented separately, whereas Fig. 5 shows the percentage of the meters failing subsequent verification is undoubtedly increasing with the age of the meter. It can be pointed out that the percentage of the meters failing subsequent verification is significant, also for the more recent meters. In fact, according to the legal metrology regulations, a meter is considered to fail when the error of indication exceeds MPE even at only one flow-rate. The test demonstrated that the 22% of the investigated meters failed only at one flow-rate, especially at Qmin. Basically, synthetic diaphragm meters present negative WMEs (i.e. in consumer advantage), whereas the natural ones present positive WMEs (i.e. in consumer disadvantage). Furthermore, the percentage of the meters presenting a systematic drift (i.e. negative
69
100% 90% 80% 70% 60% 50% 40% 30% 20%
synthetic natural
10% 0% 0
5
10
15
20
25
30
35
40
45
50
55
Age of the meter (years) Fig. 5. Percentage of the meters failing subsequent verification in respect to the age of the meter.
10.0% 5.0% 0.0%
E%
-5.0% Qmin
-10.0%
0.2 Qmax 0.5 Qmax
-15.0%
Qmax
-20.0% 8
13 18 23 28 33 38 43 50
Age of the meter (years) Fig. 6. Trend of the average error of indication at different flow-rates as function of the age of the meters.
Table 2 Error of indication of the meters grouped by year of construction. Period of manufacturing of the meter
2001–2009 1996–2000 1990–1995 1986–1990 1981–1985 1976–1980 1971–1975 1966–1970 Up to 1965 Investigated meters (all) Synthetic meters (dry gas) Synthetic meters (wet gas) Synthetic meters (all) Natural meters (all) n
Age (years)
8 13 18 23 28 33 38 43 50
Average E%n
Meters
WME
Investigated
Failing sub. ver.
%
Qmin
0.2 Qmax
0.5 Qmax
Qmax
61 89 73 29 28 22 33 34 33 402 29 194 223 179
23 29 33 12 14 14 22 18 25 190 7 78 85 105
37.7 32.6 45.2 41.4 50.0 63.6 66.7 52.9 75.8 47.3 24.1 40.2 38.1 58.7
1.4 1.7 2.3 0.7 3.5 0.4 5.1 6.1 11.6 2.9 2.1 0.8 1.8 4.4
0.2 0.9 1.0 1.6 1.9 3.0 0.7 0.4 0.8 0.6 0.8 0.1 0.7 0.6
0.4 0.1 0.3 1.3 2.3 3.3 1.7 0.7 0.7 0.6 0.0 0.7 0.1 1.5
1.1 0.7 0.3 0.6 1.5 3.1 0.5 0.2 1.6 0.1 0.8 0.2 0.7 1.1
Average errors exceeding MPE were reported in bold.
10.0%
10.0% 5.0% 0.0%
up to 1965 (33) 1966-1970 (34) 1971-1975 (33)
5.0% 0.0%
1976-1980 (22)
-5.0%
1981-1985 (28)
-5.0%
-10.0%
1986-1990 (29)
-10.0%
average
-15.0%
-15.0%
-20.0%
-20.0%
1990-1995 (73) 1996-2000 (89) 2001-2009 (61) average
Fig. 4. Average error of indication of the natural (a) and synthetic (b) diaphragm meters, grouped by year of construction.
0.6 0.1 0.2 1.1 1.9 3.2 0.8 0.3 0.7 0.4 0.1 0.4 0.2 1.2
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or positive average errors at all the test flow-rates) is not negligible (about 28.9% of the whole sample, with 16.2% presenting all positive errors and 12.7% negative ones). Among these meters, natural diaphragm are prevailing. Fig. 6 shows that a systematic negative average error in consumer advantage occurs at Qmin. This average error increases as the age of the meter increases. This is probably due to the loss of flexibility of the diaphragms and particularly for the natural ones. On the other hand, no significant behavior occurred at higher flow-rates and the average errors of the meters always lie within the legal limits in subsequent verification. Fig. 7 depicts the trend of WME against the age of the meter, for synthetic (a) and natural (b) diaphragms. It is very interesting to observe that an increasing trend of WME was observed for the more recent synthetic diaphragm meters, whereas a slight reverse trend is evident for the natural ones. This different behavior can be ascribed to the change in diaphragm materials and components of the metering unit. Furthermore, a positive effect on the old natural
diaphragms could be represented by the fact that the most part of them were operated with wet gas. Nevertheless, this effect is not the same at any flow-rate. Therefore, the meters could exceed the limits in subsequent verifications at only one flow-rate (especially at Qmin) and the percentage of meters failing subsequent verification could be high at the same time. In Table 3 the average errors of indication of the meters are grouped by manufacturer. Even if the investigation involved 7 manufacturers whose meters are more common in the distribution network of Genoa, only for three of them the sample was found statistically significant (i.e. more than 15 investigated meters). Data in Table 3 clearly highlight the different gas meter performance as function of the manufacturer, especially at Qmin. In particular, manufacturer C shows significant errors in consumer advantage at Qmin and a negative WME, whereas manufacturer A presents always positive WMEs. The more reliable meters seems to be the ones of manufacturer B, for which an average behavior is basically within the limits of subsequent verification and a low
% WME
% WME
10.0%
10.0%
8.0%
8.0%
6.0%
6.0%
4.0%
4.0%
2.0%
2.0%
0.0%
0.0%
-2.0%
-2.0% -4.0%
-4.0% 0
0
5 10 15 20 25 30 35 40 45 50 55
5 10 15 20 25 30 35 40 45 50 55
Age of the meter (years)
Age of the meter (years)
Fig. 7. Trend of the WME as function of the age for synthetic (a) and natural (b) diaphragms gas meters.
Table 3 Average error of indication of the meters grouped by manufacturer. Manufacturer
A B C Others (4)
n
a
a
Diaphragm
Natural Synthetic Natural Synthetic Natural Synthetic Natural Synthetic
Average E%n
Meters Invstigated
Failing sub. ver.
%
Qmin
0.2 Qmax
0.5 Qmax
Qmax
WME
62 41 83 114 26 17 15 44
44 12 31 41 20 7 13 27
71.0 29.3 37.3 36.0 76.9 41.2 86.7 61.4
2.6 0.5 3.3 0.5 17.7 5.9 4.8 4.6
1.7 1.3 0.5 0.8 3.6 0.1 2.4 0.1
2.5 1.0 1.6 0.1 0.7 1.1 0.6 0.4
2.2 0.8 0.5 1.3 0.1 1.3 2.0 0.1
2.2 1.0 1.0 0.4 1.0 0.9 1.4 0.2
Average errors exceeding MPE were reported in bold. Manufacturers A and C use rotary distributing valve, whereas manufacturers B uses alternative distributing valve.
10.0%
10.0%
5.0%
5.0%
0.0% -5.0%
<100 m3/year (natural diaph.)
0.0% -5.0%
<100 m3/year synthetic diaph.)
-10.0%
100
-10.0%
100
-15.0%
>500 m3/year (natural diaph.)
-15.0%
>500 m3/year (synthetic diaph.)
-20.0%
-20.0%
Fig. 8. Average error by annual consumptions for natural (a) and synthetic (b) diaphragm gas meters.
G. Ficco / Flow Measurement and Instrumentation 37 (2014) 65–72
average WME is measured. On the other hand, manufacturer B presents the higher number of not functioning meters. It is important to point out that manufacturers A and C adopt a rotary distributing valve technology whereas manufacturer B adopt an alternative one. No significant trends of the errors were found as function of the typical installation (indoor/outdoor), of the size of the meter and of the user's appliances. On the other hand, the average annual consumptions of the meter seems to influence only the error of indication of the older ones (however generally within the MPEs in subsequent/in-service verification). Fig. 8, in fact, shows that in this case average errors increase as the average annual consumption decreases. Finally, the possible variations of the metrological performance of the synthetic diaphragm meters operated with wet and dry gas was investigated. Fig. 9 shows the relative performance in terms of average errors. No particular differences are evident, even if a flattening of the error curve of the dry gas is evident with a positive drift at Qmin. Furthermore, a not-negligible percentage of the meters operated with dry gas were found not functioning. From the data in Table 4 it can be pointed out that these meters were represented by: (i) 5.0% of natural diaphragm meters; (ii) 3.6% of synthetic diaphragm meters operated with wet gas; and (iii) 13.8% of synthetic diaphragm meters operated with dry gas. It is important to highlight that the above mentioned data only considered the meters presenting errors of indication within 50%. In fact, errors higher than 50% were considered as outliers and the corresponding meters contribute to the statistics presented only as
E% 10.0% 5.0% 0.0% -5.0% -10.0%
wet gas -15.0%
dry gas
-20.0%
Fig. 9. Average errors of synthetic meters in dry/wet gas.
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meters failing subsequent verification. These latter constitute the 15.9% of the whole sample. Most of these meters showed significant negative errors at Qmin (i.e. in consumer advantage), and only very few meters presented a very poor metrological behavior (see Table 4). Some meters were found not functioning (about 4.7% of the whole sample), and this can be reasonably ascribed also to the potentially long time elapsed between removal and testing, since many meters did not work for 2–3 months before tests. It is important to point out that not functioning meters do not allow gas flow. On one hand, the percentages of not functioning meters is predictably increasing up to 40 years age of the meter. On the other hand an unpredictable peak has been found for recent synthetic meters operated with dry gas, and this could be adequately focused by manufacturers since this condition is now the most common in modern distribution networks. The author also investigated the possible correlation between significant errors of indication and other mechanical faults of the meters. All the meters tested for pressure drop (that is difference between the pressure measured at the inlet and outlet connections of the meter whilst the meter is operating) were found largely within the limit of 2 mbar predicted in [10]. Nevertheless, synthetic diaphragm meters showed average values higher than the older natural diaphragms ones. This is probably due to the lower cyclic volume of the synthetic diaphragms compared with the natural diaphragm ones. During disassembly and visual inspection no marks of tampering were found. Moreover, only few natural diaphragm meters failed the external leak tightness and the resistance to internal pressure tests. In particular: (i) 10 meters (i.e. about 2.5% of the sample) failed the external leak tightness test; and (ii) 39 meters (i.e. about 10% of the sample) failed the resistance to internal pressure test. Basically, these were natural diaphragm meters manufactured before 1990. On the contrary, the better performance of recent meters are probably due to the improved technologies applied by manufacturers. As an example, external cases are now welded whereas in older meters mechanical joints were used as well as the internal leakages of the meters were also reduced. As concerns planarity test of grid and distributing valve, the measurements showed that many older meters (about 50%) exceed the typical tolerance in production (0.020 mm). Normally, when a planarity failure is found, it occurs on both the grid and the distributing valve. Almost all the natural diaphragms meters, which failed the internal and the external leakage tests, present a significant planarity error on grid-distributing valve. Finally, the meters with a significant planarity error on grid-distributing valve
Table 4 Analysis of the main faults occurred on the outliers meters. Period of manufacturing
Investigated meters
Meters presenting significant errors at all the testing flow-rates (%)
Meters presenting negative error higher than 50% at Qmin (%)
Meters not functioning (%)
Total (%)
2001–2006 1996–2000 1990–1995 1986–1990 1981–1985 1976–1980 1971–1975 1966–1970 Up to 1965 Investigated meters (all) Synthetic meters (dry gas) Synthetic meters (wet gas) Synthetic meters (all) Natural meters (all)
61 89 73 29 28 22 33 34 33 402 29 194 223 179
1.6 4.5 5.5 0.0 3.6 0.0 3.0 8.8 15.2 4.7 0.0 4.6 4.0 5.6
3.3 1.1 2.7 3.4 0.0 4.5 21.2 11.8 24.2 6.5 3.4 1.5 1.8 11.7
3.3 3.4 6.8 6.9 10.7 13.6 3.0 0.0 0.0 4.7 13.8 3.6 4.9 5.0
8.2 9.0 15.1 10.3 14.3 18.2 27.3 20.6 39.4 15.9 17.2 9.8 10.8 22.3
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G. Ficco / Flow Measurement and Instrumentation 37 (2014) 65–72
and also failing the internal leakage test, present a significant error of indication, especially at low flow-rates.
5. Conclusions This paper presents the results of a wide experimental study carried out on 402 diaphragm meters installed in the natural gas distribution network of the city of Genoa (Italy). The study's aim was to evaluate the metrological performance of domestic diaphragms gas meters as function of different influence parameters such as age, manufacturer, and type of diaphragm. Despite in distribution networks very old meters are commonly installed (since the legal duration of such devices was only recently introduced), the results of the tests were found good in terms of consumers protection. In fact, the average error of indication of the meters is generally close to zero and the average weighted mean error (WME) of the whole population investigated is almost within the limit in initial verification. Nevertheless, it was clearly demonstrated that at minimum flow-rate (Qmin) the error of the meters is almost always negative (i.e. in consumer advantage) and it spreads as the age of the meter increases. On the other hand, in terms of metrological performance of the single meter, tests showed a not excellent stability. This condition may be critical since a mandatory 15-year legal duration of domestic gas meters is allowed in Italy. As regards the reliability of diaphragm meters, the test showed that 4.7% of the investigated meters were found not functioning and particularly synthetic diaphragm meters operated with dry gas seem to show a not good reliability. Finally, on the basis of the experimental campaign performed, the following considerations can be summarized: – the percentage of meters failing subsequent verification is significant (about 48%), also for more recent meters installed by less than 15 years (about 35%); – an increasing trend of WME as function of the age of the meter was observed for synthetic diaphragms, whereas a slight reverse trend came out for natural ones; – natural diaphragm meters operated with wet gas typically present lower WMEs; – a significant number of meters showed a systematic drift of the error, both in consumer advantage (about 12.7%) and disadvantage (about 16.2%); – the number of failures (e.g. meters not functioning or errors greater than 50%) was 15.9%; – the most significant errors occurred at minimum flow-rate Qmin, where a negative trend (i.e. in consumer advantage)
increasing with the age of the meter was observed, especially for certain manufacturers; and – the meters with alternative distributing valve presented slight better performance than the rotary ones in terms of error of indication. Future developments of this research will be focused on the comparison of the new “static” meters with diaphragm ones and the estimation of the short term drift, also to validate the actual legal duration. Acknowledgments This work was sponsored by Genova Reti Gas, the natural gas distribution company of Genoa. The author gratefully acknowledges President Eng. Paolo Del Gaudio for worthy technical discussions and for providing data and used meters for testing purposes. Furthermore, special acknowledgments to Prof. Paolo Vigo, President of the Scientific and Technological Park of Southern Lazio (PALMER), for providing facilities and laboratories to perform some of the mechanical tests. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
[18] [19] [20]
Directive 2012/27/EU on Energy Efficiency; 2012. Directive 2004/22/EU on Measuring Instruments (MID Directive); 2004. AEEG Resolution 155/08; 2008. Italian law n.99/2009, 2009-07-23; 2009. de Matos M, da Silva Ferreira V. Gas mass-flow meters: principles and applications. Flow Meas Instrum 2010;21:143–9. Buonanno G. On field characterization of static domestic gas flowmeters. Measurement 2000;27:277–85. Buonanno G, Carotenuto A, Dell'Isola M. The influence of reference condition correction on natural gas flow measurement. Measurement 1998;23:77–91. Vigo P, Cascetta F. The future domestic gas meter review of current developments. Measurement 1993;13:129–45. Dell'Isola M, Cannizzo M, Diritti M. Measurement of high-pressure natural gas flow using ultrasonic flowmeters. Measurement 1997;20(2):75–89. EN 1359, Gas meters. Diaphragm meters; 2006. EN 14236, Ultrasonic domestic gas meters; 2007. Nilsson U, Delsing J. in situ detection of inaccurate gas flow meters using a fingerprint technique. Flow Meas Instrum 1998;9:143–52. Betta G, Dell'Isola M. Optimum choice of measurement points for sensor calibration. Measurement 1996;17(2):115–25. OIML R137-1, Gas meters, 2012. ISO/IEC Guide 98-3:2008 Uncertainty of measurement – Part 3: Guide to the expression of uncertainty in measurement (GUM:1995), Geneve. 〈www.autorita.energia.it〉. Arpino F, Dell'Isola M, Ficco G, Vigo P. Unaccounted for gas in natural gas transmission networks: prediction model and analysis of the solutions. J. Nat. Gas Sci. Eng. 2014;17:58–70, http://dx.doi.org/10.1016/j.jngse.2014.01.003. Directive 71/318/EU; 1971. UNI 7988, Contatori di gas. Prescrizioni di sicurezza e metrologiche; 1986. Italian Decree n. 75 2012-04-16; 2012.