THE THERMAL FLOW METER, A GAS METER FOR ENERGY MEASUREMENT

Transcription

1 THE THERMAL FLOW METER, A GAS METER FOR ENERGY MEASUREMENT Kazuto Otakane, Tokyo Gas Co., Ltd Katsuhito Sakai, Tokyo Gas Co., Ltd Minoru Seto, Tokyo Gas Co., Ltd 1. INTRODUCTION Tokyo Gas s new gas meter, further referred to as THERMAL, is a thermal flow meter presently being developed. This flow meter was designed to replace the older conventional meters now used in Japan. In Part1 of this paper, we will describe why this meter was necessary, and about its operating principles. We will also introduce the specifications of the THERMAL, and about some of the evaluation tests that were performed. In Part2 we will describe how this meter has the potentials of usage as a calorific flow meter for energy measurement. This new type of meter is not needed at the moment, for in Japan it is required by regulations for a gas supplying company to keep the calorific value of their supplied gas within a certain range. For example, we at Tokyo Gas, by voluntary restrictions, control the calorific value of our gas within a 1% range to maintain fairness in gas pricing. This means that the calorific value of the gas is a constant value, so all we need is measurement of flow to calculate the amount of energy we have supplied. However, possibilities of new abated regulations that do not require strict calorific value control might change the situation in the near future. If this takes place, it would reduce gas production costs, but it would also mean that the calorific values of supplied gases would vary; a fact that brings doubt to current pricing practices where only the flow is measured. We would not be able to accurately calculate the total calorific value supplied, unless a gas composition analyzer is used to measure the calorific value of the gas. This would be in addition to the temperature and pressure (both gage and atmosphere) measurements also required, for current meters measure volumetric flow. The price of currently available analyzers are so expensive that it would not be very realistic to install them with flow meters to each house, or for that matter, each portion of the service area. The situation in Europe seems to be that the calorific value of natural gas tends to vary about 10% according to gas field, so calculation methods using average values and/or rough area compensation have been adopted. This obviously would have been the case in Japan as well, but for money conscious Japanese customers, who do not like to pay a single yen more for calculation errors, the THERMAL is the answer. Because the THERMAL is a mass flow meter, and mass flow is related to calorific value, it has a possibility of becoming a calorific flow meter. After several experiments, we have come to know that, with a few restrictions to the composition of the gas, this meter can precisely output calorific value rate. Concerning the several types of LNG (liquidated natural gas) used by Tokyo Gas, results showed the

2 THERMAL to have extremely high accuracy, well within the tolerances of a standard gas meter. Furthermore, because the THERMAL does not need temperature or pressure compensations, it makes the whole measuring system very simple, compact, and inexpensive. Future scopes for the THERMAL s possibility as a calorific flow meter and test results are introduced in Part2. Part 1 2. NEED FOR A NEW GAS METER 2.1 Meters now in use The THERMAL was originally designed to measure large flow of gases. Its primary target was in replacing rotary positive displacement meters, which have a 95% share of large flow gas meters within the Tokyo Gas territory. The remaining meters are vortex shedding meters and turbine meters. Rotary positive displacement meters measure the volume of gas passing through with two rotors within a casing. When the casing is filled with gas, the rotors rotate and let a certain volume through. Volumetric flow is determined by counting the number of rotations. Vortex shedding meters have a triangular bluff body within the flow. This body generates vortices at a frequency that is a function of the Reynolds number, thus making it possible to calculate amount of flow. Turbine meters have bladed rotors within its path, and calculate flow from the rotation. At Tokyo Gas, the turbine meter is not actively used because of the errors caused by momentum when there is sudden increase or decrease in flow. All three meters are volumetric flow meters, so when used in mid-pressures or above (i.e. over 0.1MPa) a pressure sensor and temperature sensor is needed to convert flow rate into standard conditions. 2.2 Problems of the Rotary Positive Displacement Gas Meters Problems concerning rotary positive displacement meters are as follows. (1) Cost Besides the meter itself being expensive, it also needs additional instruments to measure pressure and temperature. (2) Piping The meter s casing needs a certain amount of volume, so its size tends to become large and heavy, making it difficult to install. Also, in Japan, the direction of the flow is limited to a downward direction. (3) Maintenance The meter needs to be overhauled frequently, and oil for the rotor also needs to be changed or added. (4) Measuring Range The turndown ratio is 20:1, thus making measurement at low flow rates inaccurate.

3 (5) Clogging Risks There is the risk of the rotor getting jammed because of foreign substance within the piping. When this happens the meter not only stops measuring, but the supply of gas is also stopped. 3. CHARACTERISTICS OF THE THERMAL 3.1 Advantages of the THERMAL The THERMAL is a meter that solves all of the problems referred to in the previous section. (1) Cost Reduction Because of its simple shape, the THERMAL is much smaller and obviously much cheaper. Not only are the meters themselves inexpensive, they do not need temperature or pressure transmitters, thus making the total installation cost much lower. (2) Simple Installation The THERMAL is much light-weighted than the rotary positive displacement meters, making installation very easy. For example, the #600 size rotary meter weighs 118kg, while the equivalent THERMAL (4B size) weighs only 28kg. Just being 1/4 in weight is significant enough, but the THERMAL is within the range where no special lifting equipment is needed. (3) No Maintenance The THERMAL does not have any moving parts, so there is no need for maintenance such as oiling and cleaning. (4) Wide Measuring Range One of the biggest advantages of the thermal flow sensor is its ability to measure in a wide range. The THERMAL, to obtain even a larger range, uses two different models of thermal flow sensors and has achieved a 160: 1 turndown ratio. (5) Error Risks Because of its structure, there is no need to worry about the halting of gas supply. Unpredictable accidents might lead to a cease in accurate measurement, but what you have left is a normal pipe, still letting the gas through. Figure 1. The THERMAL (Right), and the Rotary Positive Displacement Gas Meter (Left) 3.2 Operating Principles and Structure The THERMAL consists of two main parts, the detector and converter. The detector is the pipe

4 where the thermal sensors are located, and the converter is where printed circuit boards are contained. (1) The Micro Flow Sensors The THERMAL uses four thermal flow sensors called micro flow sensors. These sensors detect the velocity of gases, and convert them into electric signals. The structure of this sensor is shown in Figure 2. Upstream temperature sensor (Ru) Downstream temperature sensor (Rd) Heater (Rh) Silicon chip Environmental temperature sensor (Rr) Figure 2. Structure of the Micro Flow Sensor A heater (Rh) in the center, surrounded on both sides by temperature sensors (Ru and Rd), and an environmental temperature sensor (Rr) on the corner make up most of the micro flow sensor. These are made from platinum film, placed on top of a silicon chip that is approximately 1.7mm square. Figure 3. Operating Principle of the Micro Flow Sensor The operating principle of this sensor is shown in Figure 3. The heater (Rh) is designed to keep a constant differential temperature from the gas temperature (Rr). When there is no flow, the temperature at both Ru and Rd are the same. With flow, this balance becomes unsymmetrical making the resistance value of Ru and Rd different. This difference in temperature becomes larger with increase of flow, thus making it a function of mass flow velocity. The micro flow sensor picks this signal up as a difference in resistance, which is then calculated into mass flow rate. Changing the heater temperature and the distance from the heater to the temperature sensors result in micro flow sensors with different flow velocity ranges. To cover the wide measurement ranges recommended in OIML standards, the THERMAL use two types of micro flow sensors. (2) The Detector

5 Honeycomb Micro flow sensor Wire netting Figure 4. Flow Path Structure The flow path structure of the THERMAL is shown in Figure 4. It consists of just the micro flow sensors and an internal flow straightener/conditioner. The straightening of flow is carried out with a honeycomb, while three sheets of wire netting carry out the conditioning. These two components of the straightener/conditioner are spaced apart at a certain distance, and assembled together to form a cartridge. The sole objective of this device is to make a constant ratio between average flow velocity and velocity near the inner walls where the micro flow sensors are located. This turned out to be a grueling task, as anyone might imagine, but as a result of hard work and numerous failures, we were able to come up with an ideal shape. Some evaluation test results are introduced in the following section. (3) The Converter The converter is where we keep the microprocessor. This device enables the THERMAL to have numerous new functions. To introduce just a few; a liquid crystal display showing flow rate, totalized flow, and sensing errors (when there are any), a 4-20mA flow rate output, an adjustable flow pulse output, a memory space with a month s data of daily total flow, etc. The microprocessor also chooses which of the two types of micro flow sensors to drive, depending on the flow rate. The converter also contains a battery pack, for the THERMAL is required to operate for 10 years without maintenance. The software within the meter is designed so power consumption can be kept at a very low level. 3.3 Evaluation Test Results (1) Accuracy The accuracies of two models are shown in Figure 5 and 6. Fig.5 shows the results of the 80A(3 inch flange) model, and Fig.6 shows that of the 100A(4 inch flange) model. They are both within ±0.5% throughout the entire range, satisfying our target OIML standard for maximum permissible error on initial verification (±3% or ±1.5%, depending upon flow rate).

6 Accuracy(%) Model 80A accuracy OIML Flow rate (m3/h)[293k, kPa] Figure 5. Accuracy of Model 80A Accuracy(%) Model 100A accuracy OIML Flow rate (m3/h) [293K, kPa] Figure 6. Accuracy of Model 100A (2) Pressure effect Although the micro flow sensor is a mass flow sensor, it has some minute pressure effect. Tests showed that there was about a 2.5% error at a pressure of 0.3MPa. To solve this problem, a pressure sensor was installed into the meter, and correcting calculations were performed within the microprocessor. Figure 7 shows the accuracy of the THERMAL (80A model) after using the output of this sensor to correct the flow output. Results showed the THERMAL s output to be within permissible error at all pressures. accuracy(%) Atmospheric pressure 0.15MPa 0.3MPa 0.5MPa Flow rate(m3/h)[293k, kpa] Figure 7. Pressure effect (3) Pressure loss Pressure loss was measured to confirm that it satisfied our target value (See Figure 8).

7 Pressure loss (kpa) Air, atmospheric pressure Average velocity (m/s) Figure 8. Pressure Loss (4) Severe piping conditions In Japan, installation space is very limited, making it difficult to insure adequate straight pipe length. Therefore, we studied the effect of severe piping conditions to see if it affects accuracy. Tests were performed in three different piping situations (see Figs.9and 10). (a) Solid Double Bend 1 (b) Solid Double Bend 2 (c) Solid Triple Bend Figure 9 Severe Piping Tests Accuracy (%) OIML Solid double elbow(1) Solid double elbow(2) Solid triple elbow Flow rate (m3/h) [293K, kPa] Figure 10 Influence of piping conditions Again, results showed that errors were within the OIML recommendations. This test shows that our internal straightener has the ability to correct swirls generated immediately before the meters.

8 (5) Field tests At the present time, our new flow meters are being tested in three different locations within Japan. We deliberately chose the hottest and coldest locations possible, to see how they performed in severe ambient temperatures (see Table 1 and Figure 11). To monitor the accuracy of the THERMAL, we installed them in a series with existing meters. Needless to say, we also had to install temperature, gage pressure, and atmospheric pressure transmitters to correct the output of existing meters. Using data logging equipment connected to a cell phone, we have been monitoring the outputs of all the meters, and so far have been receiving excellent results (see Figure 12). Location Kumagaya Hokkaido Chiba Meter Size 80A (3 inch) 80A (3 inch) 80A (3 inch) Pressure Rating Middle Pressure B* Low Pressure** Middle Pressure B* Existing Meter Rotary Diaphragm Turbine Piping Direction Downward Upward Horizontal Features Hottest in Japan Coldest in Japan Near Tokyo Frequent Lightning * Middle Pressure B : MPa ** Low Pressure : under 0.1MPa Table 1: Locations of Field Tests (a) Kumagaya (b) Hokkaido (c) Chiba Figure 11 Field Test Conditions

9 Figure 12. Field Test Results (Kumagaya) 3.4 Specifications The THERMAL has a lineup of four models, depending upon its flange size. Their specifications are shown in Table 2. Pipe Diameter 50A (2 inch) 80A (3 inch) 100A (4 inch) 150A (6 inch) Maximum Flow Rate [293K, kPa] 160 (m 3 /h) 400 (m 3 /h) 650 (m 3 /h) 1600 (m 3 /h) (Qmax) Accuracy 1/160 Qmax to 1/10Qmax: Within ±3% 1/10 Qmax to Qmax: Within ±1.5% Operating Pressure MPa Power Supply Lithium Batteries Battery Life Over 10 years Overall Length 220mm 340mm 400mm 500mm (same as rotary) Weight 14 kg 23 kg 28 kg 44 kg Piping Direction Horizontal or Vertical (either upwards or downwards) Table 2: THERMAL Specifications

10 Part2 4. THERMAL AS A CALORIFIC FLOW METER 4.1 Test Results Part1 showed how the THERMAL is reliable as a gas flow meter. But now we are confronted with the need for a meter that can measure the calorific value rate of a variety of gas components. To determine if the THERMAL was up to the task, further evaluation was needed. The following chapter shows results from recent tests, performed to prove that the THERMAL can surely be used as a calorific flow meter within a certain range of gas compositions. The composition of LNG, used as raw material for city gas, differ with time and place of production. It was important to find out how this difference affects heat transfer (i.e. the THERMAL s accuracy). We surveyed natural gas compositions produced at major manufacturing sites, and conducted experiments for each type. We also conducted experiments on typical gas compositions currently supplied in Japan (see Table 3). Composition Fluctuation range CH (%) C 2 H (%) C 2 H (%) C 3 H (%) i-c 4 H (%) n-c 4 H (%) i-c 5 H (%) n-c 5 H (%) H 2 0 5(%) CO (%) N 2 0 3(%) Table3: Gas Content Fluctuation The result of accuracy calculated by calorific flow standard is shown in Figure 13. Although the types of gases we tested had great variations in composition, the output of the meter was within the permissible error in service. As we mentioned earlier, the THERMAL was proven to become the ideal calorific flow meter.

11 Accuracy(%) Calorif ic rate(mj/h) Type A Type B Type C Type D Type E Type X Gas Type A Type B Type C Type D Type E Type X CH % 88.1% 89.5% 90.1% 90.0% 89.2% C 2H 4 C 2H 6 6.3% 5.0% 4.1% 5.0% 5.3% C 3H 8 4.7% 5.3% 2.7% 3.2% 10.0% 1.9% i-c 4H % 0.8% 1.6% 0.7% 0.4% n-c 4H % 0.8% 2.1% 1.0% 0.5% i-c 5H % n-c 5H % H 2 CO 2 N 2 Figure 13. Accuracy of the THERMAL s calorific output 4.2 The influence of gases without heating value Within the gases in Figure 13, Type X contains nitrogen. Nitrogen does not have any calories, but they do transfer heat. This is the reason why errors for Type X show a positive amount. Figure 14 shows how the amount of nitrogen is related to the amount of error. Type X (2.3% nitrogen) and an experimental gas with 4.13% nitrogen were used for this experiment. This result proves to show that when this meter is used as a calorific flow meter, the ratio of nitrogen within a gas directly affects the output error. This would become a problem if the nitrogen ratio increased and errors exceeded recommended values, but concerning the gases that are presently being supplied, the THERMAL s accuracy would be within limits, thus making it an effective calorific flow meter. Figure 14. Amount of nitrogen related to the amount of error 5. CONCLUSION Tokyo Gas has developed a new gas meter and confirmed its abilities for practical use. Because this meter measures mass flow, it does not need pressure or temperature compensations. Furthermore, we have also experimented and confirmed that the new gas meter has the potentials of

12 becoming a calorific flow meter, although there are a few restrictions to gas components. Tokyo Gas is ready and willing to have any experiments conducted with this meter in other gases. 6. ACKNOWLEDGEMENTS Development of this meter has been a joint project with Yamatake Corporation and Takenaka Seisakusho Co., Ltd. We deeply appreciate the guidance, cooperation, and support these firms have provided in this development. We would also like to thank Chiba Gas Co., Ltd. and Hokkaido Gas Co., Ltd for their support in the field tests.

13 REFERENCES 1. K.Nukui., et al. (2001). Development of Gas Meter with Thermal Flow Sensors Arranged on the Surface of the Rectified Fluid Path, 2001,IGRC.