Better energy management through precise flow calculation and advanced data acquisition and management. Stefan Wöhrle Madhukar Punani Application engineer Prokuct manager Energy management Data acquisition Abstract possible to come over these disadvantages and achieve a level of accuracy not In the last few years, the cost of energy has comprehended until a few years ago. increased manifold, this has made the measurement and management of energy a Data acquisition systems (DAS) allow a high key area of activity in the industry. As the degree of flexibility and mathematical demand of energy increases world wide, the calculation capabilities to better record, pressure on resources increases and this analyse, monitor data and generate early leads to the increase in costs. Even the warnings and alarms. This allows operators environment is under threat due to and plant supervisors to run the plant more increased release of greenhouse gasses. In efficiently and reduced down time through the modern competitive world, tangible costs information and diagnosis well in advance of must be reduced and efficiency optimized to impending troubles in various systems be competitive. The recently signed kyoto involved. protocol aims at reducing the emissions released through burning of fuels. It is Combining better measurement and amazing how we can achieve both goals of advanced data acquisition, the power lies in lower costs through better efficiency and your hand to improve efficiency and energy lower emissions through better energy management at your site! management. This paper comments on possible ways to manage energy in an II Flow measurement according to he improved way. More specifically we will differential pressure principle concentrate on better energy measurement The basic equation for the calculation of flow using precise differential flow measurement from differential pressure is as follows. and density calculation and increasing efficiency through recorded data Qm = C (1/1- 4) d² /4 (2 P ) management . C= Coefficient of discharge = Expansibility factor I Introduction = Diameter ratio = Density in operating We concentrate on differential flow as it is conditions one of the most widely used methods on (1/1-4) = Velocity of approach factor account of reliability, simplicity, economics d= diameter and most importantly standardization! The P= pressure two popular and accepted standards are ISO EN 5167 and AGA 3. All coefficients have dependencies to Unfortunately traditional disadvantages such pressure and temperature. as low accuracy and limited range are tolerated and this of course has a big The discharge coefficient C corrects the negative impact on energy management. theoretical equation for the influence of Through precise numerical calculations it is velocity profile, and the assumption of no energy loss between the pressure taps Flow range: 8,25 – 32,93 t/h (1:4) (caused by contraction). The velocity of (48 – 813 mbar) approach and the expansibility factor incorporate the influence on flow velocity Design conditions: and density due to the flow through the Pressure : 10 bar; primary element. For the practical Temperature : 200 °C; application this equation is simplified. It is assumed that the pressure and temperature Process condition: do not vary much from the specified Pressure : 10-12 bar conditions for design. Thus all coefficients Temperature : 190-200 °C are all combined to make a constant. As a Differential pressure : 271 mbar result one gets a flow directly dependent on the differential pressure. Thus:- Measurement as per the simplified equation Qm = C (1/1-4) d² /4 (2 P ) Mass flow: 19.41 t/h Density: 4,85 kg/m³ Qm = k (2 P) As soon as the process conditions vary from Improved or full compensation differential the design conditions, an error is introduced pressure calculation. (real flow) into the calculated flow. The discharge coefficient, expansibility Mass flow: 19.41 - 21.8 t/h factor , and especially the density vary with Density : 4,85 - 6,08 kg/m³; temperature and pressure. The error due to simplification: III Calculating errors introduced due to approx.. 11 %. simplified calculation The following examples demonstrate the Example 2: Accuracy of natural gas effect of varying process conditions on the measurement flow calculations and the magnitude of introduced error for different fluids when Orifice plate corner tap: using the simplified equation as compared to the original equation. Pipe inner diameter 150 mm ß = 0,7 As representative and widely used media we use natural gas, steam and water for the Flow range: 2075 – 8300 Nm³/h (1:4) examples (Heating, Drying, Electricity (11,9 – 202,4 mbar) generation, Distillation, Sterilization). The results can be used for media with similar Design conditions: physical state. Pressure : 3 bar; How does the variation of pressure and Temperature : 20 °C; temperature alter the flow measurement Reference: 0 °C, 1, 013 bar using the differential pressure and energy balancing? Process condition: Pressure : 2.5 bar Example 1: Accuracy of a steam flow Temperature : 30 °C measurement using a orifice plate Differential pressure : 87 mbar Reference conditions: 0°C; 1.013 bar Orifice plate corner tap: Pipe inner diameter 200 mm ß = 0,7 Measurement as per the simplified equation Tabelle 1.1 Deviation (Error) due to application of simplified DP calculation Standard volume: 5527 Nm³/h (Example 1-3) Density: 1,98 kg/m³ Discharge Expansibility Density Improved or full compensation differential coefficient factor e pressure calculation. (real flow) c Steam 0,1 % 2,9 % 25,3 % Normal volume: 4950 Nm³/h Gas 0,1 % 2,3 % 24,5 % Density: 1,59- 1,98 kg/m³ Water 1.4 % - 2,3% Error due to simplification approx. 11,6 %. Example 3. Accuracy of water flow measurement Density calculation of gasses and fluids. Orifice plate corner tab: As seen in table 1.1 (and with respect to the Pipe inner dia. 200 mm; basic equation for DPT flow calculation) the ß = 0,7 density especially for gasses is the significant factor contributing to the error in flow range 50 – 200 t/h (1:4) measurement when using the simplified (8,8 – 142,2 mbar) differential pressure (DP) flow equation. design conditions: Temperature: 100 °C; The accurate calculation of density at Density: 961 kg/m³; process condition is the critical parameter for the quality of the flow calculation at Process conditions varying process conditions. The density Temperature : 50 °C variation of a liquid can be calculated from Differential pressure : 79,84 mbar the temperature. For commonly used valuable liquids the exact density data is Meaurement as per simplified measurment compiled in the form of tables, for example Petroleum Tables (ASTM 1250; API 2540) Mass flow: 150 t/h or data provided by the manufacturer, e.g. Density: 961 kg/m³ for thermal fluid. In a simplified form one can also calculate the density for most liquids, Improved or full compensation differential relatively accurately using the coefficient of pressure calculation. (real flow) liquid expansion in a delta range of 40°C. of course certain liquids require special tables. Mass flow: 152,2 t/h Density errors introduce percent errors into Density: 983,8 flow calculations. Error due to simplification approx. 1,4 %. and the possibility to include in field devices such as flow computers are:- Gas: Redlich Kwong, Soave Redlich Kwong Natural Gas: GERG 88, AGA8 (accepted even for custody transfer applications) For water and steam: IAPWS IF 97 Standards (ASME Tables) Example: Density curve Methanol: deviation Measurement Range (flow dynamic) through calculation by coefficent of One of the most significant “disadvantage” expansion (delta T: 100 °C) 1,4 % of the traditional dp flow measurement have always been the small flow dynamic (measurement range). This is caused by the For gasses the density is a function of physical principle, i.e. the square rooted pressure and temperature. relationship of flow and differential pressure. The easiest way to calculate density is using the ideal gas equation. Qm = .....* (2 P ..) P*V = n*R*T bzw. Example: 10-50 m³/h (1:5 dynamic), dp at Z (n) p Tn (b) = (n) 50 m³/h = 250 mbar; 30 m³/h = 90 mbar, Z pn T 10 m³/h = 10 mbar. This mean 1:5 flow ratio requires 1:25 dp ratio (10-250 mbar). Tn = Temp. in Kelvin at NTP Another example 1:10 flow -> 1:100 dp. T = Temp. in Kelvin The resolution and accuracy of pressure (n) = Density at NTP transmitters in todays times is fairly limited (b) = Density at operation as leading manufacturers offer accuracies of p = Pressure in bar 0,075 % of the end value. This allows flow p(n) = Pressure at NTP ratios of 1:4 measured without any relevant influences of the dpt to the over all Increasing pressures and decreasing accuracy. (Over all accuracy < 1 %). Under temperatures ideal gas law introduces errors convenient process conditions and at design The deviation is compensated through the z- temperature and pressure, ratios up to 1:8 Factor. The deviation of a gas from the can be realised with proper results. ideal gas law is called the compressibility of Nevertheless many applications such as a gas. This factor can be calculated through day/night operations or discontinuous various methods. processes requires larger ranges. The best suited methods for high accuracy Example: Argon: Deviation from ideal gas law in the range of 0-200 bar Example 4 Other factors The deviation due to temperature in the size Orifice plate corner tap: of the pipe and the flow element are minimal Pipe inner diameter 200 mm and only when delta temperature is around ß = 0.7 50°C from the design point does it have a Flow range: 0.8 – 25 t/h (1:30) significant influence 0.37 – 456 mbar (1:225) Design conditions: Resume: Pressure : 10 bar; Through the accurate calculation of all Temperature : 200 °C; coefficients the accuracy of energy measurement can be increased manifold Process condition: enabling process supervisors to minimize Pressure : 10-12 bar errors and achieve a high degree of Temperature : 190-200 °C accuracy (1%) in flow, mass and energy measurement Of course the quality of Results (full compensated calculation): sensors is important for the overall 1 DPT (0-500 mbar): 46 % Error accuracy The accuracy of flow is directly connected to energy measurement and 2 DPT (0-30 mbar): 3 % management. 3 DPT (0-10 mbar): 1,24 % Energy calculation: Mass flow calculation acts as the basis for The example shows how a the use of calculating normal or corrected flow of gas multiple transmitters increments the flow and energy flow or potential combustion range. By use of 3 Transmitters the energy based on enthalpy or heat content of accuracy in flow measurement is less than fluid. The enthalpy of steam is calculated 1,24 % over the largest part of the based on IAPWS-IF 97. For combustible measurement range lower than 1 %. Only gasses a heating value is computed through below a DP of 0.6 mbar (1:4 ratio in dp flow gas analysis. The heating value of natural range) the error exceed 1 % up to 1.24 %. gas is also used to compute the By use of more transmitters the flow range compressibility (AGA8, Gerg 88). Due to the could be extended even further, naturally new developments in the energy world only as long as dp generation enables natural gas is becoming ever more proper flow measurement. important as an energy source. Many The switching between the transmitters networks for supply of different “types” of should be done automatically, back switch natural gas are being built and gas is being with hysteresis. traded in free market. (In developing countries such as India, which will be one of The split range technique allows to extend the largest users of energy in the future the the flow dynamic appropriate to almost all big cities are already using gas to run large applications, nevertheless it requires number of public vehicles.) Thus a instead engineering know how to design the flow of an average heating value, an exact value measurement point, e.g. adjust ranges to is found through analysis to avoid errors. the range of the dp cells (because For thermo-oils and thermal exchange fluids uncertaintainty is always referenced to the the suppliers give the heat capacity data on end value of the dp cell). request. For practical purposes, there is a limit when the costs benefit ratio is exceeded. Overall accuracy of energy measurement In the previous part we discussed the critical parameters for a precise measurement of . energy flow. The requisite complex calculations in the form of archived tables correlated with mathematical algorithms Certain field systems offer calculation and (called numerical methods) can be done by data archiving in one unit, but generally a special computational device generically speaking in interest of high accuracy and called flow computer or energy manager. easy data handling the authors recommend Differential pressure, pressure and a computation unit and an autonomous data temperature are measured using sensors acquisition system. and transmitted to a flow computer where the necessary calculation is carried out and The field units monitor the measured data energy flow computed. An overall accuracy for preset limit violations that could be useful with precise sensors, computer and Data for pre-emptive maintenance, such as too acquisition system of under 1,5 % is high flow, sudden reduction of energy value achievable. etc. Advanced units can even undertake For practical purposes the authors would intelligent totalizing where there are multi- like to recommend using a device that tier tariff systems incorporated for billing or enables important values such as efficiency calculations. Here the recording density, flow, mass flow, energy flow unit has three counters for the same energy and totalizers to be transmitted over balance and at different levels of flow different totalizers are activated. This allows analog, pulse or bus signals (such as M an easy way to keep check on peak BUS, ModBus, Profibus DP) to integrate consumption! Moreover automatic analysis into SCADA, DCS or data acquisition and comparisons of different shifts or systems. batches of production can be undertaken. The data acquired from different units can Data acquisition and management be brought to a central archive using OPC Accurate energy computation is only half the server-client architecture or propriety work done. Using this to increase efficiency software. is the real goal. Good data acquisition allows precise tracking of energy The real power lies in correct analysis of consumption and take necessary corrective data and early reaction based on deviation action in case of deviations. of efficiency. Advanced data analysis allows supervisors Data acquisition systems that enable energy to run systems at maximum efficiency and management consist of a field mounted real monitor the effects of optimization. time logging and monitoring system and an offline monitoring and archiving server. Here Also trending allows quick identification of the acquisition units are installed at various deviations. Thus when the energy produced sites, remote or in plant and are connected in comparison to fuel used drops, then it is in a network, either Ethernet or time to service the boiler or heat exchanger. GSM/Telephone line when remote. The field Or if consumption rises without increase in units maybe connected to the calculation output then the boiler firing system or quality unit through bus systems or analogue of supply need inspection! By fast corrective signals and measure acquire data such as actions enabled through trending and fuel consumption, energy, mass flow and recording one saves tremendous amounts production quantities and help establish a of money and also saves fossil fuels. If the relation between, consumption and efficiency of a Heat exchanger drops by production. Production can be monitored 10% due to hard water residue and is not through the correct measurement of end corrected there is an increased cost of product, for example in a cement factory, production and wasteful loss of energy. number of bags /shift, in a milk Saving costs through high efficiency also pasteurization factory, liters of milk per hour saves our environment! etc. Conclusion When considering cost and efficiency Stefan Wöhrle works as application optimization, look for a highly accurate flow ingenieur for flow and energy and energy computer that offers the measurment within the product center of complete compensation measurement and Endress&Hauser Wetzer GmbH. He accurate density calculation based on graduated in enviroment engeneering in accurate methods described above. Such a the university of applied science in system in coordination with a flexible Triesdorf 1998. Since his start at E&H in monitoring and data acquisition system 2000 he have been responsible for the allowing early warning and easy analysis will development of the functionality for allow you to run your energy systems at flow&energy calculation devices. high efficiency all year round and thus save high energy costs and reduce emissions. It is in our interest as responsible citizens to reduce emissions. Also it can help savings especially for large units having installed capacities above 20MW which makes it mandatory to pay certain levies in Europe. Literature: - DIN EN ISO 5167: Measurement of fluid flow by means of pressure differntial devices, 2003. - VDI 2040: Calculation Principles for the Measurement of Fluid Flow Using Orifice Plates, Nozzles and Vernturi Tubes, 1991. - IAPWS-IF 97: W. Wagner, A. Kruse, Properties of Water and Steam, Springer Verlag, 1998. - SGERG: M. Jaeschke, A.E. Humphreys, Standard GERG Viral Equation for Field Use, VDI Verlag, 1991. - AGA Report Nr. 8 (MPMS Chapter 14.2): Compresibility factors of Natural Gas and other related Hydrocarbon Gases, American Gas Association, 1994. - ASTM 1250 (API 2540): Petroleum Measurment Tables, American Petroleum Institute, 1980. - R.W. Miller, Flow Measurement Engeneering Handbook -3rd edition, Mc Graw Hill, 1996. - Poling, Prausnitz, Connell, The Properties of Gases and Liquids – 5 th edition , Mc Graw Hill 2001.
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