Better energy management through precise flow by xeniawinifredzoe

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									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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