The energy demand world wide especially in the developing countries is growing
significantly as a result of economic growth, industrial expansion, high population
growth and urbanization. Thermal power plants play a major role in meeting this
ever increasing demand. Selection of proper thermodynamic cycle plays a vital
role in extraction of power from thermal power plants. The power cycles are
investigated with an over all objective of providing high fuel conversion efficiency.
The available research related to the power plant conducted by Azhdari
et al. (2009), Naradasu et al. (2007),
Barna and Baranyai (2009) and in addition to Wazed
and Shamsuddin (2009).
Very little work has been done on measurement of condensation and flow measurements
techniques, the available investigation in this regions done by Miskam
et al. (2009), Mckeon and Smith (2002) and
Gorecki and Kubas (2005, 2003).
The lack of available experimental work in this region is the motivation for this investigation.
Systems of continuous monitoring and measurements of cooling water mass streams
for condensers of steam turbines are of extremely importance in power unit operation
(Mahesh, 2006; Gorecki et al.,
2003, 2006; Behzadi and Golnabi,
2009). They allow for reasonable water management, thus considerable savings
of water. The study describes the methods of water mass stream measurement using
averaging dynamic pressure prop and using elbow-type flowmeter. As judged by
the authors, such methods were the optimum solution as no classical measuring
method (flow nozzle) was possible due to large diameter of pipelines exceeding
Measurement signal coming from the flowmeter is converted in pressure transducer
to an analogue current signal and then sent to the power unit control room.
Therefore, the signal is converted into digital form represents the cooling
water mass stream for power unit condensers (Gorecki et
al., 2003, 2006).
MATERIALS AND METHODS
Flow meters averaging dynamic pressure: Water mass stream was measured
by dynamic pressure averaging probes located downstream the power unit condensers.
Pressure signal from probes is converted to analogue current signal (4-20 mA)
in pressure transducer and sent to the power unit control room. Here, the measuring
signal is converted into digital form and displayed as the mass stream of water
cooling the power unit condensers. Figure 1 illustrates the
model of a flowmeter which represents the averaging dynamic pressure while,
Fig. 2 shows the mounting arrangement for the probes in a
pipeline (Gorecki et al., 2003).
The measuring principle is based on the proportional relation between mass stream and a square root of differential pressure:
transducer characteristic is considered:
||Model of averaging flow meter. 1: Process pipeline, 2: Total
pressure averaging pipes, 3: Averaging pressure lines to differential pressure
transducer, 4: ΔP/I transducer and 5: Electrical signal sent to power
unit control room
||Mounting arrangement of averaging probes in pipeline
Δpd = f (I) = C1 (I-4)
where, I is the output current of differential pressure transducer (4-20 mA):
This equation forms a common application as an input signal to the measuring/control system of the power unit.
||Exemplary relation between sensitivity coefficient, k, of
averaging flowmeter and Reynolds number
||An example of an averaging flow meter characteristics
Exemplary metrological characteristics of averaging flowmeters are shown in Fig. 3 and 4.
Elbow-type inertia flowmeters: An elbow-type inertia flowmeter is based
on a curved section (elbow) of process pipeline and a transducer which measures
a difference of static pressures, Δp, between external and internal wall
of the elbow (Cengel et al., 2008).
The difference of pressures (differential pressure) results from inertia force.
The measurement principle is based on the relationship between differential
pressure and volumetric stream of flowing medium (Gorecki
et al., 2003).
Signal transmission is similar to that outlined for the flowmeter with averaging tubes. Details of elbow-type flowmeter is shown in the schematic diagram Fig. 5.
The flow meter characteristic is given by two Eq. 3 and 4:
The coefficients, C and C* are sometimes called the flow coefficients. They may be determined for specific
type of flowmeter by means of calibration using measurements of volume/mass flow, or by other method, e.g., using a high-precision flowmeter.
Determination of measuring characteristic of elbow-type flowmeter: Measuring characteristics of the elbow-type flowmeter were determined on a laboratory stand used for flowmeter calibration at Institute of Heat Engineering and Fluid Mechanics, Wroc3aw University of Technology. Figure 6 illustrates the elbow-type flowmeter under testing together with differential pressure transducer.
|| Schematic diagram of elbow-type flow meter
The flow meter was installed in a pipeline 40 mm in diameter (φ). Differential
pressure was measured by means of Rosemount type 3502 transducer with maximum
measuring range Δpmax = 6.22 kPa and output current signal 4-20
mA. Testing instrumentation includes also IBM PC with LC-020 transducer card,
LC-055-P10 digital control card and AMP-UNI-01 amplifier. The sampling period
was Δt = 1 min and the number of samples was M = 8192. Ten measurements
were taken for each measuring series (for volumetric stream of flowing water).
In parallel, the volumetric stream was measured using turbine flowmeter (Sonntag
et al., 2003) with maximum range qVmax = 16 m3
sec-1. Figure 7 shows the range of variations of
measured signals-from minimum to maximum volumetric stream of water flowing
in the system.
|| Elbow-type flowmeter under testing
|| Range of variations for measured signals
|| Diagram of the measuring system
||Flow coefficient, C, versus Reynolds number for elbow-type
Figure 8 shows the diagram of the system including measurement instrumentation.
The coefficient C was calculated from Eq. 5:
where, qvT is the volumetric stream taken by reference turbine-type flowmeter.
Figure 9 shows the relation between the flow coefficient, C and Reynolds number as found from measurements for the flowmeter under testing.
As it clear from the measurements results taken, when Reynolds number is higher than 26,000, the coefficient C is constant and equals to 7.01, hence the flowmeter characteristic is given by the equation:
This is shown in Fig. 10.
Determination was also made for the electric current characteristic of elbow-type flowmeter to allow further signals processing. Since:
||Measuring characteristic of the elbow-type flowmeter under
||Electric current characteristic of elbow-type flowmeter
This characteristic is shown in Fig. 11.
Analysis of measurement uncertainty: An exemplary uncertainty analysis
for measurement of volumetric stream in case of elbow-type flowmeter is provided
below (Central Office of Measures, 1999). It was assumed
for the analysis that water flow in the system was stable and that the predominating
is the type B standard uncertainty caused by inaccuracy of measuring instrumentation.
Thus, the extended uncertainty is given by the equation:
Following transformations and using characteristic equation, we get:
and further on:
The standard uncertainty of type B for the flow coefficient, C, can be found from the flowmeter characteristic. Assuming that relative limiting error of Δqc/C for determination of the flow coefficient equals to 2%, the standard type B uncertainty is:
The standard type B uncertainty for differential pressure on the elbow is assumed,
according to Robert et al. (2007) and Gorecki
et al. (2003) as:
||Is the resolution of differential pressure transducer, hence
0.29·1 Pa = 0.29 Pa (the resolution of differential pressure
transducer was 1 Pa)
Assuming, according to Central Office of Measures (1999),
the expansion coefficient kB (α) = 2 for the level of confidence
α = 0.95, the relative extended uncertainty is given by the equation:
||Relative extended uncertainty, uqv/qv, as a function
of differential pressure across the elbow, Δp
which is shown in Fig. 12.
The study presented alternative continuous measurement of flow streams in pipelines
exceeding 1 m in diameter using flowmeters which average dynamic pressure and
inertia elbow-type flowmeters. Such flowmeters were successfully used to measure
mass streams of cooling water for steam turbine condensers in one of Polish
power plants. Operating principles for these flowmeters and their exemplary
characteristics are also included. For the elbow-type flowmeter, the paper provides
its characteristics-according to the equation .
An average value of flow coefficient, C, was 7.01 for the flowmeter under consideration,
hence the characteristic is ,
where qV is expressed in m3 h-1 and Δp
The preliminary uncertainty analysis shows that, at the confidence level α
= 0.95, the uncertainty of volumetric stream measurement, at stable flow of
water stream and for differential static pressure over 50 Pa, quickly approaches
2.30%. The measurements taken allowed also determining the measuring range of
the flowmeter under testing. It may be used for Re >26,000, i.e., for water
flow velocity over 0.7 m sec-1. For this measuring range, the value
of coefficient C is constant. For Reynolds number less than 26,000, an additional
coefficient, C*, shall be introduced into the characteristic equation, so then
||Electric current (am)
||Coeffieint of expansion
Number of samples
Mass stream (kg sec-1)
Volumetric stream (m3 sec-1)
||Level of confidence
||Density (kg m-3)
Resolution of differential pressure transducer