Waldemar Janicki

Improved Discharge Measurement Using the Pressure-Time Method in a Hydropower Plant Curved Penstock

One of the basic flow rate measurement methods applied in hydropower plants and recommended by the International Standard IEC 60041―1999 and American National Standard ASME PTC 18―2002 is the pressure-time method, generally known as Gibson method. The method consists in determining the flow rate (discharge) by integration of the recorded time course of pressure difference variations between two cross sections of the hydropower plant penstock. The accuracy of measurement depends on numerous factors and, according to the International Standard, generally is confined within the range 1.5―2.3%. Following the classical approach, the pressure-time method applicability is limited to straight cylindrical pipelines with constant diameters. However, the International Standard does not exclude application of this method to more complex geometries, i.e., curved pipeline (with elbows). It is obvious that a curved pipeline causes deformation of the uniform velocity field in pipeline cross sections, which subsequently causes aggravation of the accuracy of the pressure-time method flow rate measurement results. The inffuence of a curved penstock application on flow rate measurements by means of the considered method is discussed in this paper. The special calculation procedure for the problem solution has been developed. The procedure is based on the FLUENT computational fluid dynamic solver. Computations have been carried out in order to find the so-called equivalent value of the geometric pipe factor F required when using the pressure-time method. An example of application of this method to a complex geometry (two elbows in a penstock) is presented. The systematic uncertainty caused by neglecting the effect of the elbows on velocity field deformation has been estimated.

Special Instrumentation and Hydraulic Turbine Flow Measurements Using a Pressure-Time Method

The objective of the work was to evaluate the efficiency of a hydraulic turbine by means of the flow measurement, for a given water head. The hydraulic turbine of 180 MW output has been in service for 20 years. The real value of efficiency was needed in order to proceed with minor/mayor modifications to improve it. In a case of a runner deterioration the pressure-time (the Gibson) method was chosen to proceed with a test for flow determination. However, to measure the pressure in the penstock no access from the external space of the penstock was found, so the special instrumentation had to be developed, which could be installed inside different sections of the penstock for determination of the pressure as required by the Gibson method. After the successful installation of the pressure transducers and a special hermetic capsule, from which a cable was laid through the manhole to the control room, the test was carried out at different loads applying the Gibson method. Simultaneously, the instrumentation for the Winter-Kennedy method was installed and calibrated during the test. In the paper all the turbine measured characteristics are given and discussed. It was concluded that the efficiency of the hydraulic turbine was still high and no modifications were necessary. Having instruments calibrated for the Winter-Kennedy method other curves can be obtained at different heads.Copyright

Experimental Investigation of Dynamic Characteristics of Swing and Tilted Disc Check Valves

The present work describes an experimental investigation of the dynamic characteristics of check valves, which means experimental examination of closing function and ability to generate pressure transients under different flow decelerations in the pipeline. Two designs of check valves are tested: a swing disc and a tilted disc check valve.The valves are mounted on discharge pipe of a centrifugal pump. The fluid transient is generated by stopping the pump motor from actual velocity to completely stop in a prescribed time.Each of the check valves is subjected to tests covering different pressure levels in the upper reservoir, initial flow rates in the pipeline, several decelerations of pump rotation, three settings of torque acting on the valve disc, three values of the moment of friction forces acting on the valve axis and finally free fall of the disc in stagnant water and air.The test stand, the instrumentation and chosen valves as well as scope and conditions for performing the experiments are described. Selected measured results like angular velocities of the discs, pressures in the pipe at different conditions, and volumetric flow rates are presented and discussed.The dynamic behaviors of the tested valves were compared with each other.Copyright

Measurement of the Flow in a 170 MW Hydraulic Turbine Recording the Pressure-Time Rise in One Section of the Penstock

The objective of the present work is to evaluate the performance of a hydraulic turbine by means of the measurement of flow using the Gibson method based on recording pressure–time rise in one section of the penstock and relate it to the pressure in the upper reservoir to which the penstock is connected. Volumetric flow is determined by integration of the time function of a differential pressure (between the section and the inlet to the penstock). Flow measurement was possible this way because the influence of penstock inlet was negligible as far as an error of the measurement is concerned. The paper presents the results obtained with this method for the case of a 170 MW hydraulic turbine. The length of the penstock was 300 m. Previous experience and a standard IEC-41-1991 were the criteria adopted and applied. An efficient and fast acquisition system including a 16 bit card was used. The flow rate was calculated using a computer program developed and tested on several cases. The results obtained with the Gibson method were used for calibration of the on-line flow measuring system based on the Winter-Kennedy principles. This last method is used for continuous monitoring of the turbine flow rate. Having calculated the flow rate and output power the efficiency is calculated for any operating conditions. A curve showing the best operating conditions based on the highest efficiency is presented and discussed. Flow simulation allowed having an estimation of a flow recirculation region size.Copyright

Elastic Water-Hammer Theory–Based Approach to Discharge Calculation in the Pressure-Time Method

AbstractAn original numerical procedure based on the elastic water-hammer theory, with special solutions of continuity and momentum equations modeling the unsteady liquid flow in pipelines, has bee...

The Effect of Boundary Layer Separation on Accuracy of Flow Measurement Using the Gibson Method

The Gibson method utilizes the effect of water hammer phenomenon (hydraulic transients) in a pipeline for flow rate determination. The method consists in measuring a static pressure difference, which occurs between two cross-sections of the pipeline as a result of a temporal change of momentum from t0 to t1 . This condition is induced when the water flow in the pipeline is stopped suddenly using a cut-off device. The flow rate is determined by integrating, within a proper time interval, the measured pressure difference change caused by the water hammer (inertia effect). However, several observations demonstrate that changes of pipeline geometry like diameter change, bifurcations, or direction shift by elbows may produce an effect on the computation of the flow rate. The paper focuses on this effect. Computational simulations have shown that the boundary layer separates when the flow faces sudden changes like these mentioned to above. The separation may reduce the effective cross section area of flow modifying a geometry factor involved into the computation of the flow rate. The remainder is directed to quantify the magnitude of such a factor under the influence of pipeline geometry changes. Results of numerical computations are discussed on the basis of how cross section reductions impact on the geometry factor magnitude and consequently on the mass flow rate.Copyright

Propagation of Structural Vibrations and Pressure Waves in the Hydropower Turbines

This article discusses two non-typical dynamic problems that occurred during the operation of a power unit located at the water power plant. The first one concerns the propagation of pressure waves in the turbine flow system, the frequency of which coincided with the excitation frequency resulting from the electric generator rotational speed. The second one is a natural mechanical vibration of the power unit (hydraulic turbine, gear and generator). It turned out that the structure that supports the generator is not rigid enough. These two dynamical problems led to the occurrence of the resonant vibrations. Apart from discussing the diagnostic methods used, the article also gives practical ways to minimize the adverse impacts of the dynamic problems described above on the machine’s performance. The effective solutions have been proposed and implemented, making it possible to achieve a significant reduction in the levels of vibration and noise.

Flow Measurement Methods Applied to Hydro Power Plants

Efficiency and maximal power output are two of the most important goals to analyze in hydraulic turbines. Turbines normally operate in variable head conditions; therefore, tests to analyze performance are frequently realized for a selected number of power plant heads. Usually it is limited to three heads: low, medium and high. The efficiency of water turbines is most frequently expressed using the weighted average efficiency or arithmetic mean efficiency, calculated from the measured results in the examined heads. To perform the calculation of efficiency is necessary to know several parameters such as kinetic and potential energy of water due to the position, because of this is required to know the flow rate entering the turbine. The flow rate of water through the turbine (Q) is determined by the volume of water flowing in time unit. The measurement of this quantity is one of most difficult tasks in water turbine tests. Three basic methods of flow rate measurement and results of them will be presented in this chapter.

Discharge measurements using the classic Gibson method with instrumentation installed inside a pipeline

The Gibson method (pressure-time method) is one of the basic methods of discharge (flow rate) measurement applied in hydropower plants. Flow rate is determined by integrating the recorded variation of pressure difference between two measuring (hydrometric) sections in a pipeline (penstock). The Gibson method in its classic version consists in direct measurement of pressure difference variation between two hydrometric sections of a pipeline. Particular difficulties, related to application of the method in its various versions, arise in conditions of no access to the hydrometric sections from the outside of a pipeline. In such cases, it is necessary to install dedicated measuring instrumentation inside the pipeline. Such instrumentation has been implemented for the purpose of efficiency tests of two Francis turbines (upgraded and not upgraded) fed from a common penstock of 10 m diameter. The hydrometric sections were furnished with pressure taps connected by means of small copper tubes (impulse tubes) and hermetic manifolds to the differential pressure transducer. The transducer was installed in a hermetic housing and its electric signal was sent from the inside of the penstock to a computer data acquisition system. Using this method, the efficiency characteristics of the tested hydraulic turbines were determined. According to the authors’ knowledge, the pressure-time method has not been used in such an application so far. The method under consideration requires transmitting pressure signals from both penstock sections to the differential pressure transducer by means of impulse tubes. This raises the question on the influence exerted by dynamic properties of the connecting pipes / transducer system on the discharge measurement results. The previously developed computational method incorporating dynamic models of the piping and the transducer has been applied in order to determine this influence. In result of calculations conducted, the piezometric tubes / transducer system has been found to exert a negligible influence on the discharge measurement results.Copyright