Litostroj Power (Slovenia)

Abstract The presence of entrapped air poses a risk to the operational safety of hydraulic pipeline systems (hydroelectric power plants, water supply systems). It can significantly increase pressure surges during transient events such as filling and emptying of pipelines, hydraulic turbomachine start-up and stop, or even in steady state operation. Air is typically entrapped at high points in the pipeline profile or close to the pipeline devices (gate, valve, turbine, pump). A simple hydraulic pipeline system containing a trapped air pocket downstream the in-line valve is studied using dimensionless analysis. Following a sudden in-line valve opening, pipeline filling can generate large pressure oscillations, which may exceed the driving pressure of the reservoir. Key parameters influencing the hydraulic response are identified and combined into dimensionless groups. For a selected range of parameters, novel dimensionless charts of pressure peaks are included to enhance the understanding of water-air interactions and to provide a practical reference for dimensionally similar hydraulic systems. Such charts can help in the initial design and risk assessment of pipelines with entrapped air without the need for detailed numerical modelling. The relationships between the dimensionless groups in this study are derived from numerical simulations of a one-dimensional rigid water column model.

A double-regulated vertical axial water turbine, known as a Saxo turbine, has become a very well-recognised product of the company Litostroj Power, with over 30 commissioned units in the North American market over the previous decade. While this turbine became popular in the North American market due to its compact design, simplified civil construction works, robustness and versatility, this has surprisingly not been the case for the European market, where the Saxo turbine has been overlooked. It seems that European investors prefer and trust more conventional turbines, i.e. classic Kaplan and tubular turbines, instead of the alternative Saxo turbines. This paper examines some of the most important design, hydraulic and economic advantages of Saxo turbines. Special attention is given to a description of the Saxo turbine andits main differences in comparison to tubular and Kaplan turbines. In addition, some of the main issues of hydraulic design and numerical investigations of the flow field in the entire water passage are described. The authors would like to draw the attention of professionals in the field of power generation to this turbine and to initiate discussions that may indicate the concerns of the European investors regarding the Saxo turbine.

This paper serves as a reference for further investigations of pipeline filling and emptying in a relatively small-scale pipeline apparatus. The apparatus consists of an upstream end high-pressurized tank, horizontal steel pipeline, four valve units positioned along the pipeline including the end- points, and a downstream end tank. The filling of an initially empty pipeline is performed by a sudden opening of the valve positioned at the high-pressurized tank filled with water. The pipeline- emptying process is accomplished by high-pressurized air supplied from the air reservoir installed at the high-pressurized tank. The high-pressurized tank is closed, and the downstream end valve is opened. Experimental results indicate that pressure fluctuations are larger for pipeline filling in comparison to pipeline emptying. It is shown that a piezoelectric transducer with a fixed relatively low discharge time constant is not entirely appropriate for accurate low-frequency pressure mea- surements.

This paper presents the modelling and simulation of transient flow in micro-hydraulic pipe systems. Liquid stream energy dissipation occurs mainly as a result of friction losses. Theoretical considerations of water hammer resulting from rapid valve closing, supported by experimental verification, were undertaken. The experimental system incorporated a straight two-meters long section of a steel pipe with an internal diameter of 4·10−3 m. An attempt was made to determine the degree of conformity of the transient flow model (previously verified in conventional pipes) to the experimental results obtained for small-internal-diameter pipes. Shear stress on the pipe wall was modelled using first a simplified quasi-steady approach and then an effective modified unsteady friction model. The pressure waveforms at the valve (at the downstream end of the pipe) were obtained for initial flow velocity, v01 = 2.39 m/s and v02 = 1.14 m/s, respectively. Experimental studies were carried out in the region of laminar flows with Reynolds numbers below 100.

Reversible pump-turbines used in Pumped Storage Power Plants are among the most cost-efficient solutions for storing and recovering large amount of energy in short time. Presented paper is focused on the pump-turbine pumping mode part-load instabilities, among them the rotating stall and the cavitating vortex in the distributor region. Rotating stall can be described as a periodic occurrence and decay of the recirculation zones in the distributor with its own rotational characteristics frequency. Unstable behaviour can result in high radial forces, high pressure fluctuations and local velocity fluctuations that can in some cases lead into the occurrence of the cavitating vortex in the distributor region, even though the distributor is located in the high pressure region. Computationally demanding calculations have been performed using commercial CFD code. Analysed results have been compared to the experimental data obtained in the ČKD Blansko Engineering hydraulic laboratory.

This paper reviews critical water hammer parameters as they are presented in official hydropower standards and guidelines. A particular emphasize is given to a number of IEC standards and guidelines that are used worldwide. The paper critically assesses water hammer control strategies including operational scenarios (closing and opening laws), surge control devices (surge tank, pressure regulating valve, flywheel, etc.), redesign of the water conveyance system components (tunnel, penstock), or limitation of operating conditions (limited operating range) that are variably covered in standards and guidelines. Little information is given on industrial water hammer models and solutions elsewhere. These are briefly introduced and discussed in the light of capability (simple versus complex systems), availability of expertise (in house and/or commercial) and uncertainty. The paper concludes with an interesting water hammer case study referencing the rules and recommendations from existing hydropower standards and guidelines in a view of effective water hammer control. Recommendations are given for further work on development of a special guideline on water hammer (hydraulic transients) in hydropower plants.

Acoustic resonance in liquid-filled pipe systems is an undesirable phenomenon that cannot always be prevented. It causes noise, vibration, fatigue, instability, and it may lead to damage of hydraulic machinery and pipe supports. If possible, resonance should be anticipated in the design process and be part of the hydraulic-transients analysis. This paper describes acoustic resonance tests carried out at Deltares, Delft, The Netherlands, within the framework of the European Hydralab III programme. The test system is a 49 m long pipeline of 206 mm diameter that is discharging water from a 24 m high reservoir through a 240 mm2 orifice to the open atmosphere. The outflow is partly interrupted by a rotating disc which generates flow disturbances at a fixed frequency in the range 1.5 Hz to 100 Hz. In previous studies [1, 2] a similar system was analysed theoretically. Herein experimental data are presented and interpreted. Steady oscillatory behaviour is inferred from pressures measured at four different positions along the pipeline. Heavy pipe vibration during resonance was observed (visually and audibly) and recorded by a displacement transducer.