Gas pockets and hydraulic jumps in pressurised pipelines
In practice, it proves to be difficult to completely degas pipelines or to ensure that free gas is not entrained in the pipeline during operation. A recent inventory among water boards in the Netherlands showed that half of the wastewater pressure lines suffered from an increased pressure loss for no obvious reason. Reduction of the system’s nominal capacity can be the result of many causes; as increased wall roughness, scaling or the occurrence of gas pockets. Gas pockets can be caused by degassing of dissolved gas or by air entrainment at the pumps’ inlet or at air valves.
The research on capacity reduction by gas pockets in pipeline systems is a part of an integral study on problems in pipelines due to occurrence of free gas. This integral study also aims at diagnosing and detecting air pockets as well as formulating up to date guidelines for design and maintenance of pressure systems and troublefree operation of gas-containing pipelines. The study is carried out by the University of Delft and Delft Hydraulics in collaboration with the majority of the waterboards in the Netherlands and two consultancy companies. For this part of the study, an experimental facility is constructed that allows air and gas flow.
The proposed paper describes the results of the experiments in one of the two facilities designed for investigation of air pockets and hydraulic jumps at high points in a pipeline. For this study, experiments have been conducted with 220 mm pipes to investigate the influence of the pipe angle, water flow velocity and air volume on the properties and location of the hydraulic jump. The head loss created by the presence of air pockets is measured by pressure transmitters in the test section.
Hydraulic jumps in pressurised pipes show a variety of different modes of air transport. The distribution of air in the hydraulic jump is determined by measuring the void fraction at several locations with optical fiber probes using a high sampling frequency.
The study resulted in a description of head loss as a function of water/air flow rate, inclination angle and length of the inclined part of the pipe (some early results were presented in the 4th SPN conference in Madeira in 2004, Lubbers & Clemens (2004)). The void fraction measurements give amongst others the local probability density function of the period of the presence of air and the mean void fraction profile perpendicular to the axis.
The objective is to obtain a better understanding of the possible flow modes in gas/water mixtures and characteristics of hydraulic jumps in pressurised pipes. The results are employed to produce guidelines for the operation of transport of gas-containing fluids through pipeline systems. Furthermore the results are applied in a further improvement of a model for pressure mains that is developed by Delft Hydraulics, WANDA.
References:
Lubbers C.L. and Clemens F.H.L.R. (2004). Air and gas pockets in sewerage pressure mains. In: Proceedings of the 4th International Conference on Sewer Processes and Networks. Funchal, Madeira, Portugal, 22-24 November 2004, 9 p.
Figure 1: Picture of a hydraulic jump in a pressurised pipe
