Gas pockets in a wastewater rising main: a case study.
This paper presents a case study of an existing wastewater rising main (WWRM) in which an extreme transient event produced by simultaneous power failure of the pumps caused the rupture of a 1.2 m (48 in) prestressed concrete cylinder pipe (PCCP), causing an important leakage of sewage. The event and the methodology followed in order to validate the diagnostics of the failure are described. The detail study included in situ observation of the system, experimental investigation in a setup, hydraulic analysis, as well as details of the structural strength of the WWRM. After the extensive investigation and several simulations of fluid transients for different scenarios and flow conditions, it was found that stationary small gas pockets accumulated at high points of the WWRM were identified as the principal contributory factor of the failure. This case study serves as clear warning of the consequences of operating a WWRM with gas pockets at its high points.
Air entrapped in gravity pipeline systems
The authors are to be congratulated for an interesting development and the systematic application of a theoretical approach to solve a problem involving entrained air in a pipeline. Nevertheless, some of the concepts and correlations need to be considered in more detail. First, the authors state that the removal of air by a hydraulic jump is essentially a velocity-dominated phenomenon and cannot be modelled accurately by Froude scaling. The discusser used the model-generated relationship of Kalinske and Robertson (1943) to predict the air flow with a hydraulic jump that filled the pipeline in three different prototype structures (Fig. D1). The data scatter at the lower end of the curve can be explained by the lack of a well-formed hydraulic jump for low Froude numbers. However, the correlation is excellent at high Froude numbers, thus showing that air removal by a hydraulic jump can be modelled correctly with Froude scaling. As the authors point out, air bubbles that are present in a sloping pipeline will either move upstream against gravity because of their buoyancy or move downstream if the drag on the bubbles exceeds their buoyancy. If the bubbles move downstream, they will tend to rise to the crown of the pipe and agglomerate into large air pockets or slugs. Again, depending upon their buoyancy and the drag, the air pockets or slugs can either move upor downstream. The upstream motion of bubbles does not cause any problems. However, the upstream motion of air pockets can damage upstream structures if the velocity of the pocket high enough. Falvey (1980) showed that the relative motion of bubbles and air pockets can be expressed as dimensionless discharge versus pipeline slope (Fig. D2). The filled boxes denote the structures that have been damaged by blowback as reported by Sailer (1955). Two structures are in the zone where blowback should not occur, but they had to pass through a blowback zone to reach the zone where all pockets are swept downstream. The condition for sweep out of all pockets for a horizontal pipeline has also been added to the plot, based on Baines and Wilkinson (1986), which was published after Engineering Monograph 42 had been prepared. The authors’ text states that “The data in Fig. 6 show that air bubbles and air pockets follow closely Eq. (7)”. However, as stated above, air bubbles and air pockets can move in opposite directions simultaneously. This bidirectional flow feature is seen in Fig. 5. Equation (7) and the data in Fig. 6 approximate the motion of air bubbles as determined by Kalinske and Bliss (Fig. D1). Thus, Eq. (7) should be used to predict the motion of air bubbles, but not of air pockets. The model used in these studies seems to be too small. Notice the dependency on the Journal of Hydraulic Research Vol. 49, No. 3 (2011), pp. 394–397 doi:10.1080/00221686.2010.547717 # 2011 International Association for Hydro-Environment Engineering and Research
Design of reinforced concrete lining in pressure tunnels, considering thermal effects and jointed rockmass
Abstract Jaeger's theory (Jaeger, Rock Mechanics and Rock Engineering , Cambridge University Press, 1972) of the design of the lining for a pressure tunnel applies to a tunnel located in an isotropic, homogeneous and elastic medium. The theory is derived from the elasticity equations for the plane stress condition. In this paper, Jaeger's theory is extended to a plane strain situation. Also included is a discussion of the effects of a uniform increase (or decrease) in the lining temperature, jointed rockmass, cracking of the tunnel lining, and cracking of the rockmass around the opening. Tables are presented for the modulus of subgrade reaction for various rockmass conditions and for rock sharing pressures for various lining conditions. The reduction factor to include the effect of joints in rockmass is derived by the analysis of a numerical model of the lined tunnel by the finite-infinite-interface element method.