Gas pockets in a wastewater rising main: a case study

Abstract

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.

Protecting a Pumping Pipeline System from Low Pressure Transients by Using Air Pockets: A Case Study

“…When this occurs, large air pockets remain stationary at high points of the line because the buoyancy of the pockets prevent them from being dragged downstream by the drag force of water flow [7][8][9]. Likewise, it is well known that the effect of large pockets on transient pressures can be beneficial [10][11][12][13][14][15][16][17]. Moreover, various researchers have demonstrated that large air pockets can act as effective accumulators suppressing the energy of pressure waves, replicating the behavior of air chambers [18][19][20].…”

“…An air pocket can accumulate at a high point of a pipeline, when the drag force of water flow is not enough to remove it and the buoyancy of the pocket prevents it from being dragged downstream [7][8][9]. These large air pocket volumes can behave as air cushions and absorb the energy of transient pressure waves [10][11][12][13][14][15][16][17]. In the same manner, Qiu [18], Gahan [19] and He [20] demonstrated through numerical studies that large air pockets can result in a positive effect during a hydraulic transient event with in a pumping pipeline, replicating the behavior of air chambers.…”

Failure of a drainage tunnel caused by an entrapped air pocket

“…Twocomponent flow with air pockets is commonly present in conduits of hydropower plants, pumping stations and sewer systems, causing many unpleasant situations and accidents during operation. In certain situations, the presence of air pockets could cause both high and low pressure fluctuations, which in turn could be of sufficient magnitude to potentially generate pipe fracture and pipe failure (Burrows, 2003;Pozos et al, 2012;Zhou et al, 2002aZhou et al, , 2002b. The incidents highlight a need for consideration of the transient wave interaction with entrapped air pockets during the design stage.…”

Investigation on critical equilibrium of trapped air pocket in water supply pipeline system

“…Consequently, this kind of local hydraulic phenomenon is also called an air lock (Greenshields and Leevers, 1995;Reynolds and Yitayew, 1995;Brown, 2006). It is a problem in irrigation and drainage systems (Zhou et al, 2004;Burch and Locke, 2012;Pozos-Estrada et al, 2015), hydraulic spillway conduits (Liu and Yang, 2013), and water pipeline systems (Burrows and Qiu, 1995;Carlos et al, 2011;Pozos-Estrada et al, 2012), since it increases the head losses, decreases the cross section, and causes pipe burst failures. According to the scale of the air lock, it can be classified as an entire air lock or a partial air lock (Brown, 2006).…”

“…To avoid the hazard of air locks, various air lock categories and their hydraulic properties have been investigated in the last few years (Pothof and Clemens, 2011;Pozos-Estrada et al, 2012). Reynolds and Yitayew (1995) studied the air lock phenomenon of a low head irrigation pipeline, with small internal diameters of 6, 8, 10, and 13 mm, and the results show that an entire air lock can form a partial or full blockage in a small-scale pipe.…”