Understanding Air Release through Air Valves

Abstract

Filling and emptying pipeline systems involve the movement of large volumes of water and air. Water transients occurring during these operations can originate large pressure peaks that can severely damage distribution networks. Entrapped air, depending on the circumstances, can have a damping or amplifying effect on these undesirable pressure peaks. Unfortunately, too often, the complexity of the phenomenon makes it difficult to obtain a fully reliable prediction on when air pockets will mitigate or accentuate water transients. For such prediction, equations governing the movement of the water column and the behaviour of the air pockets must be combined and, for practical reasons, simplified. These simplifications conventionally lead to questionable hypotheses (e.g. normal cross-section of water-air interface) required to solve the numerical model with a reasonable calculation time. Furthermore, the value of some of the parameters used in the numerical model cannot be calculated nor measured and need to be determined through a calibration process. Inevitably, this circumstance will lead to an even higher uncertainty of the results. With the aim of overcoming most of the aforementioned uncertainties, this paper summarises a complete set of tests conducted at WL | Delft Hydraulics. These tests were satisfactorily simulated by means of a tailored numerical model that includes a set of parameters whose values (the main obstacle to be overcome) were determined by means of a calibration process. The experimental setup (a large-scale facility) consisted of a single steep pipeline with an air valve installed at its top end. Air release through different air valves were tested under different conditions while the main variables (flow rate and pressure) were registered continuously at several points along the pipe. Lastly, and in order to be able to provide air valve manufacturers with helpful guidelines, the relevance of the parameters was evaluated by means of a sensitivity analysis.

Hydraulic Transients Caused by Air Expulsion During Rapid Filling of Undulating Pipelines

“…For the rigid column transient, the models (e.g., [2,8,16]) assume a sharp interface between the air pocket and the water column. The air pocket is progressively expelled from the pipeline through the orifice (or the air vent).…”

“…Other relevant results were obtained by many researchers, who analyzed air release through both orifices [8,[12][13][14] and air release valves [7,15,16]. In many cases, the valve (or the orifice) was located at the end of an ascending pipe [2,7,12,13,16] or at the end of a horizontal pipe [8,11,17].…”

“…In many cases, the valve (or the orifice) was located at the end of an ascending pipe [2,7,12,13,16] or at the end of a horizontal pipe [8,11,17].…”

Numerical Model of Air Valve For Computation of One-dimensional Flow

“…There are many possibilities. Probably, method of characteristic (or general method of characteristic) is the most popular [1], [5], [10], but Lax-Wendroff method is used in this paper. The numerical scheme is drawn in the figure 1.…”

Numerical Simulation of Transient Pressure Control in a Pumped Water Supply System Using an Improved Bypass Pipe

“…An air valve is a common, economical, and effective piece of water hammer prevention equipment that is normally installed at the local peak of a pipe. When negative pressure occurs inside the pipe, the air valve opens to allow air in; when the pressure inside the pipe increases, the air valve releases it by discharging air [4] and [5]. A check valve is an economical and reasonable water hammer prevention measure that is widely used in pumping stations [6].…”