Subatmospheric pressure in a water draining pipeline with an air pocket

ABSTRACT

An air pocket’s behaviour inside of a pipeline during transient conditions is of great importance due to its effect on the safety of the hydraulic system and the complexity of modeling its behaviour. The emptying process from water pipelines needs more assessment because the generation of troughs of subatmospheric pressure may lead to serious damage. This research studies the air pocket parameters during an emptying process from a water pipeline. A well-equipped experimental facility was used to measure the pressure and the velocity change throughout the water emptying for different air pocket sizes and valve opening times. The phenomenon was simulated using a one-dimensional (1D) developed model based on the rigid formulation with a non-variable friction factor and a constant pipe diameter. The mathematical model shows good ability in predicting the trough of subatmospheric pressure value as the most important parameter which can affect the safety of hydraulic systems.

KEYWORDS:

• Air-water
• air pocket
• pipelines emptying
• subatmospheric pressure
• transient flow
• water distribution systems

Acknowledgments

This work was supported by the Fundación CEIBA - Gobernación de Bolívar, Colombia which covered the financial support for the doctoral student, Oscar E. Coronado-Hernández.

Disclosure statement

No potential conflict of interest was reported by the authors.

Nomenclature/Notation

	=	= cross sectional area of pipe (m)
	=	= used valves to establish the boundary conditions
	=	= internal pipe diameter (m)
	=	= drain valve
	=	= Darcy-Weisbach friction factor (–)
	=	= gravity acceleration (m/s)
	=	= flow factor of the drain valve (m/s m)
	=	= length of the water column (m)
	=	= total length of the pipe (m)
	=	= polytrophic coefficient (–)
	=	= absolute pressure of the air pocket (Pa)
	=	= atmospheric pressure (Pa)
	=	= total discharge (m/s)
	=	= time (s)
	=	= valve maneuvering time (s)
	=	= number of reaches of the pipe (–)
	=	= volume of the air pocket (m)
	=	= water velocity of the water column (m/s)
	=	= length of the air pocket (m)
	=	= elevation difference of the water column (m)
	=	= density (kg/m)
	=	= refers to initial condition (e.g., initial length of the water column)