Abstract
Pipelines undergo elevated air pressures during construction and maintenance. The higher pneumatic pressures are used in caliper pigging and cleaning pigging. Pneumatic pressures are also required in special conditions during pipeline maintenance. Higher pneumatic pressures are usually released into the atmosphere through a small vent. Higher pneumatic pressure generally poses elevated risk related to safety. In the past, industry has experienced incidences where objects such as pigs are propelled due to air pressure resulting in untoward incidences.
The paper presents a theoretical approach to estimate the time required to vent the pipeline through an orifice. It is a known fact that the gas escaping through a vent has two different velocity regimes. At high differential pressure, the gas is escaped through a velocity which is known as choked or sonic velocity. The velocity usually depends upon the pressure and density of the air inside the container. At lower differential pressures, the gas escapes through a velocity known as non-choked velocity or sub-sonic velocity.
The gas escape velocity determines the rate of pressure variation inside the vessel. Similarly, the density which depends on pressure also changes. The switchover between gas velocity regimes also makes the problem significantly complex. The paper presents a detailed approach to ascertain the time of venting by solving the underlying equations. Considering the complexity of the problem, the equations are solved using numerical integration using Runge-Kutta method. The complete algorithm for solving the problem is provided in the paper. Additionally, a few cases are discussed in detail for venting of large pipelines.
