The effect of pump operation on gas accumulation in pressure mains

In the past, the pump regime of sewer pressure mains was based on the cycle frequency of the pump in combination with the storage capacity of the wet well of the pumping station. Modern pump units have higher pump cycle frequencies, enabling a reduction in size of the wet well of the pumping station. In view of construction costs of the pumping station, this trend of down-sizing is desired. A second development that took place in the Netherlands at the same time was the construction of mildly sloped, rather than steeply sloped pipe sections underneath obstacles such as railways and channels, due to a new drilling technique. A problem arises for these pressure mains when gas is present in the system. The long travel time through a mildly sloped pipe section, in combination with short pump operation, hinders the gas bubbles to travel through the sloped part. After pump stop, the gas bubbles return to the high point in the system, eventually a causing capacity reduction. The pumping station and its pump operating condition must be designed in close relation with the pipeline system. This paper describes and discusses new design aspects and proposes guidelines that have to be taken into account to avoid gas problems when designing a wastewater pressure main.