Sizing and Design of an Air Chamber — Transient Modeling Results and Field Test Comparisons

Abstract

The Wenatchee Regional Water System was constructed in 1980 to supply all domestic water demands of the Greater Wenatchee, Washington service area. The regional source facility for the system consists of four deep well pumps of various capacity adjacent to the Columbia River, a 7800 gallon air chamber for surge relief, and 10 miles of transmission piping leading to the distribution system and various reservoirs. Included in the service area is the East Wenatchee Water District, which in 1980 opted not to participate and to continue using its own wells. As a result, all system demands have been satisfied by operation of a single large pump. In 2001, increases in demand required the East Wenatchee Water District to become a partner in the regional system. To satisfy the increase in demand, it was necessary to activate operation of two source pumps simultaneously. Transient analyses of the existing system revealed that the existing air chamber was of insufficient capacity to protect the system from a power failure with two pumps operating. A numerical model for transients was used for sizing a new air chamber and to match its operation in conjunction with the existing air chamber. During the course of the study, field measurements of normal pump shutdown were obtained and used to help calibrate the transient model, in particular the simulation of the pump operation and spin down (moments of inertia, etc.). Furthermore, after the new air chamber was installed and with the full service area operating, a series of pump failure tests were conducted. Measurements of water level drawdown and pressure versus time following failure, were taken to confirm the performance and adequacy of the dual air chamber operation. Pressure and discharge readings were taken at various locations throughout the system. Air valves were also monitored for activation during the tests, which would indicate undesirable subatmospheric conditions and potential inadequate surge protection. The tests indicated that the tanks were appropriately sized, with measured results and observations comparing favorably to the transient model predictions. Acquisition of real-world data is thus shown to be useful in model development and in confirmation of the results. The presentation will summarize the transient modeling and investigations, elements of the air chamber design (including air core vortex suppressors and throttling valves set to equalize drawdown in the two tanks), and present results and comparisons of the modeled results and the measured data. This paper was presented at the 8th Annual Water Distribution Systems Analysis Symposium which was held with the generous support of Awwa Research Foundation (AwwaRF).