Surge Pressure Allowance for Thrust Restraint Design of Small Diameter Water Distribution Main

摘要

Currently, standard blocking design for thrust restraint of small diameter distribution main in the Washington Suburban Sanitary Commission (WSSC) service area is based on a design pressure of 250 pounds per sq. inch (psi), which consists of working pressure and surge pressure allowance. The surge pressure allowance, based on American Water Works Association (AWWA) 1977, ANSI/AWWA Standard C101-67, is defined at 100 psi to 120 psi for water main of 4 to 16 inches. Literature search indicated that studies on surge pressures in small main of these sizes are lacking. It was conjectured that the actual surge pressures occurring in small diameter distribution main might be lower. This study attempts to evaluate the magnitude of surge pressures that might occur in distribution main of 16 inches and smaller and how it may affect the current WSSC criteria for surge pressure allowances for thrust restraint design. Attempts were made to develop a computerized surge model representative of the entire distribution system. However, it became extremely difficult and impractical to develop such a model that could simulate surge pressures in small diameter main within an extensive and complex water distribution system. Hence, the author opted to determine if there are any system operational events that might produce surge pressures comparable to those currently adopted by the WSSC. Several scenarios that could possibly produce high surge pressures were evaluated, including rapid closure of fire hydrants and operation of water pumps on fire trucks during fire rescue. Computer modeling was performed to study the impacts of these operational events with a simplified distribution system configuration. Local fire rescue personnel were consulted to obtain information used for the analysis. From this study, it was concluded that the predicted surge pressures could be comparable to the standard surge pressure allowances currently specified by WSSC under the operating conditions of its water distribution system. Based on these findings, the author has concluded that the current criteria for standard surge pressure allowances, in accordance with AWWA C101-67, should remain to be used for thrust restraint design of small diameter distribution main for the WSSC water distribution system.