Pressure utilization in water transmission pipelines considering transient conditions

The residual pressure energy available in water transmission pipelines can be easily utilized via direct connection to the distribution system and bypassing the storage reservoirs and booster pumping stations. This study investigates the feasibility of bypassing a number of reservoirs and pumping stations in a small transmission system in AlAin City of United Arab Emirates. The system has a major transmission pumping station (AlAin Reception Pumping Station, or 'AARS') that hosts four fixed-speed pumps, a main 1200mm line, storage reservoirs and boosting pumps at three zones (Dahma, Markhania and Maqam), besides few other connection pipes. The system also has three surge vessels and air valves. The objective of the study has been achieved by conducting a comprehensive transient analysis to evaluate the potential of pressure rises associated with various operating scenarios. Such scenarios were divided into normal and abnormal operating conditions. The normal operating conditions are related to the typical water supply changes occurring during any typical day. The abnormal operating conditions represent rare and odd operating scenarios such as sudden valve closures/openings and pump trips. Limiting pressure heads of 160 m for the pipe pressure rating and 60 m for the pump shutoff head were considered to identify safe performance of the proposed bypass setting. British Standards (BS), International Standard Organization (ISO) and Abu Dhabi Water and Electricity Authority (ADWEA) Standards were adapted for the design of pipes and fittings. Simulation results associated with the normal operating conditions indicated that the proposed setting is safely performing in such conditions and has yielded safe closure times for different valves. Results of the abnormal operating conditions indicated unsafe performance in the case of sudden valve closures. To protect the system against severe transient pressures under these conditions, a number of pressure-relief valves were proposed to be installed on the upstream side of the distribution network. The proposed valves were sized and evaluated when the system pressure is maintained below 100 m. The savings in energy cost associated with the proposed bypass setting is estimated at about $850,000 per year. Such savings are found to be available after a payback period of one year and one month to cover the capital cost of the needed bypasses and pressure relief valves.