Silt Pumping Plant Rehabilitation Preliminary Design

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Preliminary Design Memorandum Silt Pumping Plant Rehabilitation surface to the pump suction bell, admission of air and loss of prime. Recommendation: Select horizontal, split-case centrifugal pumps for the following reasons: • Pump station is already designed for horizontal pumps. Therefore, structural modifications to the pumping plant substructure are not anticipated. • Modifications to the trash racks would not be required. • On-site pump and motor maintenance can continue to be provided by the District. • Split-case,horizontal pumps have been proven to be extremely reliable once primed. • Hydraulic transient mitigation can most likely be provided by increasing the rotating mass of the motors with the addition of flywheels mounted on the motor shafts in the same manner as the existing pump motors. • A very robust priming system with duplex vacuum pumps can be provided that should overwhelm the inherent vacuum leaks associated with split-case,double-suction, centrifugal pumps with dual packing boxes. The priming control system would provide modern instrumentation to assist the District in monitoring the performance of the priming system. The priming vacuum pump(s)could be sufficiently robust to prime all pumps at a time without tightening the pump packing boxes to reduce air leaks. Priming Existing Priming System: The existing priming system has been the most problematic portion of the pumping plant. The problems may have been aggravated by the complexity of the system,the relative absence of process indicators such as vacuum and pressure gauges and warning lights and possibly by delayed maintenance. The vacuum system should have been sized to evacuate the suction piping,the pump casing and the discharge piping from the pump to the discharge gate valve for the largest pump in a reasonable amount of time,typically 1 to 5 minutes. This capacity should be more than enough to easily overwhelm any normal air infiltration that might occur at the pump packing glands if the gland is properly adjusted to leak water for lubrication and cooling of the pump shaft/packing interface. The fact that the operators have to tighten the pump glands to reduce air inflow through the packing during priming is a strong indication of poor vacuum pump performance or excessive vacuum leaks elsewhere in the system. The existing vacuum pump should be disassembled and inspected for wear by a qualified technician.Any worn parts and seals should be replaced and the pump should be tested to verify performance within original specifications. The balance of the system should be inspected for vacuum leaks. The pump line 12 FOR OFFICIAL USE ONLY