Silt Pumping Plant Rehabilitation Preliminary Design

Home Browse Search

Preliminary Design Memorandum Silt Pumping Plant Rehabilitation superstructure with concrete masonry unit walls and metal roof decking. The plant has four synchronous motor-driven, horizontal, double-suction, single-stage, centrifugal pumps. A four- ton overhead, pendant-controlled,traveling crane is used to handle the equipment. The Silt Pumping Plant consists of four pumps with the following ratings (at 225 feet rated total head): Pump No. 1 — 13.00 cfs; Pump No. 2 - 13.00 cfs; Pump No. 3—6.67 cfs; and Pump No. 4 - 3.33 cfs. These pumps have been in service for more than four decades and may need to be replaced in the near future. All units are controlled by motor control equipment located in the west end of the plant. The different sized pumps allow the District a wide range of delivery capacities up to 36 cfs. The inlet canal is rated for 32 cfs when the Colorado River is at the expected minimum flow of 1200 cfs.The pumping plant discharge line and the Silt Pump Canal are also rated to convey 32 cfs. The additional capacity of the pumping plant may have been provided so that the pumps could meet the 32 cfs design flow at the maximum static lift which occurs when the inlet canal is at the minimum level and the Pump Canal is at the normal water surface for 32 cfs. Also an additional capacity of 3%is typically provided to account for pump wear when raw water is pumped. In about 2002 Public Service Company of Colorado contacted the District regarding the replacement of the pumping plant transformer. The replacement was scheduled for 2008 before the transmission lines for this area are to be upgraded from 69kV to 115 kV;however, in August 2012 the District was informed the transmission line upgrade is tentatively scheduled for 2017. ASSESSMENT OF EXISTING CONDITIONS AND OPERATIONS The pumps and motors have experienced"normal"maintenance issues. However, after nearly 50 years of service,the magnitude of the maintenance on the pumps has increased and replacement parts are difficult to find. The pump motors have been very reliable and maintenance and repair is both possible and reasonable. The vacuum priming system has been very problematic for the District. Problems are caused by vacuum leaks that are difficult to find or correcting them is difficult and sometimes only temporary. Sometimes automated switches stick or do not function at all. On a few occasions,the operators have primed the pumps by back flowing from the Discharge Pipeline. Back flow priming is possible but the discharge valve actuation system is not set up for this type of operation. Backflow priming with the current set up should not be attempted since starting the pump against a backflow large enough to spin the pump backwards has caused a significant hydraulic transient in the past that severely damaged the discharge piping and displaced the pump base. The pump discharge lines in the station appear to be very sensitive to hydraulic transient damage since unrestrained sleeve couplings were used in the original design. After the hydraulic transient event that damaged the discharge pipe on Pump No. 2,the pipe from the pump discharge flange to the flange of the discharge valve was replaced with a rubber spool piece. Like the steel pipe segment and unrestrained sleeve coupling that was replaced,the rubber spool has almost no capacity for longitudinal tension. Therefore,a 8 FOR OFFICIAL USE ONLY