Service Contract No 2007 / 147-446 - Swaziland

Based on equation 1 above the flow velocity, through the 200mm suction manifold is 2.1m/sand according to the Agricultural Research Council, ARC (2007) the ideal suction pipe flowvelocity must be 1.0 m/s, but suction pipe flow velocities up to 1.5 m/s are acceptable. Theexcessively high suction flow velocity in the suction pipe causes turbulence in the pipe, thuscausing irregular feeding of the pump (and eventually causes cavitation). As a result impellorwearing is excessive and thus explains the poor performance by this pump. Maintenancecosts are high as a result.PUMP STATION 2 – Phase Two River Pump StationThis pump station has two Howden 100-400 pumps each driven by low speed 30kW motors.These pumps supply water to a small earth dam irrigating responsible for supplying water to95 hectares. The design duty point of these pumps could not be ascertained but in order formeet sugarcane irrigation requirements these pump must generate at least 376m3/hr totalflow. Each pump therefore has a duty of at least 188m3/hr flow and using equation oneabove, flow velocity through the 250mm suction pipe is 1.11 m/s. this suction pipe is correctlysized and its flow velocity is within recommendations.PUMP STATION 3 – Phase Two Booster Pump StationThese pumps design duty point is also not known but was calculated to be the same as thatof PS 2. The two identical Howden 100-250 high speed pumps in this pump station musttherefore generate at least 188m3/hr each and the flow velocity at this flow through the250mm suction manifold is 1.11 m/s and is within recommended limit.4.1.1.2 Requirements for Fittings90º BendsThe radius (mm) of a 90º bend must be, at least, as shown in Figure 4 belowMaphobeni Farmers Association Report - 2009 Page - 15 -