The on/off operation of a sewage system means that the rising main is constantly subjected to changing pressures as the pumps are started and stopped by level switches in the sump. And typically, the surge pressures generated by pump failure are higher than the start-up case, often well in excess of the pumps' maximum discharge pressures (no-flow head).
This example shows the 33in cast iron main which runs some 2.7km from Kingston Sewage Pumping Station to Hogsmill Valley Sewage Treatment Works. The pumping station houses 6 electrically driven pumps, each fitted with a flywheel - 2 dry-weather units (rated at 1600 Igpm), 3 storm water pumps (rated at 3200 Igpm) and a standby. The actual total capacity of the station is 10890 Igpm (2970 cu.m/hr).
The pressure surge caused by the failure of all 3 storm pumps is shown on the graph above. At first the flow and pressure drop and this effect is transmitted throughout the pipeline. Then, the entire column of liquid stops and it reverses into the pumping station, impacting on the non-return valves at the pumps. This causes a local pressure rise and this is the peak pressure that you can see at 39.5s.
Our policy is to confirm the accuracy of our simulation programs by an on-going process of validation - comparison between measured and simulated results. And so this graph also compares the site measurements with the pressures predicted by our pSim program. This shows an exceptionally good level of correlation with 99.7% agreement in the magnitude of the peak and the timing of the wave is also particularly good, agreeing within 2%.
This comparison of simulation output against site data confirms the validity and accuracy of our program.
