The industry standard method for clearing blocked drains is to use High Pressure Water Jetting (HPWJ) in which a jetting head with a number of rearward facing jets is driven up the drain with high pressure water providing the thrust force to drag the jetting head and its high pressure hose up the drain whilst the water emitted from the jets acts to flush silt and debris back to the outfall.
By driving a HPW jet up the line and withdrawing it under pressure, silt is progressively flushed from the drain until the line has been cleared. Most modern drain materials will happily withstand jetting pressures of 275bar to 400bar (4,000 psi to 6,000 psi (pounds per square inch), but older materials of construction may be blown apart.
The drains we were dealing with are believed to be over 75 years of age. For this reason, high pressure water jetting of old field drains is regarded as a risky business in the trade, as it could cause collapse which would require additional excavations.
Our scaled-up “bomb jet” used in the Project had two sets of orifice jets arranged radially around the back of the bomb with angles of attack at 8o and 10o, rather than 30o and 35o in a conventional jet and a reduced working pressure of about 35 bar (500psi) vs. 125 bar (1,800psi). Although the thrust force was clearly reduced at these operating conditions there was still sufficient force to drive the bomb jet up the silted drains and to provide a gentler, yet still effective, flushing effect.
Entry submitted by Graham Construction.