State of Technology Report for Force Main Rehabilitation, Final ...
State of Technology Report for Force Main Rehabilitation, Final ...
State of Technology Report for Force Main Rehabilitation, Final ...
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Drain Rodding<br />
Simple drain rods are an obvious low-tech alternative to water jetting <strong>for</strong> small-bore pipes. For small<br />
diameter pipes, old-fashioned drain rods are still the most common tool <strong>for</strong> blockage clearance and can be<br />
very effective. The limitation is that there is little control and no feedback as to whether the blockage is<br />
adequately cleared. Another drawback is that the rods may simply be pushing the problem somewhere<br />
else. Nevertheless, this is an effective method <strong>for</strong> removing small blockages in small diameter sewers. It<br />
is less well-suited to <strong>for</strong>ce mains because they generally have higher flow rates than gravity sewers so are<br />
less likely to have loose debris in the invert.<br />
Removal<br />
Pipe cleaning and blockage clearance should involve more than just passing the problem downstream,<br />
which jetting and rodding may do. Jetters are better than rods in this respect, since they break up<br />
accumulations <strong>of</strong> debris into finer material, which is more likely to be carried in the flow and dispersed.<br />
Where there are greater volumes <strong>of</strong> debris that would almost certainly settle out further downstream, a<br />
common solution is to use a combination jetter/vacuum machine to simultaneously flush the sewer and<br />
suck out the resulting sludge. Some <strong>of</strong> the larger and more sophisticated units incorporate water filtering<br />
and recycling systems, together with compression <strong>of</strong> the solids so they take up less space and are easier to<br />
dispose.<br />
Flushing<br />
Flushing involves simply pouring a large volume <strong>of</strong> water into a sewer as quickly as possible so that the<br />
sudden increase in flow will wash away any accumulations <strong>of</strong> debris. It is an attempt to replicate the<br />
effect <strong>of</strong> a summer storm on a combined sewer. Today, such a procedure would be carried out using a<br />
large-capacity tanker-jetter, possibly with recycling and filtering facilities. Flushing is seldom used since<br />
it has been largely superseded by jetting. Jetting uses less water and is a more controllable and reliable<br />
technique. Flushing is also not well-suited to <strong>for</strong>ce mains since it does little more than repeat the normal<br />
operation <strong>of</strong> the main.<br />
Air Scouring<br />
Air scouring is flushing with a twist. The “twist” in air scouring is using alternating volumes <strong>of</strong> air and<br />
water to flush the pipe. The air and water travel along the pipe in separate discrete volumes with<br />
approximately 75% air and 25% water. The air causes the water to move with high velocity and<br />
turbulence, scouring away sediment, s<strong>of</strong>t scale deposits, and bi<strong>of</strong>ilm. Common air compressors provide<br />
the air. Both fluids are fed continuously and separately into discrete volumes by themselves. Air<br />
scouring is more aggressive than flushing and should be used with caution if the pipe’s structural<br />
condition is poor. The AWWARF research report, Investigation <strong>of</strong> Pipe Cleaning Methods, provides<br />
further details on the equipment setup <strong>for</strong> this method (AWWARF, 2003).<br />
6.1.2 Cathodic Protection. Approximately 60% <strong>of</strong> ferrous <strong>for</strong>ce mains reportedly have some<br />
cathodic protection applied. Steel pipes are more commonly protected with a cathodic protection system<br />
because the electrical continuity <strong>of</strong> welded joints makes this a practical solution. It is more complicated<br />
<strong>for</strong> DI pipe where electrical connectors (bonding) need to be placed across all bell and spigot joints.<br />
Cathodic protection is an electrochemical method used to prevent or control corrosion <strong>of</strong> buried or<br />
submerged metallic structures. They are active systems that rely on the application <strong>of</strong> electric current to<br />
control corrosion. If current is interrupted, corrosion will progress at the normal rate <strong>for</strong> the<br />
material/environment combination; if the supplied current is inadequate <strong>for</strong> complete protection,<br />
corrosion will progress at a reduced rate. After a cathodic protection system has been installed and<br />
adjusted to provide adequate protection, currents and potentials should remain relatively stable; changes<br />
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