By Graham Evans.
Having worked in Trenchless Technology for more years than I care to remember, our experience is not limited to the renovation of gravity systems.
Trenchless techniques for pressure pipes have come a long way in this time and recently many more CIPP systems are being used. The effect of this is that many companies that have been installing CIPP in gravity systems are now tackling contracts with elements of pressure pipes. Often they miss a number of the key considerations that are present in pressure pipes and pumping mains.
Many Epoxy based CIPP systems now have been tested and approved for this application and the advent of composite liners using a glass and felt combination have opened new initiatives in dealing with the higher pressures involved. Unlike gravity systems when you cut into a pressure pipe it should be with the consideration of how it will be reconnected.
Gravity lines are generally designed based on their ability to resist a safe external head of water.
Pressure pipes need to be capable of resisting considerably higher internal pressure, Higher test pressures, surges, sometimes considerable vacuum and the ability to be connected to the system to ensure that the pressure is internal and not on a bare liner end.
Some pipework, especially exposed pipe bridges may need to deal with considerable expansion and contraction. Where the coefficient of expansion is considerably different to that of the liner.
The drainage contractor that moves into this market needs to be mindful that he will come across two different types of system,
The Pumping Main, which will be under pressure but generally runs to an open end, it will suffer surges but usually not considerable vacuum, the pipe can be full and under pressure but often will be surcharged with no pump pressure and occasionally drain down and re fill with the pump cycle.
The Full Pressure Pipe, will spend all its working life under pressure, only being drained down for system maintenance. These operational considerations have very different effects on the liner material during its life.
Items that are often ignored , include the fact that as pressure pipes are not laid on a fall , water will be present and will need to be removed in order to adequately CCTV and line. The resin chosen will have bonding characteristics but this alone will not normally be enough to ensure that pressure remains inside the new line.
When you look at the original specification for a pressure pipeline, the contractor will see, Thrust Blocks, flange spools,flange adaptors , sometimes air release valves and shut off valves , all installed with provision to design against surge, pressure fluctuations and joint movement.
When considering these design criteria, the end of the CIPP liner installation will always be the weak link.
Preparatory end sealing is essential to ensure that the liner acts as part of the pressure system, through all its phases of operation.
There are a number of very good hose type trenchless systems and also many slip lining, pipe bursting and fold and form systems. With all these systems the method of reconnecting to the existing pipeline usually via a flanged connector are essential.
When quoting any CIPP pressure application this reconnection method is paramount to ensure long term stability of the system.
We believe that any contractor who cuts into a pressure system to install a CIPP Liner should always have the reconnection method planned.
The majority of CIPP material suppliers do not often supply a product for this application, but the contractor or client need to be mindful of many other considerations for pressure applications.