Double take on jetting damage
WWT invited the British Plastics Federation Pipes Group and the Concrete Pipe Association to give their views on the highly sensitive issue of pipe damage caused by sewer jetting
Later that year the WRc developed a code of practice for sewer jetting, in response to industry concerns. One of the main recommendations was that sewers which are in grade-three condition should not be jetted at pressures above 1,900psi (130bar) to minimise the risk of causing further damage. The code also recommends where the condition of the sewer is unknown, as it frequently is, it should be assumed to be in no better than grade-three condition.
It has been suggested the risk of damage through high-pressure jetting is a diminishing problem, because as sewers are replaced, particularly using traditional pipe materials, their jetting resistance increases to that recommended for pipes in grade-one condition (5,000psi). Two facts dispute the logic of this statement:
- sewer replacement is running at less than 0.5% per year - it would take 40 years to replace all existing sewers in a grade three or worse condition,
- a research project into the reliability of new sewer construction, commissioned by the Foundation for Water Research in 1989, concluded that where new sewer systems, using traditional pipe materials, were being offered for adoption the following were found: 20% required structural repair, 30% of post installation connections were defective, 80% of sites had displaced or open joints. So even brand new sewers must still be considered vulnerable to jetting at higher pressures.
All manufacturers can demonstrate the resistance of their products to a specified jetting pressure. But the knowledge that the sewer is unlikely to be in perfect condition should discourage jetting at the theoretical maximum pressure a pipe can withstand. This issue has been confused by commercially inspired arguments put forward by various parties with vested interests. The jetting resistance of different types of pipe material is largely academic, until such time that the entire sewer network is guaranteed to be in grade-one condition. A situation that, in reality, will never happen.
The new water industry specification for plastic structured wall sewer pipes
(WIS 4-35-01) requires pipes to withstand 2,600psi which is 40% above the WRc
recommended normal working pressure. From this we can conclude that there is
an urgent need for all sewer owners and jetting operators to acknowledge the
logic put forward in the jetting code of practice, enforce its recommendations
and, in doing so, significantly reduce this particular risk of damage to the
nation's sewers and drains.
Tony Calton, British Plastics Federation Pipes Group
Following years of debate regarding the serious concerns over structured wall plastic pipes, the Concrete Pipe Association last year asked: "Are the manufacturers of structured wall plastic pipes aware of damage caused by water jetting and what, if any, improvements have been made following the concerns expressed by many of the water companies?"
At the time, no satisfactory answer was forthcoming, but subsequent events have placed major problems with structured wall plastics in the public domain. It has been reported recently that a major housing developer has refused to use structured wall plastic pipes on its sites because of doubts over the material's integrity. Charles Church (Southern) the upmarket housebuilder will not use the thin wall plastic pipes after serious faults were discovered by CCTV, following water jetting, at a number of large developments in the Reading area. Video footage appeared to show the walls of buried structured wall plastic pipes with puncture holes and water pouring in through them.
Of great concern is the lack of interest by the plastics industry who have dismissed the video evidence, rather than acknowledging the damage caused, and improving their products so that it can not occur again in the future.
The Concrete Pipe Association (CPA) has obtained a copy of the video, part of which can be viewed on the CPA web site: www.concretepipes.co.uk
Following the publication of the Water Industry Specification for Structured Wall Plastic Pipes (WIS 4-35-01) many water companies still have serious reservations regarding the pipes' inability to cope with high-pressure water jetting. Unfortunately, in the WIS, the pipes' jetting resistance was only tested to a level of 2,600psi - this performance criteria is too low. Many experts in the industry believe that 4,000psi should be a minimum with 5,000psi a realistic level, a pressure the WRc jetting code of practice states traditional materials, such as concrete, can cope with.
Jetting resistance was included at such a low level in the WIS to ensure that all of the thin-walled plastic pipes produced would meet it! The WIS itself highlights the potential damage from jetting, stating that "small diameters are generally unblocked using high pressure/low volume jetting machines. These can lead to high forces on the pipe wall."
It is understood that three of the water companies, Thames, Southern and Anglian, which had suspended the use of certain structured wall plastic pipes, will continue to implement the policy.
Water companies should not be pressured into accepting and adopting products with built-in deficiencies that they will have to maintain for years.
Returning to the original questions: as the experiences of Charles Church is
now in the public domain, manufacturers of structured wall plastic pipes must
now be fully aware of cases of jetting damage to their products, but what are
they doing to improve them?
Would you be satisfied using a product so susceptible to jetting damage? Isn't it better to use a tried and tested material, concrete, that has proved its ability to cope with high pressure water jetting and in which you can have full confidence?
Keith Daniel, Concrete Pipe Association