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


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The long-running debate about sewer jetting has made little mention of the

potential threat indiscriminate use of high-pressure water jetting poses to

our ageing and often very vulnerable network of existing sewers.

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

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