Articles & Case Studies


Posted: Tuesday 3rd March 2020

Steve Handy, business development director at Sykes Pumps, discusses the remediation works being carried out to extend the service life of a sewer main in St Ives by Glanville Environmental and the overpumping solution required to facilitate the works and prevent the risk of bursts.

Once a quiet fishing village, the seaside town of St Ives in Cornwall became famous as a centre for the visual arts in the mid 20th century and now attracts hundreds of thousands of tourists to its beaches, galleries and tea shops every summer, fuelling the local economy.

The town’s resident population of around 11,000 is boosted by these visitors every summer but still relies on a limited infrastructure; including a single sewer main that runs under the primary route in and out of the resort. This 750mm concrete pipe is a gravity main, fed by the Porthgwidden pumping station, which takes sewage and rainwater run off to the nearby wastewater treatment plant.

The sewer main not only needs to cope with a fluctuating population, but must also be able to cope with heavy rainfall to avoid the risk of flooding.

The large diameter of the pipe remains more than sufficient to cope with peak flows but natural corrosion had affected the concrete pipe over time. Despite the location of the main below the road surface, this has resulted in unpleasant odours and prompted a full survey of the entire length of the pipe. The survey considered seven sections of main and revealed that four of these were compromised to Grade 5 – the most severe level on the scale of degradation – while the remaining three have an estimated 10 years of service life before remediation will be necessary.

Planning the programme

Construction and civil engineering contractor, Glanville Environmental, was assigned the task of relining the sections of the main that have been classified as Grade 5, using a no dig installation method to minimise disruption for local residents and businesses. The work was carried out over the winter months to avoid the busy tourist season, with a time critical deadline of the end of March, in time for the ‘summer embargo’ on infrastructure works on the area, which begins on 2nd April.

Each Grade 5 section of the pipe has been relined in a dedicated phase of the programme, which has involved the implementation of a contraflow traffic control for the road, along with an overpump of the sewer flow to enable the required section to undergo remediation safely. The Sykes Pumps team worked closely with Glanville Environmental to design a failsafe overpumping installation that eliminates the potential for downtime and addresses health and safety requirements, while managing variable flows to avoid any risk of sewage leaks or bursts.

No-Dig Relining

The sections of the main considered in need of immediate re-lining were section 7 (which was completed in November/December 2019), section 1 (completed January/February 2020) and sections 5 and 6 (completed in time for the start of the summer embargo).

For each section of the main, the lining material was brought to site as a flexible sleeve, pre-impregnated with resin and hardener, which was lifted into the main via manholes using a crane. High pressure water was then used to push the liner down the pipe into the relining position, followed by a U/V or hot curing process, depending on the condition of the section of pipe in question, to harden the material into a robust inner wall integral to the existing pipe. The new lining ensures that the existing main retains sufficient capacity for maximum flow rate requirements and predicted future needs within its 50-year service life. It also provides a barrier to prevent future corrosion.

In order to carry out this painstaking and technical operation, the contractor needed to be certain that the flow from the sewer would be diverted during each phase of the works and that there was no risk of leaks or bursts that could become a public health hazard during the project. The brief to Sykes Pumps was to provide an overpumping solution with the flexibility to manage variable flows, the resilience to avoid any risk of failure and the ability to be relocated for each phase of the project.

Failsafe Solution

The pump specification followed a number of collaborative meetings and a detailed site survey. The maximum flow rate requirement had been calculated to be 350 litres per second but, given the timing of the project during the winter months and Cornwall’s known vulnerability to flooding, it was decided that the overpumping solution should be capable of handling 400 litres per second of high solids content flow.

Two UV301 prime-assisted, electric surface-mounted pumps were installed, each capable of maximum flow rates of 450 litres per second. Ideally suited to sewage overpumping applications, these pumps feature an impeller to ensure effective solids handling and a back flush for blockage clearing, ensuring a robust solution for constant operation throughout each phase. The high performance pumps combine cost-effective operation with quiet functionality and a small footprint, making them an ideal choice for the route of the sewer main, overcoming the confined work area and the sensitivity of working close to residential properties.

For each section of the relining project, the pumping equipment was located at a manhole upstream of the work area and overpumped to be discharged back into the sewer downstream of the affected section of pipe. Because the UV301 is deployed at surface level, the overpump operation only needed to accommodate a 12” suction hose through the manhole to the sewer below ground, making it much easier to install and maintain the pumps. This not only provided advantages for the initial site set up but, combined with the pumps’ easily portable design, it also offered the flexibility needed to move the equipment from one location to another as the programme moved through each phase.

The UV301’s large flow capacity and their self-priming capabilities were also key elements in the pumps’ selection because one of the challenges of the typography and the gravity main is that the flow can vary dramatically between minimal amounts and gravity-induced higher flow conditions. Self-priming ensures that the pumps can cater for the varying changes in flow instantly, with the vacuum system running constantly, even when there is no flow, without any damage to the equipment. This ensures that there is no risk up of a build-up of sludge in the upstream section of the main due to a delay in priming. The UV301 was used with a variable speed drive panel to adjust the flow rate to the required level throughout the project. This ensured that pump efficiency could be optimised with real time adjustment of the flow rate to suit variable site conditions for each phase.

Installation of two surface-mounted pumps allowed one to be used as a duty pump with the other in place as a standby unit, providing a fail-safe solution. Telemetry controls were installed on both pieces of equipment to ensure automated switch to standby should a fault be detected on the duty pump. The installation also included two generators, one as the duty energy source and the other as standby, and an AMF (auto mains failure) panel was installed to automatically switch from one generator to the other should a drop in power be detected. As an additional safeguard, two 8” Sykes Super Wispaset diesel pumps were also installed for each phase of the project to provide additional capacity or an interim solution should both of the electric surface mounted pumps be compromised at the same time. Each of these pumps is capable of a maximum flow rate of 160 litres per second and was installed with a float switch to ensure the units started automatically if required. Often deployed for emergency sewer repairs, these pumps are suitable for 24/7 operation, ensuring a reliable safeguard was in place for any spikes in flow or maintenance issues with the duty and standby electric pumps.

The equipment was installed by Sykes Pumps engineers who were also present on the project throughout each phase to ensure the pumps were well-maintained and operated efficiently throughout.

To direct the flow from the site of the pump installation to the discharge location downstream, the discharge pipe was formed utilising 12” steel pipework over a length of about 400 metres per run. The layout of the pipework provided ample room for traffic, pedestrians and ongoing works. This included air valves fitted to prevent the possibility of a vacuum being created due to the pipework following the downhill route of the gravity main, there could otherwise be a possibility that a vacuum could be created during peak flows, causing the pipe to burst or experience air locks.

Safety Critical

The sewer main relining project will extend the service life of the existing sections of pipe for 50 years and has been completed within the short off-season period, allowing the less degraded sections of the main to be tackled at a later date. The failsafe pumping set-up was moved for each phase, ensuring that the safety of the contracting team and the public was protected at all times and any potential for incidental damage to the main was avoided.

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August 2021

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