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Teamwork pays off in Dorset

A major inlet works in Dorset has been replaced by on schedule and under budget by Trant Construction. James Hendersonfrom the company reveals the extent of works

Controls at the new inlet worksControls at the new inlet works

Water engineering specialist, Trant Construction, has successfully completed a large inlet works replacement at Wessex Water’s Poole sewage treatment works (STW). Poole, a large coastal town in the county of Dorset, has a population of around 170,000. Sewage treatment is provided at a site located about 1km to the north in Poole Harbour.

The existing inlet screens and screenings handling equipment was nearing the end of the asset life, causing frequent breakdowns. These regularly resulted in effluent compliance problems caused by carry-over of rag and screenings into downstream biological aerated filter (BAF) and UV disinfection processes.

Trant was appointed as an AMP5 framework contractor in 2010 and tasked to deliver a £2.4M programme of works for Wessex Water. The works consisted of band screens, compactors, new washwater facilities and intelligent motor control equipment. The equipment would be housed in the existing, odour controlled, covered inlet works building.

The project, which was completed under budget and on schedule, was achieved through multi-disciplinary teamwork and close integration on and off site between Trant Construction and Wessex Engineering & Construction Services’ (WECS) project management team. It was recognised that successful project delivery would depend on the quality of liaison with the operations team due to the inability to divert inlet flows during construction. Installation and commissioning had to take place whilst ensuring the works remained fully operational at all times, as such a detailed changeover programme was generated and agreed with all parties.

John Calnan, contracts manager for Trant Construction says: “We are delighted to have been fully involved with the AMP5 delivery team on this interesting and challenging project. Timescales were tight and maintaining flow to full treatment was critical to Wessex throughout the duration of the project.

“We achieved the client target date through excellent teamwork, sheer hard work and a can-do approach on site.”

Motor control

A new intelligent motor control centre (IMCC) for the inlet works was designed, manufactured and installed by Trant Systems Electrical (TSE), Trant Construction’s own MCC panel manufacturer. The IMCC is based upon a form 4a type 2 construction, using a certified distribution system – Busbars, rated at 400A, fault rating 30kA / 1 second.

The mechanical design process encompassed the client’s specification, Wimes 3.01 specification, BS7671 and the requirements of BS EN 60439 and 61439 with regards IP-rating requirements, clearances and creepages, protection against electric shock, temperature rise and mechanical operations.

Poole STW site has recently undergone a complete Siemens S7 400H control and monitoring system upgrade, therefore the new control equipment was required to interface directly with this. The panel consisted of a 100A incomer, three 2.2kW variable speed screen drives, three 7.5kW reversing directo online (DOL) compactor drives, two TP&N 32A feeders and an intelligent control and automation (ICA) section containing Siemens remote I/O PLC interface, ABB flow metering, and Pulsar level control. The screen drives, compactor drives and PLC I/O were connected using dual profibus networks with active terminations to the existing inlet works’ MCC.

The existing plant screened incoming flows with six Andritz 1,500mm-wide, 6mm-aperture 1D screens, situated in three channels, separated in each by a 720mm-wide concrete plinth. Three Ovivo CF200 6mm-aperture 2D band screens, with a design capacity of 1750l/s per screen, were installed to replace the existing screens, working on a duty / assist / assist basis.

By isolating one channel at a time two screens were brought offline, decommissioned and removed. The replacement screen was then installed in the existing channels which were made wider by the removal of the dividing wall between each pair of old screen channels. Due to load restrictions of the overhead beam crane, the screens were manufactured and delivered to site in two halves to allow erection and installation in situ.

The replacement screen was then commissioned and performance tested, before moving to the next channel. Using intelligent Siemens variable speed drives, each new screen operates at two different speeds, depending on the flow into the works at 34Hz (normal operation) and 50Hz (high flow operation).

Flow monitoring is via a Pulsar ultra twin differential system. Each screen has a washwater feed to the internal spray bar and its associated launder channel. Each launder channel directs flows to one of two compactors via a manual hand stop arrangement.

Compactors

Three Kuhn KWP 400/800 wash press units, including a hopper, were installed at an intermediate Mezzanine level. Each screenings handling unit is sized to handle 8m3/hr of screenings and is provided with a discharge pipe to deposit screenings into the receiving skip at ground level.

The new launder troughs are arranged such that, during normal operation, screenings from each inlet screen are transferred to its dedicated screenings handling unit. In the event of a screenings handling unit failing, screenings may be diverted to an adjacent screenings handling unit by manual operation of handstops in the launder troughs. Each of the screenings handling units is provided with an inlet hopper to allow connection of one or more launder troughs. Each compactor also requires direct washwater feed at a minimum pressure of 2.5bar. Each compactor has its own dedicated local control panel.

A duplex dual-basket filter has been installed to filter final effluent washwater to 0.5mm in two dimensions, before entering the washwater booster-set break tank. A Grundfos washwater set has been installed to supply the three inlet screens and associated launder troughs, three screenings handling units and a number of hydrants on a ring main.

The washwater system is designed to achieve a duty at the washwater set outlet of 35l/s at 41m. The set consists of three booster pumps. The pumps transfer final effluent from the break tank to inlet works equipment and hydrants connected to the washwater mains.

The pumps are variable speed driven and operate on a duty / assist / standby basis, based on the pressure in the delivery main. An accumulator is used to pressurise the washwater main and limit the number of stop/starts of the booster pumps.

Trant oversaw the production of operation and maintenance manuals via Arlosh; these are available online and on CD to the operational and maintenance team and provide a simplified route to drill down to information required. Specialised graphics were also produced providing an overview of the inlet works in an easily digestible format for anyone without the detailed knowledge of the workings of the site.

Sequencing of the changeovers and subsequent successful reliability trials of the new equipment required very careful programme management to ensure a seamless transition from old to new equipment.

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