Atkins builds UK's biggest pumping station
Work has begun on a project to drain 700km2 of land in Norfolk. John Sheppard, principal engineer of the Water Operations Team at Atkins, describes the £35M scheme, and the construction of the largest pumping station in the country
The Middle Level catchment contains natural rivers, which have been modified, and manmade drainage courses. The commission is looking after the largest and longest arterial drains, most of which are statutory navigations. Also, there are many smaller pumping stations feeding into the Middle Level system via the local land drainage network. Pumping is required as several hundred years of wind erosion and manmade drainage has caused much of the area to be below sea level.
The existing installation serves the whole of the Middle Level system, transferring all drainage to near the village of Wiggenhall St Germans. Surplus water from the system is discharged through St Germans into the tidal River Great Ouse close to its sea outlet. But, at 70 years old, the station is at the end of its useful life, and no longer meets the required standard of protection.
In April 1998, adverse climatic conditions in the catchment put the pump and drainage systems under immense pressure. At the peak of the event, the various Internal Drainage Boards were instructed to stop pumping as the Middle Level system was almost full, and the four original Gwynne's Invincible pumps at St Germans were operating at maximum capacity. Post-event analysis indicated that this had been less than a 100-year return period rainfall event and, as a result, an extensive study was commissioned.
Atkins has been working on the St Germans scheme since May 2003, working in three phases. The first, a feasibility study, reviewed three different pump type options in conjunction with seven different locations for the new pumping station. The pump options included six concrete volute pumps, eight suspended bowl pumps and six humpback (horizontal axial flow) pumps. The preferred option, which employed six concrete volute pumps just downstream from the existing pumping station, led to a successful Defra funding application.
Much of the Middle Level catchment would be at risk of being under water at certain times of the year, or following a high tide, without the St Germans pumping station. To evaluate the environmental risks to the catchment an Environmental Impact Assessment (EIA) was carried out during the feasibility and detailed design phases.
The EIA involved environmental surveys and a number of rounds of consultation
with statutory and other interested parties. The output of the EIA formed a significant element in the derivation of the preferred option.
Once complete, this new, modern station will ensure the sustainability of the Middle Level System for the long-term future, and provide security from flooding for the large areas of high-grade agricultural land, numerous communities and businesses as well as a number of environmentally sensitive areas.
The second phase consisted of detailed design, prequalification, planning and phased tendering (three distinct contracts). This took 18 months with the third phase, site implementation works, expecting to take around three years.
Water volume varies considerably throughout the year, which requires the new pumps to cope both with very low flow levels during summer months and up to the design one-in-100-year return period rainfall event. Concrete volute pumps provide the capability to satisfy the full range of pumping heads.
The existing pumping station at St Germans currently comprises four pump sets and has a total normal capacity of about 70m3/sec. The six new pump sets, each capable of raising 16.66m3/sec to a static head of 4.25m, will be required to provide this level of output at any time during its 75-year working life.
Although the new station will have the capacity to deal with extreme events, the
normal pumping regime will probably only require three pumps to run in concert.
And a system of rotation will be necessary to ensure the pumps get an equal amount of use.
A key benefit to the new station is that each of the pumps is variable speed and capable of adjusting to a wide range of flows. Each pump is able to perform in accordance with the actual flow demand, and is capable of an immediate response should circumstances change. The immediate response is aided by the pumps not needing priming when starting as is the case with the existing units.
Although the suction conditions are relatively stable, the discharge head can
vary between 0.5m and 8.0m above the suction with the possibility that there is no discharge head at all during extreme tidal events. This has resulted in careful selection of the pump impeller design and variable-speed drive control to ensure the danger points of cavitation at high and low head are avoided.
The main pump components are: the pump stator housing, shaft, upper axial/radial bearing, shaft seal, lower radial bearing, removable wear-ring and impeller. The 2.1m-diameter impellers will be mounted on 4.5m-long shafts, and will be the largest ever built by the manufacturer.
The pump stator comprises a fabricated body, upper bearing set, shaft seal, bearing column, and lower bearing set. The entire assembly will be filled with 11m3 of water, acting as a reservoir for the "liquidyne" shaft seal.
Two metering pumps are used for the seal-flushing system. Half the clean water from the flushing system is returned to the stator storage tank, and the other half flushed down the shaft between the lower seal and lower bearing. The upper bearing set holds the stator in place, absorbing the axial and radial forces imparted by the impeller. The lower bearing is non-wearing and has good heat transfer properties due to its submerged position.
Coupled to the pump shaft and supported on a concrete ring above the volute, the planetary gearbox comes complete with motor lantern. This has a maximum performance of 131rpm although typically operates at 128rpm.
The site implementation can be broken down into civil enabling works, construction of the new pumping station and the demolition of the existing station. The first part of the civil enabling works was the installation of a 1.7km long, stone-surfaced access road, running from High Road at Tilney cum Islington down to the main works area. This was completed in February 2007, and will ensure that all site traffic is routed away from the local community.
The second part will be the installation of a twin-walled, steel sheet piled cofferdam out into the tidal section of the Middle Level Drain. In addition to the cofferdam, a local widening of the drain to accommodate a bypass channel and the cofferdam will be constructed on the eastern bank of the channel.
This on-line construction of the new station will ensure no interruption to the system performance. These works started in February 2007 and are scheduled to take about seven months to complete. Installation of the cofferdam enables Phase 2 of the site works - construction of the new station - to be erected in a dry environment. Continuous Flight Augured piles provide the platform from which to build the superstructure, housing six concrete volute pumps capable of pumping 100m3 of water a second.
In the later stages, the pumps and mechanical and electrical equipment will be installed with connection to a new substation constructed for the station. Each pump is variable speed and has a dedicated 11m-long Motor Control Centre, transformer and 1700kVA standby generator.
Such is the size of each pump, the volutes and suction boxes are to be cast on site using prefabricated formwork. A principal feature of the volutes will be the flat floor and trapezoidal flow cross section. The construction of the volute, suction bell and removable cast-iron wear ring, in which the bronze impeller rotates, must be precise to conform to the required profile of the impeller.
The station will be commissioned on line, downstream from the existing station, using a sophisticated method to simulate operational conditions which requires close cooperation of all three contractors, Atkins and the client. Once the new station is commissioned, the bypass channel will be blocked off, the cofferdam removed, and the tidal sluice gates in the old station opened.
Completion of the fully operational pumping station will enable the final phase, demolition of the historic pumping station, to commence. The demolition will be carried out in two halves within another cofferdam, with completion due at the start of 2010.
The client's project manager, David Thomas, says: "The Middle Level Commissioners are pleased that this scheme has been the result of a professional partnership forged with Atkins, which has sustained the enthusiasm of those involved since the inception in 2003. In land drainage terms in the UK and even Europe, this project is a one-off. The new station will be the largest in the UK. And opportunities to be involved in projects like this don't come often."