Civil engineering and building works

9 Direct cooled circulating water systems

9.1 Civil engineering structures in direct cooling systems

AU major generation plant relies on a ‘water to air’ or ‘water$p water’ transfer of low grade heat to cool the condensers and increase the efficiency of the plant.

The water to air system is typified by cooling towers or heat exchangers/radiators and is known generically as indirect cooling (see Chapter 2).

Direct cooling systems draw water from a natural or artificial source of cool supply, pump it once through the circulating water system and condensers, and dis­ charge it where it will naturally cool before re-entering the system (see Fig 3.31). (The phenomenon known as recirculation, is discussed later.)

The source of water is usually the sea or a river, but lakes and artificial reservoirs are also used where necessary.

Once into the cooling water (CW) system the water passes in sequence through the following four main civil structures:

(a) An intake built at such a level in lhe water source as to ensure that adequate water can be abstracted under all conditions of flow or tide. It must be sited

Chapter 3

to avoid causing erosion of the bed or channel or drawing an excessive amount of debris, sill or fish into the system. coarse screen is installed lo exclude large debris either at the landward end of the channel or the seaward end of a tunnel.

(b) A screen chamber to house the fine screens which remove from the water the debris which has passed the coarse screen at the intake. These screens must similarly extend from extreme high water to extreme low water, whatever their type, while

* allowing adequate flow at both. Materials inter­ cepted by the screens are automatically jetted-off into a trough from which they are sluiced into a metal cage for disposal. Any fish arriving at these screens are diverted into by-pass channels and returned to the cold water source.

(c) A pumphouse to contain thb pumps at the lowest level in the CW system with the pipework neces­ sary to take water from its natural level in the screen chamber and raise and distribute it to the top of the condensers via the inlet culverts.

(d) An outfall to take the heated water from the condensers back into the sea dr river at a low velocity which will not cause erosion of the bed or banks or interfere with shipping. This section of the system may include a syphon recovery chamber or

seal weir whose purpose it is to keep the syphon head in the condensers and let the water stream break to gravity flow at a predetermined spot. One is required only where tide or flood levels vary substantially.

The cooling water passes through these structures in the order given. It flows via intake culverts of concrete or steel between the intake and the condensers and, alter use in the condensers, it Hows in outfall culverts to the outfall. Volumes of flow are about 55 nr1 to 60 nr'/s for a 2000 MW station, but can vary depending on envi­ ronmental constraints and the allowable temperature rise across the condensers.

Carejnust be taken in choosing the positions of the intake and outfall to ensure that recirculation of the heated outfall water directly back into the intake, and its consequent adverse effect on operating efficiency does not take place, or is minimised. Avoiding recircu­ lation can be readily arranged if the topography is right and water can be taken from a river and discharged to sea for instance. Where no such solution is possible, a physical or mathematical hydraulic model may be necessary to ensure that recirculation is minimised under all tidal and environmental conditions. Several arrangements of intake, outfall and jetty can be modelled until the best is found.

Mathematical models and physical models need data obtained from on-site surveys to produce valid and repeatable results, but once verified can be used to

of the sea or lake water.

9.2 Culverts

The route of the culverts is governed by the relation­ ship between intake ^and outfall and the axis of the turbine hall and the condensers. Because they require a wide swathe through the site and have little choice in level, they need to avoid crossing other services where possible.

Station lifetime pumping costs are affected by any. change in length or level that vary the head required after making full use of syphonic recovery. Other variables are culvert cross-section, material and water velocity which are interdependent with the material governing the maximum velocity and the velocity fixing the cross-sectional area.

Usual velocities are 2.4 m/s to 3 m/s for concrete and 2.5 m/s to 3.5 m/s for metal, although as techniques and finishes improve both are tending to rise. Glass rein­ forced plastics are also becoming competitive. Culvert areas arc usually 3 m to 4 in in diameter or the equi­ valent in square or octagonal shape.

Care needs to be taken in the design to avoid sharp corners and local changes in velocity which would set

Direct cooled circulating water systems

up turbulence in flow, to avoid areas of dead water which would encourage marine growth and silting, and to allow for the escape of trapped air from release valves or surge pipes. Man access is required at intervals for inspection and cleaning. The intake cul­ verts also have to be designed to resist the maximum discharge head of the CW pumps on a closed con­ denser, water hammer from valve failure and external and construction loads.

Outfall culverts have less onerous internal stresses, most of which are transient, partial vacuum, pressures varying with external water levels and construction and traffic loads. All the culverts must be designed for temperature stresses, overburden pressures and not be buoyant when empty. Concrete culverts in particular should avoid thick sections leading to high temperature

the turbine hall foundations or below ground level in shallow excavations. Each turbine has to be indivi­ dually served with an inlet and outlet culvert, the total calling for several being built side by side. The massive excavations needed to accommodate this multiplicity of culverting, between turbine hall and CW pumphouse, imposes a severe restriction on many other site activi­ ties and needs to be completed as early as practicable in the overall programme.

Steel or metal culverts may be up to 3.5 m diameter and where buried in the open ground require the external protection of purpose-made materials. Thejr internal protective linings have not proved reliable as they deteriorate with age and tend to peel off and clog the condensers. Concrete linings applied under control­ led conditions have been used successfully for several years and are being developed further.

Glass reinforced plastics are becoming available in suitable sizes, either for linings or providing structural properties, again further development is under way.

Reinforced concrete culverts are constructed with walls up to 600 mm thick and are rectangular or octa­ gonal in section to avoid stress concentrations. In the case of inlet culverts operating under pressure, a circular or near circular section is preferred with hoop reinforcement. Where possible they should be con­ creted in one lift with lengths in-situ of up to 10 m, or longer if precast units are used.

Significant leakage from culverts sited below the turbine hall presents very difficult repair problems and must be avoided by care in both design and con­ struction.

In-situ concrete culverts must be cast in alternate bays to allow shrinkage to take place, and ail horizontal and vertical joints require continuous flexible water bars. The main requirements of the concrete are high density and good finish on the water face to cut down leaks and head losses, other requirements are low heat or hydration and good workability, so it is usual to recommend a purpose designed mix fonculverts.

The congestion on site due to excavations for in-situ :oncrete culverts and the length of time they need to be ipen (until tested), has led to the rapid development of ilternatives in other materials and pre-cast concrete sections built round the use of large mobile cranes.

3.3 Pumphouse and screen chamber intake

In order to draw adequate supplies of water at all states T tide or river flow from an intake situated some considerable depth below site level, the CW pumps arc installed in a deep basement (Fig 3.33). This results in very deep excavations for large underground structures and causes problems in construction due to their proximity to water. Techniques for building these structures are discussed in Section 8.2 of this chapter.

The location of the pumphouse, screen chamber and intake varies with the topography, bathymetry and the geotechnical problems associated with construction and operation of the system. The structures may be com­ bined with a jetty, the intake may also be separated from the fine screens and pumps by tunnel or tube. Each site has a series of variations on possible types of structure and the final system is developed from total lifetime cost comparisons.

Construction needs can also affect the design; for example, dewatering is easier from the pumphouse than the intake, and tunnels, are more readily driven uphill than level or down. Soft ground tunnelling carries non-technical risks and may be impossible to drive with adequate safety. Submerged tubes offer an alternative but must not be buoyant if dewatered, and must not represent a permanent hazard by being insufficiently buried in the river or sea bed.

1'he option of putting the pumps close to individual condensers to save costs on pressure culverts leads to complications with layout and foundations, which often lead to its rejection in favour of a combined screen and pump chamber.

9.4 Cooling water tunnels

Cooling water tunnels are required to carry the water from the intakes to the screen and pump chambers onshore. They can be dispensed with if the pumphouse is close enough to deep water to link them by dredged channel but this is normally not possible. In order to find suitable strata for tunnelling, or one for founding a submerged tube, it is often necessary to go lower than the hydraulic optimum level.

The construction programme usually calls for work on the pumphouse, screen chambers, intake and tun­ nels to be carried out simultaneously to meet commis­ sioning dates, often in parallel with jetty works.

Providing additional tunnel faces is difficult because until the pumphouse is down to the level, the driving

shield cannot be pushed forward at the tunnel level, which occurs late in the programme. It may be found necessary to sink a temporary shaft forward of the pumphouse so that one or two faces can be opened. Rarely is it economic to drive towards the pumphouse. Achieving pumphouse excavation on programme is thus of special importance.

Because of these risks to the programme and the lack of control over tunnel driving rates, alternatives such as submerged tubes may offer a chance to avoid the imposition of fully sequential working from the pump­ house excavation.

A variety of techniques are used for tunnelling CW culverts, depending on the depth and nature of the ground. In hard rock, a pattern of holes is drilled forward, charged, fired and the rock removed. Softer rocks can be dealt with by road headers and full-face tunnel boring machines, with or without shielding. In rock, the tunnel is supported temporarily as necessary with shotcrete, rock bolts or anchors prior to final lining with reinforced concrete pumped and vibrated behind formwork in the normal manner. If needed, final back grouting is done through eyes left in the lining.

Tunnelling in rock at power station sites close to deep water is almost invariably wet. Water enters through fissures and faults and considerable pumping _ may be required. Weak rock and steeply-dipping rock may require extensive ground treatment or rock anchoring, while close to the sea or river bed com­

pressed air working may have to be adopted.

Where tunnels must be driven through soft ground" that is likely to include beds of pervious materials, it may be prudent to consolidate these layers with grouts or other means ahead of the driving face. Again, compressed air working may be required to prevent water entering the works, however thorough the pre­ treatment of the ground. Nevertheless such pre-treat- ment will reduce the volume of water entering and the amount of ffog’ in the tunnel. It may also reduce the chances of a major air escape causing a blow-out to the river or sea.

It is not recommended to work in air above the physiological limits which are equivalent to about 25 metres below high water because of long term health risks. The UK imposes such limits which form part of

for all workable pressures. Health risks to men working in air, coupled with high costs of plant and labour have led to great efforts to eliminate the need for com­ pressed air working in tunnels.

Where it is not possible to avoid using air, a large compressor plant has to be built on site to supply sufficient air to keep the workings dry. It should have a back-up prime mover, e.g., diesel and electric or gas and diesel, and be fitted with aftercoolers and an effective means of removing products of combustion from the internal compressors. Air dryers are advisable

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