Environment and Services

Peter Burberry

• Editorial: Routledge
• Año: 2014
• ISBN:
  9781317891499
• Edición: 8
• Categoría: Technology & Engineering
• Categoría: Engineering General

Environment and Services provides a comprehensive introduction to the technical aspects of building design and construction in the fields of physical environment and services installation. It explains the principles involved, the materials and equipment required, design methods and applications. The eighth edition has been brought fully up-to-date with the current building regulations and reflects recent trends by placing increased emphasis on environmental issues related to buildings.The book is suitable for undergraduate degree courses in building, building surveying, building engineering and management, and architecture. It is also suitable for HNC/D courses in building studies and building services engineering as well as CIOB and RIBA examinations.

Utility Services

9 Water supply

9.1 General standards

Water supply installations are governed by bye-laws made by water authorities and statutory water companies. The government publishes model bye-laws to ensure that, wherever it is appropriate, the bye-laws of individual water undertakings are uniform. Unnecessary variations lead to confusion and expense. In 1986 a new edition of the Model Water Bye-laws was published by the Department of the Environment.

The most significant technical changes in the 1986 edition were new requirements for backsiphonage, cross-connections, cold-water storage, water economy and pipe accessibility. A very important organizational change was the recommendation that all water authorities should make and advertise their bye-laws at the same time. Independent publication at different times could result in anomalies in the pattern of water supply legislation.

It would be difficult for individual designers and installers to evaluate whether particular fittings complied with the bye-law requirements. The Water Fittings and Materials Directory is published by the Water Bye-laws Advisory Service and gives a list of tested and approved items. A vital guide to all aspects of the subject is British Standard 6700 Design, installation, testing and maintenance of services supply water for domestic use within buildings and their curtilages.

Traditionally the Model Water Bye-laws governed hot-water installations and the Building Regulations up to and including 1976 did not address heating or hot-water installations except to ensure adequate provision for air supply, flues and hearths. The introduction of pressurized hot-water storage systems in 1985 caused a problem since the enabling legislation for water supply refers only to 'waste, undue consumptions, misuse or contamination'. Safety features could not be included in the Model Water Bye-laws. The Building Regulations, however, can deal with safety and the 1986 regulations included safety requirements for unvented hot-water storage. A similar situation arises with energy conservation. As a result, the legal performance requirements for hot-water installations are now divided between the Model Water Bye-laws and the Building Regulations.

9.2 Private water supplies

Although public utility water mains can serve the majority of buildings, it is sometimes necessary for private water supply arrangements to be made. Expert advice from specialist firms can be obtained, and the local authority public health department will require to be satisfied as to the suitability of the supply.

It is important to note that, where a private source water supply has to be provided, it is usually also necessary to make private arrangements to dispose of sewage, and care must be taken to avoid pollution of the water source.

For human consumption it is best that the water comes straight from the ground, rather than from a stream or pond which is exposed to probable pollution. A dug well of big enough diameter to admit a person carrying a spade, or a borehole of small dimensions made by mechanical means, and just big enough to admit the necessary pump or suction pipe are possible methods. In either of these the upper part should be lined to exclude surface water, as this is liable to be polluted. It is best to leave the lower part unlined if it will stand safely without collapsing, but lining is usually needed through clay zones, and some form of porous or perforated liner may be needed in water-bearing strata which will not stand up without support. Most modern wells are lined with precast concrete cylinders, and boreholes with steel tubing.

When a well is being dug the water level must be held down by pumping to enable the well-digger to work. The excavated material is shovelled into a bucket and hauled out with a rope. Great care must be taken to ensure the safety of the digger both from falling objects and from collapsing sides of the excavation. Boring operations do not involve dewatering. For most modern requirements boreholes are the most practical and convenient, but dug wells are useful in situations where the water-bearing strata do not yield water freely, and where the larger peripheral area of the well is therefore an advantage.

Dug wells for domestic or farm use are commonly 1.0-1.5 m in diameter, and boreholes 150-200 mm. For small requirements where the strata are coarse-grained gravel or soft, fissured rock, driven tube-wells are often successful. These are usually of steel tube, 30-50 mm in diameter, screwed together and having a point and perforations at the lower end.

The usual consumption of water is approximately as follows:

• 0.09-0.18 m³ per day per person
• 0.36-0.45 m³ per day per ordinary house
• 0.14 m³ per day per dairy cow
• 0.05 m³ per day per dry cow

For market-garden and pasture irrigation in season, 12mm of 'rain' per week is generally considered sufficient provision.

Figure 9.1 shows how rain soaks into porous ground and finds its way through layers of rock or sand to feed springs, wells, boreholes and rivers. Underground 'rivers' are very unusual, and the water ordinarily lies in myriads of tiny crevices in rock layers and between particles of sand. This forms a vast underground store of water, rather like a tank filled with both water and pebbles. Usually there is a movement of water through these cavities to an outlet. A simple example of this is a gravel-capped hilltop overlying clay: springs flow at the junction with the clay, and water may be obtained from wells dug in the gravel down to the face of the clay. If the catchment area is small, and the gravel clean and readily porous, the water will run out quickly and the springs and wells will fail in prolonged dry weather. If the area is extensive and the gravel interspersed with fine-grained material, the rate of flow will be diminished and springs and wells will continue to yield small supplies even in dry times. The same things happen in the more complex conditions shown in the later diagrams. An artesian borehole is one which pierces a layer of clay and enters a lower porous zone from which water rises as a gusher above ground level. A similar borehole where water rises part way but does not reach the surface is called subartesian.

9.3 Water conservation

Economy in the use of potable water has always been a key factor in the design of water supply installations. The siphon type of flushing cistern used in the United Kingdom, traditionally called a water-waste preventer, was developed and required to be used in order to limit waste. Prevention of waste is a main purpose of the UK Model Bye-laws for Water Supply. In recent years population increases and increasing standards of hygiene and equipment have led to major concerns about water economy and to studies of new ways for conservation. This is the case both in countries where water is scarce and, in those such as the UK, where rainfall is more than adequate but the provision of catchment, storage and distribution facilities present economic problems. There are several ways that offer promise of savings.

Metering

Some countries have adopted the philosophy that charges for domestic water supply should not be directly related to consumption since this might limit standards of hygiene. Where high standards of hygiene are conventional, it appears that metering of domestic supplies can reduce consumption without significant effect on health.

WCs

Supplies to WCs are the main items in water consumption. The quantities of water used at each flush vary from country to country. In the United States, flushing valves are often intended to give a 5 gallon (22 litres) flush. In the United Kingdom, 3 gallon (13 litres) flushing cisterns were widely used at one time. Nine-litre cisterns are now standard and, for increased water economy a dual-flush facility can be provided which, if no solids are present, can be operated to give a reduced 4.5 litres flush. The effectiveness of this measure depends on the users. Experience in housing areas equipped with this type of flushing cistern indicates that savings of 10% of the overall water consumption can be achieved.

In Scandinavia, in particular, WCs are in successful use with flushes of 6 litres and less, and there is every reason to believe that such designs will be used elsewhere.

Figure 9.1 Wells and boreholes for private water supplies.

Showers

Showers, particularly those delivering a fine spray, offer great economy of both water and energy when compared to baths. They can also save space and are safer in use than baths.

Urinals

The regular automatic flushing of urinal stalls consumes substantial quantities of water. The typical automatic flushing cistern delivers 5 litres per stall once every 20min throughout the 24 h, whether the urinals are in use or not. Many devices are now being used to limit the amount of flushing. They include electronic and pressure-operated systems for preventing flushing during periods when no use is made of the urinal.

The most dramatic development is the waterless urinal. Urinals are normally regarded as major sources of smell, requiring frequent flushing to control this. Recent experiments with bowl-type urinals have, however, demonstrated that it is possible for this type of appliance to perform satisfactorily without flushing. In experimental installations no trouble has been experienced from smell and the requirements for cleaning of traps and wastes have not exceeded those of conventional installations. Copper traps do not resist the corrosion of the undiluted flow, but plastic ones have performed satisfactorily.

Non-potable supplies

One anomaly of present water supply systems is that all the water supplied has to be drinkable but only a very small proportion is actually used for drinking or culinary purposes. Although in principle it seems uneconomic to provide pure water for all purposes, no practical and cost-effective system has yet been proposed to supersede the present methods.

9.4 Pumps: the three main types

There are very many types of pumps; broadly they fall into three main categories:

• Displacement pumps
  One or more plungers move backwards and forwards or up and down, each drawing water past a valve into a cylinder then forcing it out past another valve.
• Rotary pumps
  Meshing 'gear' wheels, or a rotor and a stator, are pressed together in such a way that water is pocketed, passed through the pump body, and forced out as the meshing elements meet.
• Centrifugal pumps
  A rotating impeller flings water to its periphery, where the kinetic energy is converted to pressure energy by a volute or set of diffuser vanes.

All these types a...