Basics of Strength Tube Calculation
The choice of strength and class of pipes for water lines and water supply networks should be made on the basis of a static calculation taking into account the conditions of their operation, i.e. internal soil pressure, loads from transport and atmospheric pressure in the formation of vacuum. Calculations should be made in those combinations of loads that are most dangerous for pipe material.
The calculation of the joint effect of possible combinations of loads acting on the pipeline in the conditions of its operation allows maximum use of the bearing capacity of selected pipes and choose the most economical and expedient type of pipe.
The static calculation of pipelines is carried out according to the limit states on the basis of data on the strength characteristics of the pipes used, determined by GOSTs for the manufacture and acceptance of these pipes. These data are the loads under which the pipes are tested after manufacture. Usually such a load is the test internal pressure . The calculated value of the internal pressure should be taken equal to the greatest possible working pressure in the pipeline p pa6, without taking into account the increase in pressure during a hydraulic shock or the greatest possible pressure at a hydraulic attack p multiplied by To in , , whichever is the higher.
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The coefficient K y should be taken equal to: for steel pipelines - 0,9; for cast-iron, asbestos-cement, reinforced concrete and ceramic - 1; for plastics - in accordance with the specifications for the manufacture of this type of pipe.
Steel pipelines, as well as pipelines made of polyethylene and solid polyvinylchloride, are calculated for the combined effect of internal pressure, external load, atmospheric pressure p vacuum in the formation of vacuum and external hydraulic pressure < i> p d Cast iron, asbestos-cement and reinforced concrete pipelines are counting on the combined effect of internal pressure and external load.
The value of the estimated reduced external load on the pipes when laying them in the embankment or in the trench is defined as the sum of the calculated reduced loads from ground pressure and the impact of transport or other loads acting on the surface of the soil above the pipeline (SNiP 2.04.02-84).
To facilitate the calculations of VNII VODGEO, graphs have been developed for the selection of a class of pipes, depending on the joint effect of internal pressure and external reduced load.
Before commissioning, the installed pipeline is tested for strength after backfilling of the trench. The value of the test pressure is set by the project taking into account the static calculation of pipes for strength and the design of butt joints.
Equipment and facilities on networks
To ensure the normal operation of the water lines and the water supply network, valves are installed to isolate repair areas, air inlet valves, air vent for air discharge, discharges for water discharge when the repair area is switched off and for flushing before commissioning at the end of the pipeline construction or after repair works, non-return valves or valves of other types of automatic action to turn off the network sections in the event of an emergency, timogo pressurization water hammer, standpipes, fire hydrants.
When composing the wiring diagram (network detail), the locations of the reinforcement are determined.
The gate valves divide the network into repair areas. The length of repair sections is adopted: for water pipelines - no more than 5 km, when laying water lines in one line - 3 km, for water lines - when turning off the repair site, no more than five hydrants should be cut off. In addition, the valves are installed at the beginning and end of each distribution line.
Two types of valves are adopted in our country: parallel, having parallel locking discs (Fig. 8.5, a), and wedges with one lock wedge disk 8.5, b). The cast iron casing (at high pressures - steel) with flanges.
Fig. 8.5. Gate valves with a manual drive parallel (a) and wedge (b)
In the raised points of the water lines and the main lines, the equipment for the inlet (to prevent the formation of a vacuum) and the air outlet are installed.
All air intake and exhaust devices can be divided into three types: automatic operating air vents (figure 8.6) for continuous air discharge; automatically acting anti-vacuum valves to allow air to enter the pipeline when the pressure drops below atmospheric pressure; valves for air intake and exhaust, opened directly by hand or by remote control.
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Fig. 8.6. Vantuz
Vantuzy and anti-vacuum valves are recommended to be installed in one well in the points of fracture of the profile. To allow air to enter the pipeline in small quantities, a plunger can also be used, which simultaneously serves to vent air during normal operation of the pipeline.
Check valves are generally arranged on pressure lines at pumping units to prevent water from discharging through the pump when it suddenly turns off.
Safety valves can be used as equipment to protect pipelines against unacceptable pressure rises when a hydraulic shock occurs.
Water-dispensing columns (Figure 8.7) are installed on street passages in a building area that does not have internal pipelines. The service radius of the population in one column should not be more than 100 m. Around the water columns it is necessary to arrange a blind with a width of 1 m with a slope of 0.1 from the column or asphalt, which ensures the withdrawal from the water column.
Fig. 8.7. Water Dispenser
Water is withdrawn from the water supply network for outdoor fire extinguishing through fire hydrants, which are installed along the carriageway and near the intersection of thoroughfares no closer than 5 m from the walls of buildings.
The distance between the hydrants should be in accordance with and. 8.16 SNiP 2.04.02-84.
Locking, protecting, regulating armature, vents and fire hydrants are usually placed in wells. The necessary dimensions of the wells in the plan are determined on the basis of the detailing of the units of the water supply network taking into account the sizes of the installed parts and the permissible sizes between the walls of the wells and the connections of the reinforcement with the pipelines.
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