Hydraulic calculation of sewer networks

The device of internal and external sewerage must comply with established standards. Otherwise, the drains will not be able to be sent to the main line or receiver by gravity. To ensure the free-flow operation of the collector, during its installation, the throughput of the sewer pipes, their cross-section, slope, and fullness are taken into account.

The importance of making correct calculations when designing a sewage system

Wastewater is not just a homogeneous flowing liquid. Impurities include impurities of various fractions and weights. The polydisperse substance, which is waste water, also contains colloids and suspensions. The speed of movement of sewage can periodically change. If liquid inclusions move faster, then heavy impurities in the form of sand, slags, etc. more often settle on the pipe walls, forming build-ups and blockages. As a result, the collector becomes clogged. To prevent this from happening, at the design stage of the system, a hydraulic calculation of the sewer networks is carried out.

To calculate all parameters, the estimated volume of effluents is taken as the initial data.

Basic types of calculations

When performing calculations, data from SNiP 2.04.03-85 “Sewerage. External networks and structures ", as well as recommendations from SP 40-102-2000" Design and installation of pipelines for water supply and sewerage systems made of polymer materials. General requirements".

Sewer pipeline slope

The slope is responsible for the free-flow operation of the system and the unimpeded discharge of drains into the receiver. The SNiP tables show the recommended minimum slopes for each pipe diameter:

• section 110 mm - 1 cm for each running meter of the collector;
• diameter 160 mm - 0.8 cm per meter;
• section 220 mm - 0.7 cm per meter of collector length.

For the internal part of the communication, a 1.5-2 cm slope is adhered to for each linear meter.

If there is a need to calculate the slope of the collector through the formula, use the following: d x coefficient, where d is the cross-section of the pipeline, and the coefficient corresponds to the following values:

• 160 — 0,6;
• 220 — 0,7;
• 500 mm - 1;
• 600-800 — 1,1;
• 1000-1200 mm - 1.3.

The slope is made towards the central highway or yard septic tank / cesspool.

Calculation of pipe cross-section

To lay pipes of the required diameter, you can use the SNiP data. The recommended values ​​look like this:

• the inner part of the sewage system from all plumbing fixtures - 50 mm;
• toilet pipe - 110 mm;
• public riser - 110-160 mm;
• the outer part of the sewage system - 160-220 mm (for the private sector and an apartment building);
• central highway and industrial enterprises - from 500 mm.

With the correct cross-section of the collector, the filling of the pipe will be approximately 0.3-0.5 of its total diameter.

Collector filling calculation

The filling of the pipe is calculated using the formula: Y = h / d. Here the value h is the maximum liquid level in the system, d is the internal section of the collector. Normally, the result should range from 0.3 to 0.6.

An example of calculating the filling level of a sewer pipe in the private sector: for the initial data, the filling level of the collector is taken within 60 mm, while the section of the tube is 110 mm. According to the above formula, 60/110 and 0.55 is obtained. The value is correct.

Calculation of the throughput of pipes

At the time of designing the system, the throughput of the pipeline must be calculated - the estimated costs of wastewater are determined. This parameter is calculated relative to the day, hours and seconds, depending on the purpose of the building (residential, business, etc.). The second flow rate of wastewater is calculated in liters, for daily and hourly data is calculated in m3.

Data for average costs:

• Qcp.day = n · Nr / 1000 m³ / day;
• Qcp.hour = p · Nr / (24 · 1000) m³ / hour;
• qav.sec = p · Nr / (24 · 3600) l / sec.

Where:

• n - average rates of wastewater disposal per 1 inhabitant (in liters);
• Nр - estimated number of inhabitants.

For maximum costs:

• Qmax.day = Qcp.day · kday = n · Nr · kday / 1000 m³ / day;
• Qmax.hour = p1 · Nr · ktot / (24 · 1000) m³ / hour;
• qmax sec = n1 Nr ktot / (24 3600) l / sec.

Where k are the coefficients of unevenness: kday - daily, ktot - total.

To determine the maximum throughput of the collector, use the formula: q = aXv, where the value of a is the area of ​​the free flow area, and v is the rate of waste transportation.

8 m / s (metal) and up to 4 m / s (concrete, plastic) are considered to be the norm of maximum speeds for each type of pipes (material of manufacture). If the speed actually turns out to be higher, it must be extinguished by turning the system or installing drop wells.

All hydraulic calculations must be performed sequentially. However, more often masters use not formulas for calculations, but the data given in SNiP (in the form of tables), or use an online calculator.

Determination of effective parameters of storm sewers

If a storm drain is installed in parallel with the fecal household system, for correct calculations, you need to take into account the following data:

• annual precipitation rate for the region;
• the area of ​​the serviced object;
• type of coating (concrete, soil, tiles);
• soil properties at the site;
• the mass of the drained rainwater.

All data, except for the last item, are taken from the local geodetic service. With the help of the information received, the gutters are selected according to their throughput. For example, let's calculate the data on the stormwater trays using the formula: Q = q20 ∙ P ∙ φ

Values:

• Q is the average annual volume of storm water in the region;
• q20 - coefficient for the region;
• P is the area of ​​the object from which water will be discharged;
• φ - coefficient of water absorption for different types of materials.

Having determined the capacity of the tray by this formula, the gutters are selected in accordance with their technical characteristics. As a rule, this parameter is indicated on the product.

Calculation of external sewerage

To design the outer part of the collector, the following data is calculated:

• The length of communication from the outlet from the house to the connection to a septic tank or a central highway.
• Diameter of pipes (for private construction, tubes with a section of 110, 160 or 220 mm are used).
• Waste volume. The norm is 2 cubes per person per month.
• The presence of turns and drop wells along the length of the communication.

For the device of a gravity system, a slope is observed. If, due to the peculiarities of the relief, a pressure section of the sewage system is mounted, a special caisson is mounted for the fecal pump and a siphon is arranged. It will protect the section of the system from the effects of subzero temperatures and stagnation of fecal masses.

Calculation of internal sewerage

For the internal drainage system, the number of pipes required is calculated. The diagram shows the sections of the sewage system from each type of plumbing equipment. The data is summarized and the total length of the pipeline is obtained. For accurate assembly of the system, it is important to purchase fittings, clamps, adapters, tees.

The pipeline is arranged with a slope towards the riser at the rate of 1.5-2 cm for each running meter.