Pipes connect various devices of chemical plants. They are used to transfer substances between different communications. The design includes several individual pipes, which with the help of joints form a single pipeline system.
Piping system
A pipeline is a system of cylindrical components connected by connecting elements and used to transport chemicals and other materials. Typically, underground pipelines are used in chemical plants to transport substances. As for the autonomous and isolated parts of the installation, they also apply to the piping system or network.
A stand-alone piping system configuration may include:
- Pipes.
- Connecting fittings.
- Seal connecting two removable sections.
All these elements are produced individually, after which they are connected as a single pipeline system. In addition, the pipelines can be equipped with heating and the necessary insulation from different materials.
The size of the pipes and materials for their manufacture is selected based on the requirements of the process and the dismissal established in each individual case. But to standardize the size of the pipelines, their classification and unification was carried out. The key criterion is the permissible pressure at which the operation of the pipeline is possible and safe.
Nominal inner diameter
Nominal diameter is a parameter used in pipeline systems as a coefficient of performance with which alignment of parts such as pipes, valves, fittings is performed during hydraulic calculation of the pipeline.
Nominal diameter - volumetric value, numerically equal to the internal diameter of the structure. Example of nominal inner diameter: DN 125.
The nominal inner diameter is not indicated on the drawings and does not replace the actual pipe diameters. It approximately corresponds to a clear diameter for certain sections of the pipeline during hydraulic calculation. If numerical values of nominal internal diameters are implied, they are selected in such a way as to increase the throughput of the pipeline within 40% when switching from one nominal internal diameter to the next.
The dimensions of the nominal internal diameters are set to avoid problems with the mutual alignment of parts when calculating hydraulic losses in the pipeline. When determining the nominal diameter on the basis of this value, an indicator is selected that is as close as possible to the diameter of the pipe.
Pressure rating
Nominal pressure is a value corresponding to the maximum pressure of the pumped medium at 20 ° C, which ensures long-term operation of the pipeline with the indicated dimensions. The nominal pressure — a dimensionless quantity — was calibrated based on accumulated operating experience.
When calculating the hydraulic losses, the nominal pressure for the pipeline is selected based on the pressure created in it during operation by selecting the highest value. In addition, fittings and valves must also correspond to the same system pressure level. The wall thickness of the pipes is calculated based on the nominal pressure and allows the pipe to work with a pressure equal to the nominal.
Permissible operating overpressure
Nominal pressure only applies at an operating temperature of 20 ° C. With increasing temperature, the load on the pipe decreases. At the same time, the permissible overpressure, respectively, decreases. This value indicates the maximum overpressure that can be in the pipeline system when the value of the operating temperature increases when calculating the hydraulic resistance of the pipeline.
What are pipelines made of?
When choosing materials for the manufacture of pipeline systems, characteristics such as the parameters of the medium to be transported through the pipeline and the preliminary working pressure in this system are taken into account. It should also take into account the possibility of corrosive effects of the internal environment on the wall material during the hydraulic calculation of heating pipelines.
Most piping systems are made of steel. Gray cast iron or unalloyed structures are used for the manufacture of pipelines in the absence of high mechanical stress and corrosion.
In the hydraulic calculation of heating pipelines at high working pressure and the absence of loads with the active effect of corrosion, a pipeline of improved steel castings is used.
If the average corrosion resistance is high, or stringent requirements are imposed on the purity of the product, the pipeline is made of stainless steel.
If the pipeline system must withstand the influence of sea water, copper-nickel alloys are used for its production. Aluminum alloys and metals such as tantalum or zirconium are also used.
Various types of plastics are often used as pipe materials for hydraulic calculation of pressure pipelines, due to its high resistance to corrosion, low weight and ease of processing. Such material is suitable for sewage pipelines.
Pipe elements
Plastic pipes are suitable for welding and are constructed at the installation site. Such materials include steel, aluminum, thermoplastic, copper. To connect the straight pipe sections, specially manufactured shaped elements are used, for example, splitters and diameter reducers. Such fittings are included in the system of any pipeline.
Special connections are used for mounting individual parts and fittings. They are also used to connect the necessary valves and apparatuses to the pipeline.
Connecting elements are selected depending on the following parameters:
- Materials used for the production of pipes and fittings. The main selection criterion is the possibility of welding.
- Working conditions: low or high pressure and low or high temperature.
- Production requirements for the piping system: the presence of fixed or removable connections in the piping system.
Linear expansion of pipes and its compensation
The geometric shape of objects can be changed both by force and by changing their temperature. These physical phenomena lead to the fact that the pipeline at the stage of installation in shock-free conditions and without temperature exposure undergoes some linear expansion or reduction, adversely affecting its functional characteristics, during maintenance due to pressure and temperature.
In the case when expansion is not required for compensation, the pipeline system is deformed. In this case, flange seals and pipe joints may be damaged.
Thermal linear expansion
When calculating the hydraulic resistance of the pipeline and installation, one should take into account the potential change in length due to an increase in temperature or the so-called thermal linear expansion. This value is equal to the linear expansion of pipes 1 m long with a temperature increase of 1 ° C.
Example of hydraulic calculation of pipelines: Q = (Πd² / 4) · w
Pipe insulation
When a high-temperature medium is transported through a pipeline, it should be insulated to avoid heat loss. If the low-temperature medium is transported by pipeline, the insulation is used to prevent its heating. In such cases, the insulation is made using special insulation materials wrapped around the pipes.
Typically, the following materials are used:
- At low temperatures up to 100 ° C - rigid foam (polystyrene or polyurethane).
- At average temperatures of about 600 ° C - in the form of shells or mineral fibers, such as stone wool or glass felt.
- At high temperatures around 1200 ° C - ceramic fiber (aluminum silicate).
Pipes with a nominal inner diameter below DN 80 and an insulation layer thickness of less than 50 mm are insulated, usually with molded insulating elements. To this end, two shells are wrapped around the pipe and secured with a metal tape, and then covered with a tin plate housing.
Nomogram for hydraulic calculation of pipelines
Pipelines with a nominal internal diameter greater than DN 80 must be equipped with thermal insulation having a lower shell. Such a casing comprises clamping rings, staples and a metal lining of galvanized mild steel or stainless sheet steel. The space between the pipeline and the metal casing is filled with insulating material.
The insulation thickness is calculated as the definition of production costs and losses that arise due to heat loss, and ranges from 50 to 250 mm.
Table for hydraulic calculation of pipelines
Choosing the right insulation for your pipe system solves numerous problems, such as:
Avoiding a sharp decrease in ambient temperature and, as a consequence, energy conservation.
Preventing temperature drops in gas transport systems below the dew point, which eliminates the formation of condensate and can lead to serious damage.
Avoidance of condensate emissions in steam lines.
Example:
| Material | Speed of movement, m / s |
|---|
| Liquid | Gravity flow: | |
| Viscous substance | 0.1 - 0.5 |
| Low viscosity components | 0.5 - 1 |
| Pump: | |
| Suction | 0.8 - 2 |
| Discharge | 1,5 - 3 |
Thermal insulation should be applied along the entire length of the piping system. Flange connections and valves shall be provided with molded insulating elements. They provide unhindered access to the connection points without the need to remove insulating material from the entire piping system in the event of a break in air tightness.