It often happens that the water pressure at the draw-off points in the apartment is clearly insufficient. This leads to inconvenience when using plumbing fixtures, to "freeze" or to a complete stop household appliancesconnected to the water supply system, to the incorrect operation of modern devices (showers, jacuzzi, bidets, etc.) that require a certain water pressure. Naturally, such a situation requires the adoption of administrative measures (which, alas, do not always help), or the installation of special boost pumps or pumping stations.
In order to make a claim or plan the installation of additional equipment, it is advisable to know in advance what pressure is predominantly held in the water supply system, that is, how much it differs from the standard. If there is a pressure gauge, then it will not be difficult to take readings. But what if there is no such device? It does not matter, there is a simple and accurate experimental method, under which the calculator for calculating the water pressure in the water supply system below was compiled.
Description of measurements and calculations - in the text section below the calculator.
Calculation of water pressure losses in the pipeline it is very simple, then we will consider in detail the calculation options.
For hydraulic calculation pipeline, you can use the hydraulic pipeline calculator.
Have you been lucky enough to drill a well right next to your home? Wonderful! Now you can provide yourself and your house or cottage clean waterwhich will not depend on the central water supply. And this means no seasonal shutdown of water and running around with buckets and basins. You just need to install the pump and you're done! In this article, we will help you calculate the water pressure loss in the pipeline, and already with these data, you can safely buy a pump and enjoy, finally, your water from the well.
Of school lessons physicists understand that water flowing through pipes, in any case, experiences resistance. The magnitude of this resistance depends on the flow rate, pipe diameter and the smoothness of its inner surface. The lower the flow rate and the larger the pipe diameter and smoothness, the less resistance. Pipe smoothness depends on the material from which it is made. Polymer pipes are smoother than steel pipes, and they do not rust and, which is important, cheaper than other materials, while not inferior in quality. Water will experience resistance, even moving completely horizontal pipe... However, the longer the pipe itself, the less significant the head loss will be. Well, let's get down to the calculation.
Head loss in straight pipe sections.
To calculate the loss of water pressure in straight pipe sections, he uses a ready-made table below. The values \u200b\u200bin this table are for pipes made from polypropylene, polyethylene and other words starting with "poly" (polymers). If you are going to install steel pipes, then you need to multiply the values \u200b\u200bgiven in the table by a factor of 1.5.
Data are given for 100 meters of pipeline, losses are indicated in meters of water column.
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Consumption |
Internal pipe diameter, mm |
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How to use the table: For example, in a horizontal water supply with a pipe diameter of 50 mm and a flow rate of 7 m 3 / h, the loss will be 2.1 meters of water column for a polymer pipe and 3.15 (2.1 * 1.5) for a steel pipe. As you can see, everything is pretty simple and straightforward.
Loss of pressure on local resistances.
Unfortunately, pipes are completely straight only in a fairy tale. In real life, there are always various bends, dampers and valves, which cannot be ignored when calculating the pressure loss of water in the pipeline. The table shows the values \u200b\u200bof the head losses in the most common local resistance: 90 degree elbow, rounded elbow and valve.
Losses are indicated in centimeters of water column per unit of local resistance.
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Flow speed, m / s |
90 degree elbow |
Rounded knee
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Valve
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To determine v - flow rate it is necessary Q - water flow rate (in m 3 / s) divided by S - cross-sectional area (in m 2).
Those. with a pipe diameter of 50 mm (π * R 2 \u003d 3.14 * (50/2) 2 \u003d 1962.5 mm 2; S \u003d 1962.5 / 1,000,000 \u003d 0.0019625 m 2) and a water flow rate of 7 m 3 / h (Q \u003d 7/3600 \u003d 0.00194 m 3 / s) flow rate
v \u003d Q / S \u003d 0.00194 / 0.0019625 \u003d 0.989 m / s
As you can see from the above data, head loss on local resistances quite insignificant. The main losses still occur in horizontal pipe sections, therefore, to reduce them, you should carefully consider the choice of pipe material and their diameter. Recall that in order to minimize losses, you should choose pipes made of polymers with the maximum diameter and smoothness of the inner surface of the pipe itself.
Transportation pipelines various liquids are an integral part of units and installations in which work processes related to various fields of application are carried out. When choosing pipes and configuration of the pipeline, the cost of both the pipes themselves and pipeline fittings... The final cost of pumping the medium through the pipeline is largely determined by the size of the pipes (diameter and length). The calculation of these values \u200b\u200bis carried out using specially developed formulas specific to certain types exploitation.
A pipe is a hollow cylinder made of metal, wood or other material used to transport liquid, gaseous and bulk media. Water can be used as a transported medium, natural gas, steam, oil products, etc. Pipes are used everywhere from various industries industry and ending with household use.
For the manufacture of pipes, the most different materialssuch as steel, cast iron, copper, cement, plastic such as ABS plastic, PVC, chlorinated PVC, polybutene, polyethylene, etc.
The main dimensions of a pipe are its diameter (outer, inner, etc.) and wall thickness, which are measured in millimeters or inches. Also used is such a value as the nominal diameter or nominal bore - the nominal value of the internal diameter of the pipe, also measured in millimeters (indicated by DN) or inches (indicated by DN). The nominal diameters are standardized and are the main criterion for the selection of pipes and fittings.
Correspondence of nominal sizes in mm and inches:
A pipe with a circular cross-section is preferred over other geometric sections for a number of reasons:
- A circle has a minimum perimeter-to-area ratio, and when applied to a pipe, this means that with equal bandwidth pipe material consumption round shape will be minimal in comparison with pipes of other shapes. This also implies the lowest possible costs for insulation and protective coating;
- A circular cross-section is most advantageous for moving a liquid or gaseous medium from a hydrodynamic point of view. Also, due to the smallest possible internal area of \u200b\u200bthe pipe per unit of its length, minimization of friction between the transported medium and the pipe is achieved.
- The round shape is the most resistant to internal and external pressures;
- The process of making round pipes is quite simple and easy to implement.
Pipes can vary greatly in diameter and configuration, depending on the purpose and field of application. So the main pipelines for moving water or oil products can reach almost half a meter in diameter with a fairly simple configuration, and heating coils, which are also pipes, with a small diameter have a complex shape with many turns.
It is impossible to imagine any industry without a pipeline network. The calculation of any such network includes the selection of pipe material, drawing up a specification, which lists data on thickness, pipe size, route, etc. Raw materials, intermediate products and / or finished products go through production stages, moving between different devices and installations, which are connected using pipelines and fittings. Correct calculation, selection and installation of the piping system is necessary for reliable implementation of the entire process, ensuring safe pumping of media, as well as for sealing the system and preventing leaks of the pumped substance into the atmosphere.
There is no single formula or rule that can be used to select piping for every possible application and operating environment. In each individual area of \u200b\u200bpipeline application, there are a number of factors that require consideration and can have a significant impact on the requirements for the pipeline. For example, when working with sludge, the pipeline big size will not only increase the cost of installation, but also create operational difficulties.
Typically, pipes are selected after optimizing material and operating costs. The larger the pipeline diameter, i.e. the higher the initial investment, the lower the pressure drop will be and, accordingly, the lower the operating costs. Conversely, the small size of the pipeline will reduce the primary costs for the pipes and pipe fittings themselves, but an increase in speed will entail an increase in losses, which will lead to the need to spend additional energy for pumping the medium. The speed limits fixed for different applications are based on optimal design conditions. Piping sizes are calculated using these codes for the application.
Piping design
When designing pipelines, the following basic design parameters are taken as a basis:
- required performance;
- entry point and exit point of the pipeline;
- composition of the medium, including viscosity and specific gravity;
- topographic conditions of the pipeline route;
- maximum allowable working pressure;
- hydraulic calculation;
- pipeline diameter, wall thickness, tensile yield strength of the wall material;
- number of pumping stations, distance between them and power consumption.
Pipeline reliability
Reliability in piping design is ensured by adherence to proper design codes. Personnel training is also a key factor in ensuring the long service life of the pipeline and its tightness and reliability. Permanent or periodic monitoring of the operation of the pipeline can be carried out by monitoring, accounting, control, regulation and automation systems, personal control devices in production, safety devices.
Additional pipeline coverage
A corrosion-resistant coating is applied to the outside of most pipes to prevent the corrosive effects of corrosion from the outside. external environment... In the case of pumping corrosive media, a protective coating can be applied to inner surface pipes. Before commissioning, all new pipes intended for the transport of hazardous liquids are tested for defects and leaks.
Basics for calculating flow in a pipeline
The nature of the flow of the medium in the pipeline and when flowing around obstacles can be very different from liquid to liquid. One of the important indicators is the viscosity of the medium, characterized by such a parameter as the coefficient of viscosity. Irish engineer-physicist Osborne Reynolds conducted a series of experiments in 1880, according to the results of which he was able to derive a dimensionless quantity characterizing the nature of the flow of a viscous fluid, called the Reynolds criterion and denoted Re.
Re \u003d (v L ρ) / μ
where:
ρ is the density of the liquid;
v is the flow rate;
L is the characteristic length of the flow element;
μ is the dynamic coefficient of viscosity.
That is, the Reynolds criterion characterizes the ratio of inertial forces to viscous friction forces in a fluid flow. A change in the value of this criterion reflects a change in the ratio of these types of forces, which, in turn, affects the nature of the fluid flow. In this regard, it is customary to distinguish three flow modes depending on the value of the Reynolds criterion. When Re<2300 наблюдается так называемый ламинарный поток, при котором жидкость движется тонкими слоями, почти не смешивающимися друг с другом, при этом наблюдается постепенное увеличение скорости потока по направлению от стенок трубы к ее центру. Дальнейшее увеличение числа Рейнольдса приводит к дестабилизации такой структуры потока, и значениям 2300
| Flow velocity profile | ||
|---|---|---|
| laminar mode | transient regime | turbulent regime |
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| The nature of the flow | ||
| laminar mode | transient regime | turbulent regime |
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The Reynolds criterion is a similarity criterion for the flow of a viscous fluid. That is, with its help, it is possible to simulate a real process in a reduced size, convenient for studying. This is extremely important, since it is often extremely difficult, and sometimes even impossible, to study the nature of fluid flows in real devices due to their large size.
Calculation of the pipeline. Calculation of the pipeline diameter
If the pipeline is not thermally insulated, that is, heat exchange between the transported and the environment is possible, then the nature of the flow in it can change even at a constant speed (flow rate). This is possible if the pumped medium at the inlet has a sufficiently high temperature and flows in a turbulent mode. Along the length of the pipe, the temperature of the transported medium will drop due to heat losses to the environment, which may entail a change in the flow regime to laminar or transitional. The temperature at which the regime change occurs is called the critical temperature. The value of the viscosity of the liquid directly depends on the temperature, therefore, for such cases, such a parameter is used as the critical viscosity corresponding to the point of change in the flow regime at the critical value of the Reynolds criterion:
v cr \u003d (v D) / Re cr \u003d (4 Q) / (π D Re cr)
where:
ν cr - critical kinematic viscosity;
Re cr is the critical value of the Reynolds criterion;
D is the pipe diameter;
v is the flow rate;
Q - consumption.
Another important factor is the friction that occurs between the pipe wall and the flowing stream. In this case, the coefficient of friction largely depends on the roughness of the pipe walls. The relationship between the coefficient of friction, Reynolds criterion and roughness is established by the Moody diagram, which allows you to determine one of the parameters, knowing the other two.
The Colebrook-White formula is also used to calculate the coefficient of friction of turbulent flow. Based on this formula, it is possible to build graphs by which the coefficient of friction is established.
(√λ) -1 \u003d -2log (2.51 / (Re √λ) + k / (3.71 d))
where:
k is the pipe roughness coefficient;
λ is the coefficient of friction.
There are also other formulas for the approximate calculation of friction losses during pressure flow of liquid in pipes. One of the most frequently used equations in this case is the Darcy-Weisbach equation. It is based on empirical data and is used primarily in system modeling. Friction loss is a function of the fluid velocity and the pipe's resistance to fluid movement, expressed in terms of the roughness value of the pipe walls.
∆H \u003d λ L / d v² / (2 g)
where:
ΔH - head loss;
λ is the coefficient of friction;
L is the length of the pipe section;
d - pipe diameter;
v is the flow rate;
g - acceleration of gravity.
The pressure loss due to friction for water is calculated using the Hazen-Williams formula.
∆H \u003d 11.23 L 1 / C 1.85 Q 1.85 / D 4.87
where:
ΔH - head loss;
L is the length of the pipe section;
C is the Heisen-Williams roughness coefficient;
Q - consumption;
D is the pipe diameter.
Pressure
The operating pressure of the pipeline is the highest excess pressure that ensures the specified operating mode of the pipeline. The decision on the size of the pipeline and the number of pumping stations is usually made based on the operating pressure of the pipes, pump capacity and costs. The maximum and minimum pressure of the pipeline, as well as the properties of the working medium, determine the distance between the pumping stations and the required power.
Nominal pressure PN - the nominal value corresponding to the maximum pressure of the working medium at 20 ° C, at which the continuous operation of the pipeline with the given dimensions is possible.
As the temperature rises, the load capacity of the pipe decreases, as does the allowable overpressure as a result. The pe, zul value indicates the maximum pressure (g) in the piping system with increasing operating temperature.
Permissible overpressure graph:
Calculation of the pressure drop in the pipeline
The calculation of the pressure drop in the pipeline is made according to the formula:
∆p \u003d λ L / d ρ / 2 v²
where:
Δp is the pressure drop across the pipe section;
L is the length of the pipe section;
λ is the coefficient of friction;
d - pipe diameter;
ρ is the density of the pumped medium;
v is the flow rate.
Transported working media
Most often, pipes are used to transport water, but they can also be used to move sludge, suspensions, steam, etc. In the oil industry, pipelines are used to pump a wide range of hydrocarbons and their mixtures, which differ greatly in chemical and physical properties. Crude oil can be transported a greater distance from onshore fields or offshore oil rigs to terminals, intermediate points and refineries.
Pipelines also transmit:
- refined products such as gasoline, aviation fuel, kerosene, diesel fuel, fuel oil, etc .;
- petrochemical feedstock: benzene, styrene, propylene, etc .;
- aromatic hydrocarbons: xylene, toluene, cumene, etc .;
- liquefied fuel oils such as liquefied natural gas, liquefied petroleum gas, propane (gases at standard temperature and pressure, but liquefied using pressure);
- carbon dioxide, liquid ammonia (transported as liquids under pressure);
- bitumen and viscous fuels are too viscous to be transported through pipelines, so distillate fractions of oil are used to liquefy these raw materials and result in a mixture that can be transported through the pipeline;
- hydrogen (short distance).
Quality of the transported medium
The physical properties and parameters of the transported media largely determine the design and operating parameters of the pipeline. Specific gravity, compressibility, temperature, viscosity, pour point and vapor pressure are the main parameters of the working medium that must be taken into account.
The specific gravity of a liquid is its weight per unit volume. Many gases are transported through pipelines at elevated pressure, and when a certain pressure is reached, some gases can even undergo liquefaction. Therefore, the compression ratio of the medium is a critical parameter for the design of pipelines and determination of the throughput capacity.
Temperature indirectly and directly affects the performance of the pipeline. This is expressed in the fact that the liquid increases in volume after increasing temperature, provided that the pressure remains constant. A drop in temperature can also affect both performance and overall system efficiency. Usually, when the temperature of the liquid decreases, this is accompanied by an increase in its viscosity, which creates additional frictional resistance along the inner wall of the pipe, requiring more energy to pump the same amount of liquid. Highly viscous media are sensitive to fluctuations in operating temperatures. Viscosity is the resistance of a fluid to flow and is measured in centistokes cSt. Viscosity determines not only the choice of pump, but also the distance between pumping stations.
As soon as the temperature of the medium falls below the pour point, the operation of the pipeline becomes impossible, and several options are taken to resume its operation:
- heating the medium or insulating pipes to maintain the operating temperature of the medium above its pour point;
- changing the chemical composition of the medium before entering the pipeline;
- dilution of the transported medium with water.
Types of main pipes
Main pipes are made welded or seamless. Seamless steel pipes are made without longitudinal welds with heat treated steel lengths to achieve the desired size and properties. Welded pipe is manufactured using several manufacturing processes. These two types differ from each other in the number of longitudinal welds in the pipe and the type of welding equipment used. Steel welded pipe is the most commonly used type in petrochemical applications.
Each length of pipe is welded together to form a pipeline. Also, in main pipelines, depending on the field of application, pipes made of fiberglass, various plastic, asbestos cement, etc. are used.
To connect straight pipe sections, as well as to transition between pipeline sections of different diameters, specially made connecting elements (elbows, bends, gates) are used.
| elbow 90 ° | bend 90 ° | transient branch | branching |
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| elbow 180 ° | bend 30 ° | adapter nipple | tip |
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For the installation of individual parts of pipelines and fittings, special connections are used.
| welded | flanged | threaded | clutch |
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Thermal expansion of the pipeline
When the pipeline is under pressure, its entire inner surface is subjected to a uniformly distributed load, which causes longitudinal internal forces in the pipe and additional loads on the end supports. Temperature fluctuations also affect the pipeline, causing changes in pipe dimensions. Forces in a fixed pipeline during temperature fluctuations can exceed the permissible value and lead to excessive stress, dangerous for the strength of the pipeline, both in the pipe material and in the flanged joints. Fluctuations in the temperature of the pumped medium also create a temperature stress in the pipeline, which can be transmitted to fittings, pumping stations, etc. This can lead to depressurization of pipeline joints, failure of fittings or other elements.
Calculation of pipeline dimensions with temperature changes
The calculation of the change in the linear dimensions of the pipeline when the temperature changes is carried out according to the formula:
∆L \u003d a L ∆t
a - coefficient of thermal elongation, mm / (m ° C) (see table below);
L - pipeline length (distance between fixed supports), m;
Δt is the difference between max. and min. temperature of the pumped-over medium, ° С.
Linear expansion table for pipes made of various materials
The numbers given are average values \u200b\u200bfor the listed materials and for calculating the pipeline from other materials, the data from this table should not be taken as a basis. When calculating the pipeline, it is recommended to use the linear elongation factor indicated by the manufacturer of the pipe in the accompanying technical specification or data sheet.
Thermal expansion of pipelines is eliminated both by using special compensation sections of the pipeline, and by using compensators, which can consist of elastic or moving parts.
Compensation sections consist of elastic straight parts of the pipeline, located perpendicular to each other and fastened with bends. With thermal elongation, the increase in one part is compensated by the bending deformation of the other part on the plane or by the deformation of bending and torsion in space. If the pipeline itself compensates for thermal expansion, this is called self-compensation.
Compensation also takes place thanks to the elastic bends. A part of the elongation is compensated by the elasticity of the bends, the other part is eliminated due to the elastic properties of the material of the area behind the bend. Compensators are installed where it is not possible to use compensating sections or when the self-compensation of the pipeline is insufficient.
According to the design and the principle of operation, there are four types of compensators: U-shaped, lens, wavy, stuffing box. In practice, flat expansion joints with an L-, Z- or U-shape are often used. In the case of spatial expansion joints, they are usually 2 flat mutually perpendicular sections and have one common shoulder. Elastic expansion joints are made from pipes or elastic discs or bellows.
Determination of the optimal size of the pipe diameter
The optimum pipeline diameter can be found on the basis of technical and economic calculations. The dimensions of the pipeline, including the dimensions and functionality of the various components, as well as the conditions under which the pipeline must operate, determines the transport capacity of the system. Larger pipe sizes are suitable for higher mass flow rates, provided the other components in the system are properly sized and dimensioned. Typically, the longer the length of the main pipe between pumping stations, the greater the pressure drop in the pipeline is required. In addition, a change in the physical characteristics of the pumped medium (viscosity, etc.) can also have a large effect on the pressure in the line.
Optimal Size — The smallest suitable pipe size for a specific application, cost effective over the life of the system.
Formula for calculating pipe performance:
Q \u003d (π · d²) / 4 · v
Q is the flow rate of the pumped liquid;
d is the diameter of the pipeline;
v is the flow rate.
In practice, to calculate the optimal diameter of the pipeline, the values \u200b\u200bof the optimal velocities of the pumped medium are used, taken from reference materials compiled on the basis of experimental data:
| Pumped medium | Range of optimal speeds in the pipeline, m / s | |
|---|---|---|
| Liquids | Driving by gravity: | |
| Viscous liquids | 0,1 - 0,5 | |
| Low-viscosity liquids | 0,5 - 1 | |
| Transfer by pump: | ||
| Suction side | 0,8 - 2 | |
| Discharge side | 1,5 - 3 | |
| Gases | Natural cravings | 2 - 4 |
| Low pressure | 4 - 15 | |
| High pressure | 15 - 25 | |
| Couples | Superheated steam | 30 - 50 |
| Saturated steam under pressure: | ||
| More than 105 Pa | 15 - 25 | |
| (1 - 0.5) 105 Pa | 20 - 40 | |
| (0.5 - 0.2) 105 Pa | 40 - 60 | |
| (0.2 - 0.05) 105 Pa | 60 - 75 | |
From here we get the formula for calculating the optimal pipe diameter:
d о \u003d √ ((4 Q) / (π v о))
Q is the specified flow rate of the pumped liquid;
d is the optimal diameter of the pipeline;
v is the optimal flow rate.
At high flow rates, pipes of a smaller diameter are usually used, which means lower costs for the purchase of the pipeline, its maintenance and installation work (denote K 1). With an increase in speed, there is an increase in head losses due to friction and in local resistances, which leads to an increase in the cost of pumping liquid (denote K 2).
For pipelines of large diameters, the costs of K 1 will be higher and the costs during operation of K 2 are lower. If we add the values \u200b\u200bof K 1 and K 2, then we get the total minimum costs K and the optimal diameter of the pipeline. The costs K 1 and K 2 in this case are given in the same time period.
Calculation (formula) of capital costs for a pipeline
K 1 \u003d (m C M K M) / n
m is the mass of the pipeline, t;
C M - cost of 1 ton, rub / ton;
K M - coefficient that increases the cost of installation work, for example 1.8;
n - service life, years.
The indicated operating costs are related to energy consumption:
K 2 \u003d 24 N n days C E rub / year
N - power, kW;
n ДН - number of working days per year;
С Э - costs for one kWh of energy, rubles / kW * h.
Pipeline sizing formulas
An example of general formulas for sizing pipes without considering possible additional influencing factors such as erosion, suspended solids, etc.:
| Name | The equation | Possible limitations |
|---|---|---|
| Pressurized liquid and gas flow | ||
| Loss of friction head Darcy-Weisbach |
d \u003d 12 · [(0.0311 · f · L · Q 2) / (h f)] 0.2 |
Q - volumetric flow rate, gal / min; d is the inner diameter of the pipe; hf - friction head loss; L is the length of the pipeline, feet; f is the coefficient of friction; V is the flow rate. |
| Total fluid flow equation | d \u003d 0.64 √ (Q / V) |
Q - volumetric flow rate, gal / min |
| Pump suction line size to limit frictional head losses | d \u003d √ (0.0744 Q) |
Q - volumetric flow rate, gal / min |
| Total gas flow equation | d \u003d 0.29 √ ((Q T) / (P V)) |
Q - volumetric flow rate, ft³ / min T - temperature, K Р - pressure lb / in² (abs); V - speed |
| Gravity flow | ||
| Manning Equation for Calculating Pipe Diameter for Maximum Flow | d \u003d 0.375 |
Q is the volumetric flow rate; n is the roughness coefficient; S is the slope. |
| Froude number ratio of inertial force and gravity | Fr \u003d V / √ [(d / 12) · g] |
g is the acceleration of gravity; v is the flow rate; L - pipe length or diameter. |
| Steam and evaporation | ||
| The equation for determining the pipe diameter for steam | d \u003d 1.75 · √ [(W · v_g · x) / V] |
W is the mass flow; Vg is the specific volume of saturated steam; x - steam quality; V is the speed. |
Optimum flow rate for various piping systems
The optimal pipe size is selected from the condition of the minimum costs for pumping the medium through the pipeline and the cost of pipes. However, the speed limits must also be considered. Sometimes, the size of the piping line must match the requirements of the process. Likewise, piping size is often related to pressure drop. In preliminary design calculations, where pressure losses are not taken into account, the size of the process pipeline is determined by the allowable speed.
If there are changes in the direction of flow in the pipeline, this leads to a significant increase in local pressures at the surface perpendicular to the direction of flow. This type of increase is a function of fluid velocity, density, and initial pressure. Since velocity is inversely proportional to diameter, high velocity fluids require special attention when sizing and configuring piping. The optimum pipe size, for example, for sulfuric acid, limits the fluid velocity to a value that prevents wall erosion in the pipe bends, thus preventing damage to the pipe structure.
Liquid flow by gravity
Calculating the size of the pipeline in the case of a gravity flow is rather complicated. The nature of movement with this form of flow in a pipe can be single-phase (full pipe) and two-phase (partial filling). Two-phase flow occurs when both liquid and gas are present in the pipe.
Depending on the ratio of liquid and gas, as well as their velocities, the two-phase flow regime can vary from bubble to dispersed.
| bubble flow (horizontal) | slug flow (horizontal) | wave flow | dispersed flow |
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The driving force for the fluid when moving by gravity is provided by the difference in the heights of the start and end points, and a prerequisite is the location of the start point above the end point. In other words, the difference in heights determines the difference in the potential energy of the liquid in these positions. This parameter is also taken into account when selecting a pipeline. In addition, the magnitude of the driving force is influenced by the pressure values \u200b\u200bat the start and end points. An increase in the pressure drop entails an increase in the fluid flow rate, which, in turn, allows the selection of a pipeline with a smaller diameter, and vice versa.
If the end point is connected to a pressurized system such as a distillation column, the equivalent pressure must be subtracted from the available height difference to estimate the actual effective differential pressure generated. Also, if the starting point of the pipeline is under vacuum, then its effect on the total differential pressure must also be taken into account when selecting the pipeline. The final pipe sizing is carried out using differential pressure, taking into account all of the above factors, and not based solely on the difference in heights of the start and end points.
Hot liquid flow
Process plants typically face various problems when handling hot or boiling media. The main reason is the evaporation of part of the hot liquid flow, that is, the phase transformation of the liquid into vapor within the pipeline or equipment. A typical example is the phenomenon of cavitation of a centrifugal pump, accompanied by a point boiling of a liquid, followed by the formation of vapor bubbles (vapor cavitation) or the release of dissolved gases into bubbles (gas cavitation).
Larger piping is preferred because of the reduced flow rate over smaller piping at a constant flow rate due to the higher NPSH at the pump suction line. Cavitation caused by loss of pressure can also be caused by sudden changes in flow direction or reduced pipeline size. The resulting vapor-gas mixture creates an obstacle to the passage of the flow and can cause damage to the pipeline, which makes the cavitation phenomenon extremely undesirable during pipeline operation.
Equipment / Instrument Bypass Piping
Equipment and devices, especially those that can create significant pressure drops, that is, heat exchangers, control valves, etc., are equipped with bypass pipes (so that the process is not interrupted even during maintenance work). Such pipelines usually have 2 shut-off valves installed in the line of the installation and a valve that regulates the flow in parallel to the installation.
During normal operation, the fluid flow passing through the main units of the apparatus experiences an additional pressure drop. Accordingly, the discharge pressure for it, generated by the connected equipment, such as a centrifugal pump, is calculated. The pump is selected based on the total pressure drop across the installation. While moving through the bypass, this additional pressure drop is absent, while the running pump delivers the same force flow according to its operating characteristics. To avoid differences in flow characteristics between the apparatus and the bypass line, it is recommended to use a smaller bypass line with a control valve to create a pressure equivalent to the main unit.
Sampling line
Usually a small amount of liquid is taken for analysis to determine its composition. Sampling can be performed at any stage of the process to determine the composition of the raw material, intermediate product, finished product or simply transported substance such as waste water, heat carrier, etc. The size of the piping section that is sampled will usually depend on the type of fluid being analyzed and the location of the sampling point.
For example, for gases at elevated pressure, small pipelines with valves are sufficient to take the required number of samples. Increasing the diameter of the sampling line will reduce the proportion of sampled medium for analysis, but such sampling becomes more difficult to control. At the same time, a small sampling line is not well suited for the analysis of various suspensions in which solid particles can clog the flow path. Thus, the size of the sample line for the analysis of suspensions is largely dependent on the size of the solid particles and the characteristics of the medium. Similar conclusions apply to viscous fluids.
When sizing the sampling line, usually consider:
- characteristics of the liquid to be taken;
- loss of the working environment during selection;
- safety requirements during selection;
- ease of use;
- location of the sampling point.
Coolant circulation
For pipes with circulating coolant, high speeds are preferred. This is mainly due to the fact that the coolant in the cooling tower is exposed to sunlight, which creates conditions for the formation of an algae-containing layer. A part of this algae-containing volume enters the circulating coolant. At low flow rates, algae starts to grow in the piping and after a while makes it difficult for the coolant to circulate or pass into the heat exchanger. In this case, a high circulation rate is recommended to avoid the formation of algal blockages in the pipeline. Typically, the use of highly circulating coolant is found in the chemical industry, which requires large pipe sizes and lengths to supply power to various heat exchangers.
Tank overflow
Tanks are equipped with overflow pipes for the following reasons:
- avoiding fluid loss (excess fluid enters another reservoir rather than spilling out of the original reservoir);
- preventing unwanted liquids from leaking out of the tank;
- maintaining the liquid level in the tanks.
In all the aforementioned cases, the overflow pipes are designed for the maximum permissible liquid flow into the tank, regardless of the outlet flow. Other principles of pipe selection are similar to the selection of pipelines for gravity fluids, that is, in accordance with the available vertical height between the start and end point of the overflow pipeline.
The highest point of the overflow pipe, which is also its starting point, is at the point of connection to the tank (tank overflow nozzle), usually almost at the top, and the lowest end point may be near the drain gutter, almost at the very ground. However, the overflow line may end at a higher elevation. In this case, the available differential head will be lower.
Sludge flow
In the case of mining, ore is usually mined in areas that are difficult to access. In such places, as a rule, there is no rail or road connection. For such situations, hydraulic transportation of media with solid particles is considered as the most acceptable, including in the case of mining processing installations located at a sufficient distance. Slurry pipelines are used in various industrial fields for transporting crushed solids together with liquid. Such pipelines have proven to be the most cost-effective in comparison with other methods of transporting solids in large volumes. In addition, their advantages include sufficient safety due to the lack of several types of transportation and environmental friendliness.
Suspensions and mixtures of suspended solids in liquids are kept under intermittent agitation to maintain uniformity. Otherwise, a delamination process occurs, in which suspended particles, depending on their physical properties, float to the surface of the liquid or settle to the bottom. Agitation is achieved through equipment such as a stirred tank, while in pipelines, this is achieved by maintaining turbulent flow conditions.
Reducing the flow rate when transporting particles suspended in a liquid is not desirable, since the process of phase separation may begin in the flow. This can lead to blockage in the pipeline and a change in the concentration of the transported solid in the stream. Intensive mixing in the flow volume is facilitated by the turbulent flow regime.
On the other hand, excessive pipeline size reduction also often leads to pipeline blockages. Therefore, the choice of the size of the pipeline is an important and crucial step that requires preliminary analysis and calculations. Each case must be considered individually, as different slurries behave differently at different fluid speeds.
Pipeline repair
During the operation of the pipeline, various types of leaks may occur in it, requiring immediate elimination to maintain the system's operability. Repair of the main pipeline can be carried out in several ways. This can include replacing an entire pipe segment or a small section where a leak has occurred, or patching an existing pipe. But before choosing any repair method, it is necessary to conduct a thorough study of the cause of the leak. In some cases, it may be necessary not only to repair, but to change the route of the pipe to prevent its repeated damage.
The first stage of repair work is to locate the pipe section that requires intervention. Further, depending on the type of pipeline, a list of the necessary equipment and measures necessary to eliminate the leak is determined, as well as the collection of the necessary documents and permits if the pipe section to be repaired is located on the territory of another owner. Since most of the pipes are located underground, it may be necessary to remove part of the pipe. Further, the pipeline coating is checked for general condition, after which part of the coating is removed for repair work directly with the pipe. After the repair, various verification measures can be carried out: ultrasonic testing, color flaw detection, magnetic powder flaw detection, etc.
While some repairs require a complete shutdown of the pipeline, often a temporary interruption is sufficient to isolate the repair section or prepare a bypass. However, in most cases, repair work is carried out with a complete shutdown of the pipeline. Isolation of the pipeline section can be carried out using plugs or shut-off valves. Next, the necessary equipment is installed and the repair is carried out directly. Repair work is carried out in the damaged area, freed from the medium and without pressure. At the end of the repair, the plugs are opened and the integrity of the pipeline is restored.
Why do we need such calculations
When drawing up a plan for the construction of a large cottage with several bathrooms, a private hotel, organization of a fire system, it is very important to have more or less accurate information about the transporting capabilities of the existing pipe, taking into account its diameter and pressure in the system. It's all about pressure fluctuations during the peak of water consumption: such phenomena quite seriously affect the quality of the services provided.
In addition, if the water supply system is not equipped with water meters, then when paying for utility services, the so-called. "Pipe passability". In this case, the question of the tariffs applied in this case is quite logical.
At the same time, it is important to understand that the second option does not apply to private premises (apartments and cottages), where, in the absence of meters, when calculating payment, sanitary standards are taken into account: usually this is up to 360 l / day per person.
What determines the permeability of the pipe
What determines the flow rate of water in a circular pipe? One gets the impression that the search for an answer should not cause difficulties: the larger the cross-section of the pipe, the greater the volume of water it can pass in a certain time. At the same time, the pressure is also remembered, because the higher the water column, the faster the water will be forced through the communication. However, practice shows that these are far from all the factors affecting water consumption.
In addition to them, the following points must also be taken into account:
- Pipe length... As its length increases, the water rubs against its walls more strongly, which leads to a slowdown in the flow. Indeed, at the very beginning of the system, the water is influenced exclusively by pressure, but it is also important how quickly the next portions will have the opportunity to enter the communication. Braking inside the pipe often reaches high values.
- Water consumption depends on the diameter to a much more complex degree than it seems at first glance. When the pipe diameter is small, the walls resist water flow by an order of magnitude more than in thicker systems. As a result, as the diameter of the pipe decreases, its advantage in terms of the ratio of the water flow rate to the internal area index in the fixed length section decreases. To put it simply, a thick water pipe transports water much faster than a thin one.
- Manufacturing material... Another important point that directly affects the speed of water movement through the pipe. For example, smooth propylene is much more conducive to water sliding than rough steel walls.
- Duration of service... Over time, rust appears on steel pipes. In addition, it is typical for steel, as well as for cast iron, to gradually accumulate lime deposits. The resistance to water flow of pipes with deposits is much higher than that of new steel products: this difference sometimes reaches 200 times. In addition, overgrowing of the pipe leads to a decrease in its diameter: even if we do not take into account the increased friction, its permeability clearly decreases. It is also important to note that plastic and metal-plastic products do not have such problems: even after decades of intensive use, the level of their resistance to water flows remains at the original level.
- The presence of turns, fittings, adapters, valves promotes additional braking of water flows.
All of the above factors have to be taken into account, because we are not talking about some small errors, but about a serious difference several times. As a conclusion, we can say that a simple determination of the pipe diameter from the water flow rate is hardly possible.
New ability to calculate water consumption
If the use of water is carried out by means of a tap, this greatly simplifies the task. The main thing in this case is that the dimensions of the outflow hole are much less than the diameter of the water supply. In this case, the formula for calculating water over the cross-section of the Torricelli pipe v ^ 2 \u003d 2gh is applicable, where v is the speed of flow through a small hole, g is the acceleration of gravity, and h is the height of the water column above the tap (a hole with a cross section s, per unit time passes the water volume s * v). It is important to remember that the term "section" is used not to denote the diameter, but its area. To calculate it, use the formula pi * r ^ 2.
If the water column is 10 meters high and the hole is 0.01 m in diameter, the water flow through the pipe at a pressure of one atmosphere is calculated as follows: v ^ 2 \u003d 2 * 9.78 * 10 \u003d 195.6. After extracting the square root, v \u003d 13.98570698963767 comes out. After rounding to get a simpler speed reading, this is 14m / s. The cross-section of a hole with a diameter of 0.01 m is calculated as follows: 3.14159265 * 0.01 ^ 2 \u003d 0.000314159265 m2. As a result, it turns out that the maximum water flow through the pipe corresponds to 0.000314159265 * 14 \u003d 0.00439822971 m3 / s (slightly less than 4.5 liters of water / second). As you can see, in this case, the calculation of water over the cross section of the pipe is quite simple to carry out. Also in the public domain there are special tables showing water consumption for the most popular sanitary ware, with a minimum value of the diameter of the water pipe.
As you can already understand, there is no universal, simple way to calculate the diameter of the pipeline depending on the water flow rate. However, you can still derive certain indicators for yourself. This is especially true if the system is equipped with plastic or metal-plastic pipes, and water is consumed by taps with a small outlet section. In some cases, this method of calculation is applicable to steel systems, but we are talking primarily about new water pipelines that have not had time to be covered with internal deposits on the walls.
In every modern home, one of the main conditions for comfort is running water. And with the advent of new technology that requires connection to the water supply, its role in the house has become very important. Many people no longer imagine how you can do without a washing machine, boiler, dishwasher, etc. But each of these devices for proper operation requires a certain pressure of water coming from the water supply. And now a person who decides to install a new plumbing in his house thinks about how to calculate the pressure in the pipe so that all plumbing fixtures work well.
Modern plumbing requirements
A modern water supply system must meet all requirements and characteristics. At the outlet from the tap, water should flow smoothly, without jerking. Therefore, there should be no pressure drops in the system during the analysis of water. The water running through the pipes should not create noise, have air impurities and other foreign accumulations that adversely affect ceramic taps and other plumbing fixtures. To avoid these unpleasant incidents, the water pressure in the pipe should not fall below its minimum when parsing water.
Note! The minimum water supply pressure should be 1.5 atmospheres. This pressure is sufficient for the operation of the dishwasher and washing machine.
It is necessary to take into account another important characteristic of the water supply system related to water consumption. In any dwelling there is more than one point of water sampling. Therefore, the calculation of the water supply system must fully meet the water demand of all plumbing fixtures when turned on simultaneously. This parameter is achieved not only by pressure, but also by the volume of incoming water that a pipe of a certain section can pass. In simple terms, before installation, it is required to perform a certain hydraulic calculation of the water supply, taking into account the flow and pressure of the water.
Before calculating, let's take a closer look at two concepts such as pressure and flow in order to understand their essence.
Pressure
As you know, in the past, the central water supply was connected to a water tower. It is this tower that creates pressure in the water supply network. The unit of measure for pressure is atmosphere. Moreover, the pressure does not depend on the size of the tank located at the top of the tower, but only on the height.
Note! If you pour water into a 10-meter-high pipe, it will create a pressure of 1 atmosphere at the lowest point.
Pressure is equivalent to meters. One atmosphere equals 10 m of water column. Consider an example with a five-story building. The height of the house is 15 meters. Therefore, the height of one floor is 3 meters. The fifteen-meter tower will create a pressure of 1.5 atmospheres on the ground floor. Let's calculate the pressure on the second floor: 15-3 \u003d 12 meters of water column or 1.2 atmospheres. Having made further calculations, we will see that there will be no water pressure on the 5th floor. This means that in order to provide the fifth floor with water, it is necessary to build a tower over 15 meters. And if, for example, it is a 25-storey building? Nobody will build such towers. Pumps are used in modern water supply systems.
Let's calculate the pressure at the outlet of the submersible pump. There is a submersible pump that lifts water up to 30 meters of water column. This means that it creates pressure - 3 atmospheres at its outlet. After submerging the pump into the well for 10 meters, it will create a pressure at ground level - 2 atmospheres, or 20 meters of water column.
Consumption
Consider the next factor - water consumption. It directly depends on the pressure, and the higher it is, the faster the water will move through the pipes. That is, there will be a greater expense. But the point is that the speed of the water is influenced by the cross-section of the pipe along which it moves. And if you reduce the cross section of the pipe, then the water resistance will increase. Consequently, its amount at the exit from the pipe will decrease over the same period of time.
In production, during the construction of water pipelines, projects are drawn up in which the hydraulic calculation of the water supply system is calculated according to the Bernoulli equation:
Where h 1-2 - shows the loss of pressure at the outlet, after overcoming the resistance throughout the entire section of the water supply.
We calculate home water supply
But this, as they say, is complex calculations. For home plumbing, we use simpler calculations.
Based on the passport data of the machines consumed by water in the house, we summarize the total consumption. We add to this figure the consumption of all water taps in the house. One water taps flows through it about 5-6 liters of water per minute. We sum up all the numbers and get the total water consumption in the house. Now, guided by the total consumption, we are buying a pipe with such a cross-section that will provide the required amount and pressure of water to all simultaneously operating water-folding devices.
When the home water supply is connected to the city network, you will use what is given. Well, if you have a well at home, buy a pump that will fully provide your network with the required pressure, corresponding to the costs. When purchasing, follow the pump passport data.
To select a pipe section, we are guided by these tables:
| Dependence of the diameter on the length of the water supply | Pipe throughput | |||
| Water pipe length, m |
Pipe diameter, mm |
Pipe diameter, mm |
Bandwidth, l / min |
|
| Less than 10 | 20 | 25 | 30 | |
| 10 to 30 | 25 | 32 | 50 | |
| More than 30 | 32 | 38 | 75 | |
These tables provide more commonly used pipe parameters. For a complete overview on the Internet, you can find more complete tables with calculations for pipes of different diameters.
Here, based on these calculations, and with proper installation, you will provide your water supply with all the required parameters. If something is not clear, it is better to consult a specialist.