Introduction
The hydraulic calculation of a gas pipeline network is based on determining the optimal diameters of gas pipelines that ensure the passage of the required quantities of gas at acceptable pressure drops. The calculation is based on the maximum possible gas consumption during the hours of maximum gas consumption. This takes into account hourly gas consumption for the needs of production (industrial and agricultural), municipal and household consumers, as well as for individual household needs of the population (heating, hot water supply). As a rule, when hydraulically calculating medium and high pressure gas pipelines, the estimated gas consumption by consumers is taken as concentrated loads; for low pressure networks, a uniformly distributed load is also taken into account. A distinctive feature of medium-pressure gas supply systems with the installation of gas control points at each consumer or a small group of consumers in a populated area is the applicability to them of the principle of calculating networks with uniformly distributed loads.
Hydraulic calculation of a gas pipeline.
When gas moves through pipelines, the initial pressure gradually decreases due to overcoming frictional forces and local resistance:
Depending on the flow speed, pipe diameter and gas viscosity, its flow can be laminar, that is, ordered in the form of layers moving one relative to another, and turbulent, when turbulence occurs in the gas flow and the layers mix with each other. The gas movement mode is characterized by the value of the Reynolds criterion:
where ω - flow speed, m/s; D- pipeline diameter, m; ν - kinematic viscosity, .
The interval of transition from laminar to turbulent motion is called critical and is characterized by Re = 2000–4000. At Re = 2000 the flow is laminar, and at Re = 4000 it is turbulent.
In practice, turbulent gas movement predominates in gas distribution pipelines. Only in gas pipelines of small diameter, for example in intra-house ones, does gas flow laminarly at low flow rates. The flow of gas through underground gas pipelines is considered an isothermal process, since the temperature of the soil around the gas pipeline changes little during the short time of gas flow.
There are hydraulic calculations of low pressure and medium (high) pressure networks. The development of a gas supply system for a residential building involves a low-pressure network.
When calculating a low-pressure gas supply system, a formula is used to calculate pressure losses in the area.
(3)
Where – pressure difference at the beginning and end of the gas pipeline, is the coefficient of hydraulic friction, Q is the gas flow rate, d is the internal diameter of the pipe, is the gas density, l is the length of the gas pipeline.
Specific pressure losses in sections are also determined (Pa/m - for low-pressure networks) using the formula:
– permissible pressure loss (Pa – for low pressure networks); L– distance to the most distant point, m.
The internal diameter of the gas pipeline is taken from the standard range of internal diameters of pipelines: the nearest larger one is for steel gas pipelines and the nearest smaller one is for polyethylene ones.
The coefficient of hydraulic friction λ is determined depending on the mode of gas movement through the gas pipeline, characterized by the Reynolds number,
Where, ν is the coefficient of kinematic viscosity of gas, Q is gas flow rate, d is the internal diameter of the gas pipeline pipe.
And also depending on the hydraulic smoothness of the inner wall of the gas pipeline, determined by the condition
Where, n is the equivalent absolute roughness of the inner surface of the pipe wall, taken equal to 0.01 cm for new steel pipes, 0.1 cm for used steel pipes, 0.007 cm for polyethylene pipes, regardless of operating time, and 0.001 for copper pipes cm.
Depending on the value of Re, the coefficient of hydraulic friction λ:
for laminar gas flow at Re ≤ 2000
for the critical mode of gas movement at Re = 2000–4000
(8)
At Re = 4000, depending on the fulfillment of condition (6):
for a hydraulically smooth wall (inequality (6) is true):
at 4000≤ Re ≤ 100,000
at Re ˃ 100 000
for rough walls (inequality (6) is not valid) at Re ˃ 4000
Thus, when carrying out hydraulic calculations of the gas distribution network, the material of the gas pipeline is taken into account, as well as the aging process of the pipe, which is expressed in an increase in roughness and overgrowth of steel pipes and the persistence of roughness during operation and creep of polyethylene pipes. The creep of a polyethylene pipe is expressed in an increase in the internal diameter by 5 during operation under the influence of internal pressure as a result of a decrease in the thickness of the pipe wall.
A special feature of polyethylene pipes is that they can be made from polyethylene of various densities: medium - PE 80, high - PE 63 (currently not used in gas distribution systems), and also based on a bimodal copolymer - PE 100. It is known that the inner layer of the wall of a polyethylene pipe is saturated with gas and the degree of saturation depends on the gas pressure and the density of the wall. Gas saturation leads to a change in the wall roughness, as a result of which the hydraulic resistance of the pipe changes. Creep also affects the change in the roughness of the pipe wall during operation. Together, all these factors determine the throughput of polyethylene pipes.
When calculating low-pressure gas pipelines laid in conditions of pronounced variable terrain, it is necessary to take into account the hydrostatic head, Pa,
Where h– difference in geometric elevations of the gas pipeline, m; the “+” sign is for gas flowing from bottom to top, and the “-” sign is for gas flowing from top to bottom.
Pressure losses in local resistances are caused by changes in the magnitudes and directions of gas velocities in places where a gas pipeline transitions from one diameter to another, in shut-off valves, bends, tees, etc. According to the Weisbach formula, pressure losses in local resistances, Pa,
For a number of sequentially located local resistances on a gas pipeline of the same diameter, their sum
The average values of the coefficients of some types of local resistances are given in Table 1.
Often pressure losses in local resistances are expressed through a certain equivalent length of a straight pipe section l eq, at which linear pressure losses due to friction are equivalent to losses due to a given local resistance,
Where D- internal diameter of the gas pipeline, m; l eq - equivalent length, m, of a straight section of a pipe of a given diameter, at which the pressure loss due to friction is equal to the loss in local resistance at .
Related information.
A gas pipeline is a structural system whose main purpose is gas transportation. The pipeline helps to carry out the movement of blue fuel to the final point, that is, to the consumer. To make this easier, gas enters the pipeline under a certain pressure. For reliable and correct operation of the entire gas pipeline structure and its adjacent branches, a hydraulic calculation of the gas pipeline is required.
Why is a gas pipeline calculation necessary?
- Calculation of the gas pipeline is necessary to identify possible resistance in the gas pipe.
- Correct calculations make it possible to qualitatively and reliably select the necessary equipment for a gas structural system.
- After the calculation has been made, you can best select the correct pipe diameter. As a result, the gas pipeline will be able to provide a stable and efficient supply of blue fuel. Gas will be supplied at the design pressure, it will be quickly and efficiently delivered to all the necessary points of the gas pipeline system.
- Gas lines will operate optimally.
- With proper calculation, the design should not contain unnecessary or excessive indicators when installing the system.
- If the calculation is done correctly, the developer can save financially. All work will be carried out according to the plan, only the necessary materials and equipment will be purchased.
How does the gas main system work?
- There is a network of gas pipelines within the city limits. At the end of each pipeline through which gas must flow, special gas distribution systems are installed, also called gas distribution stations.
- When gas is delivered to such a station, a redistribution of pressure occurs, or rather, the gas pressure decreases.
- Then the gas flows to the regulatory point, and from there to a network with higher pressure.
- The highest pressure pipeline is connected to the underground storage facility.
- To regulate daily fuel consumption, special stations are installed. They are called gas tank stations.
- Gas pipes, in which gas flows at high and medium pressure, serve as a kind of replenishment of gas pipelines with low gas pressure. In order to control this, there are adjustment points.
- To determine the pressure loss, as well as the exact flow of the entire required volume of blue fuel to the final destination, the optimal pipe diameter is calculated. Calculations are made by hydraulic calculation.
If gas pipes are already installed, then using calculations you can find out the pressure loss during the movement of fuel through the pipes. The dimensions of the existing pipes are also immediately indicated. Pressure losses occur due to resistance.
There is local resistance that occurs at turns, at points of change in gas velocity, and when the diameter of a particular pipe changes. More often than not, friction resistance occurs; it occurs regardless of turns and gas speed; its distribution point is the entire length of the gas line.
The gas pipeline has the ability to carry gas both to industrial enterprises and organizations, and to municipal consumer areas.
Using calculations, the points where low pressure fuel needs to be supplied are determined. Such points most often include residential buildings, commercial premises and public buildings, small utility consumers, some small boiler houses.
Hydraulic calculation with low gas pressure through a pipeline
- It is approximately necessary to know the number of residents (consumers) in the design area where low-pressure gas will be supplied.
- The entire volume of gas per year is taken into account, which will be used for various needs.
- The value of fuel consumption by consumers for a certain time is determined by calculations; in this case, a reading of one hour is taken.
- The location of gas distribution points is determined and their number is calculated.
The pressure drops of the gas pipeline section are calculated. In this case, such areas include distribution points. As well as the intra-house pipeline, subscriber branches. Then the total pressure drops of the entire gas pipeline are taken into account.
- The area of all individual pipes is calculated.
- The population density of consumers in a given area is determined.
- The gas flow rate is calculated based on the area of each individual pipe.
- Computational work is carried out according to the following indicators:
- calculated data on the length of the gas pipeline section;
- actual data on the length of the entire section;
- equivalent data.
For each section of the gas pipeline, it is necessary to calculate the specific travel and node costs.
Hydraulic calculation with average fuel pressure in the gas pipeline
When calculating a gas pipeline with medium pressure, the initial gas pressure reading is initially taken into account. This pressure can be determined by observing the fuel supply from the main gas distribution point to the conversion area and the transition from high pressure to medium distribution. The pressure in the structure must be such that the indicators do not fall below the minimum permissible values during peak load on the gas pipeline.
The calculations apply the principle of pressure variation, taking into account the unit length of the measured pipeline.
To perform the most accurate calculation, calculations are performed in several stages:
- At the initial stage, it becomes possible to calculate the pressure loss. The losses that occur in the main section of the gas pipeline are taken into account.
- Then the gas flow rate for a given section of pipe is calculated. Based on the obtained average pressure loss values and fuel consumption calculations, it is established what the required pipeline thickness is and the required pipe sizes are determined.
- All possible pipe sizes are taken into account. Then, using the nomogram, the amount of losses for each of them is calculated.
If the hydraulic calculation of a pipeline with an average gas pressure is correct, then the pressure loss on the pipe sections will have a constant value.
Hydraulic calculation with high fuel pressure through a gas pipeline
It is necessary to carry out a hydraulic calculation program based on the high pressure of concentrated gas. Several versions of the gas pipe are selected; they must meet all the requirements of the resulting project:
- The minimum pipe diameter that can be accepted within the project for the normal functioning of the entire system is determined.
- The conditions under which the gas pipeline will be operated are taken into account.
- Specific specifications are specified.
- The area in the area where the gas pipeline will pass is being studied. The site plan is thoroughly reviewed to avoid any errors in the project during further work.
- The project diagram is shown. Its main condition is that it goes around the ring. The diagram must clearly show the various branches to the consumption stations. When drawing up a diagram, make the minimum length of the pipe path. This is necessary to ensure that the entire gas pipeline operates as efficiently as possible.
- In the diagram shown, sections of the gas main are measured. Then the calculation program is executed, taking into account the scale, of course.
- The obtained readings are changed, the estimated length of each pipe section shown in the diagram is slightly increased, by about ten percent.
- Computational work is carried out to determine what the total fuel consumption will be. In this case, the gas consumption at each section of the pipeline is taken into account, then it is summed up.
- The final stage of calculating a pipeline with high gas pressure will be determining the internal size of the pipe.
Why is a hydraulic calculation of an intra-house gas pipeline necessary?
During the period of calculation work, the types of necessary gas elements are determined. Devices that are involved in the regulation and delivery of gas.
There are certain points in the project where gas elements will be placed in accordance with the standards, which also take into account safety conditions.
Shows a diagram of the entire intra-house system. This makes it possible to identify any problems in time and carry out installation accurately.
In terms of fuel supply, the number of living spaces, bathroom and kitchen is taken into account. In the kitchen, the presence of such components as a hood and chimney is taken into account. All this is necessary in order to properly install devices and pipelines for the delivery of blue fuel.
Hydraulic calculation of the intra-house gas system
In this case, as in the calculation of a high-pressure gas pipeline, the concentrated volume of gas is taken into account.
The diameter of the section of the intra-house main is calculated according to the consumed amount of blue fuel.
Pressure losses that may occur along the gas delivery route are also taken into account. The design system should have the lowest possible pressure losses. In intra-house gas systems, a decrease in pressure is a fairly common occurrence, so calculating this indicator is very important for the efficient operation of the entire pipeline.
In high-rise buildings, in addition to pressure changes and differences, hydrostatic head is calculated. The phenomenon of hydrostatic pressure occurs because air and gas have different densities, resulting in this type of pressure in a low pressure gas pipeline system.
Calculations are made of the size of gas pipes. The optimal pipe diameter can ensure the lowest pressure loss from the redistribution station to the point of gas delivery to the consumer. In this case, the calculation program must take into account that the pressure drop should not exceed four hundred pascals. This pressure drop is also included in the distribution area and conversion points.
When calculating gas consumption, it is taken into account that the consumption of blue fuel is uneven.
The final stage of the calculation is the sum of all pressure drops; it takes into account the total loss coefficient on the main line and its branches. The total indicator will not exceed the maximum permissible values; it will be less than seventy percent of the nominal pressure indicated by the instruments.
To facilitate calculations based on formulas (VI. 19) - (VI.22), tables and nomograms have been developed. From them, with sufficient accuracy for practical purposes, they determine: based on a given flow rate and pressure loss, the required diameter of the gas pipeline; for a given diameter and losses - the throughput of the gas pipeline; for a given diameter and flow rate - pressure loss; according to known local resistances - equivalent lengths. Each table and nomogram is compiled for gas with a certain density and viscosity and separately for low or medium and high pressure. To calculate low-pressure gas pipelines, tables are most often used, the structure of which is well illustrated in Table. VI.2. The range of pipes in them is characterized by the outer diameter d„, wall thickness s and inner diameter d. Each diameter corresponds to specific pressure loss D R and equivalent length Z 3KB, depending on a certain gas flow V. Nomograms (Fig. VI.3 - VI.7) are the graphic equivalent of the data given in the tables.
Table VI.2
Pressure loss Ar and equivalent lengths in for natural gas (p = 0.73 kg/m 3, v = 14.3 * 10 "* m 2 / sec, steel water and gas pipes according to GOST 3262-62)
|
d H X« (d), mm |
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|
|
|
|
|
|
Note. The numerator shows the pressure loss, kgf/m* per 1 u, and the denominator is the invivalent length, u.
A- natural woof, p - 0.73 kg/m*, v = 14.3‘Yu - * m*/sec; b - propane gas, p?= 2 Kf/m *, v "= 3.7* 10~* m"/sec.
Example 17. Through a pipe (GOST 3262-62) dH X s= 26.8 X 2.8 mm long I = 12 m natural gas of low pressure with p = 0.73 kg/m 9 is supplied in quantity V= 4 m 3 / h. A plug valve is installed on the gas pipeline and two 90° bent elbows are installed. Determine pressure loss in the gas pipeline.
Solution. G1o table VJ.2 we find that at flow V= 4 m 9 /h specific friction losses Ar - 0.703 kg/m2 per 1 m, and the equivalent length? Ek p = = 0.52 m. According to pas data. 108 we find the coefficients of local resistance: For a plug valve = 2.0 and for a bent elbow 90°? 2 = 0.3. Calculated length of the gas pipeline according to formula (VI.29) / calculated = 12 + (2.0 + 2-0.3) X 0.52 = = 13.5 m. Required total pressure loss Dr sum - 13.5-0.703 = = 9.52 kg/m2.
Example 18. Along a low-pressure steel gas distribution pipeline assembled from pipes dH X s= 114 X 4 mm, long I = 250 m natural gas is supplied with p = 0.73 kg/m 9 in quantity V- 200 m 3 /h. The geodetic elevation of the end gas pipeline is 18 m higher than the initial one. Determine the pressure loss in the gas pipeline.
Solution. According to the nomogram in Fig. VI.3 we find that at a flow rate V = = 200 m 3 / h, the specific pressure loss due to friction in the gas pipeline d H Xs = 114 X X 4 mm A R - 0.35 kg/m2 per 1 m. To take into account pressure losses in local resistances, we increase the actual length of the gas pipeline by 10%, T.V. I race Ch = 1.1 1fact = 1.1 *250 = 275 m. Total pressure loss due to friction and local resistance Lr SuI = 0.35-275 = 96 kg/m 2.
The transported gas is lighter than air, therefore hydrostatic pressure is created in the gas pipeline. According to formula (VI.24) Ar g ~ 18 (1,293 - 0,73)
*=“10 kg/m2. Then the required pressure loss in the gas pipeline is Ap* aKX = 96 - - 10 = 86 kgf/cm 2.
Example 19. Through a low-pressure steel gas pipeline d H X s = = 21.3-2.8 mm and length I = 10 m propane is supplied in quantity V== 1.2’m 8 /h. A plug valve is installed on the gas pipeline and there is one 90° bent elbow. Determine pressure loss in the gas pipeline.
Solution. According to the nomogram in Fig. VI.4 we find that at gas flow
V= 1.2 m 3 /h specific friction losses Ar= 0.75 kg/m2 per 1 m. According to the nomogram in Fig. VI.5, b for these conditions, the equivalent length of the gas pipeline /ekp = 0.41 m. According to the data on p. 108 local resistance coefficients: for a plug valve?, = 2.0, for a bent bend 90 s ? 2 = 0.3.
Calculated length of the gas pipeline according to formula (VI.29) 1 raS h = 10 + 0.41 (2.0 + + 0.3) = 10.94 11 m. The required total pressure loss Dr sum = 11 X
X 0.75 = 8.25 kg/m2.
Example 20. Through a steel gas pipeline Dy= 200 mm, 1600 m long, natural gas with a density p = 0.73 kg/m 3 is supplied in an amount of 5000 m 8 /h. Determine the excess pressure at the end of the gas pipeline if at the beginning of the gas pipeline it is equal to 2.5 kgf/cm 2.
Solution. According to the nomogram in Fig. VI.7 we find that with gas consumption
V- 5000 m 3 /h for gas pipeline Dy= 200 mm (p - pl)IL= 1.17. Hence the absolute pressure at the end of the gas pipeline
kgf/cm2. Excessive pressure at the end of the gas pipeline R,-= 2.22 kgf/cm 8,
When designing pipelines, the choice of pipe sizes is carried out on the basis of a hydraulic calculation that determines the internal diameter of the pipes to pass the required amount of gas with acceptable pressure losses or, conversely, the pressure loss when transporting the required amount of gas through log houses of a given diameter.
Resistance to gas movement in pipelines is composed of linear friction resistances and local resistances: friction resistances “work” along the entire length of pipelines, and local ones are created only at points of change in the speed and direction of gas movement (corners, tees, etc.). Detailed hydraulic calculations of gas pipelines are carried out according to the formulas given in SP 42-101–2003, which take into account both the mode of gas movement and the hydraulic resistance coefficients of gas pipelines. A shortened version is provided here.
To calculate the internal diameter of the gas pipeline, use the formula:
Dp = (626Аρ 0 Q 0 /ΔP beat) 1/m1 (5.1)
Where dp is the estimated diameter, cm; A, m, m1 - coefficients depending on the network category (pressure) and gas pipeline material; Q 0 - calculated gas flow, m 3 / h, under normal conditions; ΔРsp - specific pressure loss (Pa/m for low pressure networks)
ΔP beat = ΔP add /1.1L (5.2)
Here ΔР add - permissible pressure loss (Pa); L - distance to the most distant point, m. Coefficients A, m, m1 are determined from the table below.
The internal diameter of the gas pipeline is taken from the standard range of internal diameters of pipelines: the nearest larger one is for steel gas pipelines and the nearest smaller one is for polyethylene ones.
The calculated total gas pressure loss in low-pressure gas pipelines (from the gas supply source to the most remote device) is accepted to be no more than 1.80 kPa (including in distribution gas pipelines - 1.20 kPa), in gas inlet pipelines and internal gas pipelines - 0.60 kPa.
To calculate the pressure drop, it is necessary to determine such parameters as the Reynolds number, which depends on the nature of the gas movement, and the coefficient of hydraulic friction λ. The Reynolds number is a dimensionless ratio that reflects the mode in which a liquid or gas moves: laminar or turbulent.
The transition from laminar to turbulent regime occurs upon reaching the so-called critical Reynolds number R eкp. At Re< Re кp течение происходит в ламинарном режиме, при Re >Re kp - turbulence may occur. The critical value of the Reynolds number depends on the specific type of flow.
The Reynolds number as a criterion for the transition from laminar to turbulent flow and back works relatively well for pressure flows. When transitioning to free-flow flows, the transition zone between laminar and turbulent regimes increases, and the use of the Reynolds number as a criterion is not always valid.
The Reynolds number is the ratio of the inertial forces acting in the flow to the viscous forces. Also, the Reynolds number can be considered as the ratio of the kinetic energy of a fluid to the energy loss over a characteristic length.
The Reynolds number in relation to hydrocarbon gases is determined by the following relationship:
Re = Q/9πdπν (5.3)
Where Q is gas flow, m 3 / h, under normal conditions; d - internal diameter of the gas pipeline, cm; π - number pi; ν is the coefficient of kinematic viscosity of gas under normal conditions, m 2 /s (see Table 2.3).
The diameter of the gas pipeline d must meet the condition:
(n/d)< 23 (5.4)
Where n is the equivalent absolute roughness of the inner surface of the pipe wall, taken equal to:
For new steel ones - 0.01 cm;
- for used steel ones - 0.1 cm;
- for polyethylene, regardless of operating time - 0.0007 cm.
The coefficient of hydraulic friction λ is determined depending on the mode of gas movement through the gas pipeline, characterized by the Reynolds number. For laminar gas flow (Re ≤ 2000):
λ = 64/Re (5.5)
For critical gas movement mode (Re = 2000–4000):
λ = 0.0025 Re 0.333 (5.6)
If the Reynolds number value exceeds 4000 (Re > 4000), the following situations are possible. For a hydraulically smooth wall at a ratio of 4000< Re < 100000:
λ = 0.3164/25 Re 0.25 (5.7)
For Re > 100000:
λ = 1/(1.82logRe – 1.64) 2 (5.8)
For rough walls at Re > 4000:
λ = 0.11[(n/d) + (68/Re)] 0.25 (5.9)
After determining the above parameters, the pressure drop for low pressure networks is calculated using the formula
P n – P k = 626.1λQ 2 ρ 0 l/d 5 (5.10)
Where P n is the absolute pressure at the beginning of the gas pipeline, Pa; P k - absolute pressure at the end of the gas pipeline, Pa; λ - coefficient of hydraulic friction; l is the estimated length of a gas pipeline of constant diameter, m; d - internal diameter of the gas pipeline, cm; ρ 0 - gas density under normal conditions, kg/m 3 ; Q - gas consumption, m 3 / h, under normal conditions;
Gas consumption in sections of low-pressure external gas distribution pipelines that have gas travel costs should be determined as the sum of transit and 0.5 gas travel costs in a given section. The pressure drop in local resistances (elbows, tees, shut-off valves, etc.) is taken into account by increasing the actual length of the gas pipeline by 5–10%.
For external above-ground and internal gas pipelines, the estimated length of gas pipelines is determined by the formula:
L = l 1 + (d/100λ)Σξ (5.11)
Where l 1 is the actual length of the gas pipeline, m; Σξ - the sum of the local resistance coefficients of the gas pipeline section; d - internal diameter of the gas pipeline, cm; λ is the coefficient of hydraulic friction, determined depending on the flow regime and the hydraulic smoothness of the walls of the gas pipeline.
Local hydraulic resistance in gas pipelines and the resulting pressure losses occur when the direction of gas movement changes, as well as in places where flows separate and merge. Sources of local resistance are transitions from one size of gas pipeline to another, elbows, bends, tees, crosses, compensators, shut-off, control and safety valves, condensate collectors, hydraulic valves and other devices leading to compression, expansion and bending of gas flows. The pressure drop in the local resistances listed above can be taken into account by increasing the design length of the gas pipeline by 5–10%. Estimated length of external overhead and internal gas pipelines
L = l 1 + Σξl e (5.12)
Where l 1 is the actual length of the gas pipeline, m; Σξ - the sum of the local resistance coefficients of a gas pipeline section of length l 1, l e - the conventional equivalent length of a straight section of a gas pipeline, m, the pressure loss on which is equal to the pressure loss in local resistance with the value of the coefficient ξ = 1.
Equivalent length of a gas pipeline depending on the mode of gas movement in the gas pipeline:
- for laminar movement mode
L e = 5.5 10 -6 Q/v (5.13)
For critical gas flow conditions
L e = 12.15d 1.333 v 0.333 /Q 0.333 (5.14)
For the entire region of turbulent gas movement
L e = d/ (5.15)
When calculating internal low-pressure gas pipelines for residential buildings, permissible gas pressure losses due to local resistances, % of linear losses:
- on gas pipelines from the entrances to the building to the riser - 25;
- on risers - 20;
- on intra-apartment wiring - 450 (with a wiring length of 1–2 m), 300 (3–4 m), 120 (5–7 m) and 50 (8–12 m),
Approximate values of the coefficient ξ for the most common types of local resistances are given in Table. 5.2.
The pressure drop in the pipelines of the liquid phase of LPG is determined by the formula:
H = 50λV 2 ρ/d (5.12)
Where λ is the coefficient of hydraulic friction (determined by formula 5.7); V - average speed of movement of liquefied gases, m/s.
Taking into account the anti-cavitation reserve, the average speeds of movement of the liquid phase are assumed:
- in suction pipelines - no more than 1.2 m/s;
- in pressure pipelines - no more than 3 m/s.
When calculating low pressure gas pipelines, the hydrostatic head Hg, daPa, is taken into account, determined by the formula
H g = ±lgh(ρ a – ρ 0) (5.13)
Where g is the acceleration of gravity, 9.81 m/s 2 ; h is the difference in absolute elevations of the initial and final sections of the gas pipeline, m; ρ a - air density, kg/m 3, at a temperature of 0°C and a pressure of 0.10132 MPa; ρ 0 - gas density under normal conditions kg/m 3.
When performing hydraulic calculations of overhead and internal gas pipelines, taking into account the degree of noise created by gas movement, gas movement speeds should be taken as no more than 7 m/s for low-pressure gas pipelines, 15 m/s for medium-pressure gas pipelines, 25 m/s for high-pressure gas pipelines .
Table 5.2. Local resistance coefficients ξ for turbulent gas movement (Re > 3500)
| Type of local resistance | Meaning | Type of local resistance | Meaning |
| Bends: | Condensate collectors | 0,5–2,0 | |
| bent smooth | 0,20–0,15 | Hydraulic valves | 1,5–3,0 |
| welded segmental | 0,25–0,20 | Sudden expansion of pipelines | 0,60–0,25 |
| Plug valve | 3,0–2,0 | Sudden narrowing of pipelines | 0,4 |
| Valves: | Smooth expansion of pipelines (diffusers) | 0,25–0,80 | |
| parallel | 0,25–0,50 | Smooth narrowing of pipelines (confusers) | 0,25–0,30 |
| with symmetrical narrowing of the wall | 1,30–1,50 | Tees | |
| Compensators: | merge threads | 1,7 | |
| wavy | 1,7–2,3 | thread separation | 1,0 |
| lyre-shaped | 1,7–2,4 | ||
| U-shaped | 2,1–2,7 | ||
Gas consumption is characterized by great unevenness across months of the year, days, weeks and hours of the day.
The operating mode of the gas supply system for buildings depends on many factors: in residential buildings - on the number and type of gas appliances installed, the degree of improvement of the buildings, climatic conditions, time of year, the number of people living in the buildings; in municipal, public and industrial buildings, in addition to the listed factors - on the nature of the operation of technological equipment and technological processes, the operating mode of workshops and the enterprise as a whole.
Gas supply systems are designed to supply the maximum estimated hourly gas flow rate, which is determined by the annual gas demand.
The maximum hourly gas consumption for economic and production needs under normal conditions (pressure 0.1 MPa at 0°C) is determined by the formula
where is the annual gas consumption, m 3 /year; − coefficient of transition from annual gas consumption to maximum hourly (coefficient of maximum hourly gas consumption).
For residential and public buildings, the estimated hourly gas consumption is determined taking into account the total number of gas appliances of the same type n, the number of their types or groups of the same type m, the nominal gas consumption of one gas appliance - according to the passport or technical specifications, m 3 / h, and the coefficient of simultaneous operation of the devices , according to the formula
To calculate gas pipelines, a hydraulic calculation is performed based on the conditions of uninterrupted gas supply during hours of maximum gas consumption.
Calculation of gas network pipelines comes down to selecting pipe diameters based on calculated flow rates and gas pressure losses.
Preliminary determination of the diameters of individual calculated sections of gas pipelines is carried out according to the formula
where is the hourly gas consumption, m 3, under normal initial conditions of gas pressure and temperature (0.1 MPa and 0°C); − absolute gas pressure at the design section of the gas pipeline, MPa; – gas speed, m/s.
Next, the gas pressure drop is determined along the length of the gas pipeline and in local resistances: at turns, in connections, in fittings, fittings, etc. Taking into account the additional hydrostatic pressure of the gas, this pressure drop is compared with the permissible one. If the pressure drop exceeds the permissible value, then the diameters in individual calculated sections are recalculated in the direction of their increase.
The drop in gas pressure along the length of a low-pressure gas pipeline is determined depending on the mode of gas movement, which is characterized by the Reynolds number:
For laminar mode of gas movement at Re ≤ 2000, the gas pressure drop due to friction along the length is:
for turbulent mode at Re > 4000
where is the pressure drop, Pa; – gas consumption, m 3 /h, under normal conditions (pressure 0.1 MPa and temperature 0°C); d – internal diameter of the gas pipeline, cm; – coefficient of kinematic viscosity of gas, m 2 /s, under normal initial conditions of the gas state; – gas density, kg/m 3 , also under normal initial gas conditions; – equivalent absolute roughness of pipes: for steel pipes = 0.01, polyethylene pipes = 0.005; – estimated length of a gas pipeline section of the same diameter, cm.
For internal and external gas pipelines, the estimated length is determined taking into account the reduced length, depending on the equivalent length of the pipe, taking into account local resistance:
where is the estimated length of the gas pipeline, m; – actual length of the gas pipeline, m; − reduced length of the gas pipeline, m, equal to:
– equivalent length over which the gas pressure drop due to friction is equal to the pressure drop in local resistances at = 1; ∑ζ – the sum of the local resistance coefficients on the calculated section of the gas pipeline with a length of .
The equivalent length is determined by the formulas:
for laminar gas movement
for turbulent gas movement
For residential buildings in low-pressure gas pipelines, local gas pressure losses are determined as a part of the losses along the length, i.e. linear losses,%:
from input to riser……………………………………………………… 25
on risers………………………………………………………………………………20
on intra-apartment wiring depending on the length, %:
up to 2 m………………450 up to 7 m………………………120
» 4 m………………300 » 12 m………………50
The permissible value of pressure loss is accepted:
in internal and yard gas pipelines……………60 daPa (60 mm)
in street and intra-block gas pipelines…….120 daPa (120 mm)
Thus, the total permissible pressure loss in low-pressure distribution networks (from the gas distribution station to the most distant gas consumer) is 180 daPa.
When hydraulically calculating the gas pipeline network of a building, it is necessary to take into account the natural hydrostatic pressure of gas, which arises due to the fact that the gas density is less than the density of air, and as a result, the gas rises up the gas pipeline.
Hydrostatic head, Pa, is determined by the formula
where is the height of gas rise, i.e. difference between geodetic marks of initial and
final section of the gas pipeline, m;
And – density of air and gas, kg/m 3, under normal initial conditions
state of the gas (pressure 0.1 MPa and temperature 0°C).
As a result of the hydraulic calculation, the condition for ensuring gas supply to consumers should be checked, i.e. so that the gas pressure at the inlet is not less than the required pressure, taking into account the hydrostatic head:
The required pressure is:
where is the required gas pressure at the dictating gas device, Pa or daPa; − hydrostatic head, Pa;
∑ – the sum of pressure losses along the length and in local resistances in the network from the input to the dictating gas device, Pa.
If the inequality is not satisfied, then the pipe diameters should be increased in order to reduce the overall pressure loss.
For normal operation of household gas appliances, a nominal gas pressure of 2 (200 mm) or 1.3 kPa (130 mm) is always indicated, therefore, after hydraulic fracturing in the gas network, the gas pressure is set to 3 (300 mm) or 2 kPa (200 mm), respectively.
Thus, when calculating gas networks in buildings, the following conditions must be taken into account:
1. At the inlet, an available gas pressure is created equal to the effective (actual) pressure plus additional natural gas pressure (hydrostatic pressure), i.e.
2. The available pressure must always be no less than the required:
3. The required pressure consists of losses along the length and in local resistances and the nominal pressure for gas appliances without natural hydrostatic pressure.
4. Calculation of the gas network should be performed correctly so that the amount of permissible pressure losses in gas networks is not less than the actual losses:
The permissible value of pressure loss in gas networks is given
in table 25.1.