GB 50251-2015Design code for gas transmission pipeline engineering (English PDF)

3.1 General provisions

3.1.1 The designed delivery capacity of the gas pipeline shall be calculated according to the annual or daily maximum gas delivery specified in the design commission or contract. When the annual gas transmission volume is adopted, the design annual working days shall be calculated as 350d.

3.1.2 The gas entering the gas pipeline should meet the indicators of the second-class gas in the current national standard "Natural Gas"

GB 17820, and should meet the following regulations.

1 Mechanical impurities should be removed;

2 The water dew point should be 5°C lower than the lowest ambient temperature under the delivery conditions;

3 The hydrocarbon dew point should be lower than the minimum ambient temperature;

4 The content of hydrogen sulfide in the gas should not exceed 20mg/m3;

5 The carbon dioxide content should not exceed 3%.

3.1.3 The design pressure of gas pipelines should be determined after technical and economic comparisons based on gas source conditions, user needs, pipe quality and safety factors near the pipelines.

3.1.4 When the gas pipeline and its accessories have taken anti-corrosion measures in accordance with the requirements of the current national standards "Code for External Corrosion Control of Steel Pipelines"

GB/T 21447 and "Technical Specifications for Cathodic Protection of Buried Steel Pipelines"

GB/T 21448, the corrosion allowance of the pipe wall should not be increased.

3.1.5 Gas transmission pipelines should be equipped with pigging facilities, and the pigging facilities should be constructed in combination with the gas transmission station.

3.1.6 When the inner wall drag-reducing coating is used for the pipeline, it shall be determined by technical and economic comparison.

3.2 Process design

3.2.1 Process design should be based on gas source conditions, transmission distance, transmission volume, characteristics and requirements of users, and the relationship with the capacity and distribution of the established pipeline network and underground gas storage. Determined after technical and economic comparison.

3.2.2 Process design should determine the following contents.

1 The overall technological process of gas transmission;

2 Process parameters and flow of the gas transmission station;

3.The number and station spacing of gas transmission stations;

4 The diameter, design pressure and station pressure ratio of the compressor station of the gas pipeline.

3.2.3 The gas source pressure should be used reasonably in process design. When pressurized transportation is adopted, multi-plan technical and economic comparisons should be carried out in combination with the factors of transportation volume, pipe diameter, transportation pressure, power supply and operation management, and the station pressure ratio of the compressor station should be reasonably selected and the station spacing should be determined according to the principles of economy and energy saving.

3.2.4 The characteristics of the compressor station and the pipeline should match and meet the requirements of process design parameters and changes in operating conditions. Under normal gas transmission conditions, the compressor unit should work in the high-efficiency zone.

3.2.5 The gas transmission station with the function of distribution or distribution should be equipped with gas limit and pressure limiting facilities.

3.2.6 When the gas source of the gas pipeline comes from an oil and gas field natural gas processing plant, an underground gas storage, a coal-to-natural gas plant, or a coalbed methane processing plant, gas quality monitoring facilities should be installed on the intake pipeline of the gas pipeline receiving station.

3.2.7 The strength design of gas pipelines shall meet the requirements of changing operating conditions.

3.2.8 The gas transmission station should be provided with an over-station bypass.

3.2.9 The gas transmission pipelines entering and exiting the gas transmission station must be equipped with shut-off valves. And should comply with the relevant provisions of the current national standard "Code for Fire Protection of Petroleum and Natural Gas Engineering Design" GB 50183.

3.3 Process calculation and analysis

3.3.1 The technical design of the gas pipeline shall at least have the following information.

1 Composition of pipeline gas;

2 The quantity, location, gas supply volume and variable range of gas sources;

3 The pressure, temperature and variation range of the gas source;

4 Requirements of users along the line for gas supply pressure, gas supply volume and their changes. When it is required to use pipeline gas storage for peak regulation, the user's gas consumption characteristic curve and data shall be available;

5 The natural environment conditions along the line and the ground temperature at the place where the pipeline is buried.

3.3.2 The hydraulic calculation of the gas pipeline shall meet the following requirements.

1 When the relative height difference h<=200m of the longitudinal section of the gas pipeline and the influence of the height difference is not considered, it shall be calculated according to the following formula. In the formula. qv--the flow rate (m3/d) of gas (P0=0.101325MPa, T=293K); P1--the starting pressure (absolute) (MPa) of the calculation section of the gas pipeline; P2--The end pressure of the calculated section of the gas pipeline (absolute) (MPa); d--inner diameter of gas pipeline (cm); lambda-hydraulic friction coefficient; Z-compressibility factor of gas; --relative density of gas; T - the average temperature of the gas in the gas pipeline (K); L--The length of the calculation section of the gas pipeline (km).

2 When considering the influence of the relative height difference of the longitudinal section of the gas pipeline, it shall be calculated according to the following formula. In the formula. alpha-coefficient (m-1); h--Elevation difference between the end point of the calculation section of the gas pipeline and the start point of the calculation section (m); n--the number of sub-sections calculated along the gas pipeline. The division of the calculated pipe division is along the direction of the gas pipeline, starting from the starting point, and when the relative height difference is <=200m, it is divided into a calculated pipe division; hi--Elevation of the end point of each calculated pipe section (m); hi-1--the elevation of the starting point of each calculated pipe section (m); Li--the length of each calculation sub-pipeline (km); g-gravitational acceleration, g=9.81m/s2; Ra--gas constant of air, under standard conditions (P0=0.101325MPa, T=293K), Ra=287.1m3/(s2·K).

3 The hydraulic friction coefficient should be calculated according to the following formula, and the formula in Appendix A should be used when the process calculation of the gas transmission pipeline is performed by hand. In the formula. K--the absolute roughness of the inner wall of the steel pipe (m); d - the inner diameter of the pipe (m); Re - Reynolds number.

3.3.3 The temperature calculation at any point along the gas pipeline shall meet the following requirements.

1 When the throttling effect is not considered, it shall be calculated according to the following formula. In the formula. tx-gas temperature at any point along the gas pipeline (°C); t0--soil temperature at the place where the gas pipeline is buried (°C); t1--Gas temperature at the starting point of calculation section of gas pipeline (°C); e - the base of natural logarithm, should be taken as 2.718; x--the length from the starting point of the calculated section of the gas pipeline to any point along the line (km); K--total heat transfer coefficient from gas to soil in the gas pipeline [W/(m2·K)]; D--outer diameter of gas pipeline (m); qv--the flow rate of gas (P0=0.101325MPa, T=293K) in the gas pipeline (m3/d); cP--Constant pressure specific heat of gas [J/(kg·K)].

2 When throttling effect is considered, it should be calculated according to the following formula. In the formula. j-Joule-Thomson effect coefficient (°C/MPa); Px--the pressure drop (MPa) of x-length pipe section.

3.3.4 According to the actual needs of the project, it is advisable to conduct steady-state and dynamic simulation calculations on the gas transmission pipeline system to determine the number of compressor stations, boosting ratio, compressor power and power fuel consumption under different working conditions. The flow rate, pressure, temperature and gas storage capacity of pipelines at each node. According to the needs of system analysis, the calculation time period can be determined by hour or day.

3.4 Safe release of gas pipelines

3.4.1 The gas transmission station should set up pressure relief and venting facilities upstream of the inbound block valve and downstream of the outbound block valve.

3.4.2 Vent valves should be installed on the pipe section between adjacent line shut-off valves (chambers) of the gas pipeline, and a vent standpipe or a flange interface for leading the vent pipeline should be reserved in consideration of the construction environment. The diameter of the vent valve should be equal to the diameter of the vent pipe.

3.4.3 Pipelines, equipment and containers with overpressure must be equipped with safety valves or pressure control facilities.

3.4.4 The constant pressure of the safety valve shall be determined after system analysis, and shall meet the following requirements.

1 The constant pressure of the safety valve of the pressure vessel shall be less than or equal to the design pressure of the pressure vessel.

2 The fixed pressure (P0) of the safety valve of the pipeline shall be determined according to the maximum allowable operating pressure (P) of the process pipeline, and shall meet the following requirements. 1) When P<=1.8MPa. The constant pressure (P0) of the safety valve of the pipeline should be calculated according to the following formula. 2) When 1.8MPa< P<=7.5MPa, the constant pressure (P0) of the safety valve of the pipeline should be calculated according to the following formula. 3) When P >7.5MPa, the constant pressure (P0) of the safety valve of the pipeline should be calculated according to the following formula. 4) The constant pressure of the safety valve set by the pipeline with the strength design coefficient of

0.8 should not be greater than 1.04P.

3.4.5 The calculation of the diameter of the relief pipe of the safety valve shall meet the following requirements.

1.The diameter of the discharge pipe of a single safety valve shall be determined according to the fact that the back pressure is not greater than 10% of the discharge pressure of the valve, and shall not be smaller than the diameter of the outlet pipe of the safety valve;

2 The diameter of the discharge pipe connecting multiple safety valves shall be determined according to the fact that the back pressure generated when all safety valves discharge simultaneously is not greater than 10% of the discharge pressure of any one of the safety valves, and the cross-sectional area of the discharge pipe shall not be less than The sum of the cross-sectional areas of the relief branch pipes of the safety valve.

3.4.6 The vented gas should be safely discharged into the atmosphere.

3.4.7 The venting design of the gas transmission station shall meet the following requirements.

1 The gas transmission station shall be equipped with a vent standpipe, and a vent pipe may also be provided if necessary;

2 Natural gas at gas transmission stations should be discharged centrally through the venting standpipe, or it can be discharged in different areas. High and low pressure venting pipelines should be set up separately. unimpeded discharge;

4.1 Line selection

4.1.1 The selection of lines shall meet the following requirements.

1.The direction of the line should be based on the purpose of project construction and the distribution of gas sources and markets, combined with the current situation and planning of towns, transportation, water conservancy, mineral resources, and environmentally sensitive areas along the line, as well as natural conditions such as topography, geology, hydrology, meteorology, and earthquakes in the area along the line., through comprehensive analysis and multi-plan technical and economic comparison, determine the overall direction of the line;

2 The route should avoid environmentally sensitive areas. When the route is restricted and needs to pass through environmentally sensitive areas, it should obtain the consent of its competent department and take protective measures;

3 The location selection of large and medium-sized crossing (crossing) projects and compressor stations shall conform to the overall direction of the line. The direction of local lines should be adjusted according to the location of large and medium crossing (spanning) projects and compressor stations;

4 The route should avoid areas such as military restricted areas, airports, railway and bus passenger stations, and sea (river) port terminals;

5 Except for tunnels and bridges specially built for pipeline projects, gas transmission pipelines should not be laid in tunnels of railways or highways and on bridges. When the gas pipeline crosses under the railway or highway bridge, the hydrological conditions under the bridge should not be changed;

6 The route of the pipeline parallel to the road should be 3m away from the land boundary of the road, and the route of the pipeline parallel to the railway should be 3m away from the land boundary of the railway. If the local area with limited terrain or other conditions does not meet the requirements, the road management department consent;

7.The route should avoid the urban and rural planning area. When it is necessary to pass through the urban and rural planning area due to conditions, the consent of the urban and rural planning department should be obtained, and safety protection measures should be taken;

8 When blasting and digging trenches for pipeline routes in rockwork sections, it is necessary to avoid affecting the safety of the public and surrounding facilities;

9.The line should avoid strong interference areas such as the ground electrode of the HVDC converter station and the substation;

10 The distance between buried pipelines and buildings (structures) shall meet the requirements of construction and operation management, and the minimum distance between the centerline of pipelines and buildings (structures) shall not be less than 5m.

4.1.2 Gas pipelines should avoid geological hazards such as landslides, collapses, collapses, debris flows, and severe flood erosion, and should avoid mining goafs and Holocene active faults. When it is necessary to pass through the above-mentioned areas due to conditional restrictions, it is necessary to choose a less harmful location to pass through, and take corresponding protective measures.

4.2 Regional classification and determination of design coefficients

4.2.1 The area where the gas pipeline passes should be divided into four area levels according to the number of households and (or) the density of buildings along the line, and the corresponding pipeline design should be made according to the area level.

4.2.2 The classification of regions shall meet the following requirements.

1 Within the range of.200m on both sides of the central line of the pipeline, arbitrarily divided into several sections with a length of 2km and which can include the maximum number of households, the number of households in the designated sections should be divided into four grades. In rural villages, large courtyards and residential buildings where the population gathers, each independent household shall be counted as a building for people to live in. Regional levels shall be divided according to the following principles. 1) Level 1 and Category 1 areas. sections with infrequent human activities and no permanent residents; 2) First-class and second-class areas. sections with 15 households or less; 3) Second-level areas. sections with more than 15 households and less than 100 households; 4) Third-level areas. sections with 100 households or more, including suburban residential areas, commercial areas, industrial areas, planned development areas, and densely populated areas that do not meet the requirements of fourth-level areas; 5) Level 4 areas. areas with four or more floors (excluding basement floors) generally concentrated, frequent traffic, and many underground facilities.

2 When dividing the boundary line of regional grades, the distance between the boundary line and the outer edge of the nearest building shall not be less than.200m.

3 In places where crowds gather, such as schools, hospitals, and other public places in the first and second-level areas, design coefficients should be selected according to the third-level areas.

4 When the development plan of a region is sufficient to change the existing grade of the region, the region grade shall be divided according to the development plan.

4.2.3 The strength design factor of the gas pipeline shall comply with the requirements in Table 4.2.3. Table

4.2.3 Strength design factor

0.8 or

0.72 strength design factor can be adopted for pipelines in Class I and Class I areas.

4.2.4 The strength design factor of the pipe section crossing the road and the pipeline in the gas transmission station and valve room. It should meet the requirements in Table 4.2.4. Table

4.2.4 Strength Design Factors of Pipe Sections Crossing Roads and Piping in Gas Transmission Stations and Valve Chambers

4.3 Pipe laying

4.3.1 Gas pipelines should be laid in the ground, and earth embankments or ground can be used in special areas.

4.3.2 The minimum thickness of the covering soil layer for buried pipelines shall meet the requirements in Table 4.3.2.Protective measures shall be taken where the covering soil thickness cannot meet the requirements or the external load is too large, and external operations may endanger the pipeline. Table

4.3.2 Minimum Covering Soil Thickness (m)

1.For the section to be leveled, the level after leveling shall be used for calculation.

2 The thickness of the overburden layer shall be calculated from the top of the pipe.

3 The seasonal frozen soil area should be buried below the maximum freezing line.

4 The areas where dry land and paddy fields are rotated or where the existing dry land planning needs to be changed to paddy fields shall determine the buried depth according to the paddy fields.

5 The pipelines passing through fish ponds or ditches shall be buried not less than 1.0m below the dredging layer.

4.3.3 The slope slope of the pipe ditch should be comprehensively determined according to the soil type, physical and mechanical properties (such as cohesion, internal friction angle, humidity, bulk density, etc.), the load near the top of the slope, and the excavation depth of the pipe ditch. When there is no physical property data of the above-mentioned soil, for the pipe trench with uniform soil structure, no groundwater, good hydrogeological conditions, depth not greater than 5m and no support, the slope value of the side slope can be determined according to Table 4.3.3.For trenches with a depth of more than 5m, the slope should be slowed down, platforms should be added or supports should be added according to the actual situation. Table

4.3.3 The steepest side of the pipe trench with a depth of less than 5m...