Sun. Jun 20th, 2021

City gas distribution

Transporting future fuel

#99 DESIGN Criteria

6 min read

The pipelines shall be designed in a manner that ensures adequate public safety under all conditions likely to be encountered during operating conditions. All materials and equipment shall be selected to ensure safety and suitability for the condition of use.

The initial integrity of the pipeline is established through proper design, material selection and sound construction practices. After the pipeline has been commissioned and is in operation, a programme of condition monitoring and maintenance should be undertaken to ensure integrity is maintained.

The selection of design for pipeline system shall be based on the following evaluation of the properties and required flow rate of the fluid to be transported, together with the environment in which the pipeline is to be installed.

  1. Gas composition including hydrogen sulphide , oxygen and water vapour content
  2. Sweet or sour natural gas, single or multiphase flow conditions.
  3. Operating pressures and temperatures.
  4. Type of Pipeline System (Gathering/Transmission/Distribution/Service lines).
  5. Location class through which pipeline shall pass.

Pipeline Design

Design of Natural Gas pipeline shall be in accordance with ASME B31.8

  1. A design Life of pipeline shall be considered by the owner for designing various system and facilities beyond which pipeline system can be considered for abandonment. The life of pipeline can be extended beyond the design life subject to satisfying the comprehensive pipeline integrity test.
  2. All necessary calculations shall be carried out to verify structural integrity and stability of the pipeline for the combined effect of pressure, temperature, bending, soil/pipe interaction, external loads and other environmental parameters as applicable, during all phases of work from installation to operation. Such calculations shall include but not limited to the following:
  1. Buoyancy control and stability analysis for pipeline section to be installed in areas subjected to flooding/submergence,
  2. Crossing analysis of rivers by trenchless techniques, wherever soil data is favorable for such operation,
  3. Evaluation of potential for earthquake occurrence along pipeline route and carrying out requisite seismic analysis to ensure safety and integrity of the pipeline system.

Design Temperature
Pipelines shall be designed for the following temperature limits (i) Above ground – Maximum : 65oC ; Minimum: (-) 29oC (ii) Below ground- Maximum : 60oC ; Minimum: (-) 29oC. Design shall also consider the temperature of natural gas proposed to be transported through the pipeline. When the maximum design temperature exceeds 120oC ( 250oF), particular attention shall be given to tensile properties of the material to ensure that the derating factor for temperature is adequate as per Table below.

Gas Temperature in degree Celsius
(Fahrenheit)
Temperature derating factor (T)
Less than 120oC ( 250oF)1.000
120oC ( 250oF) to 149oC ( 300oF)0.967
149oC ( 300oF) to177oC ( 350oF)0.933
177oC ( 350oF) to 204oC ( 400oF)0.900
204oC ( 400oF) to 230oC ( 450oF)0.867

For intermediate gas temperatures, the derating factor is determined by interpolation.

Steel Pipe

The least nominal wall thickness “t” for the steel pipe, shall be calculated in accordance with ASME B 31.8. The internal design pressure shall be determined by the following formula

t = (P x D) / ( 2 x S x F x E x T )

Where
t = Nominal wall thickness, in mm;
P= Internal design pressure, in N/mm2.

D = Specified outside diameter of the pipe, in mm;
S= Specified minimum yield strength (SMYS) in N/mm2 of pipe material;
F=Design factor obtained from Table 2;
E= Longitudinal joint factor, which for electric resistance welded (ERW), longitudinal seam submerged arc welded (LSAW), helical seam submerged arc welded (HSAW) and seamless types of pipes, manufactured in accordance with API5L and considered as 1;
T = Temperature derating factor to be used in design formula is determined from Table- 1 above;

The pipe wall thickness less than 6.4 mm should not be used for cross country pipelines. Pipe diameter 4” and above shall be used in cross country pipelines.

* – Near concentration in Class 1 and 2 means places of public assembly (school, temple, church, hospital, club etc.) used by 20 or more people frequently.

Pipe Wall Thickness

Pipe wall thickness calculations shall be carried out as per above para. and a corrosion allowance as per the requirements of the owner of the pipeline may be added to the calculated thickness. Pipe thickness shall be checked and revised as required to minimize the number of field hydrostatic testing sections, considering combined testing of pipes in different class locations. In addition the selected thickness shall also be checked to ensure that the diameter to thickness (D/t) ratio does not exceed 96 in order to avoid damage to pipe during handling and transportation, unless the pipes are loaded/transported in accordance with API 5L1 or API 5LW.

Stresses & Analysis

The hoop, longitudinal, shear, bending, torsional and other stresses
shall be calculated taking into account all relevant loads. The
stress analysis shall be carried out in accordance with the appropriate
provisions of ASME B 31.8. Wherever required, the adequacy of
flexibility provided in the pipeline shall be established through appropriate design calculations as specified in flexibility requirements.

Anti-buoyancy Measure

For water crossings and marshy areas, suitable anti-buoyancy measures such as concrete weight coating shall be provided. The specific gravity of the same shall be minimum 1.2.


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