Sun. Jun 20th, 2021

City gas distribution

Transporting future fuel

#84 Design of Steel Pipeline

5 min read

Steel pipelines are trunk pipelines used for transmission of gas up to CGS & downstream of CGS. Gas is transferred from the CGS to CNG stations and DRS via. steel pipelines. The pipeline also covers the entire city in the form of ring & used for future looping & interconnections. These are high pressure pipelines operating at a pressure of 19 bar in a CGD network. Valves are installed every 2 km to control the flow. The standard code for steel high pressure pipeline is ASME B31.8.

The major criteria considered for steel pipeline are pressure & temperature ratings, gas specifications, gas velocity, pipeline sizing, stress analysis & location class.

Simulation for network hydraulics is done in the SYNERGEE software. This is basically required to derive diameter of pipeline in such a way that the required pressure at all locations can be achieved & future projections can be ascertained. Typically, new onshore & offshore gas transmission pipelines are designed & constructed to operate in a pressure range of 40-90 bar. The normal operating temperature are relatively constant for buried pipelines, except in extreme conditions, & are generally taken to be 5 degree Celsius. All pipeline materials should have adequate fracture toughness at or below the minimum design temperature.

Gas quality: The key gas quality parameters that are monitored are;

  1. Gross Calorific Value
  2. Relative Density
  3. Water Dew Point
  4. Hydrocarbon Dew Point
  5. Total Inerts
  6. Oxygen Content
  7. Hydrogen Sulphide
  8. Total Sulphur Content
  9. Impurities – dust oil & containents
  10. Wobbe Index

Gas velocity: Gas produces an abrasive effect on the equipment, so the maximum velocity is kept about 20m/s.

Pipeline sizing & pipeline stress analysis: The required diameter of pipeline depends on economics as well as on the minimum pressures & required flow capacity. The requirement of compressors, will also effect the pipeline size. Pressure & temperature as well as other operating conditions such as bending can create expansion & flexibility problem.

Roughness: Roughness depends on the type of material used & is shown below in table.

Type of MaterialRoughness (microns)
Steel – new pipe (<=10 yrs old)
With Internal coating
Without Internal coating

10-15
35-40
Steel – new pipe (>10 yrs old)
With Internal coating
Without Internal coating

15-20
40-45

Location class: The pipeline design codes use a system of area classification in order to have an objective view for allocation of design factors & other elements such as minimum proximity. The classes of location & the basic design factor for each class are given in table below:

Location classBuildingBasic Design Factor
Class 110 or fewer buildings in 1 mile section0.72
Class 210 – 46 building0.60
Class 346 or more building0.50
Class 4Areas where multi-stored building & heavy underground utilities exists0.40

Coating: Steel pipelines are provided a three-layer coating (3LPE) for improving the corrosion resistance, increasing the strength & service life. The top most layer is polyethylene to provide a protective covering. A corrosion protective layer of Fusion Bonded Epoxy (FBE) or Coal tar enamel offer very good corrosion protection & bonding with steel surface. The second layer of copolymer adhesive- maleic anhydride grafted polyethylene provides good chemical bonding to the FBE & the top layer of polyethylene.

Wall Thickness: Steel pipelines have standard values of nominal wall thickness with tolerance to allow for manufacturing process. The minimum wall thickness should be equal to or greater than the design thickness t, as shown by the following formulae,

t = (p*d) / (2*f*s)

where, t=design thickness, d=diameter of pipe, p=internal pressure, s=specified minimum yield stress of the pipe material, f=design factor < 1.0

Diameter of Pipeline: For calculating the diameter of a pipeline, it is important to understand the gas flow rate, temperature & pressure conditions & the upstream & downstream pressures. The material chosen for constructing the pipes has its own roughness & efficiency & this factor is also taken into account for designing. Weymouth formula can be used for calculating the diameter of pipelines. This formula can also be used to directly calculate the flow rate through a pipeline for given value of gas gravity, compressibility, inlet & outlet pressures, pipe diameter & length.

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