What is the purpose of gauging a pipeline?

To verify the pipeline is free from major obstructions and to ensure that any dents or ovality are within acceptable limits, typically by passing a gauging plate of 95% of the pipe's internal diameter (Clause 6.3).

What pressure is used for hydrotesting?

The pipeline is subjected to a strength test at a minimum of 1.25 times the design pressure for at least 24 hours to verify its structural integrity (Clause 6.4.4).

Why is drying necessary after hydrotesting?

To remove all residual water, preventing internal corrosion and the formation of solid gas hydrates which can block the pipeline when natural gas is introduced at high pressure and low temperature (Clause 6.5).

What is 'commissioning' in this context?

Commissioning is the final step of introducing natural gas into the pipeline after it has been cleaned, tested, and dried. It involves purging the line of air/inert gas and safely pressurizing it with natural gas (Clause 7).

IS 15663

: 2000

Design and installation of natural gas pipelines – Code of Practice: Part 3 Pre-commissioning and commissioning of pipelines

CurrentSpecializedCode of PracticeBIMMEP · Structural Engineering and Structural Sections

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This code provides the standard procedures for the pre-commissioning and commissioning of onshore natural gas pipelines. It covers essential activities like pipeline cleaning, gauging, hydrostatic testing for integrity, dewatering, drying, and the final safe introduction of natural gas (gassing-up).

Design and installation of natural gas pipelines – Code of Practice: Part 3 Pre-commissioning and commissioning of pipelines

Quick Reference Values

Minimum hydrostatic strength test pressure1.25 x Design Pressure

Minimum hydrostatic leak test pressure1.1 x Maximum Operating Pressure

Minimum hydrostatic test duration24 hours

Gauging plate diameter tolerance95% of pipe internal diameter

Recommended pigging velocity1 to 4 m/s

Typical target dew point for drying-10°C at pipeline pressure

Practical Notes

Hydrostatic testing is a critical hold point; meticulous and certified record-keeping of pressure and temperature is mandatory for acceptance.

Drying the pipeline to the specified dew point is crucial to prevent internal corrosion and the formation of solid gas hydrates during operation.

Commissioning typically involves purging the line with an inert gas like nitrogen before introducing natural gas to prevent the formation of an explosive air-gas mixture.