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Nitrogen Purging of Natural Gas Pipelines and Vessels

This course is designed for those who are required to purge and measure flammable gases in an inert atmosphere during commissioning and decommissioning operations. On successful completion and assessment, which includes theory and practical exercises, candidates will be awarded a JMS Consultants Certificate.

Course Length: 1 Day

Key Areas

  • Why purge pipelines and vessels?
  • Types of purging and definitions - direct purging
  • Types of purging and definitions - indirect purging
  • Sources and quantities of purged gas
  • Precautions when purging
  • Purging procedures, timing and purge end points
  • Monitoring purge end points using gas detectors
  • Venting of purged gases
  • Post purge arrangements
  • Planning a safe system of work
  • Theory and practical exercises

Why do we need to purge pipelines and vessels?

Prevention of hazards; flammable gas/air mixes when commissioning or decommissioning; pyrophoric scale; static electricity considerations.

Types of purging and definitions

Direct Purging
• Displacement with fuel gas or air
• Vacuum techniques
• Application where high velocity purging is possible
• Implications of stratification
• Unsuitability for vessels and suitability for pipelines.

Indirect Purging
• Types of inert medium
• Displacement purging
• Slug purging
• Pressure purging
• Dilution purging
• Branch systems; simultaneous or sequential purging and loops.

Direct and indirect purging
• Stratification
• Dilution purging
• Slug purging
• Pressure purging
• Purge end point
• Purge pressure
• Purge velocity
• Purge volume
• Double block and bleed.

Sources and quantity of purged gas

• Cylinders,‘skids’,
• Quantity of purge gas required.
• Carbon dioxide and nitrogen
• Use of single or multiple manifolds and high capacity regulators
• Need for adequate flow
• Choices of inert gas and freezing effects.
• Quantity expressed as multiples of space to be purged
• Quantities for slug displacement and dilution purging.

Precautions when purging

• Static Hazards
• Bonding
• Purge gas velocity
• Stratification
• Colour coding of cylinders to ensure correct gas
• Asphyxiation by purge gases
• Site notices: warning, prohibition, no smoking/naked light, and asphyxia hazard.
• Labelling of purged and un-purged plant
• Equipment requirements; fire extinguishers, BA and personal O2 detectors.

Purging procedures, timing and purge end points

• Use of triangular flammability diagrams as applied to natural gas/nitrogen/air purges
• Other purge gas scenarios
• Purge end points
• Effect of gas composition on triangle of flammability
• Achieving specified purge end point; use of end points as minimum standards
• Flammability of gases in air; safety margins
• Hazards of desorption from solid residues and vapours from liquid residues
• Need for rechecking
• Value of purge end points in purging pipelines and large volume vessels.

Monitoring purge end points using gas detectors

• Suitable and unsuitable gas detectors
• Recognising an appropriate detector;
• Using catalytic and thermal conductivity gas detectors
• Start-up procedure
• Calibration gas mixtures; calibrating and zeroing of detectors
• Electrical safety certification of gas detectors and other electrical apparatus.

Venting of purge gases

• Minimising inventory to vent diameter and flow in vent pipes
• Noise factors
• Vent height and distance from buildings
• Prevention of static charge build-up; use of flame traps.

Post purging arrangements

• Need for re-testing and frequency
• Implications of confined space entry regulations when man entry planned.

Planning a safe system of work

• Preparation of method statement including plant description; isolation; use of physical lock-off; methods of purging and venting; permit-to-work and sign-off procedure
• Purge: volume, velocity, pressure, end points and duration
• Risk assessment and notification of relevant bodies
• Emergency and other contingency arrangements.

Theory and practical exercises

• Group theory exercise – ‘Commissioning of a new Nuevo Pignone 15 megawatt compressor’
• Multi-choice questionnaire
• Defining the practical exercise; preparing pipeline/vessel for purging; safety precautions; inert gas and purged gas interconnections/venting; choosing, inspecting and preparing the gas detector(s) for use
• Undertaking the practical exercise.
• De-briefing from practical exercise.

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