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Nitrogen Purging of Natural Gas Pipelines and Vessels

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
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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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Additional training resources

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