Pressure Piping and Pressure Pipeline Components
Pressure piping is examined through classification - GA/GB/GC/GD - because class sets design, NDT proportion, and inspection interval. You must also know components, supports, and test pressures.
Scope
Piping with maximum working pressure ≥0.1 MPa gauge, nominal diameter >25 mm, conveying gas, liquefied gas, steam, or a flammable, explosive, toxic, or corrosive liquid (or liquid at or above standard boiling point).
Classification
- GA long-distance transmission pipelines (GA1, GA2).
- GB public utilities: GB1 town gas, GB2 heat distribution.
- GC industrial piping (GC1, GC2, GC3) - GC1 covers the most hazardous service conditions.
- GD power piping (GD1, GD2).
Components and supports
Pipes, fittings, valves, flanges, expansion joints and compensators, gaskets and sealing elements, and supports/hangers. Support condition matters: a frozen or missing spring hanger changes the stress state and can fail a pipeline that passes every wall-thickness reading.
Inspection
Periodic inspection combines annual check, comprehensive inspection, and pressure test. Comprehensive inspection interval generally does not exceed 6 years; safety condition grade 3 gives 3–6 years; grade 4 means monitored service. Hydraulic strength test at 1.5 times design pressure, pneumatic at 1.15 times, tightness test at design pressure, with medium temperature controlled and air properly vented.
Worked example
A GC1 hydrogen line, design pressure 4.0 MPa, last comprehensive inspection 2019, no records of a pressure test. Reasoning: GC1 in hazardous service cannot simply borrow the 6-year ceiling; the interval is set from corrosion/erosion rates and vibration history, and 6 years elapsed already means inspection is due. Check the support and hanger survey, flexible joints, weld NDT records against design proportions, and schedule the hydraulic test at 6.0 MPa with the medium temperature above the brittle-transition limit. Hydrogen service also raises the question of high-temperature hydrogen attack and hardness limits.
Traps that cost marks
Using hydrostatic test pressure 1.25× (that is the vessel value, not piping); forgetting GB1/GB2 split for gas versus heat; ignoring support condition in favour of wall thickness alone.