Reference
Glossary, A to Z
Terms as a test crew uses them, not as a dictionary defines them. Where a term carries a regulatory meaning, the section is named.
Air lock
A pocket of gas trapped at a high point that breaks the hydraulic continuity of a liquid column. It removes the siphon effect, so separate uphill heads add instead of balancing, and drive pressure can exceed the safe working pressure of the line.
Allowable elevation window
The vertical range a single test segment can span before the pressure spread between its high and low points exhausts the gap between the minimum required test pressure and the maximum allowable pressure. Where the window is exhausted, a segment break is required.
Balance statement
The closing arithmetic of a pressure test: measured pressure change, minus predicted thermal response, equals unexplained residual.
Barlow’s formula
The thin-wall relation between pressure, diameter, wall thickness and hoop stress, S = PD/2t, rearranged as P = 2St/D.
Batching pig
A pig used to separate two fluids in the line, holding the interface between them.
Blanketing
Maintaining a low positive nitrogen pressure on an idle line or vessel to keep air and moisture out. Its acceptance criterion is continuity over time, not a single reading.
Bleed-down
The controlled depressurisation at the end of a test. The part of the pressure cycle associated with pressure reversals.
Class location
A classification set by what surrounds the pipeline, principally the number of occupied buildings nearby. It governs test factors and, under 49 CFR 192.503, the maximum hoop stress permitted during a gas-medium test.
Compliance, system
How much a pipeline system gives under pressure — the pipe expanding plus the water compressing. It appears as the denominator in the pressure-temperature relation.
Contact hour
One clock hour of instruction. A statement of duration, not of accreditation.
Covered task
An activity identified in an operator’s operator qualification plan as requiring qualified personnel, under 49 CFR Part 192 Subpart N or Part 195 Subpart G.
Dead leg
A branch or section with no through flow, where liquid and moisture collect and where a sweep gas does not reach. A common source of a soak-test rebound that will not diminish.
Deadweight tester
A primary pressure standard using calibrated masses on a piston of known area, used on site as the reference against which working instruments are checked.
Dew point
The temperature at which water would begin to condense from the gas leaving a line. The acceptance measure for drying; lower is drier.
Drive pressure
The pressure required at the pump to move fluid or a pig through a line, comprising elevation head plus friction, plus any additive heads created by trapped gas.
Gauge pig
A mandrel pig carrying a soft aluminium plate sized as a percentage of internal diameter, run to prove the bore is clear for whatever tool follows.
Hoop stress
Circumferential stress in the pipe wall produced by internal pressure. The quantity capped as a percentage of SMYS.
Hydrostatic gradient
The pressure change with elevation in a static liquid column, 0.433 psi per foot for fresh water, scaled by specific gravity.
Kicker valve
The valve introducing flow behind a pig in a launcher barrel to drive it forward into the nominal bore where its seals engage.
Launcher
The trap from which a pig is introduced into the line, comprising closure, oversized barrel, reducer, isolation valve and associated vent and drain.
Leak test
The phase, held longer and at lower pressure than the strength test, that looks for loss of inventory rather than loss of strength.
Mandrel pig
A steel-bodied pig carrying replaceable sealing cups or discs, able to carry brushes, magnets or a gauge plate.
MOP / MAOP
Maximum operating pressure, and maximum allowable operating pressure. Test pressures are commonly expressed as a multiple of it.
Pig signaller
A device tripped by a passing pig to confirm its passage. Confirmation of arrival is not confirmation that a trap is safe to open.
Pressure ladder
The planned sequence of target pressures, hold durations and acceptance criteria for a test segment.
Pressure reversal
A defect surviving a pressure test and subsequently failing at a lower pressure, associated with the depressurisation part of the cycle.
Pressure-volume plot
Pressure plotted against cumulative volume pumped during the climb. Distinguishes healthy elastic pipe, trapped air, and the onset of yielding.
Purge
Displacing one atmosphere with another. Success is measured by sampled composition at the discharge, and the controlling variable is velocity, not volume.
Receiver
The trap at which a pig is recovered, together with whatever it has pushed ahead of it.
Residual
The part of a measured pressure change that the predicted thermal response does not explain. What acceptance is argued over.
SMYS
Specified minimum yield strength of the pipe grade. Test pressures are expressed as a percentage of it.
Soak test
A period during drying when the sweep is stopped and the line stands, allowing trapped moisture to redistribute. The rebound on resuming is the diagnostic.
Spike test
A brief excursion above the strength test pressure, applied first in the sequence, added for gas transmission by 49 CFR 192.506 in 2019.
Strength test
The phase applying the highest sustained pressure, establishing a margin above operating pressure.
Stratification
Separation of two gases by density when purge velocity is too low, which smears the interface and turns a displacement into a dilution.
Test head
The temporary closure and manifold through which a segment is filled, pressurised and instrumented. It sees full test pressure and needs a documented rating.
Tracer gas
A gas introduced to help locate a leak by detection at the surface or at a sample point.
Weighted temperature
A volume-weighted line temperature derived from several measurement points, used in the pressure-temperature analysis in place of any single reading.