Butt Welding Industry Terms & Definitions


Years experience in PE pipe machinery

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Initial melt under pressure around the circumference of the ends of the pipes or fittings which ensures that complete pipe or fitting to heater plate contact is achieved.

Bead-up pressure

Pressure exerted on the heater plate by the pipe or fitting ends during the bead-up phase of the jointing cycle
Note: The value of the corresponding gauge pressure is calculated with Formulae (1) and (2).

Bead-up size

Bead size formed on the pipe or fitting ends during the bead-up phase.

Cooling time in machine under pressure

Time period during which the butt fusion joint remains under pressure when still clamped in the machine.

Cooling time in machine without pressure or out of machine

Additional cooling period that may be required after the cooling time under pressure to ensure optimum joint strength, particularly when working at high ambient temperatures and prior to rough handling or pipe installation.

Drag pressure

Gauge pressure required to overcome, on a given machine, the sliding frictional drag force of the machine and pipe for each weld.
Note: Drag pressure, if present, can be a positive or negative value.

0 to drag pressure

Lowest possible pressure during the heat soak that will allow the pipe or fitting to remain in contact with the heater plate, up to a maximum of the current drag pressure.
Note: The drag pressure required to move the pipe or fitting may be much higher than the pressure required to maintain pipe or fitting/heater contact during heat soak.


Able to be deformed without losing toughness; pliable, not brittle.

Fusion jointing pressure / interfacial pressure

Actual pressure exerted on the pipe or fitting ends during jointing.
Note: The value of the corresponding gauge pressure is calculated with Formulae (1) and (2).

Gauge pressure

Actual pressure (bead-up pressure or fusion jointing pressure with drag pressure) in the cylinders of the butt fusion jointing machine read by the gauge.

Heat soak

Portion of the heating cycle done at 0 to drag pressure so that the heat can melt the pipe or fitting to a depth sufficient for proper mixing and fusion.

Heat soak pressure

Pressure required to maintain the pipe or fitting in contact with the heater plate taking into account the drag pressure.

Heat soak time

Time during which the heater plate is in contact with the pipe or fitting ends at the heat soak pressure.

Heater plate removal time

Time taken to separate the pipe or fitting ends from the heater plate, remove the heater plate and close the carriage in order to bring the molten pipe or fitting ends together.

Heater plate temperature

Measured temperature on the surface of the heater plate where the pipe or fitting wall cross-section makes contact.

Heating cycle

Part of the welding process in which pipe or fitting ends are in contact with the heater plate composed by bead up pressure and followed by heat soak pressure.

Jointing session

Uninterrupted series of welds made with the same parameters and same equipment.

Minimum bead size after heating

Minimum value of bead size to be attained after completing the heat soak time.
Note: Single high-pressure butt fusion procedure only.

Nominal wall thickness – en

Wall thickness tabulated in ISO 40651, corresponding to the minimum wall thickness ey, min at any point ey.

Nominal outside diameter – dn

Numerical designation of size which is common to all components in a thermoplastics piping system other than flanges and components designated by thread size.


Qualified person authorized to construct polyethylene (PE) systems from pipes and/or fittings.

Pipeline operator

Private or public organization authorized to design, construct and/or operate and maintain a pipeline supply system.

Rough handling

Any action whereby stresses are applied to the fused joint, such as tensile and bending forces.
Note: Examples of rough handling immediately after the cooling time in the machine under pressure include:
— the joint is removed from the machine by single point lifting at the joint;
— several lengths of joined pipe are pulled with the freshly made fusion joint;
— the joint is immediately subjected to a severe bending stress.

SDR – Standard Dimension Ratio

Numerical designation of a pipe series, which is a convenient round number, approximately equal to the dimension ratio of the nominal outside diameter, dn, and the nominal wall thickness, en.

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