HDPE Butt Fusion Gauge Pressure and Drag

The machine pushes on a hydraulic cylinder of fixed area and the pipe presents an annular face area that changes with every size, so the gauge reading that produces a correct joint is different for every pipe on the same machine. Two errors follow from that and both produce joints that look right. THE FIRST IS OMITTING DRAG. Some of the machine's pressure is spent moving the carriage and the pipe itself before any force reaches the joint, and that drag has to be measured and ADDED to the calculated figure. It is not a constant: it changes with the setup, with how much pipe is hanging off the machine, and with what the pipe is being dragged over, so it must be measured before every joint rather than carried over from the last one. Setting the calculated pressure without it under-presses the joint by exactly the drag. THE SECOND IS RUNNING A FIXED GAUGE NUMBER ACROSS SIZES. Face area grows roughly with the square of the diameter, so the correct gauge pressure on large pipe is several times the small-pipe figure. Using the small number on large pipe badly under-presses it; using the large number on small pipe over-presses and squeezes the melt out of the joint, which is the failure that leaves a large bead and a weak weld. Heat soak and cool times are governed by WALL THICKNESS and come from the procedure rather than from this arithmetic, and cool time is where a correct joint is most often lost at the last step: the pressure must stay on for the full cool period, and releasing early relaxes a joint that will look correct and test badly. The bead is the visual check, and its size and shape are what the operator reads. This computes pressures from an entered geometry and an entered interfacial pressure. It does not supply the interfacial pressure, the heat soak time, the cool time, or the heater surface temperature -- all four come from the pipe manufacturer's procedure and the applicable practice, and they differ between manufacturers and materials. It does not qualify a fusion machine or an operator, evaluate a bead, address pipe end preparation, alignment, high-low, or contamination, or cover electrofusion or socket fusion, which are different processes with different controls. ASTM F2620 or the manufacturer's own qualified procedure, and the operator's qualification, govern.

gauge pressure = interfacial pressure x pipe face area / machine total effective cylinder area, with MEASURED drag pressure ADDED; pipe face area is the annulus pi/4 x (OD^2 - ID^2) and scales with the square of diameter at a fixed dimension ratio.

Butt fusion pressure as ASTM F2620 practice and machine manufacturers state it, with the interfacial pressure (commonly 75 psi), the heat soak time, the cool time and the heater temperature all coming from the pipe manufacturer's qualified procedure rather than from here.

One area ratio and one addition.

Estimate. AHJ and licensed professional govern.

Field names used by the API: pipe_od_in, dimension_ratio, wall_thickness_in, cylinder_area_in2, interfacial_pressure_psi, drag_pressure_psi, alt_pipe_od_in, wall_in, face_area_in2, theoretical_gauge_psi, total_gauge_psi, alt_gauge_psi

• Interfacial pressure is entered commonly 75 psi, from the manufacturer's procedureASTM F2620 or the qualified procedure
• Heat soak and cool are by wall thickness from the procedure, not computed herethe pipe manufacturer's procedure
• Not a qualification this sizes pressure; it does not qualify machine, operator or beadthe operator's qualification