How to work out a pipe offset by hand

The triangle

The offset (Dunham calls it the set) is how far the run moves sideways, centre to centre. The travel is the centre-to-centre length of the angled pipe — always the hypotenuse (Wisconsin). The run is how far along the line the offset takes. With θ the fitting angle, travel = offset ÷ sin θ and run = offset ÷ tan θ.

1. The constants

The same figures as FM 3-34.471 Table 6-2 and the Dunham table, rounded here where the books cut the last digit off (both print 0.5773 for the 60° run, FM prints 1.7320 for the 30° run); FM’s 2.4140 for the 22½° run should be 2.4142, and its 5.0270 for the 11¼° run should be 5.0273.

2. A simple offset

FM 3-34.471’s example: a 40 in offset with 45° fittings. Travel = 40 × 1.4142 = 56.57 in, about 56 9/16 in centre to centre, as the manual prints; the run is 40.000 in.

3. A rolling offset

When the pipe moves sideways and up at once, combine the two legs into the true offset first — Dunham calls them the drop and the spread: √(drop² + spread²). His example has a 12 in drop and an 8 in spread: √(12² + 8²) = 14.422 in, and with 45° fittings the travel is 14.422 × 1.4142 = 20.40 in (the handbook prints 20.38; 1.414 × 14.42 is 20.39, so its last digit is a slip). Dunham works the example with 45° fittings and gives the factors for other fittings on p. 251; or use that fitting’s constant from the table.

4. The cut length

The travel runs centre to centre of the fittings, but the pipe stops inside them. Subtract both fittings’ centre-to-end dimensions — FM’s “fitting dimensions” — and add both pipe engagements (FM 3-34.471, p. 6-6). In the manual’s example the fittings take 9 1/4 in together and the engagements add 2 in: 56 9/16 − 9 1/4 + 2 = 49 5/16 in.

Common mistakes

• Using the run constant for the travel — at 45° they are 1.4142 and 1.0000, so the pipe comes out 29 % short.
• Working a rolling offset with one leg alone, forgetting to combine the drop and the spread.
• Cutting to the centre-to-centre travel, or taking off one fitting instead of two.
• Adding the engagement to a figure that already has it taken off: some fitting charts give a “take-out” with the engagement already deducted, so check which figure you have.
• Mixing up names: Dunham’s set is the offset of a simple offset; his drop and spread are the two legs of a rolling one.

How this matches the tool

The pipe offset calculator does simple and rolling offsets for any fitting angle, with take-outs and engagements, in millimetres or inches with sixteenths. The table and worked numbers on this page were produced by its code when the page was built.

Sources

1. FM 3-34.471, Plumbing, Pipe Fitting, and Sewerage (Department of the Army, 2001), pp. 6-6–6-7: the 45° offset constant 1.4142, Table 6-2, and subtracting fitting dimensions and adding pipe engagements
2. W. E. Hill, “Offset Calculations”, The Dunham Hand Book No. 414 (C. A. Dunham Co., c. 1925), pp. 250–251: set, length and run; rolling offsets as 1.414 × √(drop² + spread²)
3. Pipefitting Apprenticeship Curriculum (Northeast Wisconsin Technical Institute, 1989; ERIC ED330855), printed p. 40: the travel is always the hypotenuse; cosecant = hypotenuse ÷ opposite