By Tim Daly
“Offset” is a term that all pipefitters have heard, but surprisingly few fully understand—especially considering that they are encountered on practically every job in one form or another. Simply put, an offset occurs every time a line of pipe must avoid an obstacle, or when the pipe exits a space at a different point, in regards to the floor or deck as the case may be, from the point it entered the space.
For instance, if a pipe entered the south wall of a room eight feet off the floor and sixteen inches west of the center point, and exited the room nine feet off the floor and eighteen inches to the west of center, that would be an off set—in this case, a rolling offset. Similarly, a pipe running along the far west wall of the same room that had to be taken around an I-beam supporting the floor of the room above, would be offset where it went around the I-beam. This last case is more of a simple offset.
Calculating the offset of a pipe can be a tricky venture, especially when it gets very complex. Often a pipefitter or plumber running something on the idea of a single copper line to a new hose bib being installed on the side of a building, will use a hit or miss technique to make the offset around the above mentioned I-beam, or just calculate it as a straight pipe and measure out how long the two east/west pieces need to be, and throw it together with 90° elbows. However, if you were running a new twelve inch steam line through an existing boiler room, where you had to make the pipe fit around a host of other pipes and components already installed, the hit or miss method would be way too time consuming and expensive. This is where real pipefitting and pipefitter math comes into play.
Generally speaking, offsets are made using 45° angles, 22 ½° angles, or by bending the pipe as required. 90° angles tend to overly restrict flow, the fluid being carried makes much more noise going through right angles, and really, they just don’t look as professional.
While there are certainly some constants when doing the math to calculate offsets, if you don’t make the calculations on a regular basis, you’re probably not going to remember the constants. This is where a quality pipefitter’s handbook comes into play. If you don’t already have one, then the next time you venture into your local supply house, ask about getting one. Often, the supply house will give them away for the advertising as well as a good-will gesture to prospective clients. These days, most plumbing or pipe shops are equipped with computers in the office spaces, and you can find the same information online.
A simple offset is when the pipe goes around an object, but when all is said and done, the angle only changed one direction. If the pipe we mentioned in our example in the first paragraph had only changed from eight feet off the floor to nine feet of the floor, but the distance from the centerline had remained at sixteen inches west of center, it would have been a simple offset. Similarly, if the pipe had moved from sixteen inches west of center, to eighteen inches west of center but had remained eight feet of the floor instead of rising to nine feet, that also would have been considered a simple offset.
A rolling offset occurs when the pipe changes on both the horizontal as well as the vertical plane. Sometimes, a rolling offset will include the pipe completely changing direction. The change could be horizontal, vertical or both. Your best bet when trying to visualize such a thing is to draw a box to represent the space where the pipe is. Then put marks on the sides of the box to represent where the pipe comes into the box, and where it goes back out. Building a pipe with rolling offsets can be particularly difficult when you’re bending pipe instead of using fittings.
As complex as they can be to calculate, for many people rolling offsets are just as hard to visualize until they see the finished product. When I was an apprentice, our class was required to take a course based on Charles Quinlan’s Orthographic Projections. This is a class where you are taught how to find the third view of something if you have the first two views, or find the three dimensional object based on all three views. Several people in the class had a hard time visualizing the objects, and one in particular known as “Spanky,” spent the entire year before we took the course memorizing every picture in the book, just to pass the course because he knew he wouldn’t be able to see them on his own.
Jumper offset is the term used to describe offsets that take a pipe around a curved object like a cylinder or smoke stack. Often times the fitter will use two 45° elbows and one 90° elbow, or four 45° elbows. The idea is to not have so many fittings that you’re inviting leaks, but enough to keep the offset from being too far away from the object you’re piping around.
Use the image above to sketch out the jumper offset using the variables on the right. After a couple practice runs, try changing the numbers.
R – 4″
A = 3″
E = 2″
C – 20″
Step 1 – The first thing you need to find is the side of the secondary square (G). So add (R) + (A) = (G)
4 + 3 = 7″ This number is known as the secondary square (G).
Step 2 – Multiplying the secondary square by 2 will give you the length of pipe (B), and the length of pipe (F) to the centerline of the tank.
(G) x 2 = (B) so to find the end to end length of pipe (B) which ends on the center line it would be 7″ x 2 = 14″.
Step 3 – To find the length of pipe (F) will require two steps, first you have to multiply dimension (E) by 1.414 to find the length of pipe (F) below the centerline of the tank, then add that number to result you got in step two above.
2″ x 1.414 = 2.828 then 2.828″ + 14″ = 16.828″ or roughly 16 7/8″
Step 4 – To find (H) also known as the offset, multiply the diagonal (B) by the constant 0.707
(H) = 14 x 0.707 = 9.898″
Step 5 – Find dimension (J) by adding the offset (H) with (E)
(H) +(E) = (J) / 9.898 + 2 = 11.898″
Step 6 – Finding the length of pipe (D) is simple subtraction. (C) – (j) = (D)
20 – 11.898 = 8.102″
Figure 3 and accompanying chart are courtesy of www.plumbinghelp.ca
Do not be intimidated by the term offset. Most pipefitters will come across situations like this. Some will not know the math behind it, and hopefully the above charts will help them in that regard. If you’re like me, the drawings won’t be nearly as pretty, but you’ll probably—with a little practice—be able to master the concept rather quickly.