Pipe Types
The required thickness of a pipe is affected by the type of pipe selected. There are three main pipe types:
- Seamless – a pipe with no weld lines.
- Longitudinally welded – a pipe with one or more longitudinal weld lines.
- Spiral welded – spiral shaped weld.
Pipe Types: Seamless, Longitudinally Welded, Spiral Welded
Seamless
This pipe is formed by thrusting a piercing plug through a metal billet whilst it is retained between rollers, or, by pressing a mandrel into a billet that is held within a die. The result of both operations is a pipe with no weld lines (it is seamless).
Seamless Pipe Manufacturing Process
Longitudinally Welded
Flat metal plate can be hot rolled to produce the circular shape of a pipe. It is then necessary to weld the two ends of the plate together to complete the pipe shape in its entirety. Longitudinally welded pipes may have one weld, or multiple welds. As a rough guide, pipes up to approximately 1,000mm in diameter will have only a single weld, whilst larger pipes will require more than one weld.
Single Weld Longitudinally Welded Pipe
Longitudinally welded pipes are not preferred for critical systems if seamless welded pipes can be used. There are several reasons for this:
- The weld must be proven before its integrity can approach/near that of a seamless pipe. For critical systems, the aim is always to make the piping system as reliable as possible.
- Proving of a weld takes time and adds financial cost.
- The use of longitudinally welded pipe creates additional stresses within the piping system (during the design stage, stress calculations are performed extensively).
Tip – if the manufacturing process for a longitudinally welded pipe is hard to visualise, take a piece of paper in two hands and bend it lengthwise into a circular tube shape, then place tape along the joining edges to maintain the tube shape. Now imagine that the paper is metal plate and the tape is the weld, that is generally how longitudinally welded pipe is manufactured.
Spiral Welded
For systems operating with very high volumes and low pressures, a spiral welded pipe may be used. The manufacturing method is similar to that of a longitudinally welded pipe, but the metal plate is rolled at an angle which gives the pipe its spiral shape. Although this type of piping is the cheapest to manufacture, it is the least popular of the three. A typical application for spiral welded pipe would be for power station cooling water systems (non-toxic, high volume, low pressure).
Spiral Welded Pipe Manufacturing Process
Because spiral welded pipes are welded, they are also not favoured for critical systems for the same reasons as mentioned previously when discussing longitudinal welded pipes (weld integrity must be proven, cost of proving the weld, stress raising characteristics within the piping system).
Additional Resources
https://en.wikipedia.org/wiki/Nominal_Pipe_Size
https://www.archtoolbox.com/materials-systems/plumbing/standard-pipe-dimensions.html
https://www.theprocesspiping.com/nominal-pipe-size-and-schedule