Chapter 1 of 10 · 10 min read
Choosing the pipe joint
Compare welded, threaded, flanged, grooved and pressed pipe joints by installed cost, size, duty, leaks and maintenance, and find where welding starts to pay on a real pipe run.
Every pipe run is a chain of joints, and the joints decide most of what the run costs, how long it takes to build, whether it leaks, and how easily it can be changed later. The pipe itself is the cheap part. Cutting it, preparing the ends, making the joints and testing them is where the labour goes, and the choice between welding, screwing, flanging, grooving or pressing the joints is made, one way or another, before the first length is lifted.
Early welding manuals made the case for welded pipe with the arithmetic that still decides it. P. F. Willis, in 1917, set out the cost of welded joints beside threaded couplings for each pipe size, and showed the welded joint’s advantage growing as the pipe got bigger. This chapter brings that comparison up to date, looks at what each kind of joint is good for, and works out where welding starts to pay on the running case’s piping.
The kinds of joint
Five families of joint cover almost all steel and stainless piping in industry.
| How it is made | Typical use | Main limitation | |
|---|---|---|---|
| Threaded | taper threads cut on the pipe end, screwed into a fitting with sealant | small-bore water, air, gas; low pressure | thread cuts the wall; leaks; size limited |
| Butt welded | ends bevelled and welded right through | pressure, temperature, steam, process, pipelines | needs qualified welders, inspection, hot work |
| Socket welded | pipe slid into a socket and fillet welded round | small-bore high-pressure lines | crevice inside; not for some services |
| Flanged | flanges welded or screwed on, bolted with a gasket | equipment connections; parts that must come apart | cost, weight, gasket leaks |
| Grooved or pressed | groove rolled or cut, coupling clamped; or fitting pressed | fire services, cooling water, building services | pressure and temperature limits; seals |
Threaded joints are quick and need no hot work, but the thread is cut into the pipe wall, removing metal at the most highly stressed point, and the sealant between the threads is the only thing that keeps the fluid in. Butt welds, made right through the wall, are as strong as the pipe and have no seal to deteriorate. Socket welds suit small bores where a butt weld would be hard to make, at the cost of a crevice inside the joint. Flanges are for joints that must be broken, at pumps, valves, vessels and instruments. Grooved and pressed joints, made with a tool and a coupling or fitting, are fast and need no hot work, and have spread through building services and cooling water, within the pressure and temperature limits their makers set.
Cost per joint by size
The cost of a joint has two parts that change with size in opposite ways. A threaded joint’s fitting gets much dearer as the pipe grows, and threading large pipe is slow, heavy work; a welded joint’s labour grows with the weld’s length and thickness, but the fitting for a butt weld is cheap. At small sizes the threaded joint wins; at large sizes the welded one does.
| Threaded | Butt welded | Flanged pair | |
|---|---|---|---|
| DN 25 | about A$35 | about A$70 | about A$190 |
| DN 50 | about A$70 | about A$85 | about A$260 |
| DN 80 | about A$140 | about A$110 | about A$340 |
| DN 100 | about A$210 | about A$140 | about A$420 |
| DN 150 | rarely used | about A$200 | about A$560 |
In this illustration the lines cross between DN 50 and DN 80. Below that, threaded joints are cheaper; above it, welded joints are, and the gap widens quickly. Flanged joints cost far more than either at every size, because a flanged joint is really two welded or screwed joints plus two flanges, a gasket and a set of bolts. That is the pattern Willis reported more than a century ago, with the prices of the day: the welded joint’s advantage grew with size. The crossover point moves with labour rates and the price of fittings, and every contractor’s estimating records will put it somewhere slightly different, but the shape of the comparison is very stable.
A run, not a joint
Joints are not bought one at a time. A pipe run has joints between lengths and at every fitting, and the number of joints, not the length of pipe, drives the cost.
- Run
- 120 m of DN 100 in 6 m lengths
- Fittings
- 12, each with two joints
- Cost per joint
- threaded about A$210; butt welded about A$140
- joints between lengths = 120 ÷ 6 − 1 = 19; joints at fittings = 2 × 12 = 24; total 43
- threaded: 43 × 210 ≈ A$9,030; welded: 43 × 140 ≈ A$6,020
- welding saves about A$3,000 on this run, before counting leaks
The fittings contribute more joints than the straight pipe does, which is typical of plant piping, where runs turn, branch and change size often. On a long straight pipeline, by contrast, almost all the joints are between lengths, and buying longer lengths cuts their number. Willis made the same point: pipe for welding could be bought in longer lengths than the usual, so that fewer connections were needed.
Leaks over the life of the run
A joint’s cost does not stop at installation. Threaded joints are the most likely to leak, especially on lines that heat and cool or vibrate, and a leak on a compressed air line costs money every hour the compressor runs.
Ten leaking joints in this illustration waste about 10 × 0.5 × 0.11 × 6,000 ≈ 3,300 kWh a year, about A$825, every year until someone finds and fixes them. On steam lines the losses are larger, and on lines carrying hazardous or valuable fluids a leak is not just a cost but a hazard. The Davis-Bournonville course put the argument for welding plainly: a line without couplings has none to leak. A welded line can still leak, through a defective weld, but a sound weld does not loosen with time.
Strength and the threaded wall
Cutting a thread removes part of the pipe wall. On thin-walled pipe, the remaining wall at the bottom of the thread can be a small fraction of the original, which is why threaded joints are restricted to heavier wall thicknesses and lower pressures, and why piping codes set out where they may be used. A butt weld, made through the full wall with sound fusion, leaves the wall at full thickness. Willis reported tests in which welded connections carried considerably more than screwed ones of the same size. Modern codes do not rely on such tests; they set the joints permitted for each service and the qualifications of the procedures and welders that make them.
When joints must come apart
Some joints must be broken: at pumps, valves, heat exchangers, instruments and anything that will be removed for maintenance. Welding them would mean cutting the pipe each time. Flanges serve there, at a premium, and the designer’s job is to put flanges where they are needed and nowhere else. A system designed with too many flanges costs more and has more gaskets to leak; one with too few must be cut apart for maintenance. The Davis-Bournonville course argued that the need for disconnection was often overstated, and that a cutting torch could remove a welded section when changes were needed. That remains true for many services, and on lines that must be hygienic or leak-free, welded joints with a few well-placed flanges are the normal design.
Hot work and running plants
Welding needs hot work: an open arc or flame, sparks, hot metal and fume. In a new building that is a manageable hazard. In a running plant, near flammable materials, in a food factory that must stay clean, or in an occupied building, hot work needs permits, fire watches, screens and sometimes plant shutdowns, and each of those costs time and money. Grooved and pressed joints need no hot work, which is much of the reason for their popularity in building services and plant alterations. The contractor’s estimate for a tie-in to a running line includes the hot work controls, and sometimes those controls decide the joint.
Skills, qualification and inspection
A welded joint on a pressure line must be made by a welder qualified for that procedure, position and material, to a qualified welding procedure, and examined to the extent the code requires. Those requirements are a cost and a constraint as well as a guarantee. A small job may not justify qualifying a procedure, and a site may not have qualified pipe welders available. Threaded, grooved and pressed joints need trained fitters, but their qualification is simpler. The joint that is cheapest on paper is not always the one that can be made on the day with the people available.
Hygiene and the inside of the pipe
On food, dairy and pharmaceutical lines, the inside of the joint matters as much as its strength. A threaded joint has crevices where product and bacteria lodge; a socket weld has a gap behind the fillet; a flange has a gasket that can harbour residue. A full-penetration butt weld with a smooth, flush, clean inside surface can be cleaned in place as well as the pipe itself. That is why the running case’s stainless product lines are all butt welded, with hygienic clamp fittings only where parts must come apart, and why Chapter 7 is devoted to them.
Pipelines and long runs
On long pipelines, such as the running case’s raw water main, welding almost always wins. The joints are all between lengths, all the same size, and repeated hundreds of times, so welding crews work in a steady rhythm along the line. The lengths can be welded into long strings beside the trench and lowered in together, which is what Willis described for gas mains, leaving only a few welds to be made in the trench. Mechanical couplings and rubber-ring joints remain common on water mains in cast iron, ductile iron and plastics, and on steel mains where the design allows; the choice for steel mains depends on the pressure, the ground, the pipe’s lining and the client’s standard.
Time on site
Joints also differ in how long they take to make where the pipe is installed, which matters when site time is scarce or expensive. A welded joint can be made in the shop, as part of a prefabricated spool, leaving only a few field welds to join the spools on site; a threaded or grooved run is usually assembled piece by piece in place. The contractor’s choice of joint therefore shapes how much work moves off site, and Chapter 4 shows how much a shop weld saves over the same weld made in the field.
The running case: the contractor’s rules
This is an illustration. The contractor’s piping supervisor set out the joint rules for the dairy project: carbon steel steam, condensate and process lines butt welded at DN 65 and above, socket welded or threaded below where the design allowed; compressed air threaded at DN 50 and below, welded above; cooling water grooved, because much of it was tie-ins to running lines in an existing building and hot work there was costly; stainless product and cleaning lines butt welded throughout; flanges only at equipment, valves that need removal, and spool breaks for maintenance. Each rule came from the design pressure and temperature, the client’s standard, the code, and the cost comparison in this chapter, and each was written into the project’s piping specification before estimating began.
Mixed systems
Most plants end up with a mixture of joints, and the transitions need care. A threaded branch screwed into a welded main needs a welded fitting, such as a half coupling or a forged outlet, to take the thread; a grooved run joining a welded one needs a grooved end welded or rolled on the pipe; a flanged joint between different materials may need an insulating kit to prevent galvanic corrosion. The contractor marks every transition on the drawings, so that the right fittings are ordered and the welders know where each system’s rules change.
A failure: threaded steam
This is an illustration. An earlier contractor at the dairy’s old plant had installed a DN 80 steam line with threaded joints to save time on a tie-in. The line heated and cooled twice a day as production started and stopped. Within a year, several joints were weeping steam, and one had to be cut out and replaced during a shutdown after it began to leak heavily near a walkway.
The thermal cycling had worked the threads loose and the sealant had failed, and at DN 80 the threaded wall was thin at the root of the thread. Welded joints, which the dairy’s own standard required for steam above a modest pressure, would have cost about the same to install at that size and would not have loosened. The lesson the client drew was to write the joint rules into the specification, as the new project does.
Exercise: a cooling water header
A cooling water header is 60 m of DN 150 pipe in 6 m lengths, with 8 fittings, each with two joints. Using the illustrative costs of about A$200 per butt welded joint, and supposing grooved joints cost about A$180 each installed but avoid about A$1,500 of hot work permits, fire watches and screens on this tie-in, which is cheaper?
The header has 60 ÷ 6 − 1 = 9 joints between lengths and 2 × 8 = 16 at fittings, 25 joints in all. Welded, they cost about 25 × 200 = A$5,000, plus about A$1,500 of hot work controls, about A$6,500. Grooved, they cost about 25 × 180 = A$4,500.
The grooved header is about A$2,000 cheaper here, mainly because it avoids the hot work in a running building. The comparison would reverse in a new building with no hot work premium, or if the header’s pressure or temperature were outside the grooved couplings’ limits, which the coupling maker’s data would show.
