Buttweld, socket weld or threaded: choosing pipe fittings
Every pipe fitting has to be joined to the pipe somehow. There are three common ways to do it: weld the fitting to the pipe end to end (buttweld), slide the pipe into a socket and weld round the outside (socket weld), or screw the two together (threaded). The right choice depends on the pipe size, the pressure and temperature, what the line carries, and who will make the joints.
Buttweld fittings
A buttweld fitting has a bevelled end that meets the bevelled end of the pipe. The welder fills the groove with a full penetration weld, so the joint becomes as strong as the pipe itself and the bore stays smooth inside.
- Where they fit: process lines, larger bores (most lines from about 2 inch up), and high pressures and temperatures.
- What to match: the fitting's wall thickness to the pipe's schedule, and its material to the pipe's grade.
- What it takes: a qualified welder, good fit-up and, on many jobs, inspection of the weld.
Socket weld fittings
A socket weld fitting has a socket that the pipe slides into. A fillet weld round the outside holds it. The socket lines the pipe up for you, so small-bore lines go together quickly and neatly.
- Where they fit: small-bore lines, usually 2 inch and under, including high pressure lines with forged fittings.
- A detail that matters: the pipe is pulled back a little from the bottom of the socket before welding, so the joint has room to expand and the weld does not crack.
- Where to avoid them: the small gap inside the joint can trap fluid, so they are a poor choice for corrosive service and for lines that must be cleaned out completely.
Threaded fittings
A threaded fitting screws onto a threaded pipe end, sealed with tape or compound. No welding is needed, so a line can be built or changed where hot work is not allowed.
- Where they fit: small sizes and lower pressures: water, air and other utility lines.
- The trade-off: cutting a thread thins the pipe wall, and threaded joints can loosen and leak under vibration or repeated heating and cooling.
- Check the thread form: tapered pipe threads come in different standards, such as NPT and BSP, that do not mix. Tell us which one your line uses.
A quick way to choose
- Large bore, high pressure or hazardous fluid: buttweld.
- Small bore and high pressure, with welders on site: socket weld.
- Small bore, low pressure, no welding allowed or quick changes needed: threaded.
What to send for a quote
For each fitting, send the type (elbow, tee, reducer and so on), the size, the schedule or pressure class, the material grade, the end connection and the quantity. A photo of the old fitting or its markings helps.