From the measured outside diameter of the pipe, and from the length of the defect - never from the nominal bore.

Nominal size does not determine outside diameter. A pipe of a given nominal bore has a different OD in cast iron, ductile iron, steel, PVC, polyethylene and asbestos cement, and the differences are large enough that a clamp for one will not fit another.

So measure the pipe. A pi tape or a circumference measurement converted to diameter is the reliable method; calipers on a buried pipe in a trench rarely give an accurate figure. Then select a clamp whose stated OD range brackets that measurement, ideally near the middle of the range rather than at an extreme.

The clamp's band length must also exceed the defect with sound pipe either side to seal against. On older networks, identify the pipe material physically rather than from records - the material on the drawing is frequently not the material in the ground.

Many can, and it is one of the main reasons they are used - but it depends on the clamp pattern, the pressure, and the nature of the defect.

Fitting under pressure means working in an escaping jet of water, which is unpleasant, obscures the work, and washes out the excavation. On a small leak at moderate pressure it is routine. On a large split or a high-pressure main it may be impractical or unsafe, and the section is isolated first.

The excavation itself is often the limiting factor. Water escaping into a trench undermines the sides, and working in a flooded trench over a pressurised main is a recognised hazard. Dewatering and shoring come before the repair.

Check the manufacturer's instructions for the specific clamp, confirm it is rated for the main's operating pressure, and have isolation available even if the intention is to fit live.

Evenly and progressively around the clamp to the manufacturer's torque, never one bolt fully at a time.

The gasket seals by being compressed a controlled amount. Tightening one bolt fully pulls the band down on one side and lifts it on the other, so the gasket is over-compressed in one place and loose in another - which is the usual reason a clamp weeps after it is fitted.

Work around the clamp in stages as you would a flange, taking each bolt part way and returning to it, until all reach the specified torque. Use a torque wrench: 'as tight as it will go' over-compresses the gasket, can extrude it out of its seat, and on a plastic or thin-walled pipe can deform the pipe itself.

Check the gap between the clamp lugs is even all round - it is the simplest visual indication that compression is uniform. Re-check the torque after the main is repressurised.

It can be, over a localised defect on otherwise sound pipe - and it is not, over generally deteriorated pipe.

That distinction is the whole judgement. A clamp fitted over a drilled hole, an impact fracture or a single corrosion pit, on a pipe whose wall is otherwise good, is a durable repair and many such clamps stay in service for decades.

A clamp fitted over one hole in a main that is corroding generally is treating a symptom. The pipe will fail again nearby, often within months, and each repair costs another excavation.

The test is what the pipe looks like either side of the defect once it is exposed and cleaned. If the wall is sound and the failure has an identifiable local cause, close the job. If the wall is thin, pitted or flaking, fit the clamp to restore supply and raise the section for replacement. Record which of the two it was.

Stainless steel for anything buried or in aggressive ground, with the gasket matched to the fluid.

A buried clamp is inaccessible and unmonitored, and it sits in the same ground that corroded the pipe it is repairing. Ductile iron and coated steel clamps are available and cheaper, but in wet or aggressive soil the fasteners are the weak point - and a clamp whose bolts have corroded away has failed completely.

Stainless bodies and stainless fasteners are the normal specification for buried water work for that reason. In made ground, contaminated ground or near the coast, treat it as a requirement.

The gasket must suit the fluid and, on potable water, carry the local drinking-water approval. Nitrile and EPDM cover most water and general service, but they are not interchangeable - EPDM is destroyed by petroleum products.

Record the clamp's position on the as-built record. An unrecorded buried repair is found by excavation.

Yes, with clamps designed for it and with more care over tightening.

Plastic pipe deforms under the compressive load a clamp applies. A clamp designed for metallic pipe, tightened to a metallic torque, can flatten a PE or PVC pipe locally - which does not seal, restricts the bore, and introduces a stress concentration that becomes the next failure.

Clamps for plastic pipe use wider bands to spread the load, lower tightening torques, and gasket designs suited to a surface that will yield slightly. Some require a support liner inside the pipe.

PVC deserves particular caution because it is brittle: a clamp over a crack in PVC can propagate the crack as it tightens, and the correct response to a cracked PVC pipe is usually to cut out the section rather than clamp it.

Check the clamp is stated as suitable for the pipe material, and follow the torque for that material rather than the general figure.

A clamp seals a defect in a continuous pipe; a coupling joins two pipe ends.

A repair clamp is closed around an intact but damaged pipe. The pipe is never cut, the run stays continuous, and the clamp contains the pressure over the defect. It is the quicker repair and is used where the pipe is sound apart from a local failure.

A repair coupling joins plain-ended pipes. It is used when a damaged section has been cut out and a new piece inserted, when connecting to an existing main, or where the failure is too extensive to clamp.

The overlap is that some products do both - wide bolted sleeves can seal a defect or bridge a gap depending on how they are used.

Choose by whether the pipe will be cut. If the defect can be covered on an intact pipe, a clamp is faster and disturbs less. If the pipe has to come out, you need couplings and a replacement section.