From soft foam through to steel-bodied tools, chosen by what the pig has to do and by how confident you are about the line's condition.

Foam pigs are cheapest, compressible and very forgiving - they negotiate bends, tolerate diameter changes and squeeze past minor obstructions, which makes them the right first choice in an unknown or long-neglected line. Solid cast polyurethane pigs give better wiping and last longer. Disc and cup pigs use replaceable sealing elements on a mandrel and give a strong seal for product recovery. Brush pigs carry wire brushes for hard scale. Gauging pigs carry a soft aluminium plate that deforms if the bore is restricted, proving the line before anything more expensive is sent.

The practical rule is to start soft. Sending an aggressive pig into an unsurveyed line risks jamming it, and a stuck pig is a serious and expensive problem to locate and remove.

They are the chambers that let a pig be put into and taken out of a pressurised pipeline without opening it.

A launcher is an oversized barrel connected to the line through a valve, with a quick-opening closure at the far end. The pig is loaded into the barrel with the line valve shut, the closure is sealed, the barrel is pressurised and vented to match the line, and the valve is opened so product flow pushes the pig into the pipeline.

A receiver reverses that at the other end: the pig arrives into a barrel, is isolated from the line, depressurised and drained, and then removed.

Without them, pigging means breaking into the pipeline each time - draining, opening, inserting, reassembling and recommissioning - which is so disruptive that regular pigging stops happening. Permanent launchers and receivers are what turn pigging from an operation into a routine.

No - the line has to be piggable, and that is a design property rather than something you can assume.

A piggable line needs a consistent full bore throughout, valves that open to full bore, bend radii within the pig's capability, and no unbarred tees or branches where a pig can turn into a side connection and jam. Sudden diameter changes, partial-bore valves, mitred bends and protruding gaskets or weld beads will all stop a pig.

Retrofitting these features to an existing line is expensive and sometimes impractical, which is why pigging is best designed in from the start on any line where it is expected.

Before pigging an unsurveyed line, run a gauging pig to prove the bore and review the line drawings for tees, valve types and bends. A stuck pig can require locating equipment and cutting the pipeline open, and that cost dwarfs the survey.

Using a pig to push the remaining contents of a line out to their destination rather than flushing them away.

At the end of a batch or transfer, the pipeline is full of product. Without pigging that volume is either flushed to drain, which loses it and creates effluent, or left in the line, which contaminates the next batch. Sending a pig through behind the last of the product sweeps almost all of it forward into the receiving vessel.

On a long line or an expensive fluid, the recovered volume can pay for the pigging system quickly. It also reduces the cleaning and effluent burden, which is increasingly the larger saving.

In food, dairy, brewing, paint and chemical plants the driver is usually changeover rather than recovery: pigging the line between products removes the cross-contamination risk and cuts the cleaning cycle between batches, which buys production time.

With detectors at fixed points, and with transmitters on the pig for longer or more critical runs.

The simplest arrangement is a mechanical or magnetic pig signaller mounted on the pipeline that trips as the pig passes, confirming it reached that point. Several along the route give a picture of progress and quickly narrow down where a pig has stopped.

On long pipelines the pig can carry a low-frequency transmitter that can be picked up above ground, so its position can be located precisely if it stops.

The essential discipline is confirming arrival. A pig that is launched and not received is still in the line, and starting the next operation without knowing that is how a bad situation becomes worse. Count them out and count them in, record it, and investigate a missing pig before doing anything else with the line.

As often as the deposit rate requires, established by observation rather than by a general rule.

The interval depends entirely on the product and the line - wax deposition in a crude line, scale in a water main, biofilm in a process line and product residue in a food plant all accumulate at very different rates.

The practical approach is to start with a conservative interval, weigh or examine what each pig brings out, and extend or shorten the interval based on that evidence. A pig that arrives with very little suggests the interval can be longer; a pig that struggles or arrives heavily laden suggests it should be shorter.

Watch for the interval being stretched too far. A line left unpigged past the point where deposits harden becomes progressively harder to clean, and eventually the routine pig will no longer do it - at which point the recovery job is far more expensive than the maintenance would have been.

Yes, widely - the technique is associated with oil and gas but is used far beyond it.

Water utilities pig and swab mains to remove sediment, biofilm and manganese deposits that cause discolouration complaints, often as a cheaper alternative to flushing. Food, dairy, brewing and beverage plants pig product lines for recovery and changeover. Paint, ink, cosmetics and chemical manufacturers do the same, where the product is expensive and cross-contamination is unacceptable. District heating and industrial cooling systems pig to restore bore lost to scale.

The pigs differ accordingly - food-grade materials and designs that can be cleaned and inspected are standard in hygienic applications, and swabbing a water main uses soft foam rather than an aggressive tool.

What is common to all of them is that the pipeline has to be piggable, and that is decided when it is designed.