Because there is no continuous flow path through a single tube from a launch point, and the geometry is completely different.

A pipeline is one long continuous bore, and a pig is pushed along it by the product behind it over a long distance. A heat exchanger is hundreds of short parallel tubes running between two tube sheets, each only a few metres long, all fed from a common header. There is nowhere to launch a device into one specific tube under flow, and no way to keep it in that tube rather than another.

So exchanger tubes are cleaned individually and mechanically. The operator works across the tube sheet, entering each tube from the face with a gun or rod, running the head through and back, and moving on - which is why the job is labour-intensive and why marking off cleaned tubes matters.

The exchanger must be opened and drained first, so this is shutdown work rather than an on-line operation.

By the deposit first and the tube material second, and the second constraint frequently overrides the first.

Soft deposits - sludge, silt, loose biofilm - come out with nylon or polypropylene brushes and buffers. Moderate scale and mud need brass or stainless brushes. Hard scale, cement-like deposits and heavy corrosion products need scrapers or hardened cutters.

But the tube material sets the limit. Thin-walled copper, copper-nickel, titanium and stainless condenser tubes are easily scored or holed, and a steel cutter in a thin copper tube will do real damage. Tube wall thickness may also already be reduced by service.

Start with the least aggressive head that has any prospect of working and escalate only if it fails. Test on a few tubes and inspect before committing to the whole bundle. A holed tube means shell-side contamination and an outage far longer than the cleaning would have taken.

How the cleaning head is driven, and therefore what each suits.

A tube cleaning gun feeds a flexible shaft into the tube driven by water or air pressure, pushing the head through and flushing debris ahead of it. It is quick and works well on straight tubes with moderate deposits.

A rotary rod system drives a rotating flexible shaft with a brush or cutter on the end, so the head cuts as well as scrapes. It handles harder deposits and can work through longer or slightly curved tubes, but is slower per tube.

A shooter propels a projectile - a plug, brush or scraper - hydraulically through the tube at speed. It is very fast across a large bundle and suits softer deposits, with the debris and the projectile ejected at the far end.

Many jobs use more than one: a shooter for the bulk of the bundle and rods for the tubes that resist.

By measuring the actual internal diameter, remembering that a fouled tube is smaller than a clean one.

Cleaning heads are sized to the tube bore with a small interference so they scrape the wall. Too small and the head passes through without touching the deposit; too large and it jams, which on a rotary system can break the shaft or leave the head stuck in the tube.

The complication is that the tube you are cleaning has a reduced bore because of the deposit. A head sized to the clean bore may not enter at all, and the usual approach is to make a first pass with a smaller head to open the tube, then a second pass at full size to finish it.

Get the tube specification from the exchanger data sheet, and check it physically on a sample - retubed exchangers frequently no longer match their original documentation.

By marking the tube sheet as you go, because a bundle of several hundred identical tube ends is impossible to track from memory.

The standard method is a tube sheet layout drawing or a marking system on the face itself - chalk, crayon or a magnetic marker moved across the pattern - so each tube is marked as it is completed. Work systematically row by row rather than at random.

Missing tubes is the common failure. An exchanger returned to service with a proportion of its tubes still fouled underperforms, and the shortfall is blamed on the cleaning method rather than on coverage.

Record anything abnormal at the same time: tubes that would not clear, tubes that felt soft or rough, tubes already plugged. That record is what informs the next inspection and helps decide when the bundle needs retubing rather than cleaning.

Eye and face protection, control of the ejected debris, and proper isolation of the exchanger - and the hazards differ from ordinary maintenance work.

Debris leaves the far end of the tube at speed, along with water. Anyone at the opposite tube sheet is in the line of fire, so that end must be controlled with a shield or a catch arrangement and kept clear of people. Full face protection is needed at the working end too, because material blows back.

The deposit itself may be hazardous. Process residues, biological growth in cooling water including legionella risk, and heavy metals in some scales all require assessment before the exchanger is opened, and may require respiratory protection and controlled disposal.

The exchanger must be isolated, drained, vented and where necessary certified before opening, and confined space procedures apply to large water boxes. High-pressure water systems carry their own injection injury risk and are a separate competence.

Clean while the tube wall is sound and the deposit is the problem; retube when the wall itself is failing.

Cleaning restores heat transfer and flow by removing what has built up inside. It does nothing for a tube that is thinning, pitted, cracked or already leaking, and aggressive cleaning of a degraded tube can hole it.

The indicators for retubing rather than cleaning are a rising count of plugged tubes at each outage, leakage between shell and tube side, wall thickness measurements below the retirement limit, or cleaning that no longer restores performance for long.

Eddy current or ultrasonic inspection during a cleaning outage gives the wall thickness data to make that decision on evidence rather than on the plugged-tube count alone.

The economics usually favour cleaning for a long time - retubing is expensive and takes an extended outage - but the point comes where continued cleaning is deferring an inevitable job at increasing risk.