Insulation, tracing and coatings — the layers outside the pipe

Several topics on this site have referred to insulation, tracing and coatings in passing — the IH code on a line number, the weight insulation adds in pipe supports, the freezing impulse line in instrument installation, and above all corrosion under insulation.

This is the topic those all point at.

Four reasons to insulate, and they are not the same

Insulation — four different jobs, often confusedSource: General practice; the line number's insulation code says which applies


Heat conservation	Keeping process heat in, so less fuel is burned	Economics — the value of heat saved against the cost of insulation	There is an optimum thickness; more is not automatically better
Personnel protection	Stopping people burning themselves on hot surfaces	Surface temperature limit, typically around 60 °C	A guard or a barrier often does this job better, with no CUI risk
Process control	Holding a line at temperature, or stopping it freezing	Heat loss against the tracing available	Insulation and tracing are designed together, not separately
Cold and cryogenic	Keeping heat out, and stopping condensation or icing	Preventing the outer surface reaching dew point	Needs a vapour barrier, or moisture migrates inward and destroys it
Acoustic	Reducing noise from high-velocity lines and machines	Noise target at a stated distance	Uses a different construction from thermal insulation
Fire protection	Keeping a vessel or structure cool during a fire	Rating in minutes against a hydrocarbon fire curve	A different product entirely — see fireproofing, not insulation

The code on the line number tells you which of these is intended, and it matters — the thickness, the material and even whether insulation is the right answer at all depend on which job it is doing. Insulating for the wrong reason is how a plant ends up with more CUI than it needed.

The insulation code on the line number says which one applies. It matters, because the thickness, the material and even whether insulation is the right answer at all depend on the reason.

Materials and construction

Mineral wool is the general-purpose choice — cheap, easy to fit, and it holds water readily, which is exactly the CUI problem. Calcium silicate takes higher temperatures and is rigid. Cellular glass is closed-cell and does not absorb water, which makes it the usual choice for cold service and for lines where CUI risk is serious. Aerogel is very efficient and very expensive, used where space is tight.

Over the top goes cladding — aluminium or stainless sheet — which is not decoration. It is the weatherproofing, and it is where CUI starts:

laps facing the wrong way, so water runs in rather than off
open ends at terminations, valves and supports
damaged or missing sections nobody has reported
penetrations for supports and nozzles that were never resealed

An elevation of clad pipework with rain falling on it, showing three places water gets past the cladding: a longitudinal seam run along the top of the pipe, a penetration where a support passes through and was never sealed, and an open end where the cladding stops short of a flange leaving the insulation exposed. Below is a section through the layers — cladding, insulation, coating and pipe wall. — The cladding is the weatherproofing, and these are the three ways it is defeated. Note the direction of the argument: cladding keeps water out, but once water is past it, the same cladding is what keeps it in.

Insulated process pipework with bright aluminium cladding banded at intervals, a removable insulation box at a flanged joint, and one section where the cladding has been torn open and the wet mineral wool beneath is exposed to the weather. — Torn cladding with saturated mineral wool behind it. Nothing here looks urgent, and that is the problem — the steel under that wool is warm and wet, and has been for as long as the damage has gone unreported.

Tracing

Insulation slows heat loss. It does not add heat. Where a line must be held above freezing, or above a product’s pour point, it needs tracing underneath the insulation.

Heat tracing — steam against electricSource: IEEE 515 and general practice


How it works	A small steam tube run alongside the pipe, under the insulation	A heating cable run along the pipe, under the insulation
Temperature control	Crude — set by steam pressure, the same along the whole run	Precise — thermostats, or self-regulating cable
Best for	Long runs, high heat duty, sites with plenty of LP steam	Precise temperature holding, remote lines, no steam available
Hazardous areas	No electrical certification needed at all	Certified cable, glands and junction boxes throughout
Running cost	Steam is consumed continuously whether needed or not	Only draws power when the thermostat calls for it
Maintenance	Steam traps — numerous, and a known failure point	Cable degradation, and faults are hard to locate under cladding
How it fails	A failed trap floods or starves the tracer, silently	Cable fails or the breaker trips, silently

The two do the same job by very different means, and the choice usually comes down to what the site already has. What they share is the failure mode: tracing stops working silently, and the first symptom is a line that has already blocked or frozen.

The choice usually comes down to what the site already has — a plant with surplus LP steam traces with steam, one without traces electrically.

Two distinctions worth keeping straight:

Freeze protection keeps a line above 0 °C, or above the point a product waxes or solidifies. It runs when the weather demands it.
Temperature maintenance holds a line at a specific process temperature. It runs continuously and needs proper control.

Cold and cryogenic is a different problem

On a hot line, moisture tends to be driven outward. On a cold line the gradient reverses: water vapour migrates inward toward the cold surface, where it condenses and freezes.

Without a sealed vapour barrier on the warm side, insulation on a cold line gradually fills with ice. It loses its performance, and the pipe beneath it corrodes.

That is why cold insulation uses closed-cell materials, why the vapour barrier is continuous and sealed at every joint and penetration, and why a single unsealed penetration can ruin a whole section.

Coatings

Steel corrodes, so it is painted. ISO 12944 classifies the environment from C1 indoors to CX offshore, and the system follows from that — typically a zinc-rich primer, an epoxy intermediate and a polyurethane topcoat.

Galvanising — hot dip zinc — is used for handrails, gratings and small steelwork. It is sacrificial, so minor damage still leaves the surrounding steel protected.

And under insulation, the coating matters more than anywhere else, because it is the last defence once water gets in. Coating specifications for insulated lines are deliberately more demanding than for exposed ones — which is the opposite of what intuition suggests.

What to take away

Four reasons to insulate, and the line number says which. They need different answers.
Do not insulate unless there is a reason. A guard beats insulation for personnel protection.
Cladding is weatherproofing, and its laps, ends and penetrations are where CUI starts.
Tracing adds heat; insulation only slows loss. They are designed together.
Tracing fails silently. The first symptom is the line that stopped flowing.
Cold insulation needs a sealed vapour barrier, because moisture moves inward.
Surface preparation is most of a coating’s life, and it looks fine on handover either way.