Industrial and high-temperature insulation · Stone wool
ROCKWOOL ProRox PS
- Max. temp.
- 650 °C
- Density
- 120–150 kg/m³
- Thermal conductivity (λ)
- 0.037–0.038 W/m·K (50 °C)
What is personnel protection insulation and surface temperature?
In short
Personnel protection means insulating a hot pipe or piece of equipment so that the outer surface temperature is low enough that someone who touches it by accident is not burned. Saving energy is not the main aim. The surface temperature you get depends on insulation thickness, the surrounding air temperature and wind, and the emissivity of the jacket. The limit itself — how many degrees is acceptable — comes from the project specification or the plant owner. Thickness therefore has to be calculated; there is no ready-made value that fits every job.
Why limit surface temperature?
Steam pipes, hot-oil lines, furnace walls and hot tanks have surfaces hot enough to burn skin in a fraction of a second. Where people walk past, climb ladders or carry out maintenance, the surfaces within reach have to be made cooler. A warning sign alone does not remove the risk.
This aim differs from insulating to save energy or to hold a process temperature. The same pipe may end up with a different thickness under each aim — see how to choose insulation thickness.
Where does the limit come from?
ASTM C1055 is a guide that describes a process for determining acceptable surface conditions for heated systems. It is not a table of ready-made numbers. The points worth knowing:
- The maximum acceptable surface temperature depends on three things: the likely contact time, the surface system (the material and build-up of the surface touched) and the level of injury considered acceptable
- For industrial situations the guide uses a 5-second contact time for design; for consumer products 60 seconds is proposed, because children, the elderly or the infirm react more slowly
- The maximum injury level it recommends is a first-degree burn on the average person
- The guide does not define which temperature is "safe". Designers, regulators or owners set their own criteria
- It covers direct contact with hot surfaces only — not radiant heat, hot surrounding air or very cold surfaces
In practice, then, the limit is written in the project specification, company standards or local regulation. An article from the US National Insulation Association (NIA) mentions a commonly used figure of about 60 °C (140 °F), and its author suggests a lower value, about 54 °C (130 °F), when the jacket is metal, because metal passes heat to skin quickly. These figures are examples from that article; they cannot stand in for your project's requirement.
The surface material matters. A metal surface at the same temperature as a fabric or plastic one feels hotter and burns sooner, because it conducts heat into skin more readily. That is why ASTM C1055 asks for the surface system to be considered.
What does surface temperature depend on?
The outside of the insulation warms up until the heat coming through the insulation equals the heat the surface gives off to the air by convection and radiation. Anything that shifts this balance changes the surface temperature.
| Factor | Effect on outer surface temperature |
|---|---|
| --- | --- |
| Insulation thickness | Thicker → cooler surface |
| Thermal conductivity of the insulation at operating temperature | Lower → cooler surface (λ rises as the insulation gets hotter) |
| Pipe or equipment temperature | Hotter → hotter surface |
| Surrounding air temperature | Warmer air → hotter surface |
| Wind | Still air → hotter surface than with wind |
| Jacket emissivity | Shiny (low) → hotter than dull (high) |
| Pipe size and shape | Affects the calculation; a small pipe and a flat wall give different results |
Why a shiny aluminium jacket runs hotter
A bright metal jacket has low emissivity, so it gives off little heat by radiation and the surface has to run hotter to shed the same amount of heat. An NIA article gives an example of one hot pipe with the same insulation thickness: with an oxidised aluminium jacket the surface was about 53 °C, while with a high-emissivity jacket such as PVC or coated fabric it fell to about 40 °C — at the cost of higher heat loss.
It follows that:
- If the main aim is personnel protection, a high-emissivity jacket allows thinner insulation
- If the aim is saving energy, a shiny jacket loses less heat
- The calculation must use the emissivity of the jacket actually to be installed, not an assumed value
Thickness has to be calculated
Designers calculate surface temperature with standard methods such as ASTM C680 or ISO 12241 and adjust thickness until the surface stays within the limit. The NIA article recommends designing for the worst case:
- the system's maximum operating temperature
- the highest air temperature expected, indoors and outdoors
- still air
Information to prepare for the person doing the calculation: pipe or equipment temperature, pipe size or surface dimensions, location (indoors or outdoors), maximum air temperature, jacket type, and the surface-temperature limit in the project specification.
Thailand is hot and some parts of a plant have little air movement, so the air temperature used should be that of the real site, not a value from another country's tables.
Which insulation is used?
Any hot-service insulation can serve this aim. What matters is that its service temperature covers the surface temperature and that its form suits the surface.
- Pipes: pre-formed stone-wool pipe sections such as ROCKWOOL ProRox PS 960 SA
- Large pipes, vessels and curved surfaces: wired mats such as ROCKWOOL ProRox WM
- Flat equipment walls: stone-wool slabs such as ROCKWOOL ProRox SL
Each product's data sheet gives thermal conductivity at several temperatures, which the calculation needs. For choosing the form, see stone wool slab, blanket, wired mat or pipe section.
Some places should not be insulated — surfaces that must give off heat, or spots where insulation would raise the risk of corrosion under insulation. There the designer may use guards or cages to prevent contact instead, an option API RP 583 mentions.
Cautions
- This article explains principles. It does not give a "safe" temperature or a thickness you can use directly. The limit and the thickness must be set by an engineer to the project specification and applicable law
- Insulation that is wet or crushed, or damaged jacketing, makes the surface hotter than calculated. Inspect and repair it regularly
- Measuring temperature and working near hot surfaces must follow the plant's safety procedures
Sources
- ASTM C1055 — Standard Guide for Heated System Surface Conditions that Produce Contact Burn Injuries (scope; significance and use)
- National Insulation Association (NIA), Insulation Outlook — A Burning Question (December 2002)
- National Insulation Association (NIA), Insulation Outlook — Industrial Insulation Jacketing
- American Petroleum Institute — API RP 583 Corrosion Under Insulation and Fireproofing (contents: personnel protective cage)
- ASTM C680 — Standard Practice for Estimate of the Heat Gain or Loss and the Surface Temperatures of Insulated Systems (title and scope via search results)
- ROCKWOOL Thailand — ProRox PS 960 SA, WM and SL Product Data Sheets (08/25)
Related products
Categories and example products for this topic
- ROCKWOOL ProRox PS — Pipe sections for hot pipes to 650 °C, with λ at several temperatures for calculation
- ROCKWOOL ProRox WM — Wired mats for large pipes, vessels and curves: 650 / 650 / 750 °C by grade
- ROCKWOOL ProRox SL — Slabs for flat walls of hot equipment: six grades, 450–850 °C by grade
Industrial and high-temperature insulation · Stone wool
ROCKWOOL ProRox WM
- Max. temp.
- 650–750 °C
- Density
- 80–128 kg/m³
- Thermal conductivity (λ)
- 0.037–0.038 W/m·K (50 °C)
Industrial and high-temperature insulation · Stone wool
ROCKWOOL ProRox SL
- Max. temp.
- 450–850 °C
- Density
- 60–160 kg/m³
- Thickness
- 50–100 mm