**1. Introduction**

Thermal insulation is widely used in several application areas, such as in the building industry, refrigeration plants and in the process industries. In the process industries, examples of typical application areas may be temperature control, personnel protection, humidity condensation prevention or sound attenuation. In the hydrocarbon process industry, thermal insulation may be necessary in order to maintain the required production temperature [1]. A typical example may be distillation columns; in order to obtain good production e fficiency and quality of the distilled products, the temperature profiles are carefully designed. These types of process equipment may represent a potential for a major accidental hazard, as it may release large quantities of flammable material upon a potential leak. Ignition of a hydrocarbon leak may lead to severe heat loads to the exposed object, i.e., flame

temperatures in the range of 1100 to 1200 ◦C, corresponding to heat loads in the range of 250 to 350 kW/m<sup>2</sup> [2,3]. These high heat fluxes will result in a significant temperature increase in exposed production units, which may result in weakening of the steel and a possible escalation of the fire scenario, especially if the steel temperature exceeds 500 ◦C [4]. The outcome of a process equipment rupture may be a disastrous release, as evidenced by several major accidents during the last decade [5–7]. Hence, these types of units are also often protected with an additional layer of mineral-based passive fire protection.

The fire resistance of the passive fire protection is normally given as a time in minutes until an object reaches a specified critical temperature. In the oil and gas industry, the critical steel temperature is conservatively set to 400 ◦C, i.e., the protected element should not achieve temperatures above 400 ◦C within the specified time frame [8]. The fire resistance is typically given in intervals of 15, 30, 60 or 120 min [9].

Previous studies of 50 mm thick thermal insulation (ProRox PSM 971, 50 mm, Rockwool) protecting a 16 mm thick steel wall using small scale testing [10,11] rather than full-scale testing [12] demonstrated that the thermal insulation alone was su fficient to withstand 30 min of jet fire exposure. Even with only 3 mm thick steel walls, the testing showed su fficient fire protection for 20 min. During the testing, the thermal insulation sintered and partially melted in some locations. The sintering and melting of the insulation due to the heat exposure resulted in cracks/openings in the insulation mat, as shown in Figure 1b,c. The previous oven testing up to 1100 ◦C [11] showed minimal shrinking (less than 25%) of the thermal insulation. Hence, a further examination of the thermal insulation, explaining the observations in the small-scale jet fire testing [10,11] and determining the breakdown temperature of the insulation are the motivations for the present study.

**Figure 1.** Small-scale jet-fire test setup (**a**), exposed thermal insulation after the jet fire testing (**b**) and melted and sintered remains after high heat flux (350 kW/m2) fire testing (**c**) [10,11].

Sjöström et al. [13] and Olsen et al. [14], performed oven tests of a Rockwool insulation similar to that of Bjørge et al. [11], i.e., recording the temperature in the centre of the insulation. In addition, thermogravimetric analysis (TGA)/di fferential scanning calorimetry (DSC) tests and transient plane source method (TPS) measurements were performed. However, the scope of their studies [13–17] was limited to temperatures associated with building fires, i.e., at temperatures up to 1000 ◦C. Their studies focused on the performance of stone wool in such fires [13,14,16] and an analysis of the properties (measured using TGA/DSC/TPS) within the operating range of that thermal insulation [15,17].

Several numerical models have been developed that calculate the fire resistance of insulated walls or columns [18–20]. In order to account for the breakdown of the thermal insulation during fire exposure, the conductivity has, e.g., been adjusted in order to make the model fit with the performed fire tests. This has in some cases overestimated or underestimated the actual conductivity and breakdown of the insulation. The properties of the thermal insulation (Rockwool) at temperatures above the normal operating temperatures are generally missing in the literature.

The present study aimed at investigating the properties of the thermal insulation after being exposed to temperatures up to the breakdown temperature of the insulation. Cubes of the thermal

insulation (50 mm) were heat-treated in a muffle furnace at different exposure temperatures up to 1200 ◦C. To support the findings from the muffle oven tests and further investigate the properties of the thermal insulation, in-depth analyses of the material were performed. To reveal the mass loss at elevated temperatures, thermogravimetric analysis (TGA) was performed at temperatures up to 1250 ◦C. To examine the melting temperature of the thermal insulation, DSC to 1250 ◦C was performed. The ambient temperature thermal conductivity of test specimens preheated up to 1200 ◦C was measured using TPS.

The materials and methods used are explained in Section 2. Section 3 presents the results from the furnace tests, the TGA and DSC analyses and the results from the TPS measurements. Section 4 presents the discussions and Section 5 presents the overall conclusions and suggestions for future studies.

#### **2. Materials and Methods**

#### *2.1. The Studied Thermal Insulation*

In the present study, industrial-grade pipe section mat (ProRox PSM 971, thickness 50 mm, Trondheim, Norway) delivered by Rockwool, Inc., was studied as a representative industrial thermal insulation, i.e., the same thermal insulation as in previous studies [10,11]. The detailed technical data and thermal conductivity of this thermal insulation up to 350 ◦C are presented in Appendix A, Tables A1 and A2. The maximum service temperature of the studied insulation, as given by the manufacturer, is 700 ◦C, which is well below temperatures associated with fires in the oil and gas industry.Temperaturesabovetheservicetemperaturewerethereforefocusedoninthepresentstudy.

Chemically, the main components of the thermal insulation are inorganic oxides. The thermal insulation mainly consists of silica, alumina, magnesia, calcium oxide and iron (III) oxide. In addition, there are minor amounts of sodium oxide, potassium oxide, titanium oxide and phosphorous pentoxide. The detailed chemical composition, as received from the supplier, is presented in Appendix A, Table A3.

The thermal insulation is produced by melting the raw materials at 1500 ◦C before it is cooled and spun into insulation mats [21]. In addition, a dust binder is added (mineral-based oil) to make the material easier to handle when, e.g., cutting and fitting the insulation mat to equipment requiring thermal insulation. Bakelite, i.e., polyoxybenzylmethylenglycolanhydride (C6H6O · CH2O)x, is also added to give some strength to the thermal insulation up to the maximum service temperature.

As the insulation is heated, the mineral oil will gradually pyrolyse/evaporate. The degradation process of the Bakelite is dependent on the actual production conditions and the degradation process may be complicated [22]. The number of molecular cross-links will influence the degradation processes and there may be several reaction paths. Generally, the degradation of Bakelite may be expressed as the following non-balanced reaction:

$$(\text{C}\_6\text{H}\_6\text{O} \cdot \text{CH}\_2\text{O})\_\text{x} \rightarrow \text{CO}\_2 + \text{CO} + \text{H}\_2\text{O} + \text{C}\_{\text{soot}} + \text{other products.} \tag{1}$$

The number of cross-links, in addition to other components mixed into the Bakelite, will have an impact on the degradation temperatures [22].

## *2.2. Thermal Conductivity*

For materials like, e.g., thermal insulation, the thermal conductivity is limited by the pore radiation. Theoretically, it can be shown that the thermal conductivity (*k*) will be proportional to the absolute temperature to the third power, i.e., *T*<sup>3</sup> [23]. The thermal conductivity of the virgin thermal insulation as a function of absolute temperature is presented in Figure 2. It can be very well described by Equation (2), which is also presented in Figure 2.

$$k = 0.0304 + 3.11 \times 10^{-10} T^3. \tag{2}$$

**Figure 2.** Thermal conductivity of the thermal insulation (ProRox PSM 971, 50 mm) as a function of the absolute temperature. Data from Appendix A, Table A2.

The good fit of Equation (2), i.e., R<sup>2</sup> = 0.9995, with a major contribution of the temperature dependency to the third power, i.e., *T*3, clearly indicates that the thermal conductivity for this particular thermal insulation is indeed limited by pore radiation. However, when the breakdown of the thermal insulation is significant, the thermal conductivity may no longer be limited by pore radiation and is thus assumed to increase.

When exposing inorganic (ceramic) materials to elevated temperatures, sintering of the material may occur, i.e., an entropy-driven [24] physical process leading to a lower free energy, ΔG. To limit sharp edges and optimise the mix of the material, the atoms in contacting threads will diffuse across the thread boundaries. With time, theoretically, the material may approach a solid state, fusing the threads together to leave a minimum remaining pore fraction. The thermal conductivity of the thermal insulation will depend highly on the temperature exposure and the onset of crystallisation, sintering or melting of the insulation. Hence, the sintering effect will increase the thermal conductivity of the thermal insulation [25].

The sintering process may start at temperatures that are approximately two-thirds of the absolute melting temperature for ceramic materials [26]. Hence, porous ceramic materials may be expected to start the sintering process at temperatures well below the actual melting point.

#### *2.3. Furnace Testing up to 1200* ◦*C*

To investigate the thermal insulation dimensional changes and the breakdown temperature, which is defined as a considerable change in physical dimension over a limited temperature range, it was decided that a muffle furnace be used for the heat treatment. In order to minimise any elasticity issues, the thermal insulation test specimens (50 mm cubes) were pre-cut a couple of days before the heat treatment in a muffle furnace (Laboratory Chamber Furnace, Thermconcept GmbH, Bremen, Germany). The maximum temperature of the furnace was 1300 ◦C, i.e., well above the highest temperature (1200 ◦C) of interest in the present study.

One thermocouple (type K, mantel, 1.5 mm diameter, Pentronic AB, Västervik, Sweden) was inserted vertically into the centre of the 50 mm cubic test specimens to record the internal test specimen temperature. To record the furnace temperature, a second thermocouple was placed in the upper part of the furnace. The test specimen was placed on a steel plate and lifted approximately 35 mm above the 15 mm thick bottom plate, as shown in Figure 3, to allow for uniform heating of the specimen. In order to minimise any thermal radiation shadowing effects, only one test specimen was placed in the furnace for each heat exposure test.

**Figure 3.** Test setup in the muffle oven.

The heat treatment of the test specimen was performed for temperatures in the range of 700 to 1200 ◦C, as presented in Table 1. The heating rate of the oven was set to 15 K/min and the test specimens were kept at the respective holding temperatures for 30 min.


**Table 1.** Number of tests at each holding temperature.

After each heat treatment and cooling to below 100 ◦C, the test specimen was carefully removed from the furnace and the length and width were recorded at three locations at each of the four vertical faces. The average width and height were reported for each test specimen.

#### *2.4. Thermogravimetric Analysis and Di*ff*erential Scanning Calorimetry*

To support the results from the furnace testing and to ge<sup>t</sup> more detailed information about the breakdown processes, samples of the thermal insulation were tested in a simultaneous TGA/DSC apparatus (Simultaneous Thermal Analyzer STA 449F3, NETZSCH, Selb, Germany). Prior to the sample preparation, a larger sample was taken from the insulation mat, crushed and mixed well into one large batch, from which each sample was taken. This was done to, as far as possible, even out minor variations in the chemical composition of the different spun layers. The sample mass was approximately 12 mg (±1 mg). The TGA/DSC tests were run at heating rates of 5, 10, 20 and 40 K/min from room temperature to 1250 ◦C. Three tests were run at each heating rate. The tests were conducted in a nitrogen atmosphere to prevent air oxidation.

#### *2.5. Transient Plane Source Thermal Conductivity Measurements*

The thermal conductivity of the virgin thermal insulation was given by the manufacturer, as shown in Table A2. Test specimens for thermal conductivity measurements were also initially 50 mm cubes and were heat-treated in a similar way as previously described. The highest heat treatment temperature for these test specimens was 1200 ◦C. Post heat treatment, TPS [27,28] was used to record the thermal conductivity of each test specimen at room temperature. However, no thermocouple penetrated these test specimens since that would have left a hole when removed and thus disturbed the TPS thermal conductivity measurements.
