Removable Insulation Blankets are made up of a wide range of different material components: The inner and outer covers, the actual insulation material, and the fastening system. Since each material component has its own unique characteristics and temperature rating, it can be rather unclear as to what is the highest operating temperature of an insulation blanket.
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One might think that while the temperature of the component being wrapped is below the temperature rating of the insulation material and inner liner, one is in the safe operating range for the blanket. However, one must ensure that both the outer cover and the fastening system, with lower temperature ratings compared to other blanket components, are sufficiently insulated from the heat source in order not to exceed their highest temperature rating.
Outer Cover
As stated above, removable insulation blankets are typically made up of 3 layers as shown in the below diagram. Both the inner surface (hot face) that directly comes into contact with the hot component and the insulation material that rests on it should be able to withstand the maximum heat flow temperature of the component being wrapped. The outer protective cover (cold face) can be rated at a lower temperature, because it is protected by the insulation. The outer cover also contains some kind of fastening system, typically stainless steel lacing wire (but additional options are also prevalent).
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Since the outer cover typically has the lowest temperature rating of the 3 blanket layers (a traditional blanket outer cover of silicone impregnated fiberglass is rated to 500 °F), it is very important that the insulation material must be adequately effective to lower the temperature that reaches the outer cover to safe operating levels. Ineffective insulation, or high temperatures, can increase the temperatures of the outer cover beyond the set limit.
In most of the cases, the temperature of the insulated component is not sufficiently hot enough to cause the outer surface of the insulation cover to go beyond its highest operating temperature.
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“For 1 inch thick insulation, or even ½” thick, you would need extreme high temperatures of beyond 2000 °F for the outer blanket to reach a temperature beyond 500 °F, which is the temperature limit for our standard silicone impregnated fiberglass cover”, said Brett Herman, Firwin’s Vice-President of Sales & Customer Service.
Factors Affecting Blanket Performance
Besides the insulation material and heat flow temperature, there are various other factors that can influence the performance of an insulation blanket and limit its overall temperature rating:
Fastening System
Although Firwin’s removable insulation blankets use stainless steel rivets and lacing wire, which is rated to 1200 °F, as standard fastening system, other fastening systems that may have a lower temperature rating compared to the outer cover are also available. In particular, Velcro-type fastening system can begin to break down at 200 °F (there are also higher temperature versions available). This would give a lower temperature rating to an insulation blanket than it would otherwise have had with a more powerful fastening system.
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Air Clearance
A heat buildup around the outer surface of the blanket can be caused due to a lack of air space around the insulation blankets. This could potentially take the surface temperature to beyond the temperature limit.
Contact of Other Items
When the blanket’s outer surface come into contact with other items, even though not hot, it could prevent heat from leaving the blanket and cause a raise in temperature at the blanket’s outer surface.
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Overlapping Blankets
If an insulation blanket’s outer cover overlaps with another insulation blanket next to it, or if the blanket is not properly installed so that the outer cover makes contact with the hot insulation material of the blanket, there will be an increase in outer cover temperature.
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Butting Up Against Hot Component
When the insulation blanket but up against a hot component, for example, a flange, it would obviously increase the outer cover temperature.
Gap Between Blankets
Insulation blankets that are improperly installed, with a gap between blanket sections, can experience higher surface temperatures than normal when the hot air rises up between the blanket sections.
“Because of these factors, we like to play it safe and give our removable insulation blankets a temperature rating somewhat lower than the maximum it can, in theory, handle”, notes Brett. “We also need to be able to account for potential spikes in engine temperatures, which could take the exhaust temperature beyond the assumed specification.”
A case in point: “We once had a client who was running his engine with our insulation blankets. When he turned off his engine, it also disabled the fan. All of a sudden there was hot stagnant air, which shot up the ambient temperature and consequently the surface temperature of the blankets. So we are careful to always leave a nice buffer when we rate the maximum temperature of our insulation blankets.”
High Temperature Outer Fabric
Certainly, Firwin blankets can be made with high temperature outer fabrics, which would prevent most of these problems. Firwin HT is rated to 1300 °F, while Silica can reach as high as 1800 °F. The problem with these high temperature materials is that they are more expensive than the traditional insulation blankets covers; and they are non-coated fabrics, hence they lack some qualities, like such as oil resistance, that the traditional outer fabrics have (e.g. Teflon coated fiberglass / silicone impregnated fiberglass).
Certainly, just by changing the outer fabric alone would not necessarily address all the problems of a high temperature application. “With really intense heat, insulation blankets need more detailed design, as environments can vary and many factors can come into play”, notes Brett.
This information has been sourced, reviewed and adapted from materials provided by Firwin Corporation.
For more information on this source, please visit Firwin Corporation.