The Fundamentals of Fire-Protective Insulation
In discussing fire as it relates to insulation, the conversation frequently focuses on cementitious fireproofing. Cementitious fireproofing, as its name implies, is a cement-like material that is applied to structures, both building and chemical process equipment, to delay the damaging affects of elevated temperature while the fire is controlled. The term "fireproof" is a misnomer in this case, since it implies that the material renders the protected structure immune from the affects of fire. In reality these materials are applied at a specific thickness designed to provide protection for a specific length of time against a specific type of fire. If the fire is not controlled before the design period is exceeded, it is likely that the structure will fail due to weakening of the steel as its temperature rises. Conventional thermal insulation materials such as fiberglass, mineral fiber or calcium silicate do not generally have a role in protecting support structures in a chemical plant, but they do have a critical role in the protection of chemical process equipment during a fire.
Let’s consider what happens during a fire by looking at a hypothetical design example. The XYZ Chemical Company plans to manufacture a highly flammable solvent in a conventional chemical plant that will consist of feed tanks, a pressurized reactor, a distillation column and a product storage tank. Some of the process runs under pressure and above ambient temperature (225 F) and includes equipment that is constructed of carbon steel, stainless steel and copper. Cementitious fireproofing to a level 30 feet above grade protects the structural steel of the enclosed process building. XYZ’s usual insulation standards call for the use of rigid polymer foam to insulate the elevated-temperature equipment and no insulation on the ambient-temperature equipment. They normally use aluminum jacket on insulated equipment in non-flammable service. In this case, because the product to be made is highly flammable, the insulation design must be changed, and XYZ must use their fire-protective insulation standard.
Consider what would happen if the pump that transfers crude product from the reactor to the distillation column failed catastrophically, releasing a large quantity of flammable material into a pool surrounding the reactor and distillation column. If this pool ignites, an intense fire would occur at the base of pressurized process equipment that contains more fuel. While it is likely this plant would be protected by a sprinkler system, let’s assume for the sake of argument that it does not function properly. The only barrier between the surface of the process vessels and the fire is the thermal insulation; so it is critical that it remains in place for as long as possible. If it doesn’t, the pressure inside the vessels will rise along with the metal temperature, which at the least will cause the vessel to relieve and at worst may lead to failure of the vessel.
The jacket and accessories that hold the jacket in place are the first line of defense. Aluminum melts at around 650 C, well below the temperature of the fire; the usual aluminum jacket would melt quickly, exposing the underlying insulation material to the fire. The rigid foam insulation used in XYZ’s usual standard is secured by an adhesive tape that would also quickly fail when exposed to fire. When the fire company arrives to fight the fire, the first thing it does is hit the insulated vessels with a hose stream. Since the tape has failed, the hose stream will blow the insulation off the vessel, and XYZ would have an uninsulated vessel exposed to the fire.
The fire-protective insulation standard is designed to keep the insulation in place long enough for the fire to be put out in most cases. Instead of aluminum or some other low-melting-point jacket material, a metal with a higher melting point and better elevated-temperature strength is used. Often a stainless steel jacket is specified because it retains useful strength at high temperatures for a long period of time. Likewise, stainless steel bands are specified because of their superior elevated-temperature strength. By using a stainless jacket and bands, we keep the insulation in place, even when a fire hose is trained on the vessel.
The fire-protective standard must also consider the insulation material itself. The fire performance of insulation materials is measured by the ASTM E84, "Standard Test Method for Surface Burning Characteristics of Building Materials." This test exposes the insulation material to a specific type of fire and measures the rate of flame spread and the amount of smoke developed. In this case XYZ’s normally specified high-density rigid foam has an ASTM E84 flame and smoke spread rating of 15/550, which indicates this insulation will generate a large amount of smoke in a fire. In order to reduce the risk of smoke generation in the fire, the XYZ fire-protective standard calls for mineral fiber insulation instead of rigid foam because mineral fiber’s E84 flame and smoke rating is 10/0, substantially better than the foam material.
The fire-resistive insulation system thus consists of mineral fiber insulation that has a low flame and smoke spread rating held in place by stainless steel bands and jacketing. This combination has a better chance of remaining in place during a fire than the standard foam and aluminum system that XYZ would normally use. The materials selected in this example are typical of many chemical process fire-resistive insulation designs, but they are not the only materials that could have been used. Many other materials can be used in a fire-resistive design. Consult an insulation professional for advice on your specific case.
The fire-resistive insulation case also affects the extent of insulation–in other words, it affects what is insulated. In our example we stated that some of the process equipment operates at ambient temperature inside an enclosed building and would not normally require insulation. But, because of the flammability of the product, insulation is applied to all of the equipment in this case in order to protect it during a fire.
Another benefit of fire-protective insulation is its influence on relief device sizing. All pressure-containing vessels must be designed with over-pressure protection to prevent the uncontrolled rupture of the vessel in the event of uncontrolled pressurization. The temperature rise caused by a fire is a common scenario for over-pressurizing a vessel, and the pressure-relief device is designed with this in mind. The characteristics of the temperature rise are used in determining the size of the relief device that must be used. When using fire-resistive insulation, the characteristics of the temperature rise are moderated, and a smaller relief device can be used. There are American Petroleum Institute and National Fire Prevention Association standards that provide detailed information on how to take credit for the presence of insulation when designing relief systems.