THERMAL INSULATION
8.1 PROPERTIES OF THERMAL INSULATION .1 TIiermal Performance
Insulation is used to prevent heat loss or gain for process control and it is often necessary for the protected process system to function properly. For example, if a process fluid condenses or freezes or vaporizes in a line, a hazardous condition may exist, such as overpressurization, loss of process control or runaway reaction. For calculating heat transfer rates and determining simple heat loss or gain, guidelines are published (ASTM, Standard C680). Computer programs are available to aid the engineer in selecting the optimum thickness based on a predetermined set of parameters such as energy costs, local usage rates and capital costs. Insulation is also applied to protect workers from injury; however, personnel protection is outside the scope of these Guidelines.
8.1.2 Absorption of Liquids
Absorption of moisture or process liquids can lead to a hazardous condition, such as lowered thermal performance, corrosion under wet insulation or a fire if the absorbed liquid is flammable. Corrosion problems created by moisture absorption are so significant that they are discussed separately in Section 8.3.
Thermal performance is impaired when the insulation material is wet.
Moisture can enter insulation material through a break in the weather barrier, by a leak in steam trace tubing, or by a process leak in the insulated system.
When the air spaces in insulation become filled with water or other liquid, the insulation's conductivity approaches that of the liquid. For example, the conductivity of water at 7O0F is 4.1 Btu in/fr hr compared to 0.17 for air. This makes the transmission of heat across each space approximately 24 times greater when saturated with water as when dry (Malloy 1969).
While some insulation materials can regain thermal performance after being dried out, in others, such as calcium silicate, the moisture may never be completely driven away. Some insulation materials, such as some expanded perlites, may be treated with water repellents. If the liquid absorbed is a chemical product, it can create more problems than loss of thermal perfor- mance. Some chemicals can react with the resins or binders in the insulation to cause degradation. Combustibles and some flammable liquids, such as organic heat transfer fluids and other oils, may be absorbed in porous insula- tion and self-heat to the point of self-ignition (Britton 1991, Britton and Clem 1991). Test methods have been developed that can be used to determine the minimum spontaneous ignition temperature of liquid/insulation combina- tions involving isothermal heating of liquid-soaked cubes of insulation.
Systems that are heat traced with a heat transfer fluid require additional precautions to prevent the fluid from leaking into the insulation. Generally, oversize insulation covers both piping and tracer. Since the tracing lines are hidden by the insulation, leaks may go undetected.
Increased weight of wet insulation systems should be addressed in design of support structures, pipe racks, etc., since some insulation materials can absorb more than twice their dry weight in fluid.
8.1.3 Fire Safety
Fire safety is related to three major properties of insulation:
• combustibility of the insulation itself
• combustibility of absorbed liquids
• integrity during fire
For maximum safety, insulation should be noncombustible, nonabsorptive, and nonmelting. Insulation materials that increase the facility's combustibility
should be avoided. Avoid using plastic foam insulation materials of the polyisocyanurate type. Some plastic foam insulation materials that emit toxic gases when subjected to fire are prohibited in some locations. Insulation materials are tested according to ASTM E-84 for flame spread and smoke development. Insulation systems can be tested according to ASTM E-119 to determine their resistance to a slowly developing fire. The conditions specified in ASTM E-119 may not truly reflect fire exposure from burning highly flammable/combustible liquids, such as hydrocarbons. Other methods to test fire resistance have been developed (Britton and Clem 1991).
Absorption of flammable material creates a fire hazard even when the insulation itself is noncombustible. Spontaneous insulation fires may occur when a combustible liquid leaks into porous insulation and reaches a tempera- ture where runaway self-heating occurs (Britton 1991). The insulation pro- vides a large contact surface for reaction and a lower heat loss environment, where the temperature will rise until autoignition occurs, usually only smol- dering. However, sudden influx of air during efforts to remove the smoldering insulation is often the cause of a fire. Green and Dressel (1989) give an excellent introduction to the problem of heat transfer fluid fires. An option is to install nonabsorbing insulation, such as cellular glass for a short distance on both sides of locations (such as flanges) where leaks are likely to occur. Other options are to provide means to carry away leakage, or to eliminate the source of the leakage.
The abilities to withstand high temperature exposure, combustion and smoke development are desirable qualities in an insulation system. Fire resistant insulation material will not only be fire safe; it will also provide fire protection for the insulated component. In this role, the insulation minimizes the heat transfer to the protected surface and minimizes the potential for failure of the equipment and subsequent release of fuel or hazardous mate- rials. Fire resistance is an alternative to the use of other protective systems such as sprinklers or physical barriers to protect critical systems in the plant.
As used in this chapter, fire protection and fire endurance refer to the ability of the insulation system to protect equipment from an external fire. Fire resistance refers to the ability of the material to resist transfer of heat from a fire to the other side. Resistance is defined by fire resistance ratings (consult NFPA15 and NFPA 251). Insulation used for fire protection is also covered in Chapter 16, Fire Protection.
The fire envelope refers to the area where flame impinges on equipment or structures. API RP 520 and 521 define the "fire exposed area/' In addition to
"fireproofing" the structure in these areas, it is considered appropriate to use fire resistive insulation systems on critical components in these areas, even though they may not contain flammable liquids. Fire protective insulation of electrical and instrumentation cabling can be important, since loss of power
or control signals can result in disablement of emergency response equipment and controls.
Fire resistant thermal insulation may be used to protect vessels, critical equipment, and piping that is subject to exposure to external fire. The insula- tion serves to:
• Lower the rate of heat input and boiling of liquid inside piping and equipment. For nonreactive systems, this allows the use of a smaller pressure relief device and reduces the quantity of any hazardous effluent that might have to be handled and disposed of, and it allows additional time to evacuate the contents.
• Insulate heat-sensitive and/or reactive chemicals from excessive temp- erature rise.
• Protect the structural integrity of vessels and piping by limiting the maximum temperature of the outer wall, for example, the vessel wall in the vapor space, or the outer wall of a double-walled insulated vessel.
8.1.4 Fabrication
Some insulation materials perform well thermally, but are difficult to fabri- cate; they do not form well to the substrate or to adjoining insulation sections, or shrink after application and leave gaps in the system. These gaps cause " hot spots" on the jacketing surface or cold spots on hot process temperature systems. Poor insulation fit-up and the resulting problems can be reduced if the chosen insulation material is fabricated to standard dimensions and is tested for linear shrinkage and dimensional stability at the conditions for which it is being specified. In addition, allowances should be made for the differential expansion between the pipe and the insulation.
Determination of linear shrinkage and dimensional stability for high temp- erature insulation may be conducted using methods given in ASTM C356.
Other ASTM guidelines address fabrication tolerances both in the manu- facturers' shops and at the job site (ASTM C585) and various types and shapes of insulation covers. If the insulation is being fabricated at the job site, a quality assurance program is critical.
8.1.5 Durability
If the insulation does not hold up well in service, the thermal performance and ultimately the safety of the whole system can be affected. Insulation that is crushed or torn may allow a heat flow path or expose the equipment or piping surface to outside elements such as fire, moisture or corrosive atmospheres.
For example, if insulation is damaged on a high temperature line where
cabling or instrument tubing runs in close proximity, the tubing could become overheated and fail.
Insulation is frequently damaged by foot traffic. It is strongly recommended that means of access such as catwalks or manlifts be provided wherever possible to allow maintenance of equipment without damaging the insulation.
Excessive vibration affects both the insulation and the substrate. The sub- strate may also be subjected to wear by some insulation materials. Fiber-type insulation containing short fibers is prone to degradation from excessive vibration. In cellular glass and foam type insulation, wear is controlled by the application of an antiabrasive compound or a layer of fibrous insulation to the inside surface of the foam insulation before it is applied to the substrate.
Antiabrasive coatings are available in sol vent/resin types (good for low to moderate temperatures) and water-base types for use in higher temperatures.
8.2 SELECTION OF INSULATION SYSTEM MATERIALS