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A variety of variables need to be considered when designing an insulation system
Thermal insulation provides many uses in industrial (power and petrochemical) and commercial applications. In this story, we will only discuss industrial applications. In simple terms, thermal insulation reduces heat flow from one surface to another. For hot (above ambient) applications, thermal insulation reduces heat loss. On cold (below ambient) applications, the insulation generally serves to minimize heat gain.
In some cases, the application design purpose may seem unrelated to heat loss or heat gain. However the net result is that heat transfer is reduced. Examples of this are insulation for personnel protection and condensation control (sweating). In personnel protection, enough insulation is provided to keep the surface below a given temperature. For condensation control, enough insulation is provided to keep the surface temperature above the dew point. In both cases, insulation is used to control the surface temperature for a desired effect other than thermal conservation. The effect, however, is that in both cases heat transfer is reduced to maintain the surface temperature at a given design criteria.
Correctly designing and specifying an insulation system is much more involved than just selecting a particular material. An insulation system is any combination of insulation materials used in conjunction with mastics, adhesives, sealants, coatings, membranes, barriers, and/or other accessory products to produce an efficient assembly to reduce heat flow. Frequently, the design of insulation systems can either determine or direct the ultimate performance of the process. Improperly designed insulation systems are subject to damage and degradation. Degradation will compromise the material's performance characteristics, and in many cases the entire process for which the insulation system was designed.
There are many different types of insulation materials available. Each has its own set of properties and performance characteristics. And for each insulation material available, a correct application procedure and corresponding accessory material(s) or "system" application is available. The single most important thing to remember is the word "system." This refers not only to the insulation materials, but also the application and finish.
When asked to supply an insulation specification for a power plant or process plant, several questions must first be considered before design can start. Some examples are:
What are the temperature limits of the items to be insulated?
Where is the plant geographical location and what are the environmental conditions?
What fluids are being insulated?
Why is insulation required?
What type of insulation material should be used?
What type of finish is necessary?
What Are the Temperature Limits?
What are the temperature limits for insulated items? This starts the entire design and material selection. For a power plant, it's usually in a range above 32 degrees Fahrenheit (F) to about 1,200 degrees F. At an ethylene plant, the range is between minus 250 degrees F and 1,200 degrees F. This requires two very different types of design considerations, although the materials and application for the 32 degree F range and 1,200 degree F range could be the same. This also necessitates the need for expansion and contraction joints.
The design of hot service insulation expansion joints and insulation supports are quite important. In steam system design (1,000 degrees F) the piping would expand .095 inches per foot of pipe, and the insulation (calcium silicate or perlite) would contract .024 inches per foot. A total of 5.95 inches of expansion must be accounted for if the pipe length was 50 feet. The pipe expansion must still be accounted for, even though some materials will not contract (such as mineral wools). It's also important to control where the expansion will occur. On vertical piping and equipment, this is done with the use of insulation/expansion supports. Without these, all the expansion will occur at the top.
In cold insulation design, contraction joints are just as important as expansion joints are to hot insulation. If the system has an operating temperature of minus 100 degrees F, the pipe (stainless steel) will contract 0.0176 inch per foot and the insulation, depending on the material, will contract 0.01 inch per foot for cellular glass insulation to 0.102 inch per foot for polyisocyanurate insulation.
Geographic and Environmental Factors
Geographic design considerations depend on plant location. Facilities located in hot and humid climates will have different parameters than those located in a dry, cooler climate. The National Weather Bureau, the American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., U.S. meteorological services, sited data or similar service provides local weather data, which can be used in determining the minimum, maximum and average daily temperatures, wind, humidity and rainfall.
Review of the following parameters should give the necessary design data:
It's important to know if a plant location is near an industrial complex, where potentially corrosive chemicals are present, or near coastal areas, which can affect the selection of insulation and weatherproofing materials along with application procedures. Insulated equipment located near a cooling tower or ash handling equipment will be exposed to a more corrosive environment than will the other plant equipment.
Wind conditions (both positive and negative [back side negative pressure]) must be considered in insulation design. In hot service, the weatherproofing could be supported off of angle irons attached to the vessel or vessel support system. The insulation material could be rigid enough to support the positive pressure of the weatherproofing, but the attachments must be strong enough to resist the negative pressure on the backside. Corrugated metal is usually preferred on vessel sidewalls held in place with stainless steel bands on 18 inch centers and screws in the vertical overlapping seams.
What Fluids Are Being Insulated?
Insulation design for pipe and equipment that handles hazardous chemicals such as flammable or toxic materials requires special consideration in selecting insulation materials, weatherproofing materials and application methods.
Insulation materials that can absorb fluids (such as hot oils/heat transfer fluids) and cause that fluid's flash point to be reduced shouldn't be used in such service. Non-absorbent type insulation materials should be used in these services.
Non-absorbent type insulation materials may also be required for toxic services, where trapping of a toxic substance in the insulation can pose health hazards.
Why Is Insulation Required?
Why is insulation required? Because it's necessary! But the real question is, is it necessary to limit heat loss, for personnel protection, to reduce heat gain, to limit surface condensation, to provide process control or for product stabilization, freeze protection, noise control and fire protection? Each of these may require different thickness, materials, finish and extent of insulation.
Limit Heat Loss or Heat Conservation
Insulation by itself will not maintain or hold temperatures within a system. Insulation can only provide a means to limit, conserve, control, reduce, or minimize the rate of heat flow through a system. But it can't stop the process. Insulation is merely a heat flow reducer, not a barrier to heat flow.
It might be that condensate and blowdown lines to drains or holding tanks may require insulation to limit heat loss, but heat losses through valves and flanges isn't critical to the system; therefore they're not insulated (although personnel protection may be required).
When designing insulation systems for personnel protection, only enough insulation shall be used to reduce the surface temperature to an acceptable limit to prevent individuals from getting burned from the surface. Traditionally, the insulation surface's upper temperature when designing for personnel protection is 140 degrees F. To date, no mandates or statutes govern the upper limit for personnel protection. Refer to "ASTM C1055 - Standard Guide for Heated System Surfaces Conditions That Produce Contact Burn Injuries," for guidance in selecting acceptable temperature limits.
Insulation for personnel protection is generally applied only in those areas accessible to persons during normal plant operation and maintenance, and applied to a high of 7 feet above or 3 feet from platforms or work areas.
In some system designs where there's no justification for insulation, and the insulation could actually be detrimental to the process, fabrication guards may be employed to provide personnel protection.
When insulation is used for personnel protection, it's very important to flash the ends to prevent water or moisture from getting behind the insulation, and to prevent insulation deterioration and surface corrosion. Note that most mastics and sealants could have temperature limits lower than the operating or design temperature of the surface receiving the personnel protection.
In situations where solar loads are high, highly reflective metal jacketing materials reflect much of the radiant heat, thereby creating surfaces that could be too hot to touch. Dull, textured, or painted surfaces tend to absorb more of the radiant heat, creating a surface condition cooler to the touch. Gray coated metal jacketing can reduce insulation thickness for personnel protection by as much as 2 inches. As a general rule, the closer the materials emittance is to 1, the cooler the surface temperature will be.
Wind conditions also influence the selection of insulation for personnel protection. For example, in open areas in coastal regions, there's usually a prevailing wind that can be considered in the insulation design. In this situation, less insulation would be required than in an enclosed space sheltered from the wind.
Reducing Heat Gain on Cold Surfaces
In below ambient applications, the main objective of providing insulation is reducing heat gain and preventing moisture migration or water intake into the system. This type of moisture migration will have a dramatic effect on insulation performance. Cold systems are more subject to degradation from the environment than are hot systems, because of the direction of the vapor driving force. On hot insulation systems, the water vapor's driving force is away from the hot surface, and although the ingress of water into the insulation can adversely affect performance, it's generally considered to be temporary. Conversely, on cold systems, the water vapor's driving force is inward toward the colder surface.
The ingress of water into the insulation will gradually increase with time. The moisture will slowly deteriorate and eventually destroy the system. For this reason, it's extremely important that the total insulation system design be detailed and well-planned, using vapor barrier mastics, vapor barrier stops and low permeability joint sealants.
Usually, the cost of removing Btu's (heat gain) by refrigeration is greater than that of producing process Btu's (heat loss) by heat generating equipment. Therefore, the heat gain in cold processes must be kept to a minimum. The typical rule of thumb is to provide sufficient insulation to maintain an 8-10 Btu·hour/feet2 heat gain to the cold process. The design's ambient temperature and wind conditions must be utilized when calculating the insulation thickness.
In cold insulation system design, vapor barriers and vapor stops are extremely important. Vapor stops, which seal the insulation to the pipe or equipment, should be installed at all insulation protrusions and terminations. These vapor stops will prevent any failure of the insulation system from traveling along the entire system.
Limiting or Controlling Surface Condensation
Insulation systems can be designed to limit or retard condensation, but in most cases they can't be designed to "prevent condensation." In humid regions it's unfeasible to consider designing an insulation system to prevent condensation 100 percent of the time. In these areas, the required thickness of even the most efficient insulation would be unrealistic from both a financial and practical standpoint.
Insulation thickness is determined using ambient conditions and relative humidity, along with the process operating temperature and surface emittance. The insulation system should be designed to keep the surface temperature of the system above the dew point of the ambient air. This will keep condensation from forming on the outer surface of the insulation, avoiding safety hazards and preventing dripping condensate on buildings or electrical equipment. It's essential to agree on what percentage percentage of time condensation is acceptable.
In hot and humid outdoor environments and during rain, it's virtually impossible to prevent condensation 100 percent of the time. If the insulation thickness is designed to allow for an 8-10 Btu·hour/feet2 heat gain, this will be sufficient to prevent condensation the majority of the time.
Providing Process Control
Process control is a critical design parameter in many industrial applications, especially steam and critical process piping and equipment. Providing a stable temperature flow and heat loss throughout a process system in many cases is more important then any other system design.
When designing for process control, other information is also necessary, such determining what heat loss or temperature must be controlled. What's the length of pipe and size of equipment? How is the piping and equipment supported? Are they on insulated shoes, vessel skirts, legs or other components? Also, any protrusions, if any, should also be accounted for in the heat loss.
Freeze protection can be maintained by fluid flowing insulation or by insulation with some form of additional heat input. Insulation alone can't maintain a temperature. It will delay the time required for a fluid to reach a design temperature, but it can't stop it.
In the Gulf Coast region, generally most stagnant water lines in sizes 6 inches and smaller should be heat traced and insulated. Lines between 8 feet and 12 feet need insulation only.
Freeze protection could also refer to prevention of product solidification. In product solidification, most times additional heat input is required to replace the heat loss through the insulation. For example, heavy fuel oil might have to be maintained at 250 degrees F and will require additional heat input to replace the heat loss through the insulation
Environmental acoustic issues can be addressed in thermal insulation system design. However, serious noise problems should be treated as a separate and independent study.
Sound attenuation is a natural by-product of the insulation design. Because of their sound absorption characteristics, some insulation and accessory products provide greater sound attenuation than do others. Mineral fiber products are among the best thermal insulation materials for sound attenuation.
The jacketing material used to cover the insulation can play an important role in sound attenuation. A fabric reinforced mastic finish over insulation has better sound absorption properties than metal jacketing. Metal jacketing may also be purchased with a loaded mass to reduce noise.
As a general rule, insulation materials are better suited as insulation than as a fire protection product. However, the American Petroleum Institute (API) acknowledges conditions under which some insulation materials may provide "credit" in the design and sizing of pressure relief valves. API Recommended Practice 521 states insulation system requirements. Included is a requirement that the finished insulation system will not be dislodged when subjected to the fire-water stream used for fire fighting, either by hand lines or monitor nozzles. Most insulation systems used in fire protection are metal jacketed with stainless steel jackets and bands which meet these criteria.
Physical and Mechanical conditions
Physical and mechanical conditions also play an important part in insulation system design. Indoor applications generally don't require the complexity of outdoor designs. Similarly, below ambient applications are more complex than hot applications. The physical abuse and mechanical conditions that an insulation system is subject to are also important to consider during design.
Rigid insulation is resistant to deformation when subjected to foot traffic. Compressible insulation doesn't offer the same resistance to such loads. Areas that experience loads or repetitive personnel access/use will require a firmer system than inaccessible areas. Piping used as ladders/walkways and riggings hung from pipes and horizontal surfaces subject to vibration/loads are examples where rigid insulation is required. Compressible insulation is required for filling voids and closing gaps in insulation, which allows expansion, contraction, or movement of rigid insulation.
Mechanical abuse should be considered on a case by case basis. Insulated items located in high traffic areas should have a structure such as a platform or similar protection, to avoid having personnel stepping directly on insulation.
There are many types of insulation materials available for industrial application, though there are too many to discuss in detail here. A few of the most common industrial insulations and types will be described. These are:
(fiber glass and mineral wool)
The "Insulation Material Specification Guide" from the National Insulation Association's National Insulation Training Program, which may be obtained by contacting NIA (www.insulation.org), gives a quick comparison of ASTM values for these and other insulation materials.
When comparing material properties, keep in mind that ASTM test methods are usually performed under laboratory conditions and may not accurately represent field conditions, depending on process temperatures, environment and operating conditions.
Calcium silicate insulation is a rigid dense material used for above ambient to 1,200 degree F applications. This has been the industry standard for high temperature applications. It has good compressive strength and is noncombustible.
Cellular glass insulation is also a rigid dense material normally used in the temperature range from minus 450 degrees F to 400 degrees F. It's of a closed cell structure, making it preferred for low temperature application and for use on services where fluid absorption into the insulation could be a problem.
Fibrous Materials (Fiber Glass and Mineral Wool)
Fiber glass and mineral wool are actually two separate and distinct types of insulation. However, many of their applications and physical properties are the same. These products are generally not used where mechanical or physical abuse could occur. It should also be understood that although they may be used in high temperatures, some of their physical and acoustical properties maybe lessened.
Perlite insulation is generally used in the same type of applications as calcium silicate insulation. However, it's somewhat lighter in density and lower in compressive strength then calcium silicate. It's treated with a water inhibitor, preventing the material from absorbing atmospheric moisture during storage and installation.
Polyisocyanurate foam insulation is used in temperature ranges between minus 200 degrees F to 300 degrees F. It has very good thermal properties and is 90 percent close cell. In cold service application it requires multiple layer application because of its contraction characteristics.
The accessory materials used as a part of the insulation is as important as the insulation material itself. If the wrong accessory material is picked, the system will not provide the required performance.
Normally used accessory materials include acrylic latex mastic, aluminum jacketing, stainless steel jacketing, stainless steel bands and screws, hypalon mastic and electrometric joint sealers.
Metal jacketing is preferred to mastic for most outdoor applications because of its durability. Colored jacketing should be used for cold service and personnel protection insulation to reduce surface emittance from 0.01 for new aluminum to 0.8 for colored aluminum, which will reduce insulation thicknesses.
I have discussed several subjects which must be considered when designing an insulation system. I've also tried to show that there's more to designing an insulation system then picking a material and covering it with weatherproof jacketing.
This article appeared in the March 2003
issue of Insulation Outlook.
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George P. Barnett
George P. Barnett is senior advisor in the Mechanical Division for Stone & Webster Inc., a Shaw Group Company, located in Houston. He has more than 30 years experience in the design and application of insulation for both power (fossil and nuclear) and the chemical industries. He is a member of ASTM C16 Thermal Insulation and chairperson of ASTM C16.40 Insulation Systems. He may be reached at (281) 368 3336, or E mail firstname.lastname@example.org or email@example.com.