Recent laboratory test results indicate that a properly applied, high-solids-content-butyl sealant has the ability to allow for contraction or compression in a cold/cryogenic piping system as the process pipe cools from an ambient of about 75°F to the operating temperature.
These lab test implications could be significant for insulation contractors. The tests show that joints using only high-solids-content butyl sealant can be used to effectively replace conventional contraction joints in cryogenic piping systems. By using high-solids-content butyl sealant in insulation joints, the need for conventional contraction joints in operating temperatures to minus 150°F (ethylene service) can be eliminated entirely. And, in the cases of liquid natural gas, liquid oxygen and liquid nitrogen service, the number of required contraction joints could be cut in half. The results are decreased installation time and costs, fewer built-in failure points that contraction joints represent in a piping system and an overall improvement in operating system integrity.
In this paper, dimensional stability of insulation materials, the use and associated problems of conventional contraction joints and laboratory testing procedures, results and observations will be discussed.
The Importance of Dimensional Stability
Dimensional stability is a highly desirable attribute in an insulation material, particularly in cold service applications. When cryogenic systems are shut down and allowed to warm to perform maintenance, the piping system and its insulation system expand. Then, as the operating system is cooled again, the piping system and its insulation system contract. This expansion and contraction causes additional stress to the pipes and the insulation, and can cause serious damage to both systems.
Inorganic cellular glass insulation has a thermal expansion coefficient of between 3 to 5 x 10-6/degree F (5.4 to 9 x 10-6/degree K) depending upon the operating temperature range. The lower values apply to the colder or cryogenic temperature range, while the higher values apply to the above ambient temperature range. Additionally, relative to other insulation materials, cellular glass insulation has a coefficient of thermal expansion much closer to that of austenitic stainless steel (8.6 to 9.1 x 10-6/degree F or 15 to 16 x 10-6/degree F, per ASME). Thus, cellular glass insulation maintains excellent dimensional stability characteristics even under the influence of temperature and/or humidity.
If we consider a 100 foot length of pipe operating at minus 150°F (minus 101°C), the differential thermal expansion between the cellular glass and the austenitic stainless steel is about 1.2 inches, while for other materials this could be about 8 inches or greater. Because other insulation materials may have a higher thermal expansion coefficient, much greater thermal movements or contractions may result. Therefore, before considering the use of the special techniques discussed in this paper with any other insulation material, contact the insulation manufacturer.
Conventional Contraction Joints
To accommodate anticipated insulation material contraction in a cryogenic environment, effective flexible or elastic insulation joints are required. Conventional contraction joints include a cushioning layer of man-made mineral fiber (as defined by ASTM-C168) inserted between the insulation joints and cold process sealants applied between insulation layers and finished with a protective insulation jacketing (see Figure 1). This is a costly and labor-intensive technique.
These contraction joints must be properly fabricated to avoid water or water vapor intrusion, which can condense as a liquid or form as ice, into the insulation system. The problems that can result when moisture enters an insulation system are have been well documented. And, in fact, lost thermal efficiency due to moisture absorption is the single most common cause of insulation failure.
Additionally, when a system isn’t operating at its proper temperature, process control and product quality are compromised. Simultaneously, as invasive moisture causes production costs to go up and product quality to go down, the facility’s infrastructure itself can fall prey to corrosion.
In a typical pipeline transporting ethylene ( minus 150°F), design engineers normally specify that conventional contraction joints for cellular glass insulation be spaced approximately 20 feet to 40 feet apart for every 100 linear feet of piping, depending upon the pipe diameter and weight. At colder temperatures, such as a liquid natural gas facility (minus 260°F), typical specifications call for conventional contraction joints to be spaced every 20 feet to 25 feet apart for every 100 feet of piping, again depending upon the pipe diameter and weight.
However, conventional contraction joints also are built-in points of potential insulation system failure. The problems associated with contraction joints include:
- the sealant losing elasticity and becoming brittle
- loss of seal between the fabric and the insulation
- rips occurring in the flexible barrier
- tears occurring in the fabric jacketing
- condensation and/or ice problems
Moreover, it’s the seal between the flexible man-made mineral fiber cushioning material and the insulation material that’s the weakest link in a conventional contraction joint. Failed joints, in turn, can lead to moisture penetration, insulation system failure, and ultimately impact the operating system. Theoretically, reducing the number of contraction joints in a cold processing pipeline system-or eliminating them altogether-will substantially reduce the risk of insulation system failure and lower the labor costs associated with contraction joints.
Laboratory Experiments and Observation
Pittsburgh Corning Corp. has researched the problems associated with cold temperature system contraction joints. The company has conducted several internal laboratory tests aimed at reducing the number of contraction joints in a cellular glass insulation system by relying on standard sealed insulation joints to accommodate or supplement the thermal contraction. Sealants are used to prevent water vapor entry into systems on low temperature, intermediate temperature and cyclic applications. A high-solids-content butyl sealant properly applied to the outer layer of cellular glass insulation allows room for contraction to take place within the outer layer of insulation. The inner layer uses a dry joint. (See Figure 2 and Figure 3.)
As illustrated in Figure 3, the high-solids-content butyl sealant is used only on the outer layer of insulation where the service temperature is in the range of minus 70°F, and is exposed neither to the colder temperatures within the inner insulation layer, nor the colder surface temperatures of the pipe.
The results of these laboratory tests suggest that using the high-solids-content butyl sealant, applied to the outer layer of each insulation joint in an ethylene system (minus 150°F), can virtually eliminate the need for contraction joints in the insulation system. And, if spaced every 50 feet in a liquid nitrogen environment (minus 260°F), it can cut the number of required contraction joints by 50 percent.
The specific contraction in the piping system will depend upon the particular alloy used. Austenitic stainless steels will contract about 0.045 inches per 2-foot section at minus 150°F, the spacing of the sealant joints. Such an austenitic stainless steel system would contract about 0.066 inches per 2 feet at minus 260°F and about 0.077 at minus 320°F. Note that special design considerations may be necessary in terms of the potential liquid oxygen (LOX) compatibility for systems operating at or below minus 297°F (minus 183°C) such as for LOX and LN2 applications.
Another critical point is that the coverage rate of the high-solids-content butyl sealant must be sufficient to provide room for the contraction. Joints less than or equal to 1/8-inch (0.125 inch) are desirable. If a contraction of 0.077 inch per joint is necessary, then the gap or separation between pipe sections must be at least this large, and it’s also necessary to fill the surface cells of the cellular glass insulation on both sides of the joint. If more square feet per gallon are coated, then the sealant thickness is lower. Fewer square feet per gallon results in a thicker joint. The goal would be to have an actual joint thickness of about 3/32-inch; thus, the coverage rate must be between 13 ft2/gal and 15.62/gal, consistent with the product data sheet and the physical properties of this special high-solids-content butyl sealant.
The proper application of the sealant is crucial in providing for sufficient contraction, and, ultimately, in the survival of the system. Unlike other insulation materials, cellular glass insulation has a quite coarse, abrasive-like surface due to the cell size. This surface roughness, or cell size, necessitates that sealant be troweled on both surfaces the full thickness of the outer layer of pipe insulation. Through the troweling action, the sealant is pushed into the surface cells of both adjoining sections of cellular glass insulation. When these two coated surfaces are pressed together, a vapor-resistant bond is formed, and the coverage rate is consistent with that recommended previously.
Applying the sealant to just one of the surfaces, or applying in a beaded fashion, will not provide sufficient sealant to allow for the desired contraction, nor will it yield a tight, uniform joint. This creates the potential for system failure. When sealant is applied to a single surface, or too little sealant is used, the surface cells of the adjoining insulation section may not be properly filled, thus providing an opportunity for vapors to collect within the insulation joint.
Finally, during the laboratory testing procedures, there was concern whether the high-solids-content butyl sealant could withstand the expansion/contraction stresses without being excessively "squeezed out," and whether the joint would be able to continue to function properly. A series of compression tests were conducted, then additional tensile tests completed to evaluate the performance of these joints.
After conducting separate tension tests, it was determined that the sealant retained its elasticity, and that sufficient sealant was retained in the joints to operate properly, despite the number of cool-down/warm-up cycles it would see in service.
Butyl Sealant Properties Are Essential to Performance
The commercially available high-solids-content butyl sealant used in the laboratory tests is critically important to the performance of the butyl sealant joints. While sealants are commonly viewed as commodity items, and contractor selection is often based on price, utilizing the wrong sealant is a prescription for insulation system failure.
For example, water-based sealants aren’t designed to perform in low-temperature conditions. Oil-based (linseed oil) sealants, once dried out, lose their elasticity. Only solvent-based (mineral spirits) sealants and very expensive modified urethane sealants can effectively withstand the minus 60°F temperature at the interface between insulation layers in this cryogenic service and remain flexible or elastic.
Furthermore, the laboratory tests were conducted using a butyl sealant with 93 percent solids by weight, and 84 percent solids by volume. As a general rule, the higher the solids content in butyl sealants, the less shrinkage can be expected. Also, it’s important that the solids content be composed of butyl, not filler products. The alternative fillers may not have the same temperature dependent properties as the butyl, and may not remain as soft or flexible over the temperature range of concern. If the sealant possesses a lower solids content, then the applied thickness of the sealant would need to be increased to compensate for the reduced joint thickness after the loss or evaporation of the solvent.
Finally, it’s absolutely critical that the butyl sealant be designed, tested and remain satisfactorily flexible at a service temperature range of approximately minus 60°F (which is in the range of the outer layer of insulation) and 180°F.
These tests indicate that properly applied high-solids-content butyl sealant joints may accommodate approximately 0.045 inches per joint if the piping system is cooled down uniformly over a period of approximately 45 minutes. Additional contraction may be accommodated, but it becomes more sensitive to the cool down rate and the proper thickness of high-solids-content butyl sealant.
In view of this testing data, it’s possible that conventional contraction joints with man-made mineral fiber inserts are not needed for operating temperatures of minus 150°F or warmer, such as with liquid ethylene service lines.
Additionally, the evidence suggests that, in addition to the high-solids-content butyl sealant contraction joints, conventional contraction joints with man-made mineral fiber inserts capable of 1-inch movement are recommended for use for every 100 linear feet of uninterrupted straight-run piping for liquid natural gas service (minus 260°F), and every 50 to 60 feet of uninterrupted straight-run piping for liquid nitrogen service (minus 320°F). (Special design considerations may be necessary for liquid nitrogen and liquid oxygen service.)
The real-world consequences of these findings are that cryogenic service pipelines may require fewer contraction joints, thereby enhancing operational efficiency, lowering the installed cost of the insulation system and possibly even reducing potential liability to contractors.