How to Build a Reliable, Practically Leak-Free Thermal Fluid System
Thermal fluid systems utilizing synthetic organic- and silicone-based heat transfer fluids are used in industries as diverse as pharmaceutical production, petrochemicals, man-made fiber production and environmental test chambers. Operating conditions can vary substantially, from the relatively constant temperatures experienced in large vapor systems utilized by fiber plants, to dramatic, several-hundred-degree swings that can occur in the same reactor, a situation not uncommon in multipurpose production plants.
Regardless of the heat transfer fluid type, application or use temperature, all system operators share a common desire: that these systems be as reliable and as leak-free as possible. Reducing fluid leakage from a thermal fluid system has several benefits, the most obvious being economical. If the fluid stays in the system, new fluid does not have to be purchased to make up losses. Moreover, environmental and regulatory issues impacting the operation of heat transfer systems are best addressed by keeping the fluid in the system.
A leak-free system also helps minimize plant personnel exposure to the heat transfer fluid, which helps maintain the overall level of industrial hygiene in the plant. In addition, heat transfer fluid that leaks into porous insulation represents a fire risk. Eliminating leaks, as well as choosing the proper insulation, can greatly reduce the risk of these types of fires and thus help ensure worker safety. Finally, some thermal fluids have odors that plant personnel may find objectionable. Keeping leaks to a minimum while maintaining adequate ventilation will keep such complaints to a minimum.
To a large extent, reliability and integrity can, and should, be designed into a thermal fluid system. The best designs result when users focus adequate attention on selecting the proper equipment, minimizing joints and connections, and avoiding inappropriate joints and connections. The following guidelines will help you select piping, pumps, valves, flanges, gasketing and expansion joints -- all key elements essential to the design of a reliable, virtually leak-free system. While other approaches may work equally well, these practices were developed over many years and have been used successfully in many operations.
Start with the Right ConnectionsIf you are concerned only with minimizing leaks, a properly designed and installed all-welded system will provide much better leak protection than a system with flanged and screwed connections. However, the trade-off with an all-welded system is a decided lack of flexibility for maintenance operations. For example, while it is possible to repair or rebuild a valve inline, it is much easier and cheaper to remove the valve and send it out for repair. In practice, most thermal fluid systems contain a number of flanged and screwed connections -- and each is a potential leak point. For this reason (and for the sake of the system operators who will have to deal with any leaks), a thermal fluid system should be designed to include only as many breakable connections as are absolutely necessary.
Screwed connections should be avoided wherever possible, as bendable tubing with compression fittings has been found to be a satisfactory alternative. Where screwed connections are desired, however, they should be made from schedule 80 pipe and limited to pipe diameters less than 1.5". The tapered threads should be cut true-to-gauge with a sharp, clean die, washed with a good solvent, then covered with a pipe thread sealant. But, under no circumstances should you rely on pipe thread sealant to make a good joint out of a poor one.
By far, flanged connections are the best compromise you can make in terms of cost, versatility and sealing ability. As such, they are the most used breakable connections in thermal fluid systems.
Choose Flanges and Gaskets CarefullySeveral types of flanges can be used in thermal fluid systems. Your selection should be influenced by many factors, including system operating temperatures; operating pressures; purchase and maintenance cost considerations; perceived performance differences in safety and emissions; desired gasketing; and past operating experience. While ring-joint, tongue-in-groove and male-female flanges can be used in thermal fluid systems, the American Society of Manufacturing Engineers (ASME)- and American National Standards Institute (ANSI)-specified B16.5 raised-face flange is the most prevalent. But even here, there is an important choice to consider.
Typically, an ASME/ANSI Class 300 raised-face flange is chosen over an ASME/ANSI Class 150 raised-face flange. Because thermal fluid systems can operate at temperatures as high as 750oF (400oC), a Class 150 flange may not have sufficient strength for the anticipated maximum operating pressures. For example, at 750oF, a carbon steel Class 150 flange will have a maximum operating pressure of 95 psig. By contrast, a carbon steel Class 300 flange will have a maximum operating pressure of 445 to 505 psig, depending on the material group chosen. Given that the vapor pressures of most heat transfer fluids exceed 95 psig at 750oF, the unsuitability of a Class 150 flange is evident. In addition to this, ASME/ANSI B16.5 standard states: "When used above 400oF, Class 150 flanged joints may develop leakage unless care is taken to avoid imposing severe external loads and/or severe thermal gradients. For other classes, similar consideration should be given above 750oF."
At 400oF (204oC), a Class 150 flange will have a maximum operating pressure of 200 psig, which should be acceptable for most thermal fluid systems. However, even at this temperature, Class 150 flanges are more prone to leaking than Class 300 flanges.
Some thermal fluid systems will need to withstand rapid thermal cycling and the additional stresses induced by expansion and contraction forces. In general, a Class 300 flanged system is better able to maintain the minimum required seating stresses during thermal cycling, reducing the risk of leaks. Class 150 flanges can provide a suitably leak-free system at more moderate, relatively isothermal operating conditions.
To ensure proper gasket sealing, raised-face flanges must have a proper surface finish. Manufacturers of the gasket types most commonly used to join ASME/ANSI B16.5 raised-face flanges typically will specify a phonographic surface finish of 125 to 250 microinch roughness average.
Gasket choice also is determined by several factors. For starters, a gasket's construction materials must be able to withstand the system's operating temperatures and must be chemically compatible with the fluid used. Thermal fluid systems can operate at temperatures as low as -150oF (-100oC) and as high as 750oF (400oC). At these extremes, elastomers and plastics are unacceptable because of their poor mechanical properties.
Another temperature-related challenge is resistance to heat generated by an external fire. In the highly unlikely event that a fire occurs in the area of a flanged joint, the fluid should be contained within the system -- remember, thermal fluids will burn. A gasket that can withstand extreme temperatures until the fire can be extinguished could prove beneficial in an emergency situation.
Most high performance, high temperature heat transfer fluids are based on aromatic or silicone compounds. Only a few types of elastomers and plastics are compatible with these chemistries, so keep this in mind when specifying an elastomer or plastic component. Gaskets constructed with metal and graphite generally will meet the temperature and chemical compatibility and fire resistance requirements of a properly designed thermal fluid system.
In addition to a gasket's construction materials, carefully consider its physical design. A catastrophic gasket failure -- a blowout caused by unexpected over-pressurization of the system, for example -- will generate a large fluid leak that could lead to a potentially serious fire risk.
Spiral-wound and grooved-metal gasket designs resist blowouts and are constructed for leak-free operation. Probably the most commonly used in these systems, spiral-wound gaskets have a stainless steel inner ring, carbon steel centering ring, stainless steel windings and flexible graphite filler. Alternatively, grooved-metal gaskets have a core ring of grooved 316 stainless steel at least 3 mm thick. In addition, they are faced with flexible graphite and utilize a loose centering ring. So far, their use has been limited primarily to Europe.
Properly assembling the flanged joint also reduces the chance of leaks. During installation, it is critical to protect the flange surface from nicks, dents and scratches, which can cause the joint to leak.
Proper flange alignment is equally critical. Mis-alignment can result in overstressing one side of a flange while leaving the other without sufficient compression to seal the gasket. Once a flange pair is properly aligned, use studs and bolts that comply with American Society for Testing and Materials (ASTM) standards. Using ASTM A-193 Grade B7 studs and bolts with ASTM A-194 Grade 2H hex nuts will provide a fastener strong enough to supply the required sealing force. Incrementally tighten the well-lubricated fasteners in a criss-cross pattern to maintain proper alignment. Applying adequate initial preload to the flanged connection will minimize the risk of leaks after system startup.
Reduce Valve LeakagesValves tend to be a major source of leaks in thermal fluid systems, with leakage typically occurring around the stuffing box. Due to the maintenance considerations already noted, most valves are not welded into the system, resulting in additional potential leak sources at the flanged piping connections.
One way to reduce leaks from valves is to eliminate the stuffing box. This can be done by utilizing valves with a metal bellows (as the primary seal) in combination with high temperature graphite packing (as a secondary seal). Unfortunately, the cost and space requirements of bellows-seal valves have limited their use in thermal fluid systems.
When choosing packed valves, pay particular attention to the valve stems, stuffing boxes and packing specifications. In general, nonrotating rising-stem valves are preferred to quarter-turn valves. For rising-stem valves, the suggested stem-sealing surface roughness should be no more than Ra 0.8 micron (32 RMS). Also, the stem straightness should be specified within a tolerance of 0.039" (1 mm) per 39.37" (1,000 mm), length total indicated reading, and the inner wall of the stuffing box should have a maximum roughness of Ra 3.2 micron (125 RMS).
As with gasket materials, flexible graphite is the valve-stem packing material best suited for the entire operating range of most thermal fluid systems. One possible improvement to graphite-packed stem seals is to apply a live load to the packing follower. This will ensure that as the packing wears, it will remain properly compressed against the valve stem.
Globe, gate and rising-stem ball valves are the preferred choices for thermal fluid systems. General specifications for 2" or larger valve types are shown in table 1. Specifications for smaller valves will vary.
Insulate to Reduce Fire RiskOne strong driving factor behind the push to design leaks out of a thermal fluid system is reducing the risk of fire. Regardless of the connector or valve seal used, the potential for leaks still exists. To reduce the risk of fire when a leak does occur, care should be taken in valve orientation and pipeline insulation.
It is well understood that a combustible fluid can ignite at temperatures well below its published autoignition temperature if spread in a thin film. The high surface area present in many types of insulation can promote this phenomenon when soaked with a thermal fluid. To minimize this risk, use a closed-cell insulation in the immediate vicinity of likely leak points such as valves and connectors. Fibrous insulation can be used for pipe runs between connectors and valves, but install a metal drip ring to separate the fibrous material from the closed-cell insulation. A properly installed drip ring ensures that any fluid that gets under the insulation on one side of the ring cannot migrate down the pipe and soak insulation on the other side.
When installing valves, consider taking additional precautions. Because valve stems are potential leak sources, they should be installed with the stem in a horizontal position if possible (provided the valve manufacturer does not advise against this orientation). In this configuration, should a leak occur, the fluid will drip away from the valve rather than down into the insulation around the valve.
Pick the Most Effective Pump and Sealing SystemHistorically, heavy-duty API centerline-mounted pumps have been preferred for thermal fluid systems operating above 350oF (175oC). These pumps typically use either standard or cartridge metal-bellows seals.
There are several limitations to this choice of sealing system. While properly installed single mechanical seals can provide reasonably leak-free operation, by their nature, mechanical seals cannot be considered a zero-emission sealing system. Furthermore, mechanical seals are precision-engineered pieces of equipment -- sensitive to vibrations, misalignment, excessive temperatures and dirt. All of these factors must be considered during pump design, installation and operation.
In addition, accurate alignment is essential to ensure proper operation of single mechanical seals. Therefore, a pipe stress analysis must be conducted on the pump to ensure that the allowable forces on the pump flanges are not exceeded.
One positive is that many styles of single mechanical seals are available. Some can operate at temperatures as high as 800oF (427oC) without filtered, cooled seal flushes. Other seals require that the seal-face temperature be kept below a certain point to avoid heat distortion. Such seals typically require filtration and cooling of the seal flush to less than 400oF (204oC).
With seals, again, there is a trade-off to consider. Seals that do not require extra cooling and filtering cost more; seals that do require cooled and filtered seal flush are less expensive but reduce thermal fluid system efficiency.
Another choice that must be made is the seal-face construction material. There are two alternatives: hard face-to-hard face (typically silicon or tungsten carbide to silicon or tungsten carbide) or hard face-to-soft face (silicon or tungsten carbide to carbon). The best choice for a given application will depend on the specific system conditions. For systems exceeding 400oF (204oC), secondary seals typically are graphite.
Once the single mechanical seal is installed, you can take several operational steps to extend its life. Seal faces are sensitive to vibration, so promptly eliminating pump cavitation is critical. Seal faces also are sensitive to abrasion, so the thermal fluid system should be operated and maintained to generate minimal solids.
Startup can present solids problems if caution is not taken. During startup of a new system that was not cleaned, it is not unusual for mill scale or other corrosion products to break free. These corrosion products typically are very fine and cannot be removed with startup strainers. If they reach the seal faces, they could cause damage. Eliminate this problem by cleaning the system before filling it with the heat transfer fluid, or by circulating the fluid through a bypass filter that will remove the particles during startup.
Solids also can damage a seal if they form on the outside of the seal itself. All mechanical seals leak a small amount of fluid during normal operation. When this fluid escapes to the atmospheric side of the seal, it comes into contact with air. Oxygen in the air reacts with the fluid, causing a carbon buildup on the seal faces that could, if severe enough, cause the seal to fail. Putting a nitrogen purge on the stuffing-box vent eliminates oxygen, which in turn reduces coking, thus prolonging seal life (figure 1).
Dual mechanical seals also have been used on pumps in thermal fluid systems. Many of the considerations for single mechanical seals also must be confronted with dual seals, with one significant addition: A dual-seal system requires a barrier fluid. Because this barrier fluid can leak into the system, the fluid choice is important. It must not react with the heat transfer fluid yet must be as thermally stable. For these reasons, the heat transfer fluid generally also is used as the barrier fluid.
A Zero AlternativeIn contrast to pumps equipped with mechanical seals, sealless pumps can be considered zero-emission pumps. In the past, however, size and operating temperature limitations usually rendered these pumps inadequate for thermal fluid systems, so they were not used often in these applications. But now, design and material improvements are making these pumps a more viable choice for thermal fluid systems.
Sealless pumps eliminate many of the leakage issues associated with mechanical seals. The chief drawbacks of sealless pumps, either canned or magnetic-drive, are their inability to handle solids, intolerance of any cavitation, less efficient motors and higher initial cost.
A recent in-house survey pointed to four trends:
- The average repair cost of a single-mechanical-seal pump is significantly less than the average repair cost of a sealless pump.
- Sealless pumps have a much longer mean time between failure (MTBF) than pumps using single mechanical seals.
- Considering only these two influences, the single mechanical-seal pump offers the lowest long-term cost of ownership, although the gap continues to get smaller.
- When process downtime and environmental challenges associated with any failure are considered, the longer MTBF and generally leak-free failure mode of sealless pumps can outweigh the lower repair cost for single-mechanical-seal pumps (table 2).
Pipe Loops vs. Expansion JointsWhen piping is heated or cooled, it will expand and contract, and the resulting stresses in the piping system must be relieved. Steel pipe "U" bends or loops are the most common stress relievers.
If you decide to use bellows expansion joints, take care in considering the construction materials. Occasionally, austenitic stainless steel bellows joints have failed because the heat transfer fluid was contaminated by inorganic chloride ions, resulting in stress corrosion cracking. The choice of Inconel 600 or Monel 400 can reduce the risk of stress corrosion cracking. Failures in bellows expansion joints stemming from improper installation and poor maintenance also have been known to occur.
ConclusionsReliable, largely leak-free thermal fluid systems can be built and operated. Minimizing the number of connections is a key ingredient, as is proper design and assembly of flanged joints. Using sealless pumps will reduce emissions. If sealless pumps are impractical for your system, properly installed and maintained pumps with mechanical seals will give satisfactory performance.
Where possible, avoid using screwed fittings and where practical, choose pipe "U" bends over bellows expansion joints.
One additional point: While careful system design can go a long way toward ensuring relatively leak-free performance, proper maintenance over the life of the system must not be ignored. For example, while your system may last 20 years, the valve packing will not. At some point, the valves will have to be serviced.
In general, no matter how much time and effort you expend on designing reliability and integrity into your thermal fluid system, over time some leaks will occur. However, proper system design and equipment selection, coupled with regular routine system maintenance, will help keep their number, size and frequency to an absolute minimum.