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FIGURE 1. In this example with heat-transfer fluid n-hexacosane, thermal degradation occurs when excess heat drives the cracking of a straight-chain hydrocarbon (not shown is the formation of reactive free radicals, which have been omitted for clarity)
While individual system designs and variations in process and operating conditions make each application unique, all heat-transfer fluids share many common attributes, making these recommendations widely applicable. Ultimately, our goal is to educate those involved with the operation and maintenance of liquid-phase heat-transfer systems, both large and small, that use an organic-based fluid. The organics include chemical aromatics, fluids based on petroleum derivatives, silicone or glycol, the polyalphaolefins (PAO; also referred to as API Group IV-based fluids) and more. A properly designed and operated heat-transfer system can be the biggest ally in maintaining (and even increasing) productivity while reducing overall maintenance and production costs.
It starts with smart selection
The selection of the heat-transfer fluid whether at the system design phase, or on an ongoing basis after commissioning - should not be taken lightly. Fluid selection should not be dictated solely by the purchase price or any single physical characteristic. Rather, a variety of factors should be considered:
- The potential impact on workers of a given fluid, in terms of adequate training and protection that must be implemented to address hazards related to potential exposure to the fluid, in both its vapor form (inhalation risk and mist concentration) and liquid form (skin contact). In addition to direct exposure, the choice of the fluid could impact productivity engendering additional handling and paperwork protocols involving other internal resources within the company, such as the health and safety advisors, medical care personnel, personnel in the receiving department and so forth
- Freight charges related to delivery of fresh product
- Cost associated with the pickup, handling and disposal of the used oil and drums
- Proven fluid beyond fresh oil data (for instance, if vendor data is able to demonstrate the retention of fresh oil properties after some time in service, as demonstrated by extensive oxidation and thermal stability data)
- Can the current system accommodate the fluid being considered (in terms of compatibility with sealing materials, existence of a properly sized expansion reservoir, suitable match between the fluid properties and the existing hardware, such as the pump and safety-relief valve)
- Miscibility with current heat-transfer fluids if partial (rather than full) changeout is needed
- Documented success by the vendor in your type of application
- Level of liability coverage, service and expertise the fluid maker and distributor bring to the table
FIGURE 2. Excessive thermal stress often results in a breakdown of the heat-transfer fluid, and the carbonaceous by-products can build up on the inside surfaces of pipes
- Thermal degradation
- Oxidative degradation
- Process contamination
- Contamination by other materials
Each threat is discussed below, along with findings from real case studies, and practical recommendations for how to deal with these challenges.
Use the right fluid for the job. By choosing a fluid with a high thermal stability, Guyer and Brownell  suggest that most problems associated with localized or temporary temperature excursion can be prevented. Ashman  also emphasizes the importance of using a heat-transfer fluid with a suitable thermal stability for the application. Hudson, Sahasranaman [6, 7] and many others acknowledge that petroleum-based fluids of pharmaceutical quality produced by a severe hydrogenation and hydrocracking process (also referred to as “white mineral oils”) tend to have greater thermal stability compared to petroleum base oils that are produced from other refining methods [6, 7].
Use appropriate venting. Venting involves the periodic release (from the fluid and the system) of the light, more highly volatile hydrocarbons that form during thermal cracking. Venting is typically carried out by circulating some of the hot fluid to the expansion reservoir, so that those molecules with a relatively high vapor pressure can naturally migrate into the gas phase above the fluid. Then, depending on the system design, the vapors are released directly into the atmosphere or sent to a collection drum or tank, although laws governing volatile organic compounds (VOCs) and other environmental trends cause most users to collect the condensed low-boilers and properly dispose of them. Fresh fluid needs to be added periodically, to maintain the desired fluid level (to prevent pump starvation and cavitation when the system charge contracts after a shutdown). As a precautionary note, users should remember that fresh fluid must never be added directly into the hot oil stream; rather it should be added into the expansion tank or other cool reservoirs connected to the system. Venting continuously or for extended periods is not advised, because the resulting rise in the bulk fluid temperature in the expansion tank will accelerate oxidation (discussed below). We recommend the use of an oil analysis program to determine the rate of generation of low-boilers during any operation. With proper venting and analysis, users can establish how often, and for how long, the fluid must be periodically vented, in order to safely operate a high temperature system with a fluid that stays in good condition (maintaining characteristics that are similar to the fresh oil for as long as possible).
Adopt proper startup and shutdown procedures. The successful startup of any heat-transfer system is important, since the faster the heat-transfer fluid reaches its desired operating temperature, the faster the facility can produce its products and begin to fulfill orders. This becomes even more important for systems that stop and start up regularly. One may say that running the pump and the heater for a few extra hours to accommodate a slower, more-gentle startup is not cost-effective, but for many applications, such an approach pays its own dividends. For instance, by maintaining a more-gradual heating profile at startup, the fluid will be able to effectively remove heat and reduce the risk of thermal degradation, and minimizing the formation and buildup of baked-on residues. The net result will be extended planned maintenance intervals and greater component life expectancy. Shutdown procedures also impact system efficiency and fluid life. For instance, Stone  and others recommend maintaining oil circulation after the heater is turned off until it’s been cooled to 65°C (150°F). The refractory material in a furnace is designed to retain heat for as long as possible, so stopping the oil flow immediately after the heat source has been turned off provides an opportunity for the stagnant fluid to crack, forming low-boiling fractions and carbon residues. This negatively impacts the life of the fluid and the overall heater efficiency. With regard to smaller systems such as temperature-controlled units (TCUs) or extruders, many designs have improved greatly in recent years and now maintain fluid circulation for some period of time following shutdown as a common approach. An insufficient shutdown interval was the overall problem at the facility whose degraded fluid was shown in Table 1. After a service call, it was determined that the 249°C (480°F) system was shut down on Friday evening with only a short circulation period following shutdown. It was fired up again at 7:00 a.m. on Monday, to allow for production to start at 9:00 a.m. A full system shutdown and cleaning was deemed impossible by the user at that time, so the fluid was left untouched, but better future practices were implemented. The last set of results in Table 1 shows that two months after the initial analysis, the rate of generation of low-boilers had diminished (as seen in the percentage boiled below 335°C). As a direct result, the facility did not add any new oil. The increase in kinematic viscosity and flashpoint, and the fact that the strainer no longer collected carbon residues in any appreciable amount, provided evidence of improvement.
Consult your suppliers about proposed design or operational changes. Business is booming, more production is expected from the plant, more parts must be produced, and lines need to be added. Do you need to increase the operating temperature? What about the flowrate, is it adequate? What does your heater manufacturer think of the proposed addition? Operators should get as much input as possible from their system designer. manufacturers, and parts and fluid suppliers before any major changes are implemented. Stone  recommends that operators should maintain an updated list of contacts and keep it handy for questions or troubleshooting help. It is relevant to document the skin-film temperature in the current system and in the proposed operating conditions. Make certain your fluid supplier confirms your current heat-transfer fluid’s ability to handle any new operating parameters.
Maintain, inspect and perform preventative maintenance on system components. Even though liquid-phase systems commonly operate above the flashpoint of the fluid (but below its auto-ignition temperature), the risk of fire should be very low in a normal, well-designed system, especially one that is kept oil-tight, leakfree and subject to regular inspection and maintenance . For any system where heat is purposely generated to raise the fluid temperature, ensuring proper operation of the heat source is critical to achieve optimum performance. Daily inspections, using a consistent checklist of items to monitor are recommended [18, 20]. For instance, fired heaters should be inspected for flame impingement, especially if the burner is oversized or cycles frequently. In the case of flame impingement, the flame (whose temperature is typically on the order of 1,093-1,650°C, or 2,000-3,000°F) subjects the oil tubes to excessive localized heat flux, which can cause tube deformation and coking (resulting from thermal degradation, as seen in Figure 2), and leakage with increased risk of fire . In the case of systems equipped with immersed electrical heaters, excessive watt density, lack of fluid turbulence around the hot tubes, or insufficient flowrate often causes premature degradation of the fluid. Such degradation can be offset in part by proper fluid selection and maintenance practices [22, 23]. In any system, the oil-circulation pump can be compared to the heart, moving the fluid around. The pump should be well-maintained. Specifically, drive bearings on the electric motor and pump seals should receive proper attention. Centrifugal pumps should ideally operate at or near their best efficiency point (BEP), with bearings well-maintained and seals working properly. Finally, the expansion reservoir, piping, connections and valves should be selected and maintained appropriately, as part of a world-class maintenance program. Meanwhile, the life blood of the operation - the fluid itself - should be tested regularly. While further discussion of the types of tests, their significance and data interpretation is beyond the scope of this article, the American Society for Testing and Materials (ASTM) Method D5372 should be followed to properly monitor the condition of heat-transfer fluids .
FIGURE 3. These illustrations shows the type of varnish (left) and sludge (center, right) that can result from oxidation-related degradation of a petroleum-based, chemical aromatic and polyalkylene glycol (PAG) fluid
Inert gas blanketing. In closed systems, the most effective way to eliminate the potential for oxidation is to install an inert gas blanket in the expansion tank headspace. Hudson  provides details and recommendations on how to install such systems. The basic principle relies on substituting air (which contains oxygen) with an inert gas (most often nitrogen, although carbon dioxide and argon may also be considered) in the only location where warm oil can come into contact with oxygen from air — the expansion tank headspace. Displacing oxygen that might react with the fluids virtually eliminates oxidation. The pressure of the inert gas is maintained slightly above atmospheric pressure. Gas-blanketing systems, including the safety-relief valve, require ongoing inspection and maintenance to prevent inert gas leaks and limit unnecessary, costly gas consumption.
Choose a fluid formulated for the job. Oxidation-inhibitor additives are also available to enhance the performance of heat-transfer fluids. Most chemical aromatics sold today contain one or a few varieties of molecules and do not contain any performance enhancing additives such as antioxidants or rust and corrosion inhibitors. The additives that are used in heat-transfer fluids are different from the ones found in other industrial lubricants that are not subjected to such elevated temperatures. Specifically, in the case of antioxidants, some technologies combat oxidation by reacting with free radicals before they can lead to acid formation, while others attack intermediate peroxides . Fluid selection is complicated by the fact that it is extremely difficult to determine the oxidation stability of a heat-transfer fluid by its technical data sheet. Even though many of the heat-transfer fluids on the market today are unadditized, their respective marketing materials often praise their fouling resistance and promote their outstanding oxidation stability. Thus, users should assess all product claims with a critical eye. In general, systems with an enormous amount of oil tend to be more forgiving because it takes a longer time to oxidize a larger volume of fluid to a point where it raises concerns in terms of oil analysis results. In these cases, user experience, references, testimonials and competitive benchmarking studies should be evaluated in conjunction with vendor data, to assess the likely longevity of a fluid for the application at hand and avoid costly changeouts in large systems. Compared to closed or blanketed systems, open systems allow the hot fluid to come in direct contact with air, making oxidative damage a harsh reality rather than a possibility. In these cases, the importance of choosing a robust product to maintain high productivity standards becomes even more important. For example, an electronic company operating an open system at 175°C (350°F) was replacing its heat-transfer fluid every six months, after which time the fluid had become viscous and dark with a burnt odor. Switching to a fluid with better resistance to oxidation enabled longer service life. In fact, judging by the oil-analysis results, the oil properties still look like new after more than 24 months of service in these harsh conditions. This obviously saves the facility money in terms of time, labor and fluid purchases. In closed systems with no inert gas blanketing, the key is to maintain the fluid temperature in the expansion tank below 65°C (150°F), if possible. The main reason is because there is a direct relationship between the temperature and the rate of oxidation. For instance, the rate of reaction between a petroleum-derived oil with oxygen (doubles for every 10°C (15°F) increase above 80°C (175°F) (with slight variations depending on the author) , so the higher the temperature the more severe the degradation. And this does not take into account the fact that the oxidation reaction is exponential and is accelerated by contaminants such as copper or iron particles, water and other catalysts. Oxidation could occur in systems with a design that allows the oil to circulate through the expansion reservoir with full flow, either directly after the heater or on the return from the heat users. Such design exposes the hot fluid directly to oxygen from air, thereby acceleraing oxidation and greatly reducing fluid life. Using the oil-analysis results, fluid oxidation can be monitored by paying close attention to acid number (AN) and gas chromatographic distillation (GCD) results.
MINIMIZING PROCESS CONTAMINATION
Process contamination can be extremely damaging to the heat-transfer fluid and the system components. As is often the case, logic suggests that contamination is unlikely since the pressure is greater on the fluid side, but real life experience has shown on many occasions that process material can enter the heat-transfer fluid stream. The urgency required to fix a process leak really depends on the severity, the type of contaminant (chemistry), and the heat transfer media it comes in contact with. The case of contamination by water is discussed in the next section, although water is sometimes part of the process. For example, in the oil-and-gas industry, a natural-gas-extraction facility may experience an unintended leak of the process hydrocarbons into the heat-transfer fluid system. Being hydrocarbon-based, the heated gaseous molecules will mix very well with heat-transfer fluids of a similar chemistry, such as petroleum-based fluids, chemical aromatics and PAO Group IV synthetic fluids ( provides details on competing fluid types). Within a short time, the viscosity of the entire fluid charge will be greatly reduced and its overall volatility increased. In a situation such as this one, emphasis must be put into venting the heat-transfer fluid to release those light hydrocarbons into the proper collection device in order to maintain a safe operation, and if at all safely possible, to keep the unit running until the next shutdown opportunity to repair the leak.
OTHER SOURCES OF CONTAMINATION
- Hydrolysis or precipitation of oil additives (for those oils that have them)
- Accelerated rust and corrosion of system internals
- Accelerate degradation (oxidation)
- Cause pump cavitation and wear
- Create a gargling noise in the expansion tank and knocking in the hot oil piping
Discussed below are a variety of techniques for minimizing contamination that can threaten heat-transfer fluids.
Investigate and fix. All cases of contamination should be investigated and fixed, and such incidents should also be reported to your fluid supplier, for advice on the potential impact on the fluid. As mentioned earlier, sometimes the contaminant can be evacuated, boiled off or it could ruin the fluid and foul the system in a short time.
Flush new constructions or recently cleaned systems before startup. Operating companies and builders seldom factor in the cost of a system flush, since they often assume the blowing of the water will be done correctly and the contractors will not leave debris in the piping. Unfortunately, discovering such contaminants after the system is running can prove to be costly down the road. While nobody needs the extra costs of flushing a new system (especially when the fluid of choice is relatively expensive, like PAGs or silicone-based fluids), it is nonetheless a good practice. With systems filled with mineral oils, circulating a virgin base oil of the same viscosity as the heat-transfer fluid of choice is a cost-effective way to remove any potential contaminants.
Keep an eye on filters and strainers. Solids collection in the oil filters or strainers should be noted in a log book and monitored closely, preferably with photos taken. The size, texture and color of the deposits all tell a story, and such residues can be sent periodically to a laboratory with sophisticated analytical equipment for accurate identification. Keep in mind that different solids may come from more than one source, and may become discolored, so don’t jump to conclusions. Similarly solids from the previous fluids may reside in the system for a long time before an event such as pipe work or partial fluid replace-ment creates enough disturbance to loosen them. We see this in cases where a used furnace is bought and commissioned without cleaning and flushing prior to the connection to the main system. Often solids may have a familiar smell or texture that suggests an origin, but could well be something else. For example, a plant was using a heat-transfer fluid that caused valve malfunction because of deposits accumulating inside the valve spools. The black, abrasive deposits looked and felt like carbon particles (abrasive, gritty between the fingers). However, lab analysis identified the material as copper sulfide, formed by the localized chemical attack of sulfur present in the fluid’s base stock onto the copper from the brass valves. The facility could have spent several thousands of dollars in parts and labor to upgrade all the valves to more expensive stainless steel. Instead it switched to a properly formulated fluid based on highly refined API Group II base oils containing only traces of sulfur. This replacement fluid has proven to be harmless to copper components after years of service, and has had the added benefit of extending oil changes considerably, based on oil-analysis results.
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