SteamOnline - Steam Tracing

For decades, steam tracing has been an accepted practice in the heating of piping, vessels and equipment. Steam tracing has been used in industrial processing plants for over a century. Early on, steam jacketing and tubular tracing became the chosen means of keeping the contents of pipes at required temperatures. As refineries and chemical plants expanded in size and product diversity following World War II, electric-tracing methods were developed to provide thermostatic control for low-temperature and heat-sensitive materials. There are applications where one method may show to advantage over the other, but today steam tracing continues to be the most widely used method of heat tracing in industrial plants around the world.

Steam-Tracing Systems - What Are They?
Many industrial processes require the storage and transfer of fluids through piping and equipment. Often, these fluids such as liquids, gases, vapors, suspensions or slurries have inherent temperature characteristics which allow them to freeze, become viscous, or undesirably condense at normal ambient temperatures. In order to prevent these problems which typically occur in non-processing periods, it is usually required that additional heat as well as insulation be added to the piping and equipment. A process pipe requiring heat may be routed through the plant pipeworks in a complex path of turns as well as rises and drops. A tube or small-diameter pipe attached to the pipe and carrying a heating medium for the addition of heat all along the process pipe is commonly referred to as “heat tracing.” If the system of heat tracing utilizes steam in the heat delivery process, it is then termed “a steam-tracing system.” Figure 1 illustrates the steam- tracing system concept.

The Theory of Steam Tracing
The theory of maintaining temperature in an insulated line evolves around a very simple heat balance. If the process temperature is to remain constant, the heat input into the line must be equal to the heat loss (W/m or BTU/hr-ft) through the thermal insulation. Selecting a steam-tracer type with a heat output that will most closely match this heat loss is the key to efficient steam tracing. Steam tracing has always been an easy heat tracing choice for the plant engineer because the steam distribution and return system is usually an integral part of the plant energy system. That is, steam is used in the turbines to turn generators for the production of electricity, as a prime mover for pumps and other equipment, and for process heat in heat exchangers and reactors. As these steam users are usually spread throughout the process unit, the distribution headers may often be found in proximity to the piping and equipment to be steam traced.

Steam Tracing - Time-Honored Methods of the Past
Steam-tracing systems were developed during the early days of the refining industry. In the early 1900s when the first continuous-type processing plants were developed, these systems were essential in keeping petroleum residues, tars and waxes flowing. The petrochemical industry grew side-by-side with the refining industry and utilized steam-tracing systems, as well. During this era, energy conservation was of little concern. Insulation systems generally provided a minimum of thickness to allow the available steam-tracing methods to hold the process fluids at a desired temperature. Insulation thickness levels of more than 25mm (1 inch) on small lines or 40mm (1-1/2 inch) on larger lines were the exception. High-temperature rigid insulating materials were widely used, and they were generally hygroscopic. To further complicate matters, weather barriers were also less than optimal. All of these factors affected the thermal performance of the heat-tracing systems. The early steam-tracing methods included steam jacketed and gut-line systems and convection tracers using bare tubing or small-diameter pipes.

Jacketed and Gut-Line Steam Tracing Systems
Jacketed and gut-line steam-tracing systems of this era were simply extended versions of the age-old double pipe heat exchanger. In this system, one of the process streams contains flowing steam and the other stream is the process fluid which requires temperature maintenance. In tracing applications the process fluid is generally to be maintained at the desired temperature level during non-flowing periods. These systems are illustrated in Figure 2 and Figure 3. Because of the large heat transfer area between the steam and the process fluid and the relatively high-heat transfer coefficients associated with steam, the process fluid, and the metallic wall of the process pipe, these steam-tracing systems were temperature predictable. In most cases, the process fluid temperature closely approached that of the steam temperature. These systems were relatively insensitive to ambient changes or the effects of sub-standard insulation, as they simply condensed more steam to make up for the additional heat loss. Unfortunately, the majority of process fluids actually required maintenance temperatures below 100ºC (212ºF); therefore, most systems were inherently over-designed. The general over-design was not only energy inefficient but could result in over-temperature and thermal degradation of sensitive process fluids. From an installation standpoint, these systems were costly and cumbersome to install especially in complex piping systems due to the special fabrication required.

Early Convection Pipe/Tubing Tracers
Clearly, the jacketed and gut-line steam tracing systems required foresight and detailed installation planning since they must necessarily be installed at the same time as the piping and equipment. Such is not the case with the externally installed convection tracer shown in Figure 4 (see page 20). In the very early days, these convection steam tracers were typically 15mm (½ inch) or 20mm (3/4 inch) nominal Schedule 40 carbon steel pipe. In the 1930s, the availability of reliable tubing of correct diameter and thickness (and not out of round) along with better fittings, allowed the gradual change to copper or stainless-steel tubing as the preferred convection steam tracer. Except for a brief return to carbon steel pipe tracers during World War II (1940 to 1945) the trend for convection tracers was toward the use of copper and the relatively new stainless-steel tubing. Regardless of the type of material preferred for the tracer, the predominant mode of heat transfer was by natural air convection and radiation between the tracer and the process pipe. These early convection steam tracing systems provided a low-cost installation and lower heat transfer rates than the jacketed or gut-line system and improved steam tracing’s functional efficiency for a larger group of applications. Although convection tracers had heat delivery rates considerably lower than the steam-jacketed system, they would still supply too much heat for many low-temperature or heat-sensitive applications such as caustic lines and acids. To remedy the problem, small sections of wood or rigid insulation were placed between the process pipe and the convection tracer on 0.3- to 0.6-meter (1- to 2-foot) centers and held in place with tie wire. These systems had several problems that created a dilemma for the designer. The blocks would fall out of place during assembly and maintenance and also while in service due to expansion and contraction of the tracer. Sometimes the bare tubing between the spacer blocks would get pressed against the pipe during maintenance or due to unauthorized foot traffic on the pipe. However, with good installation procedures and care in performing maintenance, these systems were often successful.

Early Conduction Steam Tracers
Heat transfer compounds were developed almost 50 years ago. When this conductive paste-like material was applied to a single bare tube or small pipe tracer, it could replace multiple bare tracers and could closely parallel the heat delivery rates of a fully jacketed pipe. These thermally conductive compounds were typically hand toweled over the top of a 10mm (3/8 inch) or 12mm (½ inch) diameter tube and converted the basically “convection and radiation” steam tracer into a primarily “conduction” steam tracer. Figure 5 details a typical installation (see page 20). For pipe sizes in the 25mm to 100mm (1 inch to 4 inch) range, a single external conduction steam tracer could often provide heat transfer performance similar to the jacket and gut-line system but with the low cost and installation advantages associated with the simpler convection steam tracer. Pipe sizes above 100mm (4 inches) could require two or more conduction tracers to match a fully jacketed system. However, the number and size of conduction tracers could be adjusted to closely match the heat requirement of the system without fear of over-heating and energy waste. In addition, this system could be installed at any time in the life of an existing piping system.

Summary
In the early days of steam tracing only a few tracing methods were available. Efficiency was less than optimal because the available tracing methods didn’t provide many choices in heat transfer rates. At this time, few control options were available except for valves to reduce the steam pressure. Often, condensate was drained by manually operated valves or orifices.

Steam-jacketing and gut-line
tracing provided high-heat transfer rates while convection tracing provided medium-heat transfer rates. Multiple convection tracers had to be installed to meet the heat load requirements between the output of one convection tracer and that of a jacketed system. Later, the development of heat transfer compounds created the "conduction" tracer and expanded the available heat delivery choices by filling the temperature maintenance gap between steam jacketing and convection tracing. One conduction tracer could replace from three to five convection tracers. These early putty-like materials had to be hand toweled over the tracer but provided a much-needed service. The old "spacer block" method was the best choice of the time to supply heat transfer rates below the rates that could be provided by convection tracing. Varying heat delivery requirements called for different spacing dimensions, and accuracy of design in such a system was difficult at best. Although the old tracing systems were not very sophisticated or efficient, they were made to work by the early pioneers in the refining, chemical, pulp and paper and other industries. Their hard work and dedication have led us to here we are today. The good news is that modern thermal insulation and weather barrier systems, along with new tracing methods and design techniques, have drastically improved steam-tracing systems. Closer process temperature control, improved safety and more energy-efficient systems influencing reduced emissions are some of the advancements that have been made in recent years.

References

Sandberg, C., "Heat Tracing: Steam or Electric?" Hydrocarbon Processing, Mar. 1989.

Sandberg, C., "Heat Tracing Systems Selection." Chemical Processing, Nov. 1997.

Custom Marketing Report for Cellex Manufacturing, Inc. Industrial Resources, Inc., Houston, Texas, 1997.

ASTM Designation: C1055-92, "Standard Guide for Heated Systems Surface Conditions That Produce Contact Burn Injuries." Annual Book of ASTM Standards, 1996.

ASTM Designation: C1057-92, "Standard Practice for Determination of Skin Contact Temperature from Heated Surfaces Using a Mathematical Model and Thermesthesiometer." Annual Book of ASTM Standards, 1996.

Figures 3, 5, 6, 10 and Table 1 are from CompuTrace® SteamTrace computer software program or were developed from experiments carried out by Thermon Manufacturing Company, 1998.

Figure 9 model was developed with the use of FEA software from ANSYS, Inc.

Thermon Manufacturing Company’s series of SafeTrace tracers Form No. TSP0005-0298.

R. Knox Pitzer is product manager with Thermon Manufacturing and has been with the company for 39 years. During those years he has held the positions of vice president of sales and marketing and administrative vice president. In 1996 Knox was asked to return from retirement to help direct a renewed emphasis on the mechanical line of products. He has authored and presented domestically and internationally a number of technical articles and papers on various aspects of steam- and electric-tracing systems. Knox is a member of the Institute of Electrical and Electronic Engineers and the Society of Plastic Engineers. Contact the author at Thermon Manufacturing Co., 100 Thermon Dr., P.O. Box 609, San Marcos, TX 78667-0609, (512) 396-5801, fax: (512) 754-2416.

Steam Tracing 2

Product innovations such as "burn-safe" and "energy-efficient" steam-tracing products can help provide systems optimization for the steam-tracing user. Since the early 1900s, steam-tracing has been the primary means of keeping materials such as petroleum residues, tars and waxes flowing through pipelines and equipment in the petroleum and chemical processing industries. In recent years, however, there have been several articles published regarding the inefficiency of steam-tracing1,2. These assertions may be a result of comparisons with uncontrolled and less than optimally selected steam-tracing systems or perhaps a lack of knowledge of currently available highly efficient steam-tracing products and design technology. Today, new steam-tracing products are available which offer the design engineer a range of heat-transfer capabilities to cover freeze protection to high-temperature tracing requirements. Design methodologies for steam-tracing systems have become much more sophisticated and are typically based on a combination of extensive empirical data, mathematical analysis, and FEA/CFD numerical simulation techniques. Recent product innovations such as "burn-safe" steam-tracing products are now being used to improve operator safety. “Energy efficient” steam products are being installed that contribute to reduced plant emissions by reducing energy consumption. There has also become a greater awareness of the critical role that the installation of thermal insulation has on the overall performance of the steam-tracing. Installation practices have improved. A goal of this article is to create a new awareness of the significant advancements as well as the knowledge base that is available for the design, installation, and use of efficient steam-tracing systems.

Steam-Tracing Today
Most of the steam-tracing systems of the past have proven to be reliable when properly designed and installed. Jacketed systems, conduction tracing systems and bare convection tracing systems are still around today. The gut-line system is rarely seen anymore because of the difficulty of installing and maintaining it. However innovative and improved steam-tracing products have been developed that are now altering the course of standard steam-tracing practice.

Today’s Jacketed Steam-Tracing Alternatives
For rapid heat-up requirements on such materials as asphalt, sulfur, and heavy crude bottoms, the high heat delivery of the costly jacketed system is sometimes required. However, for cost efficiency reasons, the trend today is to specify either a conduction channel/tube system or a clamp-on jacket conduction tracing system rather than a fully jacketed system. The least costly alternative and the simplest to install is the use of a channel/tube conduction steam- tracing system. In a typical installation of this system, one or more 10 to 20 mm (3/8 to 3/4 inch) tubes are wired onto the process pipe. Subsequently, metal channels filled with heat transfer compound or pre-molded snap-on heat transfer strips are placed over the tube or tubes and the assembly is banded into place on the pipe. A typical application is shown in Figure 1. The second alternative today is the clamp-on jacket conduction steam tracer. This system consists of self contained welded steel clamp-on jackets with one side specially fabricated to fit the pipe curvature. After the application of a non-hardening heat transfer material to the curved side of the jacket, it is then banded to the process pipe. Because of the necessary short lengths of 5 m (17 feet) to 7 m (22 feet) of each of the clamp-on jackets, they are usually connected in series by steam hose jumpers for extended tracer circuit lengths. A typical tracer installation is illustrated in Figure 2. Both of these conduction steam-tracing systems exhibit similar heat transfer characteristics. For illustration purposes, a conduction steam-tracing system made up of a steel channel covering a heat transfer snap-on type strip and a 20mm (3/4 inch) tube, and then subsequently a steam-tracing system made up of a clamp-on jacket with heat transfer compound have been installed on a 200 mm (8 inch) nominal carbon steel pipe filled with oil. The pipe was insulated with 50mm (2 inch) nominal insulation thickness of fiberglass. Using a steam pressure of 411 kPa abs. (45 psig), heat up and equilibrium temperature data are shown in Figure 3 on the previous page. The heat-up times and equilibrium temperatures achieved for these two systems are quite similar.

Today’s New Convection Steam Tracers
Instead of the traditional "one type fits all" concept of the past used in applying bare convection tracers, a variety of new convection steam-tracing options are available today. The design engineer may now choose convection tracers from several levels of heat delivery such as standard heat, light heat, or extra light heat as shown in Figure 4. Each of these new types of external convection steam tracers offers a different level of heat delivery potential. In order to illustrate the heat transfer performance of this family of new convection steam tracers, heat delivery curves in a typical pipe heating application are illustrated in Figure 5. These tracers with reduced heat delivery provide more practical temperature maintenance levels in the temperature range of 10 to 100ºC (50 to 212ºF) which covers a majority of the heat-tracing applications. An example of the temperature results using these new lower output steam tracers is shown in Figure 6. In general, steam tracers run both beneath the insulation as well as extend out of the insulation at flanges, valves, equipment, and expansion joint areas. These areas can be potential burn areas for the plant personnel who maintain the facility. Due to the high thermal conductivity of bare non-insulated metallic steam tubes, an accidental touch by a plant worker can result in serious skin burns. Occupational Safety and Health Administration recordable injuries of this type are not only a personnel health hazard but can result in serious economic losses to industry. These new convection steam tracers have a unique "touch-safe" characteristic. This has been achieved by applying one or more non-thermally conductive coverings to the steam tube’s exterior surface. The addition of a non-thermally conductive surface reduces the instantaneous heat flow from the surface into the skin during an accidental touch event. The level of "touch- safeness" of these new steam tracers has been well defined thanks to the excellent work done under the auspices of the ASTM Standards C-1055 and C-1057 (which are under the jurisdiction of the ASTM Committee C-16 on Thermal Insulation). Specifically, the relative "touch safeness" is defined by the number of seconds of accidental contact prior to the epidermal layer of skin reaching a temperature level which will yield a first-degree burn during the touch event. Typical “touch-safeness characteristic” curves of these new convection steam tracers, along with that of a bare copper tube tracer, are plotted in Figure 7. In general, the normal reaction time of the typical plant worker during a burn event is in the range of a few tenths of a second to 5 seconds. For design purposes, the probable contact time for industrial situations has been established at 5 seconds. As can be seen in Figure 7, the metallic tube tracer is well above the safe temperature versus time characteristic limiting curve. The new convection tracers have also been plotted. Although the standard heat tracer with a safety jacket does not meet the full requirement of a 5-second touch event, the curve shows that it could be contacted for approximately 2 ½ seconds under the given conditions without a burn occurring. The light heat tracer falls below the threshold curve for sustaining a burn and would be regarded as touch-safe in this case. Since the extra light heat tracer has a lower heat output than the light heat tracer and therefore would fall below it on the curve, it has not been plotted.

Energy Efficiency of Today’s Steam-Tracing Systems
The opportunity for vastly improved energy efficiencies in today’s steam-tracing systems has been brought about by several factors: 1) the accuracy of today’s design methods; 2) the reliability of thermal insulation systems; and, 3) steam tracers with a wider range of heat transfer rates from which to choose. Today, it is possible to more closely match the steam tracer heat output with the specified temperature maintenance requirements of a particular tracing project. To illustrate the point, we have compiled in Table 1 (see page 29), the energy savings in freeze protection service that can be achieved by simply selecting the optimal convection tracer design versus using a traditional convection tracer.

Installation/Operational Practices Have Improved Today
Over the years, steam-tracing products and practices have made great strides. The installation and operation of steam-tracing systems have been greatly enhanced with new and improved products and practices. A few of these new and improved products and practices include the following: 1) pre-assembled steam supply manifolds and condensate return manifolds with freeze protection designs; 2) pre-assembled steam-trap stations; 3) factory pre-insulated and jacketed steam supply and condensate return lines; 4) electronic leak detection systems for steam traps; 5) a wide range of steam tracers providing light- to heavy-duty heat-transfer capabilities; 6) optimized steam-trapping distances requiring fewer trapping stations; 7) highly reliable thermal insulation and weather barrier systems; and 8) computer software for optimized steam-tracing designs. The pre-assembled steam trap stations of today have a strainer for filtering out foreign matter which can adversely affect the trap operation and a test valve so the operator can see that the trap is functioning correctly. Almost any type of trap can be used to drain tracer lines, but some lend themselves to tracing applications better than others. Small thermodynamic or inverted bucket traps work well for the majority of tracing applications. Most of today’s steam traps used for tracing service have a restricted orifice which helps the trap operate in a more energy-efficient manner under the light condensate loads found in most steam-tracing circuits. In addition, the restricted orifice reduces the steam loss through the trap during a malfunction and thus reduces the energy waste potential of an improperly maintained trap. The use of centralized steam supply and condensate return manifolds is another of today’s trends. This centralization is of great benefit to those charged with maintaining the steam-tracing system. The periodic steam system surveys are less time consuming and more likely to be carried out where supply and trap manifolds are grouped and easily accessible.

The grouping of traps at a manifold can also be beneficial for freeze protection of the traps. That is, by close coupling a group of traps together in a common manifold header, it is possible to utilize the condensate discharge of one trap to offset the heat loss and keep the entire group of traps from freezing during cold weather. Another feature of today’s steam-tracing systems is the use of factory pre-insulated steam supply and condensate return lines to transport the steam from the centralized distribution points and carry the condensate to the return header. These polymer jacketed pre-insulated lines ensure the integrity of the insulation system and keep it virtually unaffected by moisture and weather for the life of the system. Because of the minimization of field labor, these pre-insulated lines can be a factor in reducing the overall installed cost of the steam-tracing system. In order to minimize initial equipment cost as well as to reduce the size of the trap population requiring continued maintenance service, there is a greater focus today on maximizing the circuit length of the tracer between the steam supply and the trap station. One of the early "rules of thumb" used for determining the distance between supply and trap points was (and frequently is today) to limit trap distances to no more than 18m (60 feet) for 10mm (3/8 inch) O.D. tubing tracers and 30m (100 feet) for 12mm (½ inch) O.D. tubing tracers. Although this may be a conservative approach that works, it results in far too many trap and supply points in a steam- tracing system.

Today, the distance between supply and trap points is determined based on tracer steam flow rate, steam pressure, tracer tube size, the accumulated vertical tracer rise, and the resultant steam pressure drop. Figure 10 on the following page shows the trap distances that are possible for 10mm (3/8 inch) and 12mm (½ inch) outside diameter (wall thickness of 1.25mm or .049 inch) steam-tracer tubes for various heat delivery levels when using 446 kPa abs (50 psig) steam service. When calculations are done for typical water freeze protection of a steam traced line of 75mm (3 inch) nominal size with 25mm (1 inch) of fiberglass insulation in a minimum design ambient of -40ºC (-40ºF), it is clear that the design heat delivery need not be more than 38 W/m (40 BTU/hr-ft). From Figure 10, it is obvious that for this heat delivery level, the trap distance can potentially be extended from the rule of thumb value by 42m (138 feet) for a 10mm (3/8 inch) tube and by 100m (328 feet) for a 12mm (½ inch) tube.

Steam Tracing - As We Move into the 21st Century
As we move into the 21st Century, the applications for steam tracing in the process and power industries will most certainly change as the engineering world opens new frontiers of industrial development. As new applications evolve, exciting new methods and products will also evolve. In the next millennium, we would expect to see wider spread use of Finite Element Modeling (FEA) and Computational Fluid Dynamics (CFD) techniques. As a typical example of current FEA usage, a steam traced and insulated pipe can be geometrically subdivided into an assembly of finite elements as shown in Figure 9. As indicated by the color or shading of the different elements in the upper left graphic in Figure 9, this steam-traced and insulated pipe has been computationally defined by an assembly of elements each with totally defined thermal properties and dimensions. Once convection boundary conditions on the insulation surface and the fixed steam temperature associated with the steam elements are defined, the FEA method develops a set of equations for all of the elements which can be solved computationally by elimination techniques. The end result of the converged FEA solution is shown in the right graphic in Figure 9. Note that the key defines the range of temperatures associated with that color or shade. As can be seen, this solution defines temperature gradients present within the pipe itself as well as (in the lower left corner) the temperature gradients throughout the insulated and traced pipe cross-section. While this particular example is rather simple, it is clear that this FEA technique can be applied to almost any geometry and application, providing of course that the heat transfer mechanisms are well understood and an accurate model translation is achieved and verified. Many of the steam-tracing products in use today have benefited from the continuing advancements in materials technology. There is however, an ongoing research effort (as well as field test programs) to apply some of the even more advanced materials to the application of steam tracing. The use of some of these materials and processes are being targeted at reducing installation costs while still maintaining the predictability and reliability that the steam-tracing user has come to expect.

Summary
In the past, engineers had a limited range of design tools and tracing methods available for designing steam-tracing systems. Since steam tracing seemed simple and flexible it was often installed at the plant level without any particular design review involved. Since steam was available as surplus around many industrial processing units, the need to be energy efficient was easily forgotten. It was not considered possible to provide a “burn-safe” steam tracer. Today, however, energy conservation which results in lowered boiler emissions for steam users is a primary goal in the refining and petrochemical industries where millions of meters (feet) of steam tracing exists. Some of these new products are being introduced in today’s steam-tracing designs and will help to minimize energy consumption and thus fuel consumption and emissions. Energy saved is energy that does not have to be produced. Further, some of the new steam-tracing products are also helping to reduce the risk of accidental burns for plant workers maintaining steam-tracing lines. In a recent study3, over 70 percent of the refineries and chemical plants surveyed stated that steam-tracing is their primary means of heat tracing. Therefore, it is prudent to design and install energy-efficient and safe steam-tracing systems that will optimize the user’s cost of ownership.

References:

Sandberg, C., "Heat Tracing: Steam or Electric?" Hydrocarbon Processing, Mar., 1989.

Sandberg, C., "Heat Tracing Systems Selection." Chemical Processing, Nov., 1997.

Custom Marketing Report for Cellex Manufacturing, Inc. Industrial Resources, Inc., Houston, Texas, 1997.

ASTM Designation: C1055-92, "Standard Guide for Heated Systems Surface Conditions That Produce Contact Burn Injuries." Annual Book of ASTM Standards, 1996.

Figures 3, 5, 6, 10 and Table 1 are from CompuTrace® SteamTrace computer software program or were developed from experiments carried out by Thermon Manufacturing Company, 1998.

Figure 9 model was developed with the use of FEA software from ANSYS, Inc.

Thermon Manufacturing Company’s series of SafeTrace tracers Form No. TSP0005-0298.

Roy E. Barth is vice president of research and development and Charles Bonorden is product development specialist with Thermon Manufacturing Co., San Marcos, Texas. For more information, contact the authors at Thermon Manufacturing Co., 100 Thermon Dr., P.O. Box 609, San Marcos, TX 78667-0609, (512) 396-5801, fax: (512) 754-2416.