Certain hot process piping services require thermal maintenance devices to keep stream temperatures within design limits. In such cases, conventional pipe jacketing has been regarded as sufficient but often too expensive. An alternative to pipe jacketing, tube tracing, does not effectively prevent problems caused by temperature variations along the process pipe wall. Frequently, tube tracing is used as an economical way to try and compensate for heat loss. In low temperature applications with broad temperature envelopes, steam tracing can be effective. However, in those applications, the position of the tracers and the temperature distribution in the pipe material are not considered important.
As an alternative, a bolt-on heating system can be used to maintain process temperature or ensure uniform pipe wall temperatures. Consisting of contoured, bolt-on trace elements, this system is positioned after heat dynamics modeling is performed for the operating piping system.
In dealing with the actual problems caused by unwanted heat variation, frequently, the key principle is ensuring uniform pipe wall temperature. To ensure uniformity, it is necessary to manage the heat distribution in the pipe wall. Proper management of heat distribution depends on the number and placement of heating elements. Placement, in turn, depends on accurate modeling of process thermodynamics. Therefore, use of accurate process modeling software is a good way to ensure uniform pipe wall temperature.
A typical result of this type of modeling is shown in figure 1. Heat from steam at 318°F (159°C) is applied from a contoured heating element to a pipe wall at defined intervals. Directly under the element, the wall temperature is 297°F (147°C). As the distance from the element increases, the wall temperature decreases until a minimum of 270°F (132°C) is reached. At this point, heat conducted through the wall from another heating element causes the wall temperature to rise again. The wall temperature varies within a defined range, and the warmest and coolest points also can be readily identified.
Contoured pipe tracing designed using thermal analysis has been employed to prevent problems caused by mixed vapors in sulfur processing and in a common anhydrous vapor process. In the first case, the system prevents condensation and associated pipe corrosion. In the second case, it prevents pipe blockage by precipitated material.
Case 1: Sulfur RecoverySulfur recovery units in refineries or gas plants usually include tail gas or degas vapor lines. These lines commonly are heated to prevent the condensation of either sulfur or water. In most cases, the gas in the line is at an elevated temperature and can be assumed to be at or above dewpoint temperatures. There are two dewpoints: one for water and one for sulfur. Common practice has been to employ conventional tube tracing (or, in some cases, electric tracing) to heat the line. Despite this practice, condensation and resulting corrosion still can occur. The cause is uneven or inadequate pipe heating.
Faced with this problem, a gas plant operator can choose to use a bolt-on system to achieve more uniform pipe heating. A detailed thermal analysis of the pipe, gas, heating elements and insulation should be performed before the bolt-on temperature management system is installed. Thermal analysis may reveal that the flowing process gas actually cools the pipe wall. Conventional steam or electric tracings usually do not heat the pipe uniformly, resulting in cold spots and condensation. Condensation can be prevented using the bolt-on steam tracing material properly distributed around the pipe.
A look at an actual installation at a natural gas plant sulfur degassing unit in central Alberta, Canada, shows the potential benefits.
The degas vent piping at the Alberta plant had been in place for about six years and had suffered serious corrosion, particularly in and around the low spots. The original steam tracing had been constructed with conventional 0.625" (1.5875 cm) tube tracing employing 500 kPag saturated steam. Vent line sizes were 20, 24 and 30" (50.8, 60.96 and 76.2 cm) in diameter, running about 181.5' (55 m) from the degas pit to the incinerator. They had been installed with 10 tracing tubes on the 20" line and as many as 13 tubes on the 30" line. The tracing generally was located around the bottom of the pipe in horizontal runs and on a convenient side on vertical runs. The analysis model showed that 13 tubes were the absolute minimum that could be used, and they must be spaced at equal intervals around the 30" pipe.
Because the six-year lifetime of the original pipe was considered economically unsound, plant management sought a better way to maintain pipe temperature. They reviewed the experience of similar plants equipped with bolt-on heating systems. In some instances, these systems had been effective in comparable service for more than ten years.
The staff at the gas plant analyzed the various parameters involved in heating and cooling the pipe. They used a proprietary finite-difference computer model developed by the bolt-on system supplier to create a model for gas flow through the pipe. Then, they generated piping system temperature profiles under various operating conditions, varying the number of heating elements and their spacing, steam temperature, insulation thickness and flow conditions (from maximum expected to zero flow). They also determined the optimum number of elements and their placement so that pipe wall temperatures would prevent process condensation.
Plant management then conducted a material cost and installation expense analysis specific to the Alberta plant. It revealed that even though the material cost for a bolt-on system was higher than for tube tracing, the installed cost was expected to be significantly less. Because the management staff of the Alberta plant expected longer pipe lifetime with the bolt-on system, both the performance and economic criteria were theoretically met. Management awarded a contract to the bolt-on heating system supplier for the necessary thermal modeling, design and fabrication of the new bolt-on tracing system. Installation was completed by the plant and plant contract personnel.
Effective Heat TransferThe bolt-on heating elements are drawn from SA 178 carbon steel into a rectangular shape. One side is curved to fit the outside contour of the pipe. The elements used on the sulfur pipeline are nominally 1 x 2" (25 x 50 mm). They are registered in Canada with a pressure rating of 2,380 kPag at 644°F (340°C). Heat exchange between the steam and the pipe is enhanced by the use of a thin film of heat transfer mastic. This results in improved and more predictable performance. At the Alberta plant, the bolt-on system uses fewer elements (tubes) than traditional round tubing and is more mechanically stable (figure 2).
At the gas plant, the sulfur pipeline continued to operate during installation. In spite of this, the actual installation cost was less than predicted originally. Although some of the bolt-on tracing was shop mounted prior to pipe hanging, most of the installation was done from scaffolding. The bolt-on system was supplied prefabricated into rings and headered panels.
Following installation, the system was insulated. Insulating over the bolt-on system was no more difficult than insulating similarly sized pipe outfitted with tube tracing. Steam system trapping was based on plant standards and steam loads taken from the thermal modeling program. Interconnection of steam between individual components was done with hard piping in accordance with plant standards.
At startup, pipe wall temperatures were checked with a surface pyrometer. Temperatures quickly came to values in the expected range. Since then, periodic pipe wall temperature measurements have been made, with all values falling in the expected range.
Case 2: Anhydrous Vapor ProcessA major producer of a common anhydrous material used as feedstock for production of a commodity plastic material faced a potential blockage problem. Switch condensers had been installed to capture the process material entrained in a gas stream. Inside the condensers, vapor entered the solid phase, causing an accumulation of heat. The heat melted the solid flake particles so the material would flow into a holding tank.
The condensers were not completely effective, allowing some of the anhydrous material to enter the main waste gas header. There, it solidified and caused plating to build up inside the header. The plating conducted heat poorly and created conditions that continuously aggravated the problem.
To prevent plating in the waste gas header, the header's wall temperature had to be uniformly maintained at 280°F (138°C) minimum. Precipitation would occur at 268°F (131°C), and it was important to prevent it. So, the specified performance left a margin of only 12°F (7°C).
Conventional solutions were not sufficient for maintaining the wall temperature of the waste header within this limit. Tube tracing at the required density would be difficult to fabricate and expensive to install. Also, because performance demands were greater than typical for tube tracing, there was no certainty it would work.
The solution was installation of bolt-on heating elements drawn into shape from A-106 carbon steel pipe and rated for service at 600 psig at 600°F (316°C). The elements were attached as needed to various diameter pipe sections and elbows comprising the header (figures 3 and 4).
Coverage requirements of the heating elements were determined using the proprietary thermal management and modeling software (table 1). Similarly, all of the 4 and 6" process transfer lines -- used to interconnect the condensers and transfer the molten process to a large storage area -- specified the use of single and dual element bolt-on heating systems. The valves and other process equipment in the piping system are heated with similar bolt-on heating technology. The successful operating history experienced by other processors of anhydrous feedstock over the last decade suggest similar results will be encountered in this case application.
In conclusion, uniform temperature distribution in pipeline walls can be achieved by applying bolt-on heating elements positioned using accurate thermodynamic process modeling software. The elements should be designed to efficiently transfer heat and accommodate convenient installation. A well-designed system protects the process from interruptions caused by corrosion or blockage of the pipeline. Bolt-on tracing should be considered whenever temperature variations outside of defined limits must be prevented.