VTI

Pipeline heating is a common requirement in many industrial processes across a broad spectrum of industries, including petroleum, plastics, chemicals, pharmaceuticals, power generation and food processing. Pipeline heating applications, commonly referred to as heat tracing systems, provide heat both to prevent pipeline freezing and to replace process heat loss. Often, electrical heating is the most practical and lowest cost way of achieving pipeline heat tracing.

VTI's tcm120 Controller used in its Resistance Inference Mode with self-regulating heating cable, provides modern, closed-loop temperature control without the need for conventional temperature sensors or field mounted control devices of any kind. Consequently, the Val-Tech approach produces the easiest to design, easiest to install, easiest to maintain, most energy efficient, and most cost effective overall solution to pipeline heating applications.

Self-regulating heating cables produce heat from the flow of current through a polymeric heating element located between two parallel copper wires. The heating element possesses a positive change of resistance with temperature, thus limiting internal power dissipation and preventing destruction due to self heating. It is this relationship between the heating element resistance and temperature that can be used to control pipeline temperature in lieu of dedicated sensors such as RTD's, thermostats, and thermocouples.

A self-regulating heating cable can be visualized as an array of parallel resistors with each individual resistor having a value inversely related to its temperature. As a result the measurement of pipeline temperature by sensing cable resistance produces a value averaged over the length of the cable being used. The averaging effect is a desirable feature when one considers that individual heating zones can exceed 200 feet in length, and that pipeline temperature will vary along its length due to variations in thermal losses caused by such effects as valves, pipe supports, damp insulation, etc. The problem of positioning a single external sensor in the correct location along a pipeline is thus avoided.

A typical pipeline heating system consists of self-regulating cable strapped to the upper quadrant of the pipe with both pipe and cable jacketed under a thick insulating blanket to minimize thermal losses to the environment. The inside diameter of the insulation should be larger than the pipe/cable diameter to provide an air chamber which acts via convection to warm the pipe. In a steady state condition the heat produced by the cable balances the heat which is lost by the system to its ambient.

The cable temperature must exceed the pipe temperature if heat is to flow from cable to pipe. At equilibrium the heat being lost to the environment equals the product of the thermal resistance ( Ø ) and the heat energy produced per unit time or watts. In order to estimate the pipe temperature both the cable (equation 1) and the thermal loss term must be evaluated. By using manufacturers data and theoretical analysis of the cable connection, an analytical method has been developed to infer cable from cable resistance. Similarly, the value for the loss term has been evaluated for manufacturer recommended installation guide-lines.

The plot of cable temperature vs. resistance for a 20 watt/ft. cable is shown in (figure 2). The curve labeled delta is the difference between the heating cable and the pipe caused by losses to the ambient. Therefore, the pipe temperature is the difference between the cable temperature and delta curves.

Since pipe temperature is proportional to cable resistance, the pipe temperature can be held constant by maintaining a constant cable resistance. Figure 3 is a block diagram for a theoretical temperature controller. The inner loop is a power controller. Its function is to maintain a constant power output whose reference is established by the resistance loop- The outer loop is the resistance loop. The cable model is used to compute the resistance at which the set point temperature will be achieved. This resistance is the reference for the resistance loop.

In actual practice, the user of self- regulating cable has the option of selecting from various wattage per foot cable (generally ranging from a low of about 3 watts/foot up to 20 watts/foot) and from either low temperature or high temperature range operating characteristics. Of course, the material costs of low wattage/low temperature range cable has the advantage of being $4 to $5 per foot less than high wattage/high temperature cable, however the installation cost is exactly the same. In addition, using high wattage/high temperature range cable has numerous other benefits that mitigate this material cost differential quite quickly:

High temperature range cables will not be destroyed if the pipeline is steam-cleaned or if a steam lance is used to remove a blockage.

The full output of the high wattage cable will be available to prevent freezing or melt-out of the pipeline if there is an insulation breakdown either in local areas or across the complete circuit, thereby reducing process downtime.

The only engineering required when using high wattage cable is a quick check of manufacturer supplied data to insure that the high wattage cable has sufficient watts/foot to overcome the worstcase heat loss of the pipeline. This normally results in excess heat capacity being available, but this excess capacity allows the system to compensate for the losses at flanges and valves, where it is more difficult to install enough cable on the pipeline. Then only a rough check of the circuit length for each zone circuit is needed. The actual length is not critical unless the circuit length exceeds the maximum allowable.

By using high wattage/high temperature cable, only one type of cable need be stocked on site, thereby eliminating selection mistakes during installation. Field installation becomes a matter of mounting only one type of cable on the pipeline, cutting length at either end of the circuit, and connecting to the controller. In addition, savings are achieved from larger volume purchases of a single item and, as pipeline size goes up, the price differential between cable types becomes smaller or disappears.

VTI also recommends that the following procedures be followed in pipeline heating design to insure the most reliable, safe, maintenance-free system:

Use Highest Quality Insulation System

• Polyisocyanurate for temperatures > 250°F
• Perlite expanded calcium sulfate for temperatures < 250°F
• Oversize insulation by one size
• Outside load bearing pipe supports
• All insulation penetrations in lower 180° quadrant (to minimize water penetration into insulation gaps)

This was a very basic introduction to heat tracing system design; there are many technical issues and considerations which must be addressed in the design of any specific system. VTI's expertise in this area is unmatched; please contact us for more information or consultation. We would be happy to come to your facilities and do an on-site, thorough presentation.

VTI's Temperature Control Systems can be used in many Heat Tracing applications. Please find more information on the following temperature control products:

• VTI Model itc5000 Heat Tracing Control System
• VTI HPP42 Heater Power Panel
• VTI TS10 Series Temperature Switches