Time to prepare for winter

By Dan Dvorak

Chemical Processing magazine

Winter can hamper delivery of process fluids, but steam tracing can assure smooth operations during cold weather. Here are guidelines for properly designing and installing such tracing.

Winter can cast a chill over plant operations. Delivering a process fluid at specific conditions can become difficult when significant temperature changes negatively impact fluid physical properties such as freeze point, viscosity and moisture content. Likewise, the performance of utility streams such as water, compressed gases and organic heat-transfer media can suffer at ambient temperatures below their freezing or dew points. To avoid such problems, plants must take adequate steps to maintain specific temperatures of fluid systems.

Maintaining these temperatures demands an understanding of process heat loss and methods used to limit or make up for such a loss. Insulation plays a key role, but so does heat tracing. In this article, we will specifically focus on the right way to provide steam tracing.

Insulation isn't enough
Insulation is important; it's a relatively inexpensive way to reduce heat loss. Always keep in mind, though, that insulation reduces , but does not eliminate , heat loss. In process fluid applications, this heat loss will result in lower fluid temperatures over time. Even with insulation, outside utility service systems subject to cold ambient conditions can suffer freeze-ups during periods of intermittent or reduced fluid flow.

Heat loss for a specified process pipe size, insulation type and thickness varies with the temperature differential between the process fluid and ambient conditions. For a 3-in., Schedule 40 pipe with 1.5-in. calcium silicate insulation carrying fluid at 175 Degrees F and subject to an ambient temperature of 0 Degrees F, heat loss might amount to 63 Btu/hr-ft. That doesn't seem like all that much. In reality, however, a large manufacturing operation that relies on a process fluid whose temperature control is somewhat critical would have to make up considerable amounts of heat loss to simply keep this system in operation. In addition, this incremental heat, in today's volatile energy market, comes at considerable cost.

This is where heat tracing comes in. Typical heat-tracing systems use steam, heat-transfer fluids or electricity to overcome incremental heat loss. Such systems perform two vital functions: They replace incremental heat loss through insulation and provide freeze protection by maintaining a temperature above the freeze point.

Heat tracing can clearly impact both operability and cost. Depending upon the application, the stake associated with poor heat-tracing system design, improper installation or inadequate operating and maintenance practices can be significant. Ultimately, results of poor heat-tracing system performance can include:

* Unscheduled area or complete site outages.

* Increased system maintenance costs.

* Poor process control.

* Process safety or environmental-related incidents.

Given the potential business impact, it's important to understand and respect the role heat tracing systems and components play in maintaining safe, reliable and cost-efficient operations.

Steam's attraction
The availability of steam at most plant sites, coupled with its thermodynamic properties and relative ease of use, make it an attractive heat-transfer media for many heat-tracing applications. Steam-tracing system designs use pipe jacketing, relatively small-diameter pipe or tubing that is attached to process piping or vessels. Theoretically, steam flowing through the tracer pipe or tube will give up its latent heat to the heated pipe and condense at constant temperature. Although process temperatures may require application of high-pressure steam, pressures found in steam tracer systems commonly are less than 250 psig. Heat transfer can occur via either conduction or convection.

Jacketed piping, direct-contact bare tubing and tubing that is bonded to the surface by solid heat-transfer material are examples of conduction-type steam tracing. Applications requiring critical temperature control, high temperatures or uniform pipe-wall temperatures often rely on fixed jacketed piping or "bolt-on" versions.

Tubing-type steam tracers bonded to the surface with a solid heat-transfer material have proved to be extremely cost-effective alternatives to the jacketed design. The bonding material acts to increase both the heat transfer coefficient and heat transfer area. It allows greater heat input compared to a plain tracer in contact with the surface. Generally speaking, these types of tracers are used when temperature requirements exceed 175 Degrees F. Typically, tubing materials are copper for temperatures up to 400 Degrees F, and stainless steel for higher temperatures; tubing sizes usually are in the ³/₈-in. to ³/₄-in. range, although larger tubing and even pipes sometimes are used.

Convective-type steam tracers generally employ tape instead of heat-transfer material to attach the tubing to the surface. Air convection movement between the tracer and the surface transfers the heat. Bare tubing can be used, typically with spacers to keep it off the surface. An alternative design uses tubing with some form of covering or jacket that reduces thermal conductivity. This "light" or "isolated" design can closely match input requirements and avoid localized hot spots and issues of spacer location, attachment and movement during installation. These types of tracers usually suit applications with lower temperature requirements, say, under 175 Degrees F.

Figure 1 shows a typical steam-tracing installation for maintaining a specified process-fluid temperature range. The installation consists of steam-supply piping, manual isolation valves, control valves, tracer tubing, the heated fluid piping system, insulation, steam trap and condensate return piping.