 Neil Malone, chairman of Heat Trace Ltd., the global technological leaders in electric heat tracing, provides an introduction to heat tracing, discusses the various systems available and gives an overview of the use of heat tracing across all process industries.
Heat tracing (or trace heating) has developed from a small niche activity in the 1960s into a highly sophisticated, technology-led and commodity-style industry today.
A method of applying heat to a body or a product (liquid, powder or gas), contained within a system (pipework, vessel or other equipment) for storage or transportation, using an adjacent steam pipeline or electrically heated cable, heat tracing is largely used to: prevent liquids from freezing, and to reduce viscosity to enable pumping, compensate or balance loss of heat from pipelines, tanks, vessels and hoppers to surrounding atmosphere, raise temperature of pipelines, tanks, vessels and hoppers following short shutdown periods, eliminate condensation that could result in dry powders clogging, prevent hydration of gases, due to a drop in pressure across fittings such as valves, prevent formation of hydrates and waxes in hydrocarbon liquids (oil, etc),
Electricity or steamTypically, the energy source used for heat tracing is either electricity or steam.
In the past, process-generated steam was frequently seen as the preferred means of heat tracing, particularly across the petrochemicals and petroleum industry.
Although process steam may be perceived as free, the reality, however, is quite different. As steam tracing is rarely controlled, it may typically deliver many times the volume of heat required to protect a pipeline from freezing.
In light of its obvious disadvantages, such as poor temperature control, condensation issues, leaks, poor heat transfer (due to limited contact area between the steam tracer and the pipeline) and high energy costs, steam tracing has fallen increasingly out of favour, with industry preferring the convenience offered by today’s self-regulating (or self-limiting) cable-type electric heat tracers, which can be cut to length at site and are usually temperature safe.
Even where excess steam is freely available, however, organisations are frequently finding that the most efficient course of action is to use that excess steam to generate electricity which, in turn, is then used as the energy source for a controlled, highly efficient electric heat tracing system. The purpose of heat tracingHeat tracing is usually provided to maintain a product or equipment at a temperature that will prevent processing problems. For example: - temperature maintained above 5°C – to freeze-protect water or aqueous solutions,
- temperature maintained above 50°C – to prevent oil from becoming too viscous to pump,
- to maintain surfaces of powder storage vessels above dew point temperatures, below which condensation could form and cause blockages.
Heat raising capabilitiesHeat tracing may also be used to ‘heat raise’ products from cold to a required maintain temperature. A case in point would be where a pipeline is used infrequently to deliver fuel oil from an off-loading berth into a plant area. In such an instance, the pipeline and its contents may be raised from the ambient temperature to the fuel oil pumping temperature over a period of, for example, 24 hours prior to the delivery of the fuel oil.
Understanding the need for heat tracingWhenever the contents of a pipe or equipment are maintained at a process temperature exceeding the ambient temperature, there will be a flow of heat from the product or equipment through the thermal insulation to the external air. The rate of heat loss varies directly with changes in ambient temperature.
To prevent the temperature of the product falling below its required level, this variable heat loss must be compensated for by heat tracing the pipeline, vessel or equipment involved.
A typical electric heat tracing systemA heat tracing system normally comprises a heater, together with some form of temperature control and monitoring system. Advances in electronics and microelectronics have brought about a quantum leap in temperature control and monitoring techniques. A typical system will embrace:
- heating cable(s) together with termination components,
- ancillary items, such as junction boxes and fixing materials,
- temperature control devices (sometimes/optional),
- monitoring/alarm technologies (sometimes/optional),
- power distribution/circuit protection facilities.
The types of electric heat tracing technologyThere are four generic types of heat tracer technology. They are:
- parallel (self-regulating),
- parallel (constant power) zonal,
- parallel power limiting,
- series resistance,
- skin tracing.
Parallel self-regulating (often called self-limiting) tracers (Fig.1) remain the most popular option. This is because they can be conveniently cut to length on site and are often inherently temperature safe, due to the positive temperature-coefficient heating matrix. As a result, temperature control is not usually needed to provide temperature safety.
Until recently, parallel self-regulating cables were limited to low/moderate power applications. Recent developments, however, have resulted in the introduction of semi conductive tracers able to withstand temperatures up to 300°C. This means that self-regulating heat tracers can now fulfil 80-90% of all applications within industrial heat tracing.
Parallel Constant Power zonal heat tracers (Fig. 2) can also be conveniently cut to length on site. They are, however, less popular as they often require thermostatic control to ensure temperature safety. That said, sometimes a calculated temperature-safe stabilised design is possible.
Until recently, most constant power tracers were polymeric and were therefore limited in temperature capability. As a result of the introduction of a patented parallel resistance cut-to-length metal sheathed, mineral insulated heater - known as AHT - having a withstand temperature of 425°C, there is now a solution for applications that cannot be handled by parallel self-regulating heat tracers.
 Parallel Power Limiting heating cables have emerged in recent years, but the term “power limiting” is marketing jargon and they offer no technical benefits. Such heating cables are not inherently temperature safe and are closer to parallel constant power heaters than self-regulating heaters.
Series Resistance heat tracers (Fig. 3) are disadvantaged by virtue of the fact that they need to be individually designed into particular length/load configurations, meaning they are not quite as versatile as the parallel derivatives already mentioned. Their advantage, however, is that extremely long circuit lengths are possible, with electric supply points at multi-kilometre intervals. Known as ‘Longline’ heat tracers, typical operating limitations are as follows:
- up to 1000 Volts (3 phase),
- up to 230°C withstand temperature,
- up to 60 W/m.
 Skin-tracing (Fig. 4) is an induction-resistive method of heat-tracing based on skin and proximity effects of an AC current within a ferromagnetic tube. In this type of heat tracing, the heating element comprises a carbon steel tube into which is inserted an insulated non-magnetic conductor. The conductor and steel tube are connected together at one end. At the other end, an AC voltage is applied between the conductor and the tube. The relationship of conductor/tube sizes and voltage determines the outward power developed. The skin effect of the magnetic tube results in the current being concentrated towards the tube’s inner surface, the potential to the outside being zero.
The advantage of using skin-tracing is that extremely long circuit lengths are possible, with a pipeline of up to 30km being heated with a single electric supply point, making this type of induction-resistive trace heating most suitable for cross-country pipelines.
Selecting the most suitable heat-tracing solution
As the different product types described above illustrate, different heat tracers suit different environmental and plant conditions.
Heat tracers for in-plant areas are typically selected according to the maximum temperature to which the tracer will be subjected, and the power required from the tracer.
Heat tracers for long pipelines are usually selected according to pipe length and the ability of the chosen tracer to minimise the number of electrical feed points as well as distribution costs. In calculating heating loads for pipelines, the potential for heat loss from fittings such as valves, flanges, strainers filters and pumps have to be accounted for as they can typically equate to an additional 25% of pipework heating load requirements. Additionally, pipe supports, which are rarely detailed on engineering drawings can also account for considerable heat losses unless the supports are thermally insulated.
Creating a safe system that worksThe prime objective in considering a heat tracing installation should be providing the highest levels of safety. This is achieved by ensuring temperature safety, over-current protection and earth leakage protection.
Temperature safety is achieved by preventing the surface of the heat tracer from exceeding the limiting temperature; this limiting temperature may be the maximum rating of the tracer, the maximum design temperature of the pipe and/or product ,or the temperature classification (T-Class) of the area where the installation is within an hazardous location.
Temperature safety can be provided in the following number of ways:
- use of inherently temperature-safe heat tracers,
many self-regulating tracers fulfil this criteria, their power output reducing with rising temperature and thereby providing the highest levels of safety;
the favoured form of temperature safety where inherently temperature-safe heat tracers are not available. A calculation is made to ensure that, under worst-case conditions, the heat tracer always operates at below the limiting temperature, without need for additional temperature control;
this method is necessary where a stabilised design cannot be assured. The safety of the system relies on the correct functioning of the controller and the location/operation of the temperature sensor, making this the least safe option;
Temperature control may also be incorporated into the heat tracing system to ensure temperature safety, or to provide process temperature accuracy. It is recommended that, when using temperature controllers to ensure limiting temperatures are not exceeded, they should only be considered when the use of inherently safe heaters, or a stabilised design, is not possible. In such instances, it is required that:
- in Safe Areas, a controller provided for process temperature control may also act as the over temperature controller,
- in Zone 2 Areas (where explosive gas-air mixtures may be present only under abnormal conditions), an over temperature controller is required which must be in addition to the process temperature controller, and
- in Zone 1 Areas (where explosive gas-air mixtures may be present), it should be as for Zone 2 , but the over-temperature controller should either be a manually re-settable lockout type, or be fitted with an over temperature alarm. If an alarm is used, there must be adequate monitoring.
The sensor of the over-temperature controller must be fitted to the pipe or workpiece to limit the pipe to a temperature level at which the heater will not exceed the maximum limiting temperature. It should NOT be sited on the actual heater itself!
Other safety considerations include the provision of over-current protection for each heating circuit and earth leakage protection.
Conclusion
In conclusion, it is perhaps wise to emphasise the importance of energy efficiency in relation to heat tracing control systems. All heat tracing systems are designed to cope with worst case situations and, as such, are almost always over-powered, given their average operating conditions. Considerable savings in operating costs can be achieved by using specifically designed, energy-efficient control systems. In some cases, energy savings of 80-90% are possible when compared with conventional ON/OFF switching controls.
A typical refinery, for example, or large processing plant in northern Europe, might contain upwards of 10-12 Megawatts of heat tracing. Therefore, by using the right control system, plant operators can make significant reductions in energy usage with meaningful savings in plant operating costs.
Heat Trace Ltd
Frodsham
Cheshire
Can be contact on
Tel: 01928 726 451
Fax: 01928 727 846
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Web: www.heat-trace.com
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