Trace heating in hazardous areas

Gary Ashburner, Managing Director of Bartec (UK), explains how choosing the right trace heating system can dramatically reduce operating costs.

Predominantly, the need for trace heating in the UK is to maintain process temperatures in pipework, vessels and plant, and more recently on hot water pipes in the building industry, for instant hot water.

Now with the general acceptance of global warming, increasingly here in the UK, and in other countries with much colder climates, these requirements extend to freeze protection, snow melting and ice prevention.

Over the years, electrical trace heating has undergone dramatic developments, resulting in the production of a wide range of electric heating cables and sophisticated control and monitoring devices becoming available to plant designers.

The constant need for safety in hazardous areas has also produced a complete range of approved equipment for safe use in the (Ex) explosion protection environments, found in the oil, gas, coal, petrochemical and other industries.

Electrical trace heating can now be used with confidence in hazardous areas thanks to the availability of a variety of approved products, usually complying with the requirements for explosion protection type "e" "increased safety" equipment.

The following provision must be adhered to when trace heating with electrical heating cables having type "e" protection: heating tapes shall have a metallic covering with provision for earth connection, and provide both a high degree of mechanical protection and protection against ingress of moisture; there must be a means of isolating the system from the main incoming supply; the system must be provided with over current and earth leakage protection for each heating zone; and the system must be protected against overtemperature either by the heater stabilising at a safe temperature, or alternatively by means of temperature limiting controls.

Of particular importance in the hazardous area is the surface temperature of the heater, which must not exceed the T-class of the environment.

As the use of heating tapes and cables for industrial and building applications increases annually by thousands of kilometres, an ever growing proportion of installations are in hazardous areas where modern systems have consistently proven themselves to be cost effective, reliable and above all, safe, when compared with steam.

More and more engineers are turning to electrical trace heating because of the many benefits it offers.

Electrical systems cost less to install, are cheaper to run and have almost no maintenance costs.

Temperatures on pipelines and vessels can be controlled much more accurately, avoiding potentially costly or dangerous over or under heating of processes.

Time is saved during startup, as it is instantly available and process efficiency is improved by maintaining the product at its optimum temperature in vessels, or as it flows along pipelines.

Trace heating is also used for anticondensation purposes to avoid rapid corrosion of plant and guards against product contamination.

The advantages of electrical trace heating are as follows: more accurate control; lower capital, maintenance and operating costs; better efficiency; improved plant safety; and increased life of process plant.

Today, when designing electrical surface heating systems, the range of heating cables available allow the heating engineer to choose one or a combination of heater tapes and cables to perfectly match the parameters of a plant, thus balancing optimum performance with safety.

Heating cables or tapes can be divided into four main types: self limiting parallel resistance heating tapes operating at both high and low temperatures; constant power parallel-resistance heating tapes; single-core polymer-insulated heating cables; and single-core mineral-insulated heating cables.

From the original on/off mechanical thermostats, a new generation of energy management devices and circuit health monitoring and alarm equipment has evolved, that improve energy efficiency, reliability, temperature, safety and system life.

Today, trace heating systems can have programmed cycles for heat raising or melt-out, maintaining temperatures, cool down cycles, soft-start facilities etc, with energy delivered by the system matched precisely to the process requirements.

Changes in process system usage are accommodated simply by the reprogramming of controls.

Mechanical thermostats have given way to electronic controllers that are more accurate, more efficient and much more reliable.

The ability to site controls remotely and the development of devices providing energy management, circuit health monitoring/alarm etc have brought about centralised control and monitoring for installations requiring high system integrity.

The step from centralised control to Scada control and data acquisition was a logical one.

Mimic displays indicate the system status, and enable calibration for optimum performance, efficiency, maximum demand, safety and system life.

Reliability has come with electronic controllers having no moving parts, except the switch itself.

Relays and contactors used to switch traditional mechanical thermostats enjoyed a reasonable life as the switching action was infrequent due to a wide switch differential.

Accurate electronic controllers, switch much more frequently and contact life can be unacceptable unless switching takes place at zero voltage, rendering it spark free.

This increases contact life by 1000 times, effectively increasing life from 35 days to 100 years, based on two switching operations per minute.

In hazardous areas, compliance with the Ex standards, as described, and special observance of the T-class rating are imperative.

For all these applications, the question of energy control arises.

The extent of control is often determined by economics - the larger the system, the greater the tendency to optimise control.

But control should not be understated.

For each of the applications mentioned, energy management (as opposed to on/off control) is likely to produce a reduction in operating costs of: freeze protection - up to 90% plus; process temperature maintenance - 40 to 70%; snow melting/ice prevention - up to 80%; and hot water heating - up to 40%.

So, in this time of industrial soul-searching about climate change and the environment, where energy efficiency is becoming more and more of an issue, the spotlight on energy saving systems has never been more keenly focused.

It is therefore worth stating that in addition to its obvious increase in effectiveness, the acceptance of innovative energy management technologies by industry will not only bring about vast improvements in safety, production reliability and life, but in addition can help reduce carbon dioxide emissions, reducing the rate of climate change that is inflicting irreversible damage on the planet.

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