Explosive atmospheres — Part 30-2: Electrical resistance trace heating — Application guide for design, installation and maintenance ICS 29.260.20 12&23... 8.4 Installation of control an
General
This standard supplements the requirements of IEC 60079-14 and IEC 60079-17
When installing trace heating systems in explosive gas atmospheres, it is essential to specify the hazardous area classifications according to IEC 60079-10 This specification must include the designated zone (1 or 2), gas group (IIA, IIB, or IIC), and temperature classification as per IEC 60079-0 Additionally, any special considerations or challenging site conditions should be clearly outlined in the trace heating specification.
When installing trace heating systems on mobile equipment or interchangeable skid units, it is essential to ensure that the specifications for these systems are designed to withstand the most challenging conditions they may encounter.
Where any parts of the trace heating system are likely to be exposed, those parts should be suitable for the environment.
Corrosive areas
It is essential to assess all components of electric trace heating systems for compatibility with corrosive materials encountered throughout their lifespan, as these systems are more prone to failure in corrosive environments Corrosion can exacerbate the deterioration of thermal insulation, particularly when weather barriers fail and leaks occur in pipelines or vessels Special attention must be paid to the materials used in piping systems and electric trace heating systems, especially concerning the effectiveness of earth-leakage and ground-fault return paths The use of non-metallic, lined, or coated piping can complicate these return paths, necessitating careful consideration Additionally, earth-leakage and ground-fault return paths established during installation may deteriorate due to corrosion over time.
1 IEC 60050-426, International Electrotechnical Vocabulary (IEV) – Part 426: Electrical apparatus for explosive atmospheres
Process temperature accuracy
A Type I process requires maintaining the temperature above a minimum threshold, with ambient sensing control being a viable option A single control device and an electrical distribution panel board can manage large power blocks, although heat input may sometimes be excessive, allowing for wide temperature fluctuations Implementing dead-leg control techniques can enhance energy efficiency.
A Type II process is one for which the temperature should be maintained within a moderate band Control by pipeline sensing mechanical thermostats is typical
A Type III process necessitates precise temperature control within a narrow range Electronic pipe-sensing controllers, utilizing thermocouples or resistance-temperature detectors (RTDs), enable field calibration and offer flexibility in temperature alarm and monitoring functions These systems can provide heat input to preheat an empty pipe or elevate fluid temperature within a defined range and time frame Strict adherence to flow patterns and thermal insulation systems is essential for Type III systems.
Installation considerations
A trace heating system is a critical component of the overall process, as any failure in its parts can lead to safety or process issues The requirements for temperature control and circuit monitoring in an application should be established based on the temperature control types outlined in section 4.3, along with the criticality of circuit monitoring detailed in Table 1.
Desired accuracy of process temperature control
Is trace heating a critical component of the process? Above a minimum point
Type II Within a narrow band
In critical trace heating applications, it is essential to implement circuit monitoring for proper operation, malfunction alarms, and redundant trace heaters Specifying spare controllers that activate automatically during faults enhances system reliability This redundancy allows for maintenance or repairs without necessitating a process shutdown, ultimately improving overall system dependability.
General
The selection, installation, and maintenance of thermal insulation are crucial for the efficiency of an electrical trace heating system A well-designed thermal insulation system minimizes heat loss, while the trace heating system compensates for any remaining loss Consequently, issues with thermal insulation can significantly affect the overall performance of the system.
Thermal insulation primarily serves to minimize heat transfer from surfaces with temperatures differing from the surrounding environment, effectively reducing energy loss and lowering operating costs.
Before conducting a heat loss analysis for electrically traced pipelines, vessels, or other mechanical equipment, it is advisable to review the insulation system selection Key areas to consider include the effectiveness of the insulation material, its thickness, and the overall design to ensure optimal thermal performance.
– selection of an insulation material;
– selection of a weather barrier (cladding);
– selection of the economic insulation thickness;
– selection of the proper insulation size.
Selection of insulating material
The following are important aspects to be considered when selecting an insulation material These factors should be considered and the selection optimised according to the operator’s criteria:
– thermal conductivity, λ, of the insulation;
– chemical compatibility and corrosion resistance;
– toxicological properties when exposed to fire;
Insulation materials commonly available include:
When using soft insulants like mineral fibre or fibreglass, it's important to tightly band the insulation around the actual pipe size, ensuring that the trace heater is not buried within the insulation to avoid damage and ensure proper heat transfer Alternatively, the next largest pipe size insulation that can comfortably enclose both the pipe and the electric trace heater is acceptable For rigid insulation types such as calcium silicate or cellular glass, pipe-size insulation can be used if board sections are cut to fit the longitudinal joint, a method known as extended leg installation Alternatively, selecting the next largest insulation size to accommodate the trace heater is also permissible It is crucial to clearly specify the insulation size and thickness in all cases.
Selection of weather barrier (cladding)
The effective functioning of an electrically trace heated system relies on the insulation being dry, as electric tracing typically does not generate enough heat to dry wet thermal insulation Additionally, certain insulation materials may fail to restore their original properties even after being removed from the piping and force dried.
Straight piping can be effectively weather-protected using metal jacketing, polymeric materials, or a mastic system When opting for metal jacketing, it is essential that the surface is smooth and features formed, modified “S” longitudinal joints Additionally, the circumferential end joints must be sealed with closure bands and applied with sealant on the outer edge or at overlapping areas.
Overlapping jacketing or unsealed joints are ineffective in preventing moisture intrusion Even a single unsealed joint can lead to significant water leakage into insulation during rainstorms.
The type of weather barrier used should, as a minimum, be based on a consideration of the following:
– corrosive nature of chemicals in the area;
3 metal jacket insulated pipe 7 movement
Figure 1 – Thermal insulation – Weather-barrier installation
Selection of economical thickness
When evaluating insulation from an economic perspective, it is essential to compare the initial costs of materials and installation with the energy savings achieved throughout the insulation's lifespan It's important to recognize that actual insulation thicknesses may not always align perfectly with nominal thicknesses Additionally, when selecting insulation size, one must consider whether the actual pipe-size insulation can adequately accommodate both the pipe and the trace heater.
Double insulation
The double insulation technique is utilized when pipe temperatures exceed the maximum allowable limits of trace heaters, particularly to prevent condensate freezing in inactive high-temperature steam lines This method involves placing the trace heater between two layers of insulation around the pipe The key to this technique is selecting the appropriate types and thicknesses of inner and outer insulation to achieve a suitable interface temperature for the trace heater, taking into account maximum ambient temperature conditions.
2 inner insulation layer 7 interface temperature
3 heat tracer 8 outer insulation surface temperature
4 outer insulation layer 9 ambient temperature
Introduction
Trace heating applications require careful design to meet specific temperature demands and maintain them under defined conditions These systems must effectively integrate with other components, including thermal insulation and the available electrical supply To ensure optimal performance, it is essential to understand and control the values of these interfacing elements during the design process.
The design of trace heating systems must adhere to all IEC standards for electrical equipment, as well as the specific requirements of this standard It is essential to consider system and process equipment maintenance, energy efficiency, and the testing of installed systems to ensure operational satisfaction and safety.
When designing trace heating systems for use in explosive gas atmospheres, additional constraints are imposed due to the requirements and classification of the area under consideration
Persons involved in the design and planning of electric trace heating systems should be suitably trained in all techniques required.
Purpose of, and major requirement for, trace heating
When selecting and installing trace heaters, it is essential to ensure they provide adequate power for three key purposes: a) compensating for heat loss to maintain a specified temperature of a workpiece at the minimum ambient temperature, as detailed in calculation method 6.3; b) raising the temperature of a workpiece and its contents within a designated time frame, following calculation method 6.4; or c) fulfilling a combination of both a) and b).
The system heat requirements should then be multiplied by a safety factor as determined on the basis of 6.5
When selecting trace heaters, it is essential to determine the maximum system temperature under worst-case conditions as outlined in IEC 60079-30-1 This temperature can be lowered by adjusting system parameters, utilizing multiple tracers to decrease power output per unit length, or choosing an appropriate temperature control system It is crucial that the installed power exceeds the required power without introducing unacceptable risks in explosive gas atmospheres, even after evaluating worst-case scenarios.
Heat loss calculations
The heat loss of a workpiece can be simplified and calculated using the formula \$q = k \Delta T\$, where \$q\$ represents the heat loss per unit length of pipe in watts per meter (W/m) In this equation, \$\Delta T\$ denotes the temperature difference between the desired maintenance temperature (\$T_p\$) and the minimum design ambient temperature (\$T_a\$) in degrees Celsius (°C) The thermal conductivity of the system, represented by \$k\$, is treated as a constant value (W/m K) for simplification purposes.
Factor k is influenced by the thickness, size, and type of thermal insulation layers, as well as the mean temperature of the insulation and the convective film coefficients of both the workpiece contents and the external environment Consequently, the accuracy of the calculations relies heavily on the precise definition of these system parameters.
To calculate heat loss for pipes and tubes accurately, detailed calculations are necessary The equation presented in (1) is modified to incorporate the conduction parameters.
T T q K (2) where q is the heat loss per unit length of pipe, in watts per metre (W/m);
T p is the desired maintenance temperature, in degrees Celsius ( °C);
T a is the minimum design ambient temperature, in degrees Celsius ( °C);
D 1 is the inside diameter of the inner insulation layer, in metres (m);
D 2 is the outside diameter of the outer insulation layer, in metres (m);
K is the thermal conductivity of the inner layer of insulation evaluated at its mean temperature (W/m K)
To enhance the precision of the heat loss equation, it is essential to differentiate the characteristics of various system layers and include the convective parameters, as illustrated in the subsequent equation.
D 2 is the outside diameter of the inner insulation layer, in metres (m), (inside diameter of the outer insulation layer when present);
D 3 is the outside diameter of the outer insulation layer when present, in metres (m);
K 1 is the thermal conductivity of the inner layer of insulation evaluated at its mean temperature (W/m K);
K 2 represents the thermal conductivity of the outer layer of insulation, assessed at its mean temperature (W/m K) The inside air contact coefficient from the pipe to the inner insulation surface is denoted as h i (W/m 2 K), while h co refers to the inside air contact coefficient from the outer insulation surface to the weather barrier (W/m 2 K) Additionally, h o indicates the outside air film coefficient from the weather barrier to the ambient environment (W/m 2 K), with typical values ranging from 5 W/m K to 50 W/m K for low-temperature applications below 50 °C.
Vessel heat losses often require a more complex analysis to determine total heat loss The trace heating supplier should be consulted
For ease of product selection, trace heating suppliers will often furnish simple charts and graphs of heat losses for variously maintained temperatures and insulations, which usually include a safety factor.
Heat-up considerations
In specific plant operations, it is essential to ensure that the trace heating system can elevate the temperature of a static product within a designated timeframe The heat delivery requirements for a trace heated system on piping can be assessed using a specific equation.
U is the heat loss per unit length of pipe per degree of temperature difference o 3 co 3 2
The thermal time constant, denoted as H, represents the total energy stored in the mass of the pipe, fluid, and insulation for each degree of temperature It is calculated by dividing this energy by the heat loss per unit length for each degree of temperature differential.
P 1 is the density of the product in the pipe (kg/m 3 );
C p1 is the specific heat of the product (J/kg K);
V c1 is the internal volume of the pipe (m 3 /m);
P 2 is the density of the pipe (kg/m 3 );
C p2 is the specific heat of the pipe (J/kg K);
V c2 is the pipe wall volume (m 3 /m);
P 3 is the density of insulation (kg/m 3 );
C p3 is the specific heat of the insulation (J/kg K);
V c3 is the insulation wall volume (m 3 /m);
T i is the initial temperature of the pipe, in degrees Celsius ( °C);
T f is the final temperature of the fluid and the pipe, in degrees Celsius ( °C);
T a is the ambient temperature, in degrees Celsius ( °C);
T p is the desired maintenance temperature, in degrees Celsius ( °C); t is the desired heat-up time, in seconds (s);
U is the heat loss per unit length of pipe per degree of temperature (W/m K);
H is the thermal time constant, in seconds (s);
T sc is the temperature at which phase change occurs, in degrees Celsius ( °C); h f is the latent heat of fusion for the product (J/kg); q c is the output of the trace heater(s) (W/m)
The relationships discussed assume that system densities, volumes, thermal conductivities, and heat losses remain constant across the relevant temperature range It is important to note that certain products do not experience a phase change during heating While the model effectively represents a straight pipeline, it does not account for additional equipment like pumps and valves.
Insulation for valves, flanges, pumps, instruments, and uniquely shaped equipment can be tailored to fit specific configurations This insulation can be made from blocks, segments, or flexible removable covers.
Non-insulated or partially insulated pipes and equipment need extra heat to offset increased heat loss To address this, insulating cements or fibrous materials should be utilized to seal cracks and joints When insulating cements are applied to irregular surfaces, a thicker layer may be necessary to ensure effective insulation.
Heat-loss design safety factor
To ensure accurate heat-loss calculations, it is essential to apply a safety factor to the theoretical values, as these do not account for real-world installation imperfections This safety factor, typically ranging from 10% to 25%, compensates for various tolerances in the trace heating system Key considerations for determining the safety factor include thermal insulation degradation, supply voltage variations, branch wiring voltage drop, trace heater voltage drop, increased radiation and convection in high-temperature applications, and the quality of thermal insulation installation.
Selection of trace heater
When selecting trace heaters for a specific application, it is essential to consider several key design characteristics Firstly, the maximum withstand temperature of the trace heaters must exceed the highest possible workpiece temperature, which may surpass the normal operating temperature Secondly, the heaters should be appropriate for the specified environmental conditions, such as corrosive atmospheres or low ambient temperatures Lastly, it is crucial that trace heaters are certified for use in the designated explosive gas atmosphere.
In any application, it is essential to adhere to the maximum allowable power density for trace heaters to prevent damage to the workpiece or its contents, especially in sensitive situations like lined pipes or vessels with caustic soda This maximum power density must be documented in the system records Additionally, multiple tracing or spiraling of a single trace heater may be necessary to ensure optimal performance.
Site-fabricated trace heaters are permissible provided:
• installation personnel are competent in the special techniques required; trace heater(s) pass the field (site work) tests specified in 8.5.5; trace heater(s) is/are marked in accordance with 6.1 of IEC 60079-30-1
Trace heaters that meet the specified criteria are technically appropriate for the application; however, it is essential to establish the maximum allowable power density for each unit This determination depends on factors such as the heater's construction, maximum withstand temperature, maximum operating temperature, and the maximum permissible temperature of both the workpiece and the thermal insulation.
The maximum allowable power density for each trace heater must be determined from the manufacturer's data, following the tests outlined in Clause 5 of IEC 60079-30-1 This value should be selected to ensure that the maximum withstand temperature is not surpassed The limiting maximum allowable power density is defined as the lower value between the manufacturer's data and the specifications for the process, with the possibility of further restrictions due to the requirement for multiple tracing.
The designer must choose the appropriate type, length, size, and loading of the trace heater, ensuring that the installed load meets or exceeds the design loading while maintaining a power density that does not exceed the specified limits It is essential to document the type of trace heater along with the values of installed load and power density in the system documentation.
6.6.1 Specific types of trace heating
The series and parallel types of trace heating are generally defined by their electrical characteristics
Series trace heaters utilize electrical conductors as heating elements, making voltage supply and circuit length crucial in their design For long circuit lengths, series trace heaters with polymeric insulation are ideal, while those with mineral insulation and metallic sheaths are best for maintaining very high process temperatures In contrast, parallel trace heaters consist of two parallel conductors with a separate heating element, suitable for freeze protection and maintaining process temperatures in complex piping systems The constant wattage type features a spirally wound metallic heating element.
The PTC type features a polymeric heating element extruded between conductors, while the power-limiting type offers higher output at elevated temperatures compared to the PTC type, yet operates at lower temperatures than the constant wattage type.
6.6.2 Trace heater performance and equilibrium conditions
Evaluating heat tracing systems at equilibrium conditions is essential, particularly for systems without controls, those with ambient sensing controls, and those intended for explosive gas environments Power output curves for constant wattage trace heaters and PTC (positive temperature coefficient) trace heaters reveal distinct characteristics The heat loss line indicates conditions at the lowest ambient temperature, demonstrating that the constant wattage trace heater, with the highest output of 32 W/m, maintains the workpiece at the maximum temperature of 80 °C In contrast, the PTC trace heater with the steepest slope, which has the lowest output of 23 W/m, keeps the workpiece at a minimum temperature of 50 °C, resulting in the lowest operating temperature.
Constant wattage PTC-2 PTC-1 Heat loss
Figure 3 – Equilibrium conditions for workpiece maintenance
Figure 4 illustrates the evaluation of upper limits, where the heat loss line is adjusted to the maximum ambient temperature The intersection points indicate the maintained temperature and relative power outputs under these conditions Notably, the PTC-1 trace heater achieves a higher maintained temperature of 78 °C; however, its output level decreases to 18 W/m due to the reduced slope of the output curve This methodology can also be applied to assess the upper limit operating conditions for the stabilized design approach.
Po we r ou tp ut W /m
Constant wattage PTC-2 PTC-1 Heat loss
Figure 4 – Equilibrium conditions for upper limit evaluation
Manufacturers usually provide the power output levels of various trace heaters in their product literature or design programs Typically, the output curves for PTC trace heater types are established based on empirical data obtained from test fixtures, similar to those outlined in IEC 60079-30-1, section 5.1.10.
The output of series type trace heaters is typically defined from its electrical parameters by using the following formula:
Q is the power output of the trace heater (W/m);
V is the system voltage (V); r s is the specific resistance of each conductor (ohm/m); l is the length of each conductor (m)
Note that the resistance of the conductor is a function of the conductor temperature, as given by the equation:
The resistance of a conductor at 20 °C, denoted as \$r\$, is influenced by the temperature difference between its operating state and the reference temperature, represented by the equation \$T_{rrs} = +\alpha \Delta T\$ Here, \$\alpha\$ is the temperature coefficient specific to the conductor's material, measured in 1/°C.
For a successful trace heating system installation, it is crucial that the output of the trace heater(s) exceeds the system's heat loss, incorporating an appropriate safety factor This can be accomplished by utilizing a single trace heater with adequate output, employing multiple passes, or spiraling as necessary to maintain a low output level Additionally, any potential voltage deviations or changes in system parameters over time must be assessed and accounted for within the safety factor Finally, the upper limit of the system should be carefully evaluated for applications requiring precise process temperature control, those exposed to a wide range of ambient temperatures, or systems lacking control or relying on ambient sensing control.
Maximum temperature determination
Determining the maximum operating temperature of trace heaters is crucial for applications in explosive gas atmospheres This includes scenarios such as non-metallic piping, where the trace heater's temperature may approach the maximum withstand temperature of the workpiece or thermal insulation Additionally, applications lacking control or ambient sensing can lead to high tracer sheath temperatures at equilibrium In critical applications requiring precise process temperature accuracy, and in environments where the controlling thermostat may fail, it is essential to ensure that the tracer sheath temperature does not exceed the T-rating of the location.
To safeguard the tracing system from surpassing the high-limit temperature, three effective measures can be implemented: utilizing the PTC characteristic of the trace heater, incorporating a limiter or control device, and conducting stabilized design calculations.
Trace heaters can be assigned temperature classes through testing, as outlined in IEC 60079-30-1 sections 4.4.2, 4.4.3, and 5.1.13, which significantly reduce power consumption with increased temperature In many cases, additional temperature limiting controls are unnecessary if the trace heater's temperature class is lower than the application's specified temperature However, implementing limiters and stabilized design measures can help maintain the system within a narrower range of process temperatures.
6.7.2 Use of a temperature limiter/control device
A temperature limiter or control device is essential for ensuring that the trace heater does not surpass its high-limit temperature This is typically achieved by monitoring various factors, including the temperature of the workpiece or other components, the surface temperature of the trace heater, and additional parameters such as current.
The temperature limiter/control system is designed to interrupt the circuit in the event of a malfunction in either the sensor or the limiter/control device For detailed specifications regarding controlled designs, refer to section 4.4.3 of IEC-60079-30-1.
Stabilized design involves calculating the maximum temperatures of workpieces and trace heater surfaces under worst-case conditions, where heat input equals system heat loss These conditions include a maximum ambient temperature of 40 °C, still air, conservative thermal conductivity values for insulation, lack of temperature control, operation of the trace heater at 10% above its stated voltage, and operation at the upper limit of manufacturing tolerance or minimum specific resistance for series trace heaters.
Figure 4 illustrates the set of circumstances related to stabilized design testing as defined in IEC 60079-30-1 The maximum surface temperature of the trace heater is typically calculated using equations based on empirical data or through a theoretical approach Additionally, design programs can be utilized to determine the maximum surface temperature based on worst-case parameters.
6.7.4 Theoretical sheath temperature calculations – Metallic applications
The maximum achievable pipe temperature is determined by the highest ambient temperature while the trace heater is continuously powered This calculation is based on a rearrangement of the heat loss formula.
T pc is the maximum calculated pipe temperature ( °C);
NOTE The maximum process pipe temperature may exceed the calculated value
The trace heater output, denoted as Q sf, is adjusted to account for 110% of the rated voltage and the maximum output tolerance from the manufacturer (W/m) when determining temperature classifications for stabilized design Additionally, k represents the thermal conductivity of the insulation at its average temperature (W/m K).
Equation (3) defines additional terms, while equation (9) may require iterative techniques for calculating T pc, as the thermal conductivity of the insulation and the output of the trace heater can depend on the temperature of the pipe.
The sheath temperature of a trace heater may be calculated as follows: sf pc sh T
T sh is the trace heater sheath temperature (°C);
C is the trace heater circumference (m);
U is the overall heat transfer coefficient (W/m 2 K)
The overall heat transfer coefficient (U) varies by trace heater type, installation method, and system configuration, encompassing conductive, convective, and radiation heat transfer modes U values can range from 12 for cylindrical trace heaters in air, which rely mainly on convection, to 170 or higher for heaters using heat transfer aids, focusing primarily on conduction Trace heating suppliers can provide the U-factor for specific applications or offer calculated or experimentally determined sheath temperatures upon request.
The power output Q sf of the trace heater selected shall provide the stabilized design and shall not exceed the temperature classification or any other maximum temperature limitations listed above
6.7.5 Theoretical sheath temperature calculations – Non-metallic pipe applications
When using non-metallic pipes, it is crucial to account for the thermal resistance of the pipe wall, as these materials exhibit significantly lower thermal conductivity—up to 1/200 that of steel This can lead to a considerable temperature difference across the pipe or tank wall, influenced by the power density of the trace heater Consequently, two major risks arise: exceeding the maximum allowable temperature of the non-metallic pipe and surpassing the maximum allowable temperature of the trace heater.
The sheath temperature of the trace heater during normal operation is primarily determined by equation (10) It is essential to consider the thermal resistance of the pipe wall when calculating the overall heat transfer coefficient for plastic pipes.
U p is the overall heat transfer coefficient for a non-metallic pipe (W/m 2 K);
U m is the overall heat transfer coefficient for a metallic pipe (W/m 2 );
L is the pipe wall thickness, in metres (m); k p is the thermal conductivity of pipe wall material (W/m K)
The non-metallic pipe wall introduces additional thermal resistance, resulting in a temperature difference between the outside pipe wall and the fluid inside Unlike metallic pipes, this variation necessitates careful consideration of the fluid temperature.
For non-metallic pipe, then p f sh T
T = W + (12) where T f is the fluid temperature (°C)
Equation (12) is a conservative simplification of a complex problem that involves criteria beyond the scope of this standard The individual trace heating manufacturer shall provide sheath temperature data for specific applications
The power output of the trace heater selected shall provide the stabilized design and shall not exceed the temperature classification or any other maximum temperature limitations.
Design information
For an effective trace heating design, it is essential to provide current process and piping information, along with updates reflecting any specification or drawing revisions related to the trace heating system This includes relevant process parameters that may apply.
3) Equipment detail drawings (pumps, valves, strainers, etc.)
4) Ignition temperature of gas or vapour involved
5) Process procedures that would cause elevated pipe temperatures, that is, steam out or exothermic reactions b) Piping parameters
1) Equipment layout drawings (plans, sections, etc.)
2) Pipe drawings (plans, isometrics, line lists, etc.)
3) Piping specifications c) Thermal insulation requirements and data
4) Bill of materials d) Electrical parameters
1) Electrical drawings (one lines, elementaries, etc.)
4) Equipment installation and instruction manuals
6.8.2 Isometric or trace heater configuration line lists and load charts
Each trace heater circuit design should include the following information: a) Design data
1) Area classification, including the lowest ignition temperature for each area if applicable
2) Piping designation or line number
3) Pipe size, rating and material
4) Thermal insulation type, nominal size, thickness, and k-factor
5) Trace heater designation and/or circuit number
9) Maximum exposure temperature (when applicable)
10) Maximum sheath temperature (when required)
11) Heat loss at desired maintenance temperature per unit length of pipe
12) Heat-up parameters (when required)
13) Watts per unit length of trace heater at desired maintenance temperature
15) Circuit current, start-up and steady state
16) Bills of materials b) Installation drawings
2) Piping location with line numbers
3) Location of power connections, end seals, and temperature sensors as applicable
5) Trace ratio of trace heater per length of pipe
7) Extra length of trace heater applied to valves, pipe supports, and other heat sinks
9) Power distribution panel number or designation, the alarm and control equipment designation, and set points
Power system
Trace heater branch circuit protection must effectively interrupt both high-impedance earth faults and short-circuit faults, as outlined in IEC 60079-30-1 This is achieved through an earth-fault protective device with a nominal trip rating of 30 mA or a controller equipped with earth-fault interruption capabilities, used alongside appropriate circuit protection For adjustable devices, the trip level is generally set at 30 mA above the inherent capacitive leakage characteristics specified by the trace heating supplier.
In environments where maintenance and supervision guarantee that only qualified personnel service the installed systems, uninterrupted circuit operation is crucial for the safe functioning of equipment or processes Therefore, earth-fault detection is permissible without interruption, provided that it is alarmed in a way that ensures a prompt and acknowledged response.
Start-up at low ambient temperatures
When selecting protective devices for trace heating systems, it is crucial to ensure that their ratings and characteristics are suitable for environments with low ambient start-up conditions For further guidance and specific recommendations, consult the trace heating supplier's instructions.
Long trace heater runs
In long runs of parallel circuit trace heaters, the power density at the end may be lower than at the beginning due to voltage drop This factor is crucial when assessing the output of trace heaters and positioning temperature sensors.
Flow pattern analysis
In applications requiring critical temperature control, it is essential to evaluate all potential flow conditions within the piping network to determine the appropriate trace heating circuit segments For instance, as depicted in Figure 5, three distinct trace heating circuits with individual controls are vital for maintaining the piping system at the desired maintenance temperature When heated product flows from the tank through pipe A, circuits one and two are de-energized, while circuit three, which heats the non-flowing line, remains active If these circuits are merged into a single control system, the non-flowing line A or B may become de-energized, causing it to fall below the required maintenance temperature.
A bypass around a control valve is another common occurrence where additional circuits are needed, as shown in Figure 6
Effective circuit design is crucial for piping systems, particularly in the case of dead legs and manifold systems These setups necessitate meticulous placement of trace heating devices and their corresponding controls to ensure optimal performance.
4 cold end termination 8 hot end termination
1 circuit no 1 4 hot end termination
2 circuit no 2 5 cold end termination
Dead-leg control technique
The dead-leg technique is utilized for temperature control in complex piping networks and manifold systems, allowing for a reduction in the number of temperature controllers while achieving some energy savings This method involves creating a section of pipe that maintains static flow conditions and matches the heat loss of other controlled piping Consequently, all sections receive adequate heating regardless of flow conditions, ensuring that areas with static flow are properly heated as ambient temperatures fluctuate However, sections with flow may be heated unnecessarily The primary advantage of this approach lies in balancing energy savings with initial cost reductions, though caution is advised when applying it to temperature-sensitive products.
Ensure that the dead-leg section for control is sufficiently long to prevent temperature fluctuations caused by adjacent piping flow Additionally, position the temperature sensor in a thermally independent area to maintain accurate readings unaffected by flow conditions.
Chimney effect
In long, vertical piping systems requiring precise temperature control, multiple control circuits may be necessary This is due to the significant temperature variation that can arise from the convective circulation of hot fluid, leading to a notable difference between the bottom and top of the vertical run The optimal length of each control circuit is influenced by the maintenance temperature tolerance and the specific characteristics of the fluid within the pipe.
General
A control and monitoring system must be implemented that aligns with the application requirements of this standard, tailored to the specific considerations of different process types based on their criticality and the accuracy of process temperature, as outlined in sections 4.3 and 4.4.
Control and monitoring equipment is crucial for maintaining safe temperature levels, preventing high-limit temperature exceedance, and ensuring the integrity of the trace heater circuit by detecting faults Additionally, it provides over-current and residual current protection, along with isolation features It is vital that any specific requirements outlined by the trace heating system designer are adhered to, ensuring both operational efficiency and safety compliance.
Mechanical controllers
Mechanical controllers, like thermostats, operate on two main principles: a bimetallic element or the expansion of a fluid within a bulb or bulb and capillary Temperature variations cause positional displacement, which activates electrical contacts to either complete or interrupt the circuit.
Mechanical controllers are rugged; however, the short sensing element prevents remote panel mounting, and field calibration is cumbersome Thermostats are mounted in the field
When choosing a temperature sensor for a mechanical controller, it is essential to consider the sensor's maximum temperature rating, its components, and any potential corrosive environments it may encounter.
Field located capillary and bimetallic type thermostats shall be provided with a type of protection suitable for use in the explosive gas atmosphere classification appropriate to the installation.
Electronic controllers
Electronic controllers utilize various temperature sensing devices, including resistance temperature detectors (RTDs), platinum resistance thermometers (PRTs), thermistors, and thermocouples (T/Cs) These controllers are often positioned in control and distribution panels, which can be several hundred meters away from the sensors, ensuring convenient access for operators and maintenance personnel.
These controllers electronically process the sensor signal in order to switch an electro- mechanical relay or solid-state device for on-off or phase control.
Application suitability
Freeze protection systems with a Type I process temperature accuracy requirement may only need a basic ambient air sensing control system However, for enhanced energy efficiency and greater process temperature accuracy, it is advisable to consider alternative or additional air-sensing or pipe-sensing controls for Types II or III systems.
Most process temperature applications are Types II or III, requiring sensing of pipe temperature and are often provided with at least a mechanical thermostat
For critical applications requiring precise temperature control (Type III), it is essential to implement alarm functions that notify users of high and low process temperatures, as well as trace heating circuit failures When specified, electronic controls are recommended to enhance system reliability Additionally, systems should include continuity, earth fault, and diagnostic alarms, along with high-limit temperature switching Depending on specific system needs, high-limit signals can be set up to trigger alarms and/or activate circuit protection devices.
Location of controllers
Electronic controllers should be housed in a cabinet that meets the explosive gas atmosphere classification requirements It is advisable to position these controllers outside hazardous areas whenever possible Additionally, temperature controllers should be installed in locations that facilitate easy access for maintenance and calibration.
Location of sensors
The number and location of sensors are determined by the requirements of the process design criteria
Sensors should be positioned at points that are representative of the maintain temperature
Where two or more trace heating cables meet or join, sensors should be mounted 1 m to 1,5 m from the junction
For optimal performance in a trace heating circuit that incorporates both piping and in-line heat sinks or sources, it is essential to position the sensor approximately 1 to 1.5 meters away from these heat sources on a section of the pipe.
In situations where a pipeline heating circuit traverses areas with varying ambient temperatures, such as the interior and exterior of a heated building, it is essential to utilize two sensors along with corresponding controls to effectively manage the temperatures within the pipeline.
In complex piping systems, the material flow patterns must be evaluated for all possible circumstances before selecting the sensor location Detailed information on this evaluation is given in 6.12 and 6.13
To ensure accurate temperature readings, the temperature sensor for control must be positioned away from direct heat influence of the trace heater Additionally, it is essential to securely mount the sensor to guarantee optimal thermal contact with the workpiece.
Certain process materials and piping types may require both a control and a high-limit temperature device due to their temperature sensitivity The control sensor should be positioned at least 90° from the trace heater's circumference, while the high-limit sensor can be placed next to the trace heater, set to the maximum allowable temperature of the material or system, minus a safety margin.
When implementing a high-limit temperature sensor to control sheath temperature in explosive gas environments, it can be positioned either directly on the trace heater or at a distance to prevent it from acting as a heat sink If the sensor is placed away from the trace heater, the set point must be adjusted to a lower value than the maximum sheath temperature to account for the expected difference between the pipe temperature and the sheath temperature.
Alarm considerations
The main purpose of an alarm circuit is to notify personnel when the trace heating system is potentially operating beyond its design limits, prompting a necessary inspection for corrective measures The specific type and function of alarm systems vary based on process requirements Additionally, these alarms can be integrated into data-logging equipment, with the characteristics of the most common devices detailed in sections 7.7.2 to 7.7.4.
A trace heating circuit alarm is essential for detecting issues such as loss of current, voltage, or continuity within the circuit It typically includes a current-sensing device that triggers an alarm if the current falls below a predetermined minimum while the temperature switch is closed Additionally, a voltage-sensitive device monitors the voltage at the end of the trace heater or on a return wire within the system Furthermore, resistance or continuity-sensing devices check the trace heating circuit when the system is de-energized by sending a low-voltage signal or pulse into the trace heater for monitoring.
Temperature alarms serve critical functions in monitoring piping systems A low-temperature alarm signals when the temperature drops below a predetermined minimum, indicating potential issues with cooling that may exceed acceptable operating limits Conversely, a high-temperature alarm alerts when the temperature surpasses a set maximum, suggesting that heating could also exceed safe operational criteria These alarms can be integrated with temperature controllers or installed as standalone devices to ensure optimal process management.
Various alarms are available for monitoring trace heating circuits, including: a) Auxiliary contact alarms, which indicate when a contactor is closed and power is supplied, but do not guarantee the trace heating circuit's operation if a secondary contactor is open or if continuity is lost; b) Residual current protective devices that operate at 120/240 V a.c and trip when earth-leakage current exceeds a set level, with options for alarm contacts or alarm-only functions; c) Switch-actuated alarms, typically triggered by an auxiliary contact on the temperature controller; d) Current-sensing apparatus, which includes a temperature control bypass switch and an ammeter or current-sensitive relays; and e) Diagnostic alarms, activated by a diagnostic circuit in the electronic controller to signal failures in internal control or data processing logic.
Integrating trace heating system control and alarm circuitry with a central control and monitoring system is essential It is crucial to select equipment that is compatible for both control and supervisory functions to ensure reliable data transfer and system performance.
Introduction
Each electric trace heating system is tailored to the specific needs of the process and plant It is crucial to verify that the plant parameters used for the design remain valid during the installation of the trace heating system and that all components are installed correctly Regular testing and maintenance are vital to ensure optimal performance and safety.
Preparatory work
All preparation should be conducted in accordance with the design documentation, and every item listed in the following should be checked after completion
Before installing the trace heating system, ensure that all piping and equipment have been pressure-tested and that all instrumentation is in place The surface for the trace heater must be clean and free from rust, grease, and oil, with any sharp edges or protrusions removed Additionally, all coatings on the heated surfaces should be appropriate for their intended use Coordination between the trace heating installation, thermal insulation, and instrumentation is essential to meet the project timeline Thermal insulation installation should only begin after the electrical trace heating has been fully installed and tested.
To ensure accuracy, the on-site equipment requiring trace heating must be verified against the design drawings, confirming that the lengths of piping and the quantities of vessels, valves, flanges, and components match The amount of trace heating needed is directly related to these quantities Additionally, any modifications made to the equipment necessitate a review of the trace heating materials schedule.
Upon receiving trace heating components, it is essential to perform a thorough inspection to confirm the correct type and quantity of materials and documentation This includes verifying the catalogue type, product and package markings, power rating, voltage rating, quantity, and any special characteristics of the trace heaters Additionally, it is important to ensure that installation instructions and the certificate or declaration of conformity from a notified body are included, as required.
Materials should be stored in protected, dry areas Materials are to be released only as required on the jobsite, so as to avoid any unnecessary handling and inadvertent damage
Proper training is essential for individuals involved in the installation and testing of electric trace heating systems Installation must be supervised by a qualified electrician with additional training in electric trace heating systems for explosive gas environments Only specially trained personnel should perform critical tasks, including the installation of connections and terminations.
Installation of trace heating circuits
For effective installation of trace heating circuits, it is essential to have the workpiece drawings and design data readily available Before proceeding, verify that the as-built piping and equipment align with these drawings, as any discrepancies may require adjustments to the trace heaters Additionally, coordinating the installation with piping, thermal insulation, and instrumentation disciplines is crucial to meet the scheduled completion date.
The trace heating system supplier must deliver detailed instructions for the trace heaters and their various components It is essential that the guidelines for components and trace heaters designed for unclassified and explosive gas atmospheres are clearly specified.
The pre-installation checklist must document essential tests to verify that the trace heating design aligns with installation conditions First, trace heaters should undergo a visual inspection for damage, followed by continuity and insulation checks, with insulation resistance measured as per section 8.3.4 Additionally, individual controls need to be tested for proper calibration, including set points and operating temperature ranges Furthermore, vendor-fabricated control panels must come with documentation confirming the accuracy of wiring, layout, and functionality, and a general inspection should be conducted upon receipt at the work site to ensure no damage occurred during transit.
The trace heaters shall be completely free of physical damage Connections preassembled at the factory must be sufficiently rugged to withstand normally expected conditions during installation
Insulation resistance must be measured from trace heater conductors to any metallic braid, sheath, or equivalent conductive material using a minimum test voltage of 500 V d.c It is advisable to utilize higher test voltages, with mineral insulated trace heaters tested at up to 1,000 V d.c and polymeric insulated trace heaters at 2,500 V d.c The insulation resistance should not fall below 20 MΩ.
Trace heating system components can be substituted under certain conditions: a) components explicitly listed in the supplier's installation or maintenance instructions cannot be replaced with similar parts unless they are included in the certification; b) other components mentioned in the supplier's instructions may be replaced with any suitably rated component; c) components within the wiring system that powers the trace heater can be substituted with any suitably rated component approved by the authority having jurisdiction.
8.3.6 Determination of power supply location
Before installing trace heating, it is essential to determine the location of the power supply Junction boxes must be installed to ensure that the trace heater is protected from damage between its exit from the insulation and its entry into the junction box.
Items to be checked Remarks
1 Is the workpiece fully erected and tested and all temporary supports removed? Is the surface to be heated free from sharp edges, weld spatter and rough surfaces?
Any welding or pressure testing after the installation of a trace heater could damage the device See 8.2.2, 8.3.7.1 and 8.4.1
When applying a trace heater, it is essential to determine if the surface is normal steel or non-metallic Special precautions are required for polished stainless steel, very thin-walled pipes, or any non-metallic surfaces.
3 Do the items to be heated correspond in size, position, etc with the design?
It is sometimes difficult to be sure that the correct pipe is being heated A suitable line numbering system may be of assistance
4 Has it been specified that metallic foil be installed before the application of the trace heater?
This may be used to aid heat distribution
5 Has it been specified that metallic foil be installed after the application of the trace heater?
This may be used to prevent insulation from surrounding the trace heater or to aid heat distribution
6 Can flow of product under normal or abnormal conditions reach temperatures greater than those that the trace heater can withstand?
This would normally be covered in the design stage; however, further discussion with staff at the plant may show that incorrect or out-of-date information has been used
7 Is the trace heating system most recent documentation
(working drawings, designs, and instructions) available?
No change shall be contemplated without reviewing the trace heating system documentation, as careful calculations are necessary to ensure safe operation
8 Can pipes or surfaces expand and contract so as to cause stress on any part of the trace heating installation?
In this case precautions are necessary to avoid damage
9 Can sensors of temperature controllers be affected by external influences?
An adjacent heating circuit could affect the sensor
10 Is the trace heater to be spiralled or zig-zagged onto the workpiece, according to the design?
Check design loading per unit length of pipe (or surface area) to determine if spiral or zig-zag application is necessary
Items to be checked Remarks
11 Are cold leads, when fitted, suitable for contact with the heated surface?
If the cold lead is to be buried under the insulation, it has to be able to withstand the temperature
12 Is the pipework hung from a pipe rack? In this case, special precautions are required to ensure the weatherproofing of the insulation at points of suspension
13 Does pipework have its full complement of supports?
The addition of intermediate supports at a later stage could damage the heating system
14 Are sample lines/bleed lines, etc at the plant but not on drawings?
These could obstruct or prevent the fitting of the trace heater, and a review of the trace heating system documentation may be necessary
15 Are other parameters used in the design of the equipment, such as pipe supports, specified by the design documentation?
When selecting trace heaters, controllers, junction boxes, switches, and cable glands, it is crucial to ensure they are appropriate for the classification of explosive gas atmospheres and the specific environmental conditions Additionally, these components must be adequately protected against corrosion and the ingress of liquids and particulate matter.
Trace heaters must be installed on clean, smooth sections of piping and equipment, following the supplier's guidelines It is crucial to position them carefully around flanges, valves, and fittings to prevent damage from sharp edges and to mitigate risks from impact, abrasion, or vibration Additionally, it is important to verify that the trace heater, terminations, and cold leads can handle the movement and vibration of the piping and equipment.
Understanding the significance of the tracing system is crucial for installers to ensure uniform heating of piping and equipment Equipment with higher mass or heat sinks necessitates additional tracing To achieve optimal heating, trace heaters should be installed with close contact to the surfaces In cases where direct contact is not feasible, such as on valves, using a heat-conductive covering made of temperature-rated metal foil or other conductive materials is recommended.
To ensure optimal performance, trace heaters must not be folded, twisted, or allowed to overlap, cross, or touch themselves unless explicitly stated in the supplier's instructions It is crucial to adhere to the supplier's specified minimum bending radii.
In the installation of trace heating systems, only genuine components may be used Otherwise the system certification will not apply
8.3.7.2 Straight tracing runs on pipe
To ensure effective heating, multiple straight trace heaters must be evenly distributed around the pipe's circumference Additionally, extra lengths of trace heaters should be included to address the increased heat losses at pipe supports, hangers, and anchors.
8.3.7.3 Spiral tracing runs on pipe
Before applying the trace heater, it is essential to mark the spiral pitch on the pipe and equipment, starting from the power supply point Ensure that the trace heater is applied in a uniform spiral while maintaining slight tension It is crucial to adhere to the minimum spacing requirements specified by the supplier.
When applying spiral tracing, it is essential to ensure that valves and other components can be easily removed or replaced To maintain a consistent spiral design, adjustments to the spiral pitch should be made if there is an excess or insufficient trace heater at the end of the heated section.
NOTE While spiral tracing may be more convenient on short runs or piping and equipment, multiple straight tracing runs may be preferred for ease of installation and maintenance
8.3.7.4 Inline equipment such as valves
In the design of trace heating systems, it is essential to incorporate extra lengths of trace heater to account for increased heat losses at components such as valves, flanges, strainers, and pumps These additional lengths should be implemented following the guidelines provided by the supplier.