XICON INTERNATIONAL LIMITED
GUIDELINES FOR FURNISHING INFORMATION IN THE RFQ FORMAT
The following write up is furnished with a view to help the personnel filling up the RFQ format, clarify the various terminologies used the choices available and their impact on the design aspects of electric heat tracing.
1. MAINTENANCE TEMPERATURE (TM)
Maintenance temperature is the minimum temperature to be maintained by the heat tracing system, usually for the following reasons:
a. Winterisation. e.g. maintain +5°C when the ambients are in the range of (-) 60° C... + 5°C
b. Temperature maintenance i.e. maintain the fluid 5°C to 15 °C above the solidification or condensation temperature of the process medium
c. Process trace heating To compensate for heat loss
2. DESIGN AMBIENT TEMPERATURE (TA)
Design Ambient temperature is the lowest temperature that may be expected in the vicinity of the heat traced pipework. If a value is not given in the enquiry documentation, then climatological data may be searched or an assumption may be made.
Design Ambient temperatures need to be considered with care. This worst case temperature creates the temperature difference against which the required heat compensation is calculated and so determines the amount of heat tracer that is required. A low ambient temperature declaration could result in an over engineered system with a consequent cost penalty.
Xicon recommends that an average monthly value for the coldest month of the year be considered.
Xicon can recommend / select design ambient temperature from climatological data based on the location.
3. EXPOSURE TEMPERATURE (TEXP)
The exposure temperature is the maximum temperature the heat tracer will be subjected to under any of the following conditions.
a. Steam flushing temperature
b. Process fluid temperature under operating condition. Operating fluid temperatures can be higher than the maintenance temperatures specified.
c. Failure of any cooling medium in the process.
4. ENVIRONMENT
Following environments can be considered corrosive :
a. Along sea coast
b. If corrosive gases are present in the atmosphere.
5. INSULATION
Insulation reduces the heat loss by 70 to 80%. Reducing heat loss below 70% to 80% is economically and practically not feasible. The balance 20% to 30% heat loss is made by the heat tracing system.
a. Insulation Type:
The choices available in insulation are listed below:
a. Glass Wool
b. Mineral Wool
c. Polyurethane
d. Polyisocyanurate
e. Ceramic Wool
f. Cellular Glass
g. Calcium Silicate
The insulation are available in one of the following forms:
a. Mattress
b. Pipe Section
c. Slabs
b. Insulation Thermal Conductivity
Heat retention property of thermal insulation is a property of its thermal conductivity.
Thermal conductivity of various insulation materials are listed below:
Insulation material / Thermal Conductivity (w/m ° C)@ 50 ° C
Glass Wool / 0.036
Mineral Wool / 0.043
Polyurethane / 0.023
Polyisocyanurate / 0.023
Cellular Glass / 0.058
Calcium Silicate / 0.054
The thermal insulation of various insulation material vary with their temperature.
c. Insulation Thickness
Insulation thickness may be decided on any one of the following criteria:
I. Optimisation of thickness after the investment cost Vs power saving cost study.
ii. Based on clearance available with respect to other pipeline running alongside.
iii. Based on saddle support height.
iv. Based on a cladding surface temperature on personnel safety consideration.
Xicon can recommend insulation type and thickness in case the customer desires so.
Xicon can recommend insulation thickness after carrying out an optimisation of insulation thickness considering investment and energy costs.
6. POWER SUPPLY
Heat Tracers are generally available for use at the following voltage levels:
a. 110 V 1 Phase
b. 240 V 1 Phase
c. 415 V 3 Phase
d. 600 V 3 Phase
Generally 415 V 3 Phase 3 Wire or 415 V 3 Phase 4 Wire supply is normally available in industrial sites.
240 V 1 Phase power supply is devised by tapping power from phase & neutral.
In case only 415 V 3 Phase 3 Wire supply is available, a delta star transformer can be introduced in the panel to convert to 415 V 3 Phase 4 Wire and 240 V power supply tapped by connecting between phase & neutral.
7. PIPING & TANK BILL OF MATERIAL
This information is arrived at from the piping drawings / equipment drawings and is used for working out the heat loss and heat tracer bill of materials.
8. DRAWINGS
Flow Diagram & P & I Diagram
This information helps in deciding the quantity of accessories, number of circuits and consequently the number of feeders in the panel.
Piping Drawing
This information is used to locate the accessories and consequently to work out the cable length and size knowing the panel location.
9 LOCATION OF POWER SUPPLY:
This information is required to check out availability of required power and the size and length of cables from the supply point of the distribution panel.
10. LOCATION OF ELECTRICAL DISTRIBUTION PANEL
Preferred electrical distribution panel location needs to be indicated. This information has an implication on the size and lengths of cables from the distribution panel to the circuits.
11 CORRELATION OF INFORMATION ON DESIGN ASPECTS
The table and write up below correlate the use of information furnished vis-à-vis the various aspects of heat tracing system design.
Description / HeatLoss Calcu-lation / Selection of type of tracer / Suitability for hazardous area application / No.of Circuits/ Feeder in Panel / Cable Length / Cable Size / Power Requi-rement / Heat Tracer & Accesso-ries
Pipe Size (NB) / Ö / Ö
Maint. Temp. (°C) / Ö / Ö / Ö / Ö
Ambient Temp.
(° C) / Ö / Ö / Ö
Thermal Conductivity (Ke) / Ö / Ö / Ö
Insulation Thickness (mm) / Ö / Ö / Ö
Exposure Temp. (° C) / Ö
Environment / Ö
Area Classification / Ö / Ö
Gas Group / Ö / Ö
T Rating / Ö / Ö
Piping Bill of Material / Ö / Ö
Piping Drawing / Ö / Ö
Flow Diagram / Ö / Ö
Panel Location / Ö / Ö
AREA CLASSIFICATION / GAS GROUPING & T RATING
NEC APPROACH
The National Electrical Code (NEC) devotes Article 500 to the introduction and definition of Hazardous (Classified) Locations. the varied interpretations for heat tracing help make the subject easily confused, and is a basis for this article.
The following “ Summary of Classifications Chart” provides a good overview of this information:
CLASS / DIVISION / GAS GROUP1. Hazard Exists / A / Acetylene
Always Present in Atmosphere. / B / Hydrogen & Manufactured Gases containing Hydrogen
C / Petrochemicals
I / GAS / D / Petrochemicals
2. Potential Hazard / A / Acetylene
May be present in Atmosphere / B / Hydrogen & Manufactured Gases containing Hydrogen
C / Petrochemicals
D / Petrochemicals
1. Hazard Exists / E / Conductive & Combustible Metal dust.
Always Present in Atmosphere. / F / Combustible Carbon Dust
G / Non Conductive Carbon Dust.
II / DUST / 2. Potential Hazard / E / Conductive & Combustible Metal Dust.
May be present in Atmosphere / F / Combustible Carbon Dust
G / Non Conductive Combustible Dust.
1. Production Areas / Combustible
III / FIBRES / 2. Handling or Storage Areas / Combustible
Notes
1. More detailed descriptions of area classifications can be found in Appendix B. Appendix B outlines circumstances that help distinguish between Divisions 1 & 2.
2. The anticipated influence of European and International Terminology will further complicate the subject. For additional information on this, see Appendix A of this report.
Once the components of a volatile atmosphere have been defined, its still necessary to determine the circumstances of its presence. No matter how infrequently the hazard may be present, if it is possible that it can exist under normal circumstances the area becomes classified as Division 1.
ELECTRICAL ENCLOSURES
Most arcing devices (thermostats, circuit breakers etc.,) to be installed in any Class I area must be housed in NEMA 7 (explosion proof) enclosures. Hermetically sealing the arcing contacts or purging an enclosure with inert gas or instrument air can sometimes preclude the need for the NEMA 7 enclosures.
NOTE: Various NEMA enclosure ratings are described in Appendix C of this report.
In an explosion proof enclosure is used to house an arcing device, a permanent “Seal-off” must be present on all entries and exits to a junction box before operation. This prevents the propagation of a flame to the conduit system if ignition occurs inside the enclosure. Seal offs may be installed in the field during conduit installation and actually “poured” with the sealing compound after all electrical conductors have been brought into the enclosure and terminated.
Note : Seals are installed on all NEMA enclosures; even if only a “terminal box”.
In the case of electric heat tracing, installation in hazardous areas is more involved than the use of approved enclosures. As a source of thermal energy, as well as electrical energy, heat tracing is addressed in several sections of the NEC.
FIXED HEATING DEVICES
NEC Article 427 addresses “Fixed.... Heating Equipment for Pipelines....” separately from heating equipment for other applications. All of the information pertains to pipe heating (including tracing cables), but Article 427 does not address cable sheath temperatures, critical to heat tracing design in hazardous areas.
NEC Article 500 specifically states that in no case shall any exposed surface temperature be greater than the ignition temperature of the specific gas, vapor, or dust. Further, Article 501-10 for heaters limits the maximum temperatures to 80% of the ignition temperatures for the area, measured in ° C .
Approvals testing involves determining the cable runaway temperature under the following conditions:
a. Continuously energized heater
b. 10% over voltage and
c. an ambient temperature of 104 ° F with no wind.
The resulting temperatures of the heating cable are considered to be the maximum possible for the system, and are to be below 80% of the ignition temperature of that atmosphere. In some cases a high temperature limit switch with a manual reset function may be introduced to the system in lieu of the 80% of the temperature requirement to prevent unacceptable sheath temperatures.
All heating cables approved for use in hazardous areas have, as a minimum, a continuous metal braid or sheath over the entire heater cable surface. Many may have a thermoplastic overjacket to increase the cable durability and provide protection for the braid in corrosive environments.
As of this report, it is necessary that the manufacturer review and approve the design of all heat tracing systems for Class I Division 1 areas.
The approvals of some systems go beyond the cable, enclosures and termination’s. For Class I Division I hazardous areas, approvals agencies require that each branch circuit be protected by a Ground Fault Circuit Interrupter (GFCI).
Note: 1. The use of approved components will not ensure that the assembled system is approved for the area in which it is to be installed. Most approvals reports call out the specific components and accessories with which the system is approved.
2. GFCI or Ground Leakage Circuit Breakers (GLCB) are also known as Equipment Protection Devices (EPD). These have trip ratings of 30 ma ground leakage or greater and are not to be mistaken for Ground Fault Interrupters (GFI) for personal protection with 5 ma trip ratings.
“T” CLASSIFICATION
Until the 1970’s it was necessary that the lowest ignition temperature of any given “Group” was to be the temperature limit of all materials in that group. [ Reference NEC Article 500 - 3 (6)]. In most cases, the specific atmosphere under consideration had an ignition temperature well above that for the group with which it was assigned. More often than not, it was difficult for a customer to commit to a specific list of gases thought to be in the area.
The Introduction of the “T” Classification (also called the “T rating”) of specific heating products or systems now allows the customer to select a product suited to his plant. No longer are equipment manufacturers restricted by the grouping of volatile gases that encompassed their customers’ plants.
The following information regarding the NEC “Identification Numbers” is taken directly from NEC Article 500.
ARTICLE 500 - HAZARDOUS (CLASSIFIED) LOCATIONS 70-427
Table 500-3(b) Identification Numbers
Maximum Temperature / Identification Number° C / ° F
450 / 842 / T1
300 / 572 / T2
280 / 536 / T2A
260 / 500 / T2B
230 / 446 / T2C
215 / 419 / T2D
200 / 392 / T3
180 / 356 / T3A
165 / 329 / T3B
160 / 320 / T3C
135 / 275 / T4
120 / 248 / T4A
100 / 212 / T5
80 / 185 / T6
CONCLUSION
The subject of electric heat tracing in hazardous (classified) areas is more complex than outlined herein. With approvals agencies and the authoring committees of the codes continuing to review this subject, it is expected to change in the years ahead.
APPENDIX A
EUROPEAN APPROACH
In recent years there has been an interest in expanding the Division 1 area classification. The International Electrotechnical Commission (IEC) and European approvals agencies have long used Zone 1 and 2 in much the same way Divisions 1 and 2 are referenced in North America. But the IEC also provides for a Zone 0 classification. An example of such an area is in the vicinity of gasoline pumps, the vapour space of a tank, a paint spray booth, or sumps where volatile gases can collect.
This subject is addressed here because of growing interest in the US (NEC) and Canada (CEC) for a similar Division 0 area classification. More on the correlation between the IEC classifications and those presently utilized in North America are shown below.
IEC / CENELEC / U.S.A. (to NEC) andCanada (to CEC)
Zone 0 / Intrinsically safe apparatus of category apparatus, both specifically approved Zone 0 / Class 1
Division 1 / (Many users recognize the Zone 0 principle without using the name and would only install apparatus suitable for Zone 0 in such areas).
Zone 1 / Apparatus for use in Zone 0, and Apparatus of type protection:
- ‘d” flameproof enclosures
- “p” pressurized apparatus
-”I” intrinsic safety
- “o” oil immersion
-”e” increased safety
-”q” powder filling / Apparatus of types of protection:
- Explosion proof enclosures
- Purging
- Intrinsic safety
- Oil immersion
Zone 2 / All equipment certified for Zone 0 or 1.
Requirement for Zone 2 apparatus are in preparation. / Class 1 Division 2 / All equipment certified for Division 1 Apparatus unable of creating sparks or hot surfaces capable of ignition in “general purpose” enclosures type of protection “n” (in preparation)
Note: Above table taken from IEEE / PCIC paper number PCIC-90-25 “Non Metallic Equipment for Hazardous (Classified) Locations - The IEC Approach”, by H.A. Bockle, J.I.Cohen & A.C. Milelr.