Final DRAFT – 10/21/99

IPC-1331 Voluntary Safety Standard for Electrically Heated Process Equipment

1. Scope

This voluntary standard establishes the minimum requirements for the design, insafe installation, and operation of electrically heated process equipment in order to minimize electrical hazards and prevent fires that may occur in combustible tanks, tank liners and drying equipment. It is intended to cover both liquid and gas (e.g.i.e air) heaters used in the manufacture of printed wiring boards (PWBs) and printed wiring assemblies (PWAs). Minimum requirements are indicated by the use of the terms "must" or "shall". Optional enhancements or alternate approaches to the minimum requirements are indicated by the use of the terms "can" or "may".

Please note: standard does not purport to address all of the safety issues associated with its use. It is the responsibility of the users of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitation prior to its use.

2. Reference documents

2.1 This Standard is intended to be compliant with the following regulations and codes:

2.1.1 29 CFR 1910 Subpart S-Electrical

2.1.2 ANSI C2-1997 National Electrical Safety Code

3. NFPA 70 National Electrical Code, Article 427 of National Electric Code
   
   
4. NFPA 79 National Fire Prevention Association
   
   
5. UL 499 Electric Heating Appliances
   
   
6. CSA C22.2 No. 88-1958 Industrial Heating Equipment
   
   
7. CEmark

1. Electric immersion heaters are generally used to heat aqueous or semi-aqueous solutions.

They are not recommended for use with flammable solutions, and shall not be used with class 1 or 2 flammable liquids.

2. Users must verify with equipment and/or heater manufacturer and chemical supplier(s) that heater sheath material is compatible with theuser’s intended solution solution before installation and use. If user changes chemical composition, user must ensure that heater sheath material is compatible with the new solution by checking with equipment and/or heater manufacturer.
   
   
3. Electric immersion heaters may ignite combustible tanks or liners. To prevent ignition, heating elements shouldmust be securely mounted in a manner that prohibits direct contact with the tank or tank liner. Heating elements must be located at least one inch (25 mm) from the tank bottom and side walls or minimum distance recommended by manufacturer, whichever is greater.
   
   
4. Electric heating elements, used in both liquid and gas heating applications, shall be equipped with appropriate and adequate supports to prevent contact with combustible surfaces as a result of heating element deflection/sag due to use.
   
   
5. Electric immersion heaters must be protected from physical damage. They must be shielded from [physical damage]. [They must be shielded or located to prevent physical damage from contact with other items entering tanks such as anodes, cathodes, racks, product, concentrated chemicals, and/or electrically charged components. Protection is generally accomplished by proper placement and appropriate guarding.
   
   
6. Heater elements must be allowed to cool before they are removed from equipment for service or replacement.
   
   
7. Tanks must not be drained until heater element has cooled.
   
   
8. Electric heating devices must be built and tested to comply with a nationally recognized independent testing laboratory specification such as Underwriters Laboratories (UL), Factory Mutual (FM), and ? (ETL) , listedfor their intended application.

3. Design and Installation Requirements

Electrically heated process equipment shall comply with the following design and installation requirements:

4.1 Control Circuit Design

1. A corrosion-resistant temperature-sensing element, such as a thermocouple, thermistor, or resistance thermal device (RTD), shall be used in conjunction with a for the temperature-indicating controller (see §4.1.2). The use of a thermostat with set point indication is also permitted. All sensing elements must be compatible with the environment in which they are used (i.e., bath chemistry, fumes, or vapors, etc) or be housed in a chemically compatible thermowell.
   
   
2. A temperature-indicating controller with both sensor break and short protection that provides an indication of process set point shall be used. The controller shall allow the power control device (see §4.1.3) to be switched off when the tank temperature reaches the set point or when the system is not being used. A controller that provides a visual display of both process set point and actual process temperature may be preferred as a control enhancement for certain applications.
   
   
3. An appropriately sized power control device, such as a contactor relay, solid state relay (SSR), or silicone controlled rectifier (SCR), shall be used for controlling the availability or the amount of electrical power to the process heater.
   
   
4. All liquid heater elements shall contain a thermal limit device, such as a fusible link,n bimetallic thermostat, or other temperature regulating device, to detect an over-temperature condition in all installations where the possibility of combustion exists. The device shall disable power flow to the heating elements in the event of an over-temperature condition. This device may be non-resettable, manually reset, or automatically reset. If the device automatically resets, power flow to the heating elements must be restricted from automatically resuming through use of manual latching circuit. Please note that the type of thermal limit device used will depend upon the technologies employed.
   
   
5. A temperature-sensing device must be provided to protect electrically heated process equipment from over-temperature conditions. This device shall disable power flow to all heating elements in the event of an over-temperature condition. This device must be unique to and redundant with the temperature-sensing element specified in §4.1.1. This device may be a pre-set temperature switch or a temperature sensing element, such as a thermocouple, thermistor, or resistance thermal device (RTD) and its attendant hi-limit controller. The set point of this over-temperature device must be set at a temperature value that is less than the maximum temperature limit of the equipment (i.e., when reached, no damage will occur to the chamber, lining, vessel or any other component contained within).
   
   
6. If an adjustable device or controller is used, the maximum adjustment position that is available must be no greater than the equipment’s maximum temperature limit (i.e., when reached, no damage will occur to the chamber, lining, vessel or any other component contained within).
   
   
7. If exothermic chemistries are present in the process vessel, the over-temperature device and associated cooling equipment must be set at a temperature value to ensure that the exothermic action is controlled. The over-temperature set point for such chemistries shall be lower than the maximum temperature limit dictated by the materials of construction of the vessel or chamber used (i.e., when reached, no damage will occur to the chamber, lining, vessel or any other component contained within). Please note that the over-temperature limit must be non-resettable to prevent resetting to an unsafe temperature, such as a temperature that is above an exothermic temperature.
   
   
8. An over-temperature controller for the over-temperature sensing device specified in §4.1.5 shall disable power flow to all heating elements in the event of an over-temperature condition. This over-temperature controller may be non-resettable, manually reset, or automatically reset. If the controller automatically resets, power flow to the heating elements must be restricted from automatically resuming by the use of a manual latching circuit. The over-temperature controller shall activate a visible light and/or audible alarm to signal that an over-temperature condition exists.
   
   
9. For liquid tank heaters, a low-level switch must be included in the electrical control circuit to disable the heater load whenever the process liquid level drops to no less than one inch above the heater elements’ hot zone. This switch shall also be equipped with a visible light and/or audible alarm to indicate that the switch has been activated. It is recommended that users choose low-voltage, low liquid sensors to prevent risk of shock.
   
   
10. For gas (e.g., air) heaters, a low-flow switch must be included in the electrical control circuit to disable the heater load whenever the process gas (e.g. air) flow drops to a value just above the minimum flow required by the heating elements’ manufacturer for gas (e.g. air) heaters. This switch shall also be equipped with a visible light and/or audible alarm to indicate that the switch has been activated.

A separate safety interruptrelaywith a contactorcoil must be included in the electrical control circuit and be wired in series with the thermally-used heater elements specified in §4.1.4 and activated by the temperature-sensing device §4.1.5, the over-temperature controller specified in §4.1.6, the low liquid level switch specified in §4.1.7, or the low flow switch specified in §4.1.8. This contactor is separate from the power control device sepcified in §4.1.3 and will interrupt power flow to all heating elements in the case of an over-temperature condition.

11. An enable switch or process set point must be included in the electrical control circuit to disable the heating control (not the safety interrupt controls) should process cooling be utilized in addition to process heating.
    
    
12. A safety interlock must be included in the electrical control circuit and connected to a process exhaust fan if the process generates flammable fumes that require ventilation. This will prevent generation of possible flammable and toxic vapors when the exhaust fan fails, and the buildup of process vapors to a level above 25% of their lower flammability limit.
    
    
13. For liquid heaters, Ground Fault Circuit Interrupters must be employed whenever risk of contact with electrically charged solutions is possible.
    
    
14. Electric heater elements must be equipped with a ground wire of sufficient size to carry any fault current. The construction of both heater and ground should be approved by a nationally recognized testing laboratory.
    
    
15. All installation work must be implemented in accordance with the latest edition of the National Electric Code.

 

1. Control System Installation

2. For conveyorized equipment, the low-level switch shall be mounted to allow testing of the low-level switch sensing element without draining the sump level whenever possible.
   
   
3. The temperature-sensing element for the thermal over-temperature switch shall be mounted to facilitate testing without draining the tank or sump contents on liquid heaters.
   
   
4. Thermostat temperature indicating sensors and hi-limit sensing devices shall be secured in a location below the minimum liquid level and above the bottom of the heating element.
   
   
5. Electrical wiring shall be designed and installed so as to minimize condensation and facilitate drainage of condensation away from electrical connections to prevent arching, bridging and corrosion on the electrical connections.

2. Testing

1. Testing of all temperature sensing elements, limit controls and liquid level devices shall be performed by trained personnel on a periodic basis to ensure that all components are working properly and to assure the integrity of the safety interrupt circuit. Monthly testing is recommended. Any failure must be addressed by the user’s corrective action policy.
   
   
2. All testing shall be documented. Records including corrective action shall be retained for a minimum of one year or per the record retention policy of the user until at least the next test cycle is completed
   
   
3. Any replacement of a temperature-controlled circuit element should be calibrated and proper operation verified prior to placing into operation.
   
   
4. Calibration of the temperature controller specified in §4.1.2 should be performed per the manufacturer’s recommendation, but no less than semiannually.
   
   
5. Calibration of the over-temperature controller in §4.1.6 should be performed as recommended by the manufacturer, but no less thanon annually.

2. Heating System Maintenance

1. All heater elements should be inspected and cleaned frequently to remove any deposits, which may inhibit heat transfer. It is recommended that inspection and cleaning occur on a monthly basis or more frequently if used in additive operations.
   
   
2. All electrical power must be turned off and proper personal protection equipment worn prior to removing and inspecting heating elements.
   
   
3. All electrical service to heating elements must be locked out during idle periods of more than one eight-hour shift.
   
   
4. The interchanging of parts, which were not included in the original heating system design, shall be limited unless advised by the equipment/heating manufacturer. Alternate parts can lead to failures resulting in either fire or physical injury.