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2. Weighing Processes in Explosive Hazardous Environments
Steven Wise
Mettler Toledo
September 2012
Information contained within this presentation is presented for informational purposes only. It may not represent the general requirements and installation requirements for your classified area. Review all applicable rules and regulations prior to installing any equipment in hazardous areas.
Manufacturers and System Integrators do not classify areas. It is the responsibility of the owner/customer to classify their areas.

3. Hazardous Area Weighing Agenda
Hazardous Area Overview
Weighing Technologies
Example System Designs
Information contained within this presentation is presented for informational purposes only. It may not represent the general requirements and installation requirements for your classified area. Review all applicable rules and regulations prior to installing any equipment in hazardous areas.
Manufacturers and System Integrators do not classify areas. It is the responsibility of the owner/customer to classify their areas.

4. Definition What is a hazardous area?
A hazardous area can be defined as an area in which an explosive atmosphere is present, or may be expected to be present, in quantities such as to require special precautions for the construction, installation and use of potential ignition sources.

5. Certifying Agencies
Regional bodies govern the certification of equipment design and installation
Globally, there are several guidelines and standards. In the US, we tend to use the National Electric Code guideline. The two most well-known testing bodies in the US are UL and Factory Mutual. Be aware that if your equipment is being installed in other regions, other rules and regulations may apply.

6. NEC ARTICLE 500 National Electric Code
“Locations shall be classified depending on the properties of the flammable vapors, liquids, or gases, or combustible dusts or fibers, that may be present and the likelihood that a flammable or combustible concentration or quantity is present.”
(NEC 500.5)
In the US, NEC article 500 defines the guidelines for hazardous areas.

7. Hazardous Classified Locations
Ratings Overview
Division
Quick Definition
Division 1
Explosive hazard is normally present
Division 2
Explosive hazard MAY be present if a fault occurs
Unclassified
Explosive hazard is not present or not defined
Division 2 Area
Division 1 Area
Hazardous Process
Air
Ventilation Fans
Define Div 1, Div 2, and unclassified.
Area classifications are defined for a specific installation. For example, in the graphic shown here we have a div 1 area immediately around the process. A div 2 area extends out for some distance beyond that. Finally, there is an unclassified area in the rest of the structure. Note that ventilation and air flow is considered when defining the areas.

8. Hazardous Classified Locations
US Ratings Overview
Classification
Group
Quick Definition
Example
Class 1
Group A
Gases & Vapors
Acetylene
Group B
Hydrogen
Group C
Ethylene
Group D
Propane
Class 2
Group E
Dust
Metal Dust
Group F
Coal Dust
Group G
Grain Dust
Define Classes and Groups.
Class 3
N/A
Fibers & Flyings
Textiles

9. Hazardous Area Classification
Classification of maximum equipment surface temperature produced under fault conditions at an ambient temperature of 40.0 C °
Temperature Classifications
As defined by NEC Article 500
T1 = 450 C°
T2 = 300 C°
T3 = 200 C°
T4 = 135 C°
T5 = 100 C°
T6 = 85 C° T2 T3 T4 T5 T6
The rating means that the equipment is designed such that no surface or circuit in the classified equipment will reach this temperature.

10. Definitions Circuit Definitions Incendive Non-Incendive
Circuit carries enough energy to ignite flammable material
Non-Incendive
Under normal operating conditions, circuit is not capable of igniting flammable material
Intrinsically Safe
Under normal or fault conditions, circuit is not capable of igniting flammable material
Non-incendive: A circuit, other than field wiring, in which any arc or thermal effect produced under intended operating conditions of the equipment is not capable, under specified test conditions, of igniting the flammable gas-air, vapor-air, or dust-air mixture.
Normal operation includes opening, shorting, or grounding the field wiring.
Intrinsically Safe differs in that it is tested under abnormal conditions as well as normal conditions. 9

11. Methods for Protection
Unclassified Area
Move the equipment or application to safe area.
Division 2 Solutions
Enhanced Electrical Designs and Approvals
Additional Safety & Installation Requirements
Division 1 Solutions
Intrinsically Safe Equipment
Explosion Proof Enclosure
Purged Enclosures
First, if at all possible move the application to the unclassified or safe area. Lower risk, generally simpler installation, and usually cheaper. Use barriers where possible to move equipment to safer areas.
But, if we have to put the equipment in the area: How do we protect equipment such that it can be used in these classified areas?

12. Division 2 Protections
Requires less stringent protections than Div 1 areas
Can be a combination of incendive and non-incendive circuits, but incendive circuits must be protected
Rigid sealed conduit
Dust-tight enclosures
Generally, additional installation requirements are necessary beyond what may be required in unclassified (safe) areas.
Div 2 means the hazardous material is only present in abnormal operations.
Div 2 equipment must meet the requirements as defined in the NEC guidelines.
Mixing Div 1 and Div 2 equipment in a div 2 installation is not allowed. Appropriate barriers and connections must still be used. Div 1 equipment may normally be used in a div 1 area, but must be installed to the div 1 requirements.
Non-incendive field wiring parameters normally applied for connected equipment.

13. Intrinsically Safe Systems
Ideal solution for Division 1 Areas
Equipment design limits energy available
Limit
Voltage
Current
Stored Electrical Energy
Inadequate energy for ignition, regardless of the state of the system
Designed to handle Normal and Fault conditions
Broad definition of Intrinsically Safe
Stored Electrical Energy can be in the form of capacitance (voltage) or inductance (current)

14. Explosion Proof Enclosures: Type 7/9
Designed to contain an explosion, not to make the electronics safe
Conservative approach
OK for small electronics, but prohibitive for larger devices
Openings have very tight tolerances
Designed to protect the area from the explosion, but equipment inside is not protected from a potential explosion.
For larger devices, this can be quite expensive.
For small electronics in which no operator access is required this can be a viable solution.
Not very practical if access to many keys is required for the operation of equipment.
Enclosures have very tight tolerances and must be installed carefully. Including tightening all bolts to specific tolerances.

15. Purged/Pressurized Enclosures
Purge reduces the Classification within the Enclosure
Benefits include the ability to use normal (unclassified) electrical equipment in the area.
Three Common Types
X-Purge, Y-Purge & Z-Purge
Works by maintaining a positive pressure inside the enclosure, keeping the explosive material out. Requires continuous monitoring to ensure the positive pressure is maintained.
Tends to be quite expensive comparatively, but you can use a broader array of electronics solutions inside the enclosure.
There are ongoing costs to a purged solution:
Air (or other non-hazardous gas like nitrogen)
When maintaining the equipment, must take down entire line (reset time a factor)
X-purge Div 1 to non-hazardous
Y-purge: Div 1 to Div 2
Z-purge: Div 2 to non-hazardous
front panel devices such as keypads are covered by the purge!!! The are technically in the haz loc and need a form of barrier between them and the internal devices. So the example shown in the picture is either IS, or has a barrier to anything else inside the box.

16. Control Drawings Control Drawing
"A drawing or other document provided by the manufacturer of the intrinsically safe or associated device…that details the allowed interconnections between the intrinsically safe and associated apparatus or between the nonincendive field wiring apparatus or associated nonincendive field wiring apparatus" NEC Article 500.2
The control drawing is a critical piece of the safety of the device and the installation. The installation of equipment must follow the control drawing to the letter. No deviations are allowed or safety could be compromised. 15

17. Entity Concept Entity Parameters Voltage Current Power Capacitance
General Application
IND560x
Voltage
Current
Power
Define entity parameter concept.
Compare to System Approval
Equipment and load cell designers have a specific parameter target value so that their equipment can coexist in a system that meets overall system requirements.
Allows for the interconnection of various devices while maintaining the safety of the overall system.
Capacitance
Inductance

18. Communication and Circuit Protection
Barriers
Zener / Shunt Diode
Galvanic Isolation
Transformer
Optic
Zener diode or shunt diode barriers maintain a physical wired connection, but add circuit elements to prevent hazardous energy levels from entering the hazardous area. Zener diodes limit the voltage. Current limiting resistors limit the current. Fuses limit the total power. These barriers operate to failure before letting in hazardous energy. They are not repairable on failure.
Galavnic isolation is a concept to separate the two circuits so that there is no direct connection. Most use transformers to pass the data through without a direct wired connection. A light based system using an optical transmitter and receiver are also used. It may be possible to repair some parts of these barriers.
Galvanic systems have lower MTBF (more components), may require their own power supply, more expensive, dissipate more energy
Sizing a barrier: ensure the barrier you install meets entity parameters, is suitable for the application, and is capable of carrying the signal you need…
For example, if you need an excitation voltage of 10V into the hazardous area, you need to make sure you don't get a barrier that clamps at 5V.
W&M approvals…when used with load cells, some barriers maintain W&M approvals and some don't. Be careful!
Barriers primary function is to prevent dangerous energy from passing into the hazardous area. 17

19. Cables for I.S. Current Loop vs. Fiber Optic
Current Loop transmits data on copper conductors
Fiber Optic uses light to transmit data over plastic or glass fibers.
Do not assume any cable is ok
Entity parameters and cable specifications must be matched to the device.
Even fiber optic cables transmit energy!
There are additional I.S. solutions besides current loop. But the basic concepts of a metal cored conductor still apply.
Recommend mechanical protection (rigid conduit), but not necessary for electrical safety.
Both current loop and "glass" fiber optic can run long distances (for our systems 1000ft). Plastic core tends to only be capable of shorter distances.
CL (or any conductor based cable): relatively robust, but thicker
FO: Easy to damage, but thin
FO cable is often more expensive than copper wire cable 18

20. Hazardous Area Weighing Agenda
Hazardous Area Overview
Weighing Technologies
Example System Designs

21. Scale Types for Hazardous Areas
Measuring Technologies
Mechanical
Analog Load Cell
High Precision (EMFR)
Mechanical Scales -No electronics. However, accuracy can be questionable. Connections to other systems or devices is essentially unavailable. Rely on operators to read each weight directly from a fixed dial.
Strain Gauges -Electronics used which must be managed for hazardous areas
EMFR
Explain how strain gauge (analog) and EMFR work. EMFR very precise (300,000 divisions). Analog is more like 10k-15k divisions max.
Technology selection depends on application and precision. 20

22. Associated Apparatus: AIS
Device is usually installed in unclassified (safe) area
But…connected to intrinsically safe circuits in hazardous area
Requires an AIS (associated intrinsically safe) approval
Note the Blue cables for the IS circuits. 21

23. Peripheral Devices Common Connected Systems and Devices Concerns PLC's
PC's
Remote Displays
Printers
Barcode Scanners
Keyboards
Concerns
Does the control drawing indicate this type of connection?
Where will the attached device be located?
Does the device need an approval?
How will I connect this other device?
With IS equipment, peripheral devices must be connected using entity parameters. However, many of these devices would require installation in safe areas with barriers. 22

24. Peripheral Devices – Discrete I/O
Discrete I/O Devices
Operator Buttons
Lights
Valves
Passive and Active Connections
Passive I/O Example
Haz Area
Safe Area
120V in
120V switched out of amplifier
Ex controls
Switch Amplifier
Entity Parameters!
Use peripherals with approvals for the area. 23

25. Hazardous Weighing Complexities
Weights & Measures Approvals
For legal-for-trade applications, system must carry hazardous and W&M's approvals
Static or Mobile Weighing
AC power must be brought into the area using some protection method, but provides a fixed continuous power supply.
Batteries avoid fixed AC power lines and add mobility, but require additional approvals for the battery and add challenges for charging.
W&M: be careful with legal for trade applications. Some IS equipment doesn't have a W&M approval. Some barriers may negate W&M approvals (mainly due to temperature compensation). If your application is legal for trade, work closely with the manufacturer to ensure your system meets the necessary requirements.
Charging usually occurs in safe/non-classified area.

26. Hazardous Area Weighing Agenda
Hazardous Area Overview
Weighing Technologies
Example System Designs

27. Simple Floor Scale Installation
Simple terminal and analog load cells
Note the blue cables. Note the loadcells, wiring, and jbox under the scale.
The power supply is not pictured here.

28. System Design Example 1 Checkweighing Hazardous Area Safe Area
I.S. Terminal and Analog Weighing Platform
Power Supply
Rigid sealed conduit
For simple manual checkweighing, the system looks very much like the floor scale.
This could also be used with a roller top system.
It could also be connected to peripheral devices.
Main Power

29. Hazardous Area CheckWeighing
Example of a mechanical and I.S. solution for weighing cylinders. The I.S. electrical system adds automated control and cutoff for the filling system.

30. Hazardous Area Vehicle Scales
Vehicle Scale in Hazardous Area – Electrical Layout
Hazardous Area
Safe Area
Weighing Terminal
I.S. Barrier
Truck Scale Platform, Load Cells, and junction box.

31. System Design Example 2
Vehicle Scale in Hazardous Area – Physical Layout
Division 2
Division 1

32. System Design Example 3 Process Filling With PLC Hazardous Area
I.S. Terminal With Discrete I/O
Safe Area
PLC
A.I.S. Accessory
I.S. valve
Power Supply
Main Power
Filling Stand On Load Cells

33. Hazardous Area Filling

34. Summary Know Your Application Know your Area Know Your Options
What am I trying to accomplish?
Know your Area
What is your area rated?
Know Your Options
What are my options for safe system design?
What are my options for changing the application or location?
Customer always classifies the area. Manufacturers of I.S. or other rated devices do not classify the area or the application.

35. Further Resources Agencies Manufacturers
NFPA and the NEC (
Factory Mutual (
Underwriters Limited (UL) (
International Electrotechnical Commission (IEC) (
Manufacturers
Rockwell Automation (
Mettler Toledo International (
Pepperl & Fuchs (
To contact Mettler Toledo, North America
Phone: North American Headquarters
Polaris Parkway
Web: Columbus, OH 43240