Electrical+Classification.ppt

1、Electrical Classification - Training Objective,Provide a basic understanding of what is electrical classification Show how to determine the electrical classification of a general area Does not equip participants to determine extent of the classified area.,Electrical Classification- Definitions,Autoi

2、gnition Temperature (AIT). The minimum temperature required to initiate or cause self-sustained combustion of a solid, liquid, or gas independently of the heating or heated element. Class I, Division 1 Area. A location where (1) ignitable concentrations of flammable gases or vapors exist under norma

3、l operating conditions; or (2) ignitable concentrations of such gases or vapors may exist frequently because of repair or maintenance operations or because of leakage; or (3) breakdown or faulty operation of equipment or processes might release ignitable concentrations of flammable gases or vapors a

4、nd might also cause simultaneous failure of electrical equipment.,Electrical Classification- Definitions,Class I, Division 2 Area. A location (1) in which volatile flammable liquids or flammable gases are handled, processed, or used, but in which the liquids, vapors, or gases will normally be confin

5、ed within closed containers or closed systems from which they can escape only in case of accidental rupture or breakdown of such containers or systems, or in case of abnormal operation of equipment; or (2) in which ignitable concentrations of gases or vapors are normally prevented by positive mechan

6、ical ventilation, and which might become hazardous through failure or abnormal operation of the ventilating equipment; or (3) that is adjacent to a Class I, Division 1 location, and to which ignitable concentrations of gases or vapors might occasionally be communicated unless such communication is p

7、revented by adequate positive-pressure ventilation from a source of clean air and effective safeguards against ventilation failure are provided.,Electrical Classification- Definitions,Combustible Liquid. A liquid having a flash point at or above 100F (37.8C). Combustible liquids are subdivided as fo

8、llows: Class II liquids are those having flash points at or above 100F (37.8C) and below 140F (60C). Class III liquids are those having flash points at or above 140F (60C), and are subdivided as follows: Class IIIA liquids are those having flash points at or above 140F (60C) and below 200F (93.4C).

9、Class IIIB liquids are those having flash points at or above 200F (93.4C).,Electrical Classification- Definitions,Combustible Material.* A generic term used to describe a flammable gas, flammable liquid produced vapor, or combustible liquid produced vapor mixed with air that may burn or explode. Fla

10、mmable Liquid. A liquid designated as Class I, having a flash point below 100F (37.8C) and having a vapor pressure not exceeding 40 psia at 100F (37.8C). Class I liquids can be further subdivided. Flash Point. The minimum temperature at which a liquid gives off vapor in sufficient concentration to f

11、orm an ignitable mixture with air near the surface of the liquid, as specified by test.,Electrical Classification- Classification of Hazardous Areas,Article 500 of the NEC designates as hazardous (classified) any area in which a combustible material is or may be present in the atmosphere in sufficie

12、nt concentration to produce an ignitable mixture. Article 500 designates three major categories of hazardous areas: Class I, Class II, and Class III. In a Class I hazardous area, the material present is a flammable gas or vapor. In a Class II hazardous area, the material present is a combustible dus

13、t. In a Class III hazardous area, the material present is an ignitable fiber or flying.,Electrical Classification- Classification of Hazardous Areas,The Class I category is further subdivided into either Class I, Division 1 or Division 2 as follows: Class I, Division 1 in which the combustible mater

14、ial is present normally or frequently Class I, Division 2 in which the combustible material is present as a result of infrequent failure of equipment or containers Areas not classified either as Class I, Division 1 or Division 2, are “unclassified” areas,Electrical Classification- Behavior of Gases/

15、 Vapors,Lighter-than-Air (Vapor Density Less than 1.0) Gases. These gases tend to dissipate rapidly in the atmosphere. They will not affect as great an area as heavier-than-air gases or vapors. Except in enclosed spaces, such gases seldom accumulate to form an ignitable mixture near grade level, whe

16、re most electrical installations are located. Heavier-than-Air (Vapor Density Greater than 1.0) Gases. These gases tend to fall to grade level when released. The gas may remain for a significant period of time, unless dispersed by natural or forced ventilation.,Electrical Classification- Behavior of

17、 Liquids,Flammable Liquids. When released in appreciable quantity, a Class I liquid will begin to evaporate at a rate that depends on its volatility: the lower the flash point, the greater the volatility; hence, the faster the evaporation. The vapors of Class I liquids form ignitable mixtures with a

18、ir at ambient temperatures more or less readily. Even when evolved rapidly, the vapors tend to disperse rapidly, becoming diluted to a concentration below the lower flammable limit. Until this dispersion takes place, however, these vapors will behave like heavier-than-air gases. Class I liquids norm

19、ally will produce ignitable mixtures that will travel some finite distance from the point of origin; thus, they will normally require area classification for proper electrical system design Combustible Liquids. A combustible liquid will form an ignitable mixture only when heated above its flash poin

20、t.,Electrical Classification- Behavior of Liquids,With Class II liquids, the degree of hazard is lower because the vapor release rate is low at the normal handling and storage temperatures. In general, these liquids will not form ignitable mixtures with air at ambient temperatures unless heated abov

21、e their flash points. Also, the vapors will not travel as far because they tend to condense as they are cooled by ambient air. Class II liquids should be considered capable of producing an ignitable mixture near the point of release when handled, processed, or stored under conditions where the liqui

22、d may exceed its flash point. Class IIIA liquids have flash points at or above 140F (60C) but below 200F (93.4C). These liquids do not form ignitable mixtures with air at ambient temperatures unless heated above their flash points. Furthermore, the vapors cool rapidly in air and condense. Hence, the

23、 extent of the area requiring electrical classification will be very small or nonexistent. Class IIIB liquids have flash points at or above 200F (93.4C). These liquids seldom evolve enough vapors to form ignitable mixtures even when heated, and they are seldom ignited by properly installed and maint

24、ained general purpose electrical equipment. A Class IIIB liquid will cool below its flash point very quickly when released. Therefore, area classification is seldom needed.,Electrical Classification- Conditions Necessary for Ignition,In a Class I area, the following three conditions must be satisfie

25、d for the combustible material to be ignited by the electrical installation: A combustible material must be present. It must be mixed with air in the proportions required to produce an ignitable mixture. There must be a release of sufficient energy to ignite the mixture,Electrical Classification- Di

26、vision 1 Classified Areas,A condition for Division 1 is whether the area is likely to have an ignitable mixture present under normal conditions. For instance, the presence of a combustible material in the immediate vicinity of an open dip tank is normal and requires a Division 1 classification. Norm

27、al does not necessarily mean the situation that prevails when everything is working properly. For instance, there may be cases in which frequent maintenance and repair are necessary. These are viewed as normal and, if quantities of a flammable liquid or a combustible material are released as a resul

28、t of the maintenance, the area is Division 1. However, if repairs are not usually required between turnarounds, the need for repair work is considered abnormal. In any event, the classification of the area, as related to equipment maintenance, is influenced by the maintenance procedures and frequenc

29、y of maintenance.,Electrical Classification- Division 2 Classified Areas,The criterion for a Division 2 area is whether the area is likely to have ignitable mixtures present only under abnormal conditions. The term “abnormal” is used here in a limited sense and does not include a major catastrophe.

30、As an example, consider a vessel containing liquid hydrocarbons (the source) that releases combustible material only under abnormal conditions. In this case, there is no Division 1 area because the vessel is normally tight. To release vapor, the vessel would have to leak, and that would not be norma

31、l. Thus, the vessel is surrounded by a Division 2 area.,Electrical Classification- Relative Magnitude,Relative Magnitudes of Process Equipment and Piping Handling Combustible Materials:,Electrical Classification- Procedure for Classifying Areas,Step OneDetermining Need for Classification. The area s

32、hould be classified if a combustible material is processed, handled, or stored there. Step TwoGathering Information. Proposed Facility Information: for a proposed facility that exists only in drawings, a preliminary area classification can be done so that suitable electrical equipment and instrument

33、ation can be purchased. Plants are rarely built exactly as the drawings portray them, so the area classification should be modified later based upon the actual facility.,Electrical Classification- Procedure for Classifying Areas,Step TwoGathering Information (cont.) For an existing facility: the ind

34、ividual plant experience is extremely important in classifying areas within the plant. Both operation and maintenance personnel in the actual plant should be asked the following questions: Have there been instances of leaks? Do leaks occur frequently? Do leaks occur during normal or abnormal operati

35、on? Is the equipment in good condition, questionable condition, or in need of repair? Do maintenance practices result in the formation of ignitable mixtures? Does routine flushing of process lines, changing of filters, opening of equipment, and so forth, result in the formation of ignitable mixtures

36、?,Electrical Classification- Procedure for Classifying Areas,Step TwoGathering Information (cont.) Process Flow Diagram: a process flow diagram showing the pressure, temperature, flow rates, composition and quantities of various materials (i.e., mass flow balance sheets) passing through the process

37、is needed. Plot Plan: a plot plan (or similar drawing) is needed showing all vessels, tanks, trenches, lagoons, sumps, building structures, dikes, partitions, levees, ditches, and similar items that would affect dispersion of any liquid, gas, or vapor. The plot plan should include the prevailing win

38、d direction. Fire Hazard Properties of Combustible Material (e.g. flash point, AIT, %LFL, %UFL, vapor density, vapor pressure, etc.),Electrical Classification- Procedure for Classifying Areas,Step Three Selecting the Appropriate Classification Diagram. Review the quantities, pressures, flow rates to determine relative magnitude of equipment Select appropriate classification diagram(s). Step Four Determining the Extent of the Classified Area. The extent/boundaries of the classified area may be determined by using sound engineering judgment to apply the methods given and t