METHODOLOGY FOR HAZARDOUS AREA CLASSIFICATION (HAC) STUDY-IS
OBJECTIVE
The purpose of the Hazardous Area Classification study is to determine the extent of the hazardous areas around equipment handling or storing flammable fluids, combustible fluids and combustible dusts, both liquid and vapors in terms of Zone 0, Zone 1, Zone 2 and Division 1 and Division 2
The aim of hazardous area classification is to avoid ignition of those releases that may occur from time to time in the operation of facilities handling flammable liquids and vapors. The approach is to reduce to an acceptable minimum level the probability of coincidence of a flammable atmosphere and an electrical or other source of ignition occurring. In Hazardous Area Classification the extent of the hazardous areas around equipment handling or storing flammable fluids, both liquid and vapors is determined in terms of Zone 0, Zone 1, and Zone 2. The hazardous area classification around the equipment handling flammable/explosive dust is determined in terms of Division 1, Division 2.
2. Scope
The Hazardous area classification is carried out as per:
Classification of the Hazardous area for explosive dust is carried out as per IS 61241
Classification of the Hazardous area for flammable liquids/ gases/ vapors is carried out by IS 5572
3. HAC is carried out using Guidelines from following Standard’s latest updated revisions
IS 61241
IS 5572
4. ORDER OF PRECENDENCE
IS 61241/ IS 5572
As per Client Guidelines
Other international standards
5. Proposed Methodology
5.1. Introduction
Hazardous Area is defined as a place/an area where concentrations of flammable gases, vapors, or dusts occur and persists in that place/area.
A hazardous area is defined as a three-dimensional space in which a flammable atmosphere may be expected to be present at such frequencies as to require special precautions for the design and construction of equipment, and the control of other potential ignition sources.
5.2. Methodology
For flammable liquids/ gases/ vapors
To determine the type of electrical installation appropriate to a particular situation, the hazardous areas have been divided into three zones, namely, Zone 0, Zone 1 and Zone 2.
The procedure of HAC involves following steps:
i. Identification of the Point Source or Source of Release:
For purpose of HAC the point source or source of release is defined as a point from which a flammable gas vapor or liquid may be released in the atmosphere. Typically this includes equipment that has the potential to leak (e.g. various items of plant equipment, pipe work, flow lines, flanges, valves, seals, instrument connections, nozzles) or part of a system or equipment that may release hydrocarbons to atmosphere (e.g. sumps, pits, vents, bunds, drains)
ii. Determine the Grade of Release:
Three grades of release are defined in terms of their likely frequency and duration (definitions reference is taken from Model Code of Safe Practice -IP Part 15, Area Classification Code for installations handling flammable fluids):
Continuous Grade release (C): A release that is continuous or nearly so, or that occurs frequently and for short periods.
Primary Grade release (P): A release that is likely to occur periodically or occasionally in normal operation, that is, a release which, in operative procedures, is anticipated to occur.
Secondary grade release (S): A release that is unlikely to occur in normal operation and, in any event, will do so only infrequently and for short periods, that is, a release which, in operating procedures, is not anticipated to occur.
The grade of release is dependent solely on the frequency and duration of the release. It is completely independent of the rate and quantity of the release, the degree of ventilation, or the characteristics of the fluid, although these factors determine the extent of vapor travel and in consequence the dimensional limits of the hazardous zone.
Most of the releases are considered as secondary grade releases i.e. releases are from valves, flanges, sampling and drainage point, instrument connections and pumps.
Grade of release is calculated as per INDIAN STANDARD (IS) 5572: 2009 – Classification of Hazardous Areas (Other than Mines) having Flammable Gases and Vapors for Electrical Installation. The first step in Hazard Area Classification is preparation of summary of characteristics of fluid and dust.
The release of vapor in tank/vessel is from vent & leakages of nozzles / instrument nozzles / flange joints. The release from vent is during transferring operation which is periodical in nature, so the release from vent is primary Grade Release (P). The release from leakages is not the part of procedure / operation & not anticipated to occur, so the release from vent is Secondary Grade Release(S).
iii. Determine the Class of the Fluid:
Flammable Liquids vary in volatility and have a flash point below 930C and a Vapor Pressure not exceeding 2.81 kg/cm2 at 37.80C. These are divided into three classes, as follows,
Class A : Flammable liquids having flash point below 23° C
Class B : Flammable liquids having flash point 23° C and above but below 65°C
Class C : Flammable liquids having flash point 65° C and above but below 93°C
iv. Area Classification (As per IS 5572):
Hazardous areas are subdivided into zones. Based on the likelihood of occurrence and duration of a flammable atmosphere Areas are classified into Zone 0, Zone 1, Zone 2 and Non-hazardous Zone.
Zone 0 Areas: An area in which an explosive gas atmosphere is present continuously or is present for long periods
Examples are vapor space above closed process vessels, storage tanks or closed containers, areas containing open tanks of volatile, flammable liquid.
Zone 1 Areas: An area in which an explosive gas atmosphere is likely to occur in normal operation
Zone 2 Areas: An area in which an explosive gas atmosphere is not likely to occur in normal operation and if it does occur it will exist for a short period only
Methodology for Flammable liquid, vapors and Combustible Dusts
The procedure of HAC involves following steps:
Review Process Flow diagram
Prepare separate table for liquid and solid chemical along With flammable/combustible Properties
Classification of material as per NFPA-497
·Selecting the Appropriate Classification Diagram
Determine the Zone classification
Determine Extent of the Classified Location
Review and recommendation
To enable selection of electrical equipment appropriate to the zone classification, the apparatus sub group and temperature class is then determined based on the flammable substances that can be release.
For Explosive dusts
General
The procedure for identifying zones of risk
The first step is to identify the material characteristics, whether it is combustible and, for the purposes of apparatus selection, particle size, moisture content, cloud and a layer, minimum ignition temperature and electrical resistivity.
The Second step is to identify where dust containment or sources of dust release can be present. It may be necessary to consult a process drawings and plant layout
The third step is to determine the likelihood that dust will be released from those sources and thus, the likelihood of explosive dust/air mixtures in various parts of installation
It is only after these steps that the zones can be identified and their extents defined. The decisions on the zone types and extent and the presence of dust layers shall be recorded on the area classification drawing. (The drawing is to be used subsequently as the basis for the selection of equipment).
i. Sources of release for explosive dust atmospheres
Explosive dust atmospheres are formed from sources of dust release. A source of dust release is a point or location from which combustible dust can be released or raised, such that an explosive dust/air atmosphere can be formed. This definition includes layers of combustible dust capable of being dispersed to form a dust cloud.
ii. Identification of sources of release
The conditions need to be identified in which process equipment; process steps or other actions expected in plants can form explosive dust/air mixtures or create combustible dust layers. It is necessary to consider separately the inside and outside of a dust containment.
iii. Sources of release
Many factors can influence the area classification. Where higher than atmospheric pressures are used within the dust containment (e.g. positive pressure pneumatic transfer) dust can easily be blown out of leaking equipment. In the case of negative pressure within the dust containment, the likelihood of formation of dusty areas outside the equipment is very low. Dust particle size, moisture content and. where applicable, transport velocity, dust extraction rate and fall height can influence release rate potential.
Grade of release are as follows
Primary grade of release, e.g. close vicinity around an open bag filling or emptying point;
Secondary grade of release, e.g. man-holes that need to be opened occasionally
iv. Area Classification (As per IS 61241)
Based on the likelihood of the formation of potentially explosive dust/air mixtures, the area scan be designated according to Table 1
Table 1 Explosive dust area classification as per IS 61241
Zone 20: A place in which an explosive atmosphere, in the form of a cloud of combustible dust in air, is present continuously, or for long periods or frequently for short periods
Zone 21: A place in which an explosive atmosphere, in the form of a cloud of combustible dust in air, is likely to occur occasionally in normal operation.
Zone 22: A place in which an explosive atmosphere, in the form of a cloud of combustible dust in air, is not likely to occur in normal operation but, if it does occur, will persist for a short period only.
Methodology for Combustible solids
The procedure of HAC involves following steps:
Need For Classification
Facility information & history
Classification of material as per NFPA-499
Selecting the Appropriate Classification Diagram
Determine the Zone classification
Determine Extent of the Classified Location
Review and recommendation
To enable selection of electrical equipment appropriate to the zone classification, the apparatus sub group and temperature class is then determined based on the flammable substances that can be release.
5.3. Project Deliverables
Table 1:HAC Study
5.4. Software to be used
AutoCAD
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