industrial safety of equipment and plants

7/30/2019 Industrial Safety of Equipment and Plants

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Government industrial safety regulations [1] require that every potentially hazardous industrial plant should be sub-

ject to an objective evaluation of the safety of the technology, process plant, and the installation as a whole in the form of an

industrial safety declaration, which should be drawn up and presented to the appropriate government inspectors.

A managerial principle in the industrial safety provision is the categorization and classification of industrial plants

in terms of their potential hazards.Categorization and classification methods: probabilistic or deterministic (Table 1). The first of them [2, 3] requires

one to calculate the probability of an explosion from the available statistical data. In accordance with the appropriate state

standard [3], the probability of an explosion for = 8760 h is

P() 106 yr1. (1)

An explosion requires the simultaneous occurrence of at least two independent factors: a combustible mixture and

an initiating factor (Fig. 1), so the explosion probability can be put as

Q() = Q1()Q2(), (2)

where Q1() is the probability of an explosive mixture occurring, in yr1; and Q2() is the explosion initiation probabil-

ity, yr1.

One can represent Q1() and Q2() as the products of the probabilities of occurrence for fuel and oxidizer Q1 and

the characteristics of the initiating factor Q2. If an explosion is possible without the occurrence of some factor, its value is

taken as 1. One proceeds in that way, for example, in calculations on the production of ethylene oxide, which under certain

conditions can explode in the absence of an oxidizing agent, and also when certain factors occur at a given energy or time,

e.g., a lightning strike as an initiating factor. It is clear that each of the factors given in the lower part of Fig. 1 is of itself the

result of considering all the probabilities in the tree of events leading to the explosion. The regulatory documents [3] allow

one to perform these calculations from simplified formulas.

In the design stage, one assumes an exponential distribution, with the event probability given by the theoretical for-

mulaQ1() = 1 exp(), (3)

where is the event intensity in sec1 or h1.

Chemical and Petroleum Engineering, Vol. 38, Nos. 78, 2002

PRINCIPLES FOR CATEGORIZING AND

CLASSIFYING PLANT EXPLOSION AND

FIRE HAZARDS

A. G. Vetoshkin

INDUSTRIAL SAFETY OF EQUIPMENT AND PLANTS

Penza State Architecture and Building Academy. Translated from Khimicheskoe i Neftegazovoe Mashinostroenie,

No. 8, pp. 4144, August, 2002.

0009-2355/02/0708-0490$27.00 2002 Plenum Publishing Corporation490


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Estimating explosion hazardQ(t)

Estimating scope for

combustible mixture to formQ1(t)

Estimating scope for

explosion initiationQ2(t)

&

Fig. 1. Components in estimating explosion hazard.

TABLE 1

RegulatoryPurpose

Conditions Calculation Objective of Parameters Documentdocument envisaged type calculation determined purpose

GOST 12.1.010-76 Defining general Occurrence of Probabilistic Comparison with p() 106

yr1

; Demonstrating[2] specifications and combustible and, in standardized m mper for need for additional

GOST 12.1.004-91 principles mixture (CM), applications, values p pper protection from

[3] mass m of it, and deterministic X; Y;Z explosions and

occurrence of initi- fires (for staff,

ating factor (IF) main document)

NPB 105-95 [4] Categorizing m, CM Deterministic Assigning rooms p > 5 kPa; Selecting standards

production and and buildings A, B, C1C4, and rules for

storage buildings to previously D, E constructional

and rooms defined categories X; Y;Z measures and also

safety rules SR

PU [5] Classifying zones m, CM Deterministic Assignment to C-I, C-II, C-Ia, Demonstratingin terms of previously C-IIa, C-1b, C-Id, choice of

explosion and determined P-I, P-II, P-IIa, equipment, rules,

fire hazards classes P-III, mixture and standards for

production safety

PB 09-170-97 [6] Categorizing m, CM, IF Deterministic Category I, II, III; m, Qa Demonstrating the

process definition from need to upgrade

units and stages in potential production

terms of explosion process energy equipment and

and fire hazards technology

PB 09-170-97 [6] Estimating possible m, CM Deterministic Determining p;R Selecting safety

damage, injuries, pper scope for standards and

and hazard factors reducing hazard rules:

factors in demonstrating

damage zone need for additional

and seriousness protection

of consequences measures


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For existing plant, the probability of an analogous event is given by

(4)

where Ks is the safety factor (Ks = 1 if = 1), w is the working time in sec, j is the time for which the cause of the event

has existed in sec, and is the number of causes of the events.

If one provides the regulated probability for the absence of explosion, one can take the plant as being explosion-pro-

tected.

However, at present one cannot perform such calculations because we lack reliable statistical data. Some compo-

nents of the event tree such as the probability of explosion initiation by lightning strike can be determined [3].One not only calculates probabilities but also envisages determining the maximum permissible mass mper of com-

bustible components in a mixture representing an explosion hazard, which is such that its explosion does not lead to mechan-

ical destruction of the companys buildings when it produces an excess pressure p at the shock-wave front or in a closed

building equal to or less than the permissible pper.

The mass m of combustible components released on emergency failure is compared with mper, and then one derives

the coordinatesX, Y, andZin the space within which the pressure may exceed the permissible value. That pressure pper is

taken from the companys standards, while m is taken in accordance with the worst emergency conditions. This part of the

QK

j

j

i l

1

1

( ) ,

=

=

=

sw

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TABLE 2

Room or building Rooms Buildings, structures

category p, kPa Characteristics of substances or materials present Lacking automatic With automaticor formed in room fire extinguishers fire extinguishers

Explosion A 5 Combustible gases or flammable liquids with flashpoints not SA > 5% SA > 25%

and more than 28C. Substances and materials capable of or >200m2 or >1000m2

fire exploding or burning on interacting with water, atmospheric

hazards oxygen, or one another

B 5 Combustible dusts or fibers, flammable liquids with (SA + SB) > 5% (SA + SB) > 25%

flashpoints over 28C, and combustible liquids in amounts or >200m2 or >1000m2

such that potentially explosive dust-air or vapor-air mixtures

may form

Fire C1C4 5% (SA + SB +

hazard substances and materials (including dusts and fibers), or for SA = 0 and + SC) > 25%

substances and materials capable of reacting with water, SB = 0 or >3500m2

oxygen from the air, or one another only by burning, subject SC > 10%

to the condition that the rooms in which they are present or

formed do not belong to categories A and BD 5% + SC + SD) > 25%

accompanied by the release of radiant heat, sparks, and or >5000m2

flames; also, combustible gases, liquids, and solids that can

burn as fuels

E


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categorization is envisaged by the obligatory application of the state standard [3], and it provides a single-valued relationship

between p and m, so the calculation can be taken as deterministic.

The deterministic method involves comparing parameters with preset values. One can restrict the calculations to the

worst case of events leading to an explosion if one states the detailed conditions assumed and any other possible assumptions,

which must be justified from the comparability of the results. The basic regulatory documents for such calculations are pro-

vided by interdisciplinary standards and rules [46].

We now consider the procedure for use in deterministic calculations designed to categorize and classify fire and

explosion hazards. One begins by categorizing rooms and buildings in accordance with industrial safety standards 105-95 [4].The basic test in assigning any room as representing an explosion or fire hazard on the basis of the regulations [4] is the excess

pressure produced by an explosionp that exceeds the regulated value (one assumesp = 5 kPa for any plant). At most indus-

trial plants, the mechanical strength of the structures is much greater than 5 kPa, but that is not considered in this case. With-

in the explosion-hazard categories, one performs an additional subdivision on the basis of the properties of the materials or

products arising (Table 2). For example, in the complete set of categories A, B, C1C4, D, and E, only the first two (A and B)

represent explosion and fire hazards, while categories C1C4 represent a fire hazard.

In the electrical installation design rules [5], the test is based on the relative volume of mixture representing an explo-

sion hazard (Table 3). If that relative volume exceeds 5%, then the entire zone is taken as representing an explosion hazard

(classes C-I, C-II, C-Ia, and CIIa), but otherwise the zone representing an explosion hazard is taken as that out to a distance

of 5 m from the combustible mixture source (plant) in the room or else the distance specified in the regulations [5].

In some cases, if the mixture volume is less than 5% of the free volume, the entire room can be assigned to classC-Ib. In rooms in categories A and B on the basis of the industrial safety rules 105-95 [4], there must be equipment protect-

ed from explosion and designed in accordance with the electrical installation design rules [5]. That is, categorization in accor-

dance with the industrial safety regulations 105-95 serves to define not only the rules and standards for safety engineering

and the specifications for the building installations, but also the equipment, which must be in accordance with the electrical

installation design rules.

A somewhat different approach is used in categorization from the explosion and fire hazards in accordance with

safety rules 09-170-97 [6]. The basis is the total potential energyEof the engineering process. It is used to calculate the clas-

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TABLE 3

ClassExplosive mixture

Plant position Substances and products, type of occurrence Note numbervolume, %

V-I >5 Within room Combustible gases, vapors, flammable liquids FL, under normal conditions

V-Ia >5 Ditto The same, but in accidents or faults

V-Ib 5 and 5 The same, but less dangerous than in C-Ia 1V-Id Outside Combustible gases and FL vapors 2; 3

V-II 5 Within room Combustible dusts and fibers under normal conditions 3

V-IIa 5 Ditto The same, but in emergencies

P-1 0 Combustible liquids (flashpoints over 61C)

P-II 0 Combustible dusts and fibers (LICL > 65 g/m3)

P-IIa 0 Solid combustible substances 3

P-III Outside Combustible liquids (flashpoints over 61C) and solid combustible substances 3

Notes: 1. Combustible gases have high LICL (lower ignition concentration limit, >15%), or gaseous hydrogen for which the combustiblemixture volume is less than 5% of the free volume, with the height of the explosion-hazard zone 75% or more of the height of the room,and laboratory and other zones provided that the mixture volume is less than 5%. 2. Outside plant is classified in terms of the distance

from the possible point of occurrence of a combustible component. 3. Outside plant is not classified as regards dust.


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sification parameters: effective mass m (in general, not equal to the mass of combustible components in calculations from the

regulatory documents [4]) and the relative energy potentialQe. The parameters are closely correlated and one may be derived

from the other, but it is best to calculate them independently and define one of the three possible categories (I, P, or TIT). If

a plant is assigned as representing an explosion hazard from calculations in accordance with industrial safety rules 105-95

[4] or in accordance with the electrical installation design rules [5], then it is obligatory to define the category in accordance

with safety rules 09-170-97 [6].

In those rules [6], it is also recommended to determine the trotyl equivalent, from which one can derive the distance

R corresponding to pper, which characterizes the stability of adjacent units or structures. Then one examines all the results

and plans measures to meet the standards of 09-170-97 [6] and to reduce the explosion hazard for that unit.

Categorization in accordance with rules 105-95 [4] is based on the assumption that mixtures representing an explo-

sion hazard can occur only as a result of emergencies. The electrical installation design rules [5] classify zones in terms of

the scope for combustible mixtures to occur not only in the normal state but also in emergencies. One classifies zones after

categorization in accordance with rules 105-95 [4] on the basis of the calculated massm, from which one determines the vol-

ume Vm of the combustible mixture (m3) and the volume fraction in % Cm:

Vm = m/LICL; Cm = 100/ (VmK),

where m = WFeTin kg; W= 10

6

psM

1/2

is the evaporation rate in kg/sec; is a coefficient [4];ps is the saturation vapor pres-sure in kPa;Mthe molecular mass of the combustible substance in kg; Fe = ml is the evaporation area in m2; the specific

evaporation area in m2/kg; ml the mass of combustible liquid in kg; T= ml/WFe 3600 sec; Vfthe free volume of the room in

m3; Kthe air exchange factor; andA in h1 the capacity of the emergency ventilation (A = 0 in the first calculation stage).

IfCm > 5%, the entire zone is taken as representing an explosion hazard. IfCm < 5%, the explosion-hazard zone is

taken as the space at a distance of 5 m along the horizontal and vertical from the source of likely combustible mixture (equip-

ment). Also, if the amounts of combustible gases or flammable liquids are such that a combustible mixture cannot be pro-

duced in a volume Cm > 5%, such zones are assigned to the category with least explosion hazard (Table 3).

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TABLE 4

Explosion hazard Relative explosion Total effective mass m in kgcategory hazard energy potential Qe of combustible vapors or gases

I >37 >5000

II 2737 20005000

III


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The electrical installation design rules [5] envisage explosion-hazard zones C-I, C-Ia, C-Ib, and C-Id for mixtures

of combustible gases or flammable liquid vapors with air (oxidizing agent), or zones C-II and C-IIa for dustair mixtures.

The fire hazard zones P-I, P-II, P-IIa, and P-III are classified only from the presence of combustible materials.

One characterizes the energy potential of any plant, stage, or unit showing explosion hazards from the sum of the

adiabatic expansion energy of the vaporgas phase and the energy from the complete combustion of the vapor already pre-

sent and that formed from the liquid together with the internal energy and the external energy (from the environment) on sys-

tem failure:

E =E1* +E2* +E1** +E2** +E3** +E4**, (10)

whereE1* is the sum of the adiabatic expansion energy and the combustion energy of the vaporgas mixture VGM directly in

the failed unit (if the excess pressure is less than 0.07 MPa and the product of the excess pressure and the volume is less than

0.02 MPam3, one neglects the adiabatic expansion energy because it is relatively small);E2* is the combustion energy of the

VGM coming from adjacent units or plant;E1** is the combustion energy of the VGM formed by the heating of the liquid

phase to a temperature above the boiling point at atmospheric pressure;E2** is the combustion energy of the VGM formed

from the liquid phase as a result of the heat from ongoing exothermic reactions;E3** is the combustion energy of the VGM

formed from the liquid phase as a result of heat influx from external media; andE4** is the combustion energy of the VGM

formed from the liquid phase spilt on a solid surface.

One uses the total explosion hazard energy potential E to determine the other parameters characterizing the unit

explosion hazards: total effective mass in kg of the combustible vapors (gases) in the vaporgas cloudm = T/4.6104;

relative energy potential Qe = E1/3/16.534 (Eis the total energy potential in kJ and 4.6104 kJ/kg is the specific

combustion energy unit).

From m and Qe, one classifies (categorizes) the various units (Table 4).

The effective mass of combustible vapor (gas) provides an approximate definition of the possible damage zones. One

calculates the mass in kg of vapor (gas) participating in the explosion:

m* =zm,

wherez is the fraction of the effective mass of combustible vapor (gas) m that participates in the explosion; in an unclosed

space,z = 0.020.1, while in an enclosed space,z = 0.5 (for combustible gases) orz = 0.3 (for flammable liquid vapors).The mass of condensed explosives participating in the explosion is determined by the total mass present in the unit.

The trotyl equivalent in kg for a vaporgas mixture is

WT= (0.4/0.9)(q*/qT)zm,

where q* = 4.6104 kJ/kg; qT = 4520 kJ/kg is the explosion energy of trinitrotoluene TNT; 0.4 is the fraction of the energy

consumed in generating the explosion shock wave from the vaporgas mixture; and 0.9 is the fraction of the energy consumed

in producing the shock wave from the explosion of a condensed explosive.

The trotyl equivalent in kg for a condensed explosive is

WT= (qc/qT)Wc,

where qc is the specific explosion energy of the condensed explosive in kJ/kg and Wc is the mass in kg of the condensed

explosive.

The complete-destruction radius is

R0 = WT1/3/[1 + (3180 /WT)]

1/6;

and for m > 5000 kg,R0 = WT1/3.

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The radii of the damage zones are defined by

Ri = KiR0,

where Ki is the damage zoning coefficient (Table 5).

When the unit has been categorized, one can if necessary draw up proposals for reducing its explosion hazard.

REFERENCES

1. Federal Law Industrial Safety for Hazardous Production Plant dated July 21, 1997, No. 116-FZ.

2. GOST 12.1.010-76 SSBT.Explosion Safety: General Specifications [in Russian].

3. GOST 12.1.004-91 SSBT. Fire Safety: General Specifications [in Russian].

4. Industrial Safety Rules 105-95. Fire Safety Standards: Defining Categories for Rooms and Buildings as Regards

Explosion Plus Fire or Fire Hazard[in Russian].

5. Electrical Installation Design Rules [in Russian], nergoatomizdat, Moscow (1998).

6. Industrial Safety 09-170-97. General Rules for Ensuring Explosion Safety for Chemical, Petrochemical, and

Oil-Refining Processes Representing Explosion and Fire Hazards [in Russian], PIO OBT, Moscow (1999).

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