Industrial Process Safety and Risk Analysis - Appunti, esercizi e domande per l'esame

TOXICITY PROPERTIES

The toxicity of a material is a property describing the effect on biological organisms.

The associated hazard expresses the damage to biological organisms based on exposure (like

ingestion, inhalation, injection, dermal absorption).

These types of exposures can be controlled by applying proper hygiene techniques and

protective devices (PPE, personal protective equipment).

Inhalation is the easiest to quantify by directly measuring the concentration of toxic materials

in air.

DOSE and TOXICITY

The dose is the amount of toxic material absorbed, computed as the product of the amount of

toxic material and its exposure time → = ( ,

)

The effect of toxic materials is the response of an organism to a given dose.

The response distribution is shown in the cumulative distribution curve → dose-response

relations.

We use this type of graphs because the response depends on the target so it’s difficult to

predict the consequences of a certain dose:

●​ x axis: dose

●​ y axis: toxic response i.e. the percentage of the population that will experience a

response to a certain dose

Generally, in these graphs there are different curves:

●​ ED - Effective Dose: regards minor and reversible effect on health

●​ TD - Toxic Dose: irreversible effects

●​ LD - Lethal Dose: lethal effect

For example, in the following graph:

●​ ED is the dose that causes 50% of the population to experience minor effects

50

●​ LD is the dose that causes the death of 50% of the population

50

If several chemicals are involved, the toxicants might interact:

●​ additively → the effect is the sum of the individual ones

●​ synergistically → combined effect is more than individual ones

●​ potentiately → presence of one chemical increase the effect of another

●​ antagonistically → both counteract each other

One of the most toxic chemicals is phosgene .

2

●​ chronic toxicity: describes effect of a substance on a person after many exposures

over an extended period (occupational exposure, even with very low concentrations)

●​ acute toxicity: describes effect of a substance on a person after a single exposure (or

several in a short amount of time) → is the case of very toxic materials with low lethal

doses

MODELS for RESPONSE-DOSE

Most used model is based on the Probit functions:

= + ·

where:

●​ Pr is the Probit variable that, using a Probit table, is connected to a certain percentage

of a given population with specific response

●​ a and b are constants

●​ V is the causative variable and depends on the event that is causing the effect.

○​ In the case of toxic compounds, V is the exposure factor

= ∆

where n depends on the substance

Probit table:

EXAMPLE

Determine the likely percentage of fatalities from a 15 minute exposure to 10 ppm of nitrogen

dioxide. According to TNO Yellow Book, Appendix A.3 the Probit constant for lethal scenario

of NO are:

2

=− 16, 192

=1

= 3, 7

with c in ppm and in min.

∆

Applying Probit equation: 3,7

( ) ( )

= + · = + · ∆ =− 16, 192 + 1 · 10 · 15 = 3, 56

Using the table, a Pr between 3,52 and 3,59, between 7-8% of the population would die if

exposed to this event.

Vulnerability of human beings to toxic compounds depends on the nature, properties of the

material and its dose. Different threshold values for the maximum tolerable dose are

assumed in different situations:

●​ occupational exposure limits

○​ for example: TLV-C (Threshold limit value-ceiling): concentration at which a

worker cannot be exposed

●​ emergency scenario: typical time for emergency is 30 minutes

ZONING

The area potentially affected by an accident can be

divided into different zones according to the type of

hazard, intensity of effects and vulnerability of

people and property:

●​ intervention zone (IZ): if an accident occurs,

in this area application of protective measures

must be immediate (for example zones with

high percentage of lethal concentrations)

●​ alert zone (AZ): in this area protective measures are non mandatory, except for

protecting critical groups of people

●​ domino effect zone (DZ): potential accident propagation in this zone

●​ moderate consequences zone: out of this zone is the “safe zone”

SPECIFIC HAZARDS

●​ chemical/material hazards: depend on material itself (intrinsic) → comes with the

use of such materials

●​ process hazards. depend on operating conditions or external factors (extrinsic) like

temperature, pressure, external temperature, materials of construction

Process hazards are hard to recognize and may lead to significant consequences if not

mitigated. Most common process hazard is due to loss of containment, which could cause the

release of hazardous materials.

Process conditions could:

●​ increase the effect of hazards, for example:

○​ processing monomers at high T may cause self polymerization and therefore

uncontrolled reactions)

○​ at vacuum pressures, air could enter the system

○​ adding too much additive in batch operations could lead to extreme exothermic

operations

●​ help to reduce or eliminate significant hazards, for example:

○​ treating at ambient temperature materials with high flash point.

Every process that has been proposed is based on specific knowledge, consisting partly on

data of all chemicals that are used or produced (including intermediates) in the plant:

●​ list of flammable materials

●​ list of toxic materials

●​ list of hazardous reactions → dangerous reactions occur when certain materials

(compatibility charts) come in contact with each other resulting in uncontrolled

situations

●​ ecc

Compatibility charts are consulted to make sure that two materials can come in contact with

each other. The interaction matrix helps to identify incompatibility between two different

chemicals or chemicals under specific process conditions also with the addition of footnotes

for further explanation.

In this matrix, every interaction box can be filled with:

●​ X if there is potential incompatibility

●​ ? for unknown capability

CRW 4 is a chemical reactivity worksheet used to find potential reactive hazards of chemicals

or mixture of chemicals.

HAZARD IDENTIFICATION (HAZID)

The first step in risk analysis consists in collecting:

●​ Properties of materials

○​ Hazardous properties

○​ Compatibility properties with other substances and construction materials

●​ Properties of the process

○​ Operative conditions (pressure, temperature, reactions, etc.), even anomalous

○​ Aim of each unit operation

○​ Specific hazardous operations

○​ Monitoring and control

●​ Other parameters/factors

○​ Historical accidents, reports, documentation

○​ Near-misses (or quasi-accidents)

People involved in hazard studies must have a basic understanding of hazards and potential

hazardous events: every hazard must be identified and managed.

In fact, every hazard (any source of potential damage) could lead to its manifestation in a

specific place and time causing an industrial accident.

During the hazard identification, three approaches can be applied and the choice depends on

the context, available information, phase of plant life:

●​ knowledge and experience

●​ literature

●​ applying dedicated techniques

KNOWLEDGE and EXPERIENCE

It’s the most powerful approach. It exploits a person that identified or experienced an

accident in the past and it’s aware of a particular hazard.

Organizations, typically, have no memory of such events.

The approach consists of:

●​ collect data on all accidents and near misses of the site

●​ train people on reported accidents

●​ share lessons to avoid the same accidents

●​ communicate information to staff that is concerned

LITERATURE and DOCUMENTATION

Knowledge is supported by the availability of many sources of information, like articles

and books. This approach consists in:

●​ cross-checking validity of available information

●​ analysis of the gained informations

DEDICATED TECHNIQUES

This approach is required to study more complex installations or to identify specific

hazards.

However, experience and the actual goal, the identification of hazards, are the pillars of every

approach.

It consists in:

●​ focus on the aim (finding hazards)

●​ refer to important resources

●​ applying the HAZID technique systematically

Hazard identification is the critical quantitative risk assessment procedure step, carried out at

the design stage to change hazardous plant configurations into safer ones. The advantage is

that identifying hazards before the actual plant construction provides inputs to project

decisions which have minimal cost implications compared to changing an already existing

plant.

Different strategies can be adopted:

●​ experience

●​ engineering codes

●​ checklists

●​ hazard index techniques

●​ what if analysis

●​ hazard and operability analysis

●​ failure models

●​ preliminary hazard analysis

Before starting hazard identification studies we need information about:

●​ process and non process material

●​ equipment

●​ infrastructures of the site

●​ nearby zones

●​ weather and geotechnical data

→ the quality of hazard evaluation depends on the quality of the information available.

After gaining this type of information, we should choose a criteria for screening the hazards

since considering all possible scenarios would be time-wasting. The criteria with which we

neglect some hazards is related to the technique we will use.

Hazards should not be neglected a priori simply because:

●​ their frequency is very low

●​ they didn’t happen previously

●​ they are considered “controlled” by existing measures

Transferring a laboratory scale plant to a real scale plant, the amount of data relevant to

process safety progressively increases.

PROCESS LIFETIME

Risk analysis consists in different actions based on the step in the process lifetime.

1.​ Research and development: the beginning of a new possible process

a.​ risk engineers tries to identify possible chemical reactions and interactions

b.​ requesting additional process data

2.​ Conceptual design: flow diagram

a.​ identify possible safety measures

b.​ Compare hazards

c.​ provide inputs and advices to process desi