Process Safety and Design - Process Design Practice - Lecture Slides, Slides of Process Engineering

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Process Safety and Design

Importance of Process Safety

–^

The safety record of the chemical process industry isthe responsibility of all of us in the profession.

-^

Process

safety

is^

important

for

employees,

the

environment, the general public, and it’s the law.

-^

As

process

design

engineers

we

are

tasked

with

reducing the risk of operating a chemical manufacturingprocess to a level acceptable to employees, regulatoryauthorities, insurance underwriters and the communityat large.

-^

Recent

chemical

plant

disasters

underscore

the

importance of this point in terms of both human andfinancial losses.

•^

Hazard vs. Risk –^

HAZARD

is^

a^

measure

of

the

severity

of

the

consequences

of

a

catastrophic

failure

of

a

given

process

or

system,

regardless

of

the

likelihood

and

without considering safeguards.

-^

RISK

is the combination of both the severity of the worst

case

consequence

and

the

likelihood

of

the

initiating cause occurring.

-^

In short, for an

EXISTING PROCESS

, we have little

influence on the

HAZARD

, but through the application

of safeguards, we can reduce the

RISK

of operating

the process.

Process Safety Terminology

Process Hazard Analysis

–^

Process

Hazard

Analysis

(PHA)

is

a

technique

for

determining the RISK of operating a process or unitoperation.

-^

PHAs

are

required

by

law

for

process

handling

threshhold quantities for certain listed Highly HazardousChemicals (HHC) or flammables.

-^

Approved techniques for conducting PHAs:•^

HAZOP (Hazard and Operability)

-^

What If?

-^

FMEA (Failure Mode and Effects Analysis)

-^

In general, a PHA is conducted as a series of facilitated,team brainstorming sessions to systematically analyzethe process.

Mitigating Process Risk

–^

The operating risk is determined by the PHA using anappropriate Risk Assessment Methodology.

-^

This

risk

is^

mitigated

through

the

application

of

safeguards that reduce the risk to an acceptable level.

Process Risk

InherentRisk

OperatingSafeguards

Level ofAcceptableOperating Risk

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Layer of Protection Analysis

• LOPA is a quantitativetechnique for reducing theRISK of a process.• The theory of LOPA is basedon not “putting all your eggsin one basket”.• The layers mitigate theprocess RISK as determinedby the PHA.• Each layer reduces the RISKof operating the process.

Core Process1st Layer ofProtection 2nd Layer ofProtection 3rd Layer ofProtection

Each layer must be: Independent;Effective; Reliable; Auditable.

Inherently Safe Process Design

–^

Inherent safety is a concept based on eliminating thecauses and/or reducing the consequences of potentialprocess upsets.

-^

Inherently Safe Process Design is a technique appliedduring the conceptual phase of process design.

-^

Inherently Safe Process Design targets the

HAZARD

rather than reducing the

RISK

after the fact.

–^

This

technique

is

based

on

making

inherently

safer

design choices at a point in the process developmentwhere the engineer has the most influence on the finaldesign.

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Inherently Safe Process Design

•^

Definitions –^

Inherently safe process design can be grouped into 5categories

-^

Each of these inherently safer design choices is appliedin the conceptual phase of development.

1

Intensification

Continuous reactor vs. batch reactor

2

Substitution

Change of feedstock

3

Attenuation

Alternate technology

4

Limitation of effects

Minimization of storage volume

5

Simplification

Gravity flow vs. pumping

Category

Example

Inherently Safe Process Design

•^

Traditional Process –^

Sample Risk Assessment using What If? Methodology

-^

Consider what types of safeguards would be required tomitigate the Process Risk due to these scenarios. What If…?

Initiating Cause

Consequence

1.^ There is higherpressure in theEntrainmentVessel?

1.1^ External fire in theprocess area.

1.1^ Potential increased temperature and pressure leading topossible vessel leak or rupture. Potential release offlammable material to the atmosphere. Potential personnelinjury due to exposure.

1.2^ Pressure regulator forinert gas pad fails open.

1.2^ Potential for vessel pressure to increase up to the inert gassupply pressure. Potential vessel leak or rupture leading torelease of flammable material to the atmosphere. Potentialpersonnel injury due to exposure.

2.^ There is higherlevel in theEntrainerVessel?

2.1^ Vessel level transmitterfails and indicates lowerthan actual volume.

2.1^ Potential to overfill vessel with cyclohexane. Potential toflood vent line with liquid leading to flammable liquidreaching the vent gas incinerator. Potential to overwhelmincinerator leading to possible explosion. Potentialpersonnel injury due to exposure.

Inherently Safe Process Design

•^

Azeotropic Distillation vs. Pervaporation

AzeotropeColumn

Pervaporation

Unit

SolventColumn

1

2

3

4

Stream s: 1 Solvent Feed2 Azeotrope3 Waste Water4 Solvent Rich Phase5 Water Rich Phase6 Recovered Solvent

5

6

Inherently Safe Process Design

•^

Inherently Safer Process (Cont’d) –^

Based on this risk comparison, it is clear that multipleindependent

protection

layers

would

be

required

to

mitigate the operating risk of the traditional process.

-^

This risk can be reduced by designing an inherentlysafer, ie, less hazardous process.

-^

Although

a

complete

economic

analysis

would

be

required, this example has illustrated that the need forindependent

protection

layers

is^

reduced

in^

the

inherently safer process design.

Summary 1:

•^

Conclusions –^

Clearly, process safety is a critical component of processdesign.

In

industry,

no

process

is

put

into

service

without a comprehensive risk assessment.

-^

It^

is^

important

to

realize

that

the

management

of

operating risk is the key focus of process safety.

As

design engineers, we have responsibility for and themost influence on the overall hazard of a process.

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