Lect 3 Safety

7/31/2019 Lect 3 Safety

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INHERENTLY SAFE DESIGN

•

PROCESS RISK MANAGEMENT METHODS USEDDURING THE DESIGN PHASE CAN BE PUTINTO 4 CATEGORIES: – Inherent

–

Passive – Active

– Procedural

• TARGET IS A FAIL-SAFE INSTALLATION

• FROM: Dennis C. Hendershot and Kathy Pearson-Dafft, Safety ThroughDesign in the Chemical Process Industry: Inherently SaferProcess Design , AIChE Process Plant Safety Symposium,27OCT98



INHERENT SAFETY DESIGN

• Inherent — Eliminating the hazard by usingmaterials and process conditions which are non-hazardous.

– Minimize — Reduce quantities of hazardous substances

– Substitute — Use less hazardous substances

– Moderate — Use less hazardous process conditions, lesshazardous forms of materials, or configure facilities tominimize impact from hazardous material releases or

uncontrolled energy release – Simplify — Configure facilities to simplify operation



PASSIVE SAFE DESIGN

• Passive — Minimizing the hazard by processand equipment design features which reduceeither the frequency or consequence of the

hazard without the active functioning of anydevice.

– Location of facilities – separation of ignitionsources and fuels from other facilities

– Design equipment for design pressure in excess of the adiabatic pressure from a reaction.



ACTIVE SAFE DESIGN

• Active — Using facilities to detect and correctprocess conditions:

– controls

– safety interlocks

– monitoring systems for hazards that develop overa long term

– and emergency shutdown systems to detect andcorrect process deviations.



PROCEDURAL SAFE DESIGN

• Procedural — Prevention or minimization of incident impacts using:

• Safe operating procedures and operator

training• Administrative safety checks

• Management of Change

• Planned emergency response



DESIGN IN OVERALL SAFETY MANAGEMENTArt M. Dowell, III, Layer of Protection Analysis, 1998 PROCESS PLANT SAFETYSYMPOSIUM, October 27, 1998 Houston, TX



DESIGN OF SAFETY INSTRUMENTED SYSTEMS

• ACTIVE INHERENTLY SAFE DESIGNPROCEDURE (Separate instrumentationand control component in CHE 165

Design)• First Level – Alarm systems for out of

range situations and operator action

•

Second Level – Interlock systems toautomatically activate safety devices

• Third Level – Devices to minimize impact

of out of control conditions



USE OF HAZAN AND HAZOP

• PHA’s (Process Hazards Analysis) Areused to define areas of concern

• HAZAN and HAZOP provide a summary

of the type of risk associated withvarious process locations and operations

– Frequency should be determined

–

Intensity should be determined



OVERPRESSURIZATION EXAMPLE

• OVERPRESSURIZATION IS THE SUBJECT OFNUMEROUS CODES & REGULATIONS

– AIChE Design Institute for Emergency Relief Systems (DIERS)

– OSHA 29 CFR 1910.119 – Process SafetyManagement of Highly Hazardous Chemicals

– NFPA 30 – Flammable & Combustible Liquids

– API RP 520 and API RP 521 – Pressure Relieving

Devices and Depressurization Systems – ASME Boiler & Pressure Vessel Code

– ASME Performance Test Code 25, Safety & Relief Valves



SOURCES OF OVERPRESSURIZATION

• API 521 LISTS THE FOLLOWINGCATEGORIES OF SOURCES

API RP521 Item

No.

Overpressure Cause API RP521 Item

No.

Overpressure Cause

1 Closed outlets on vessels 10 Abnormal heat or vapor input

2 Cooling water failure to condenser 11 Split exchanger tube

3 Top-tower reflux failure 12 Internal explosions

4 Side stream reflux failure 13 Chemical Reaction

5 Lean oil failure to absorber 14 Hydraulic expansion

6 Accumulation of noncondensables 15 Exterior fire

7 Entrance of highly volatile material 16 Power failure (steam, electric, or other)

8 Overfilling Storage or Surge Vessel Other

9 Failure of automatic control



FIRST LEVEL DESIGN

• HOW ARE SOURCES ADDRESSED FOR ASTORAGE TANK?

• Item 1 in previous list - Closed outlets on vessels

– Would be a concern for a nozzle used for pressure control

in the tank, during filling operations.• Perhaps a temporary blind flange would have been left in place after a

maintenance operation.

• A pressure relief valve may malfunction.

–A PAH pressure switch (

ΔP) could be installed if there wasmeasurable difference between the Normal Operating

Pressure and the Maximum Allowable Working Pressure.



SECOND LEVEL DESIGN

•

HOW ARE SOURCES ADDRESSED FOR ASTORAGE TANK?• Item 1 in previous list - Closed outlets on vessels

• Add a pressure relief valve to allow gas to leave thetank and be directed to an appropriate flare orscrubber.

• Set point needs to be at or slightly above theMaximum Allowable Working Pressure

• Need an interlock to: – Alarm to indicate valve has been activated and receiving

unit (flare or scrubber) is activated. – Shut down a valve in the tank fill line and/or shut off a

pump used for filling.



THIRD LEVEL DESIGN

• HOW ARE SOURCES ADDRESSED FOR ASTORAGE TANK?

• Item 1 in previous list - Closed outlets on vessels

• Add a rupture disc to relieve to either a flare orscrubber.

• This level is to protect the equipment from failureon a major scale

• Need to have an indication that the rupture dischas opened – typically a wire across the disc

• Need to determine actions necessary when the

disc opens –

stop filling, start flare, etc.



OTHER DESIGN CONSIDERATIONS

• A large storage tank is filled manually byan operator opening and closing a valve.Once a year, the tank overfills as the

operator is distracted by other activities. A high pressure alarm is added to thetank. After the alarm is added, the tank is typically overfilled twice a year.

• Why?



EXAMPLE 1

• After the alarm was installed, theoperator relied on it to indicate a highlevel and did not supervise the filling

closely. The alarm loop turned out tohave a failure rate of twice per year, sothe system was not as reliable as themanual operation.



OTHER CONSIDERATIONS – EXAMPLE 2

•

Fail-safe valves are either Air-to-Open or Air-to-Close, which equate to Fail Closed and FailOpen, respectively. Recommend the correctvalve for the following processes:

1. Flammable solvent heated by steam in a heatexchanger. Valve is on the steam supply line.

2. Exothermic reaction. Valve is on the reactantfeed line.

3. Endothermic reaction. Valve is on thereactant feed line.

4. Gas-fired utility furnace. Valve is on the gassupply line.



EXAMPLE 2 - CONTINUED

• SPECIFY EITHER FAIL-CLOSED OR FAIL-OPEN FOR THE VALVES IN THESE SYSTEMS

5. Remote-operated valve on the drain for astorage tank.

6. Remote-operated valve on the fill line to astorage tank.

7. Gas-fired Combustion furnace. Valve is on

the air supply line.8. Steam supply line. Valve controls the

downstream steam pressure from the boiler.



EXAMPLE 2 – SOLUTIONS 1

1. Valve to FAIL-CLOSED to preventoverheating the solvent

2. Valve to FAIL-CLOSED to avoid a

runaway reaction3. Valve to FAIL-CLOSED to avoid reactor

thermal stresses.

4. Valve to FAIL-CLOSED to stop gas flowto uncontrolled combustion.



EXAMPLE 2 – SOLUTIONS 2

5. Valve to FAIL-CLOSED to preventdraining material from tank

6. Valve to FAIL-CLOSED to prevent

overfilling tank 7. Valve to FAIL-OPEN to maximize air

flow to furnace

8. Valve to FAIL-OPEN to avoid localizedoverpressure of line



EXAMPLE 3

• 4 kg of water is trapped in between inletand discharge block valves in a pump.The pump continues to operate at 1 hp.

–

What is the rate of temperature increase inC/hr if the cP for the water is constant at 1kcal/(kg C)?

– What will happen if the pump continues to

operate?



EXAMPLE 3 SOLUTION - 1

• Assume adiabatic conditions for thecalculations: Set up a heat balance:Q m Cp T Tref

Take the derivative with respect to time and

rearrange to getdQ

dtm C

p

dT

dt . And

resolving to getdT

dt

1

m Cp

dQ

dt

Using conversions:1 hp 0.178kcal

sec

m 4 kg dQ/dt 0.178kcal

sec Cp 1

kcal

kg C

dT/dt1

m Cp

dQ/dt dT/dt 160.2C

hr

3 SO O 2



EXAMPLE 3 SOLUTION - 2

• Allowing the pump to continue to runwill eventually result in high pressuresteam formation. This could result in the

pump exploding.• Adding a thermal switch or a high

pressure switch to shut down the pumpcan prevent this from occurring.

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Lect 3 Safety