Basics Of Hazop

Basics of HAZOP

6-1

CHAPTER 6

Basics of HAZOP What Did we Do Before HAZOP Came Along? We relied upon: Good engineering practices Codes of practice such as ASME, API, NFPA, etc. Informal safety reviews So what slips through the cracks? Interface problems between equipment and systems Abnormal conditions not envisioned during design Human error in design, operation, maintenance

How Do We Know If a Plant Is Safe? Historical record of plant incidents and near misses History of incidents on similar plants Record of onstream time Can apply some risk ranking, e.g., Dow/Mond indices Ask: What guarantee do we have that there isn't a serious accident about to happen?

If there is NO guarantee: What can we do about it?

1=3 Increase Hazards & Risk Awareness by: Process Safety Management Program.

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Basics of HAZOP

6 -2

HAZOP Methodology HAZOP Acronym for HAZards and Operability Analysis. Originated by Imperial Chemical Industries (ICI) in Mond Division.

Basic Concept Simulate abnormal behavior by considering deviations and disturbances due to causes likely to impact immediate and surrounding plant resulting in consequences. Then decide whether the design has adequate features (i.e., safeguards) that can prevent occurrence or limit the consequential effects. If no such safeguards exist, then consider what actions are needed to remedy the situation. High Deviation Normal (Design Intention) Low Deviation

Other Deviations typically include: Reverse of what was intended. What else can happen? System only partially functions. What additional things can occur?

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6 -3

Basics of HAZOP

Methodology for Generating Deviations

Guide

+ Property = Deviation

W o r d

For example:

When Property

High Low More

+ + +

When Property

No Less

=

+ +

Parameter:

Flow

-

High Flow

Pressure

-

Low Pressure

Transfer

-

No Transfer

Empty

-

Residue Remaining

Steam

-

No Steam

Diluent

-

More Diluent

Reaction =

Greater Reactivity

Operation:

When Property = Material:

No More

+ +

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Basics of HAZOP

What Type of HAZOP Should You Use? Parametric Deviation (e.g., High pressure, Low temperature, etc.) Good for continuous processes. Most widely used in world today.

Critical Examination The approach examines: Material

= Activities Sources and Destinations Good for batch operations, start-up, shut down.

Procedural Methodology Useful far HAZOPing: Operating manuals, procedures Batch operations Start-up, shut down

Knowledge Based HAZOP (more like 'What i f with established Checklist) Mainly applicable to: Well established (continuous) processes Organizations with very high quality engineering practices & standards

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6-4

Basics of HAZOP

6-5

Steps in the HAZOP Process 1. Preparation Assemble: P&IDs (Full size and reduced copies for the team) PFDs plus material and energy balances Equipment specifications Layout drawings

2. Facilitator and Process Engineer Break P&IDs down into Nodes. Nodes are equipment items (or numbers of items). If nodes are too small you can loose sense of analysis and incur excessive repetition. If nodes are too large, hard to handle, becomes confusing. Question: How do you size a node? Answer: Based on system function. Example: Reactor feed system may consist of Pump

+ Line + Exchanger.

3. Prepare HAZOP Outline with List of Deviations 4. Assemble HAZOP Team

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5. Facilitator Explains The facilitator or one of the team members explains the purpose and scope of the HAZOP and sets the rules for the study.

6. Process Engineer Explains Process in general Immediate Node being HAZOPed

7. HAZUP Each Node Using Deviations Listed in Outline Working Through

the P&ZD. Produce HAZOP worksheet recording the following: Causes Consequences Safeguards Actions/Recommendations Remarks

8. At the End of HAZOP, the Facilitator Issues Preliminary HAZOP Report (issuance is optional) consisting of Attendance Outline Detail Report Action/Recommendations Register

9. Issue Final Report Giving Full Details A sample of table of contents is given in page 6- 11.

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6-7

Basics of HAZOP

Variations in HAZOP Types Three basic types: Guide Word HAZOP Knowledge Based HAZOP "Creative Checklist" The Guide Word method is the most accepted method. There are five main variations: Cause-by-cause Consequence-by-consequence Deviation-by-deviation Exception only Action/Recommendation item only

Cause-By-Cause Methodology Correlates Consequences, Safeguards and Actions to each particular Cause of a Deviation. Precise method Reduces ambiguity Detail print-out can be followed, is fully auditable Example:

Deviation: Line Rupture

Cause #I

Consequence

Safeguard

Action

Line overstressed

Flammable release, fire

Pipe stress analysis

Check fire protection

Cause #2

Consequence

Safeguard

Action

Brittle fracture

Flammable release, fire

Charpee tested steel

Check fire protection

Cause #3

Consequence

Safeguard

Action

Vehicular impact

Flammable release, fire

None

Provide crash barrier

Cause-By-Cause provides full cross-referencing.

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Basics of HAZOP

6-8

Consequence-By-ConsequenceMethodology Correlates Consequences, Safeguards and Actions to each particular Consequence of a Deviation. Precise method Reduces ambiguity Detail print-out can be followed, is fully auditable Example: Deviation: Line Rupture Cause #I

Consequence

Safeguard

Line overstressed

Flammable release

Pipe stress analysis

Fire

Action Checkfire protection

Cause #2

Consequence

Safeguard

Brittle fracture

Flammable release

Charpee tested steel

Fire

Action CheckJre protection

Cause #3

Consequence

Safeguard

Action

Vehicular impact

Flammable release

None

Provide crash burrier

Fire

Checkfire protection

Consequence-By-Consequence providesfull cross-referencing.

Deviation-By-DeviationMethodology All Causes, Consequences, Safeguards and Actions are related only to a particular Deviation. Fairly simple to execute Some ambiguity Fairly rapid Detail print-out hard to follow

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Basics of HAZOP

6-9

Example: Deviation: Line Rupture Causes

Consequences

Safeguards

Actions

Line overstressed

Flammable release, fire

Pipe stress analysis

Checkfire protection

Charpee tested steel

Provide crash barrier

Brittle fiacture Vehicular impact

Deviation-By-Deviation provides no cross-referencing.

Exception Only Methodology (Not Recommended) Includes only those deviations for which team believes there are credible causes. Reduces time Cannot be audited Citations have been issued by OSHA in the USA against covered facilities using this method.

ActiorulCecommendations Item Only Methodology (Not Recommended) Only suggestions that team makes for action items are recorded. (No proper analysis). Not auditable

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6-10

Preparation of HAZOP Reports Basic Report Should Consist of 1. HAZOP Outline Nodes Deviations Guide Words Parameters Design Intent Design Conditions

2. Detail Report Lists output of sessions. For each Node and Deviation lists: Causes Consequences (+ Risk Ranking) Safeguards Actions/Recommendations Remarks

3. Attendance Register Facility, Unit Location Team members and expertise Attendance (Present/Absent/Not required/Part-time)

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6-11

Basics of HAZOP

4. Action/Recommendations Register

Action/Recommendations Item Person(s) responsible for follow-up Prioritization Status Target date for completion (Resolution) Other Risk ranking Categorization

Final Report 1. Executive Summary 2. Introduction

3. Process Description

4. Hazards of the process 5. Hazard and Operability Methodology

6. Conclusions and Recommendations Appendices o Outline of Hazard and Operability Study o Drawings o Project Information Report o Drawing Report o Team Members Report o Risk Matrix Report o Worksheet Report o Action~RecommendationsReport o Computer files

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HAZOP Example Table 6-1: Scope of HAZOP and Process Description E?&&pslqk XYE Processing Corporation t

LPenibw byplace

Pm$a3 IR: , A X W l t

Start Date: If2IReMI;I; I '

r

end'rnk- 1

C-t:

LIGHT ENDS RECOVERY UNIT (See Figures 6- 1 and 6-2)

To imuam the li&hta& pottion of a liquid feed stream containing 50%, by weight, of light 4 s us& a d m M 6 m t ~ . The feed rate is 100,000 lbhour and the intent is ta r-wr 81% af the light em& hF in ,orderto prpduue a1W h ,by weight, light ends distillate.

w ,k

RYPOess Description:

The lmit is W &QXII aq uptream &d dnxm,V- 101. Liquid feed is sqpplied at a btw@ MI pdgdat 22Q P. The f& Is pre-heated by heat exchange with the light emis wippwr bmggp&&m that &&&M&m ~~b EX-101, before it enters the light ends stripper, C-101.. The light end$ t@l~per+ C-lQl, is a 22 plate fractionation column, ushg valve-type h y s w l i h b ~ P m d on plate 12.

@ 3QQF,

in

ImWd

Overhead vapors from the light ends stripper, C- 101, pass to the light ends condenser, EX-102, which is water cooled. Fluids from this condenser flow to the reflux drum, V-102. Non condensible vapors entering the reflux drum, V-102, are vented to the flare system while the condensed liquids pass to the reflux pump, P-101 or spare. The total distillate is split so that product distillate is sent directly to storage while the main portioo is reflusedbaak to the top of the light ends stripper, C- 101. At the base of the light ends stripper, C-10 1, there is a vertical thermosiphon reboiler, EX-103,which is heated by 300 psig steam on the shell side. Bottoms liquid from the base of the light ends stripper, C-101, is pumped by a bottoms pump, P-102 or spare, to the tube side of the feedhottoms exchanger, EX-101, in which it is cooled by feed before passing to storage. Process Controls: The feed to column is under flow control via loop FRC-10 1.

The reflux flow is under flow control via loop FRC-116. Distillate withdrawal is under level control via loop LlC-107, from the reflux drum level. Non-condensibles bleed off under pressure control via loop PIC- 106 based on the overheads column pressure.

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The rate of bottoms withdrawal is under level control via loop LIC-119 based on the column bottom level. The steam flow to the reboiler is under composition control via temperature control loop TRC-126 based on the process side of the reboiler outlet. Protective Devices: Relief valve PSV-105, protects against overpressuring of the light ends stripper and connected components. Relief valve, PSV-106, protects against thermal expansion on the cooling water side of the light ends condenser. High and low level conditions, LAH-120, LAL-12 1 and LAH & LAL-107, are alarmed respectively on the light ends stripper and the reflux drum. The low level condition, LSL-12 1, on the stripper is also interlocked to stop the bottoms pump. The low low level condition on the reflux drum stops the reflux pump. In event of failure of the bottoms pump, the spare pump is started by a low low pressure switch, PSLL-125. The same arrangement, for the reflux pump, is also supplied by a low low pressure switch, PSLL-109. High or low column pressures are alarmed by PAH & PAL- 106 respectively. Loss of reflux is alarmed by FAL- 116. Loss of steam to the reboiler is alarmed by TAL- 126.

A minimum flow bypass on the bottoms pump protects against the no flow condition. Remotely operable motor operated valve, MOV-122, can be manually initiated in an emergency, such as bottoms line leaklfiacture, to prevent significant flammables inventory loss and fire. The instrument air failure positions of the control valves are indicated as F.C. (fail close) or F.O. (fail open). Car seal open (CSO) valves are as indicated. Assumptions: During a normal HAZOP you would normally have access to full equipment specifications, plant layout drawings, piping specifications, line lists, tie points and other pertinent documents. As this sample demonstrates PHA-Pro, rather than being an exercise in design, such documents are not included. Therefore make whatever assumptions you think reasonable if you wish to modify or extend the HAZOP as shown.

Normal Operating Conditions: Stream #1, Feed @ 220 F, 90 PSIG, 100,000 LbIHour, 50% Light Ends Stream #2, Overhead @ 200 F, 75 PSIG, 135,000 LbIHour, 90.2% Light Ends Stream #3, Bottoms @ 300 F, 120 PSIG, 50,000 LbIHour, 9.5% Light Ends Stream #4, Reflux @ 200 F, 75 PSIG, 85,000 LbIHour, 90% Light Ends Stream #5, Non Condensibles @ 200 F, 75 PSIG, 5,000 LbMour, 95% Light Ends Stream #6, Distillate @ 200 F, 150 PSIG, 45,000 LbIHour, 90% Light Ends Stream #7, Reboiler Feed @ 300 F, 80 PSIG, 185,000 LbIHour, 9.5% Light Ends Stream #8, Steam Flow @ 420 F, 300 PSIG, 25,000 LbIHour

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Basics of HAZOP

T-l.I*--"

.--,

6-14

",--'-'.""'"

Heat Exchanger Duties:

" l . l -

l-.l...-..

.

Y1

Condenser, EX- 102: 19.5 MMBTUIHR FeedlBottoms Exchanger, EX-] 01 : 1.9 MMBTUIHR Reboiler, EX-103: 19.9 MMBTUIHR

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Basics of HAZOP

6-15

Figure 6-1: P&ID of Light Ends Process

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Basics of HAZOP

6-16

Figure 6-2: Process Flow Diagram

FOR L I G H T S W O V E B Y I m

HEATCONDENSER: FEEDIBCYITOMS EXCHANGER:

REBOILER.

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19.5 MMBTUl?W 1.9 MMBTUlRR 19.9 MMBTUIISR

6-17

Basics of HAZOP

Table 6-2: List of Deviations Node: 1. Light Ends Recovery Unit '

I

Types: Centrifugal Pump, Column, Heat Exchanger, Line, Maintenance problems, Vessel ' ~ ~ u i ~ r nID: e nDistillation t unit with heat recovery from bottoms heating feedstream

Drawings: 1. Process Flow Sketch of Light Ends Recovery Unit; 2. Piping & Instrumentation Diagra~ # PCD-A1

7

r

Design ConditionsIParameters: Design conditions are listed in Process Flow Sketch of Light Ends Recovery Unit ~rovided Guide Word

Deviation 1.1. High Flow

High

1.2. LowINo Flow

LowINo

Parameter Flow

r

Sessio In 1

/

I

I

Revision # Design Intent

---I----

,

1

I1 lo --7"-7;

1.3. ReverseJMisdirected Flow ReverseIMisdirecte d 1.4. Other than Flow

Other than

-

I

Flow

O

1

As per Process Flow Diagram As per Process Flow Diagram As per Process Flow Diagram As per Process Flow Diagram

1.5. High Temperature 1.6. Low Temperature 1.7. High Pressure 1.8. Low Pressure I

1.9. High Level 1.10. Low Level I

1.11. Cavitation 1.12. Column Flooding

---

I

1.13. Low Tray Level

1.14. High Concentration of Impurities t

1.15. Leak

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Basics of HAZOP

-

.

--

6-18

- -

I

Dev~ation

t

l Parameter

G u ~ d eWord

--

1 Sessio

- - -

1

-

Revision # Design Intent

I

1.16. Rupture

I

1.17. Start-~ip1Shutdown Hazards .

I Other

1 start-

than

/ ~~plshutdown

1

.

1.18. Maintenance Hazards &2-w-sik-&&

Other than

&>

bSa-&"@&$~&&SY R~&QSI ,>a3k i b d & k , It L

12

Maintenance

Other than I

$53

s ,wLk%**~~Q,&SI&&-

2

I

$-&&V.!~V&

,0

1 As per Operating

,O

1 1 As per Maintenance

fik& "bbAi3ba2&.&#*&

I Instructions 1 Data &&~-~t%%&

"*>ihaw-

&k

2)

-

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6-19

Basics of HAZOP

Table 6-3: Sample Worksheet Node: 1. Light Ends Recovery Unit r

Types: Centrifugal Pump, Column, Heat Exchanger, Line, Equipment ID: Distillation unit with heat recovery from bottoms heating feedstream r

Design ConditionslParameters: Design conditions are listed in Process Flow Sketch of Light Ends Recovery Unit provided

Drawings: 1. Process Flow Sketch of Light Ends Recovery Unit, 2. Piping & Instrumentation Diagram # PCD-A I

1.1. High Flow

2. Investigate recycling line and/or additional

101 (dwg. no. PCDAA1) to trip FV- 101 closed using solenoid

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Basics of HAZOP

-.-

- --

Causes 4. Control valve or ' fails CV

TV-126 open or ' bypass left open

6-20

* *-

1 Safeguards 1 S

' Consequences j 4.1. High bottoms ' temperature

'

i;ff-spec

14.1. PSV-105 j I

Products

I

I

1

I

I

'

j

14.3. Over-pressuring of column

I

Phillip control valve TV- 126 Smith so that on fill1 opening column will be in likely to flood due to excess vapor flow

I

I

14.2. TI- 1 17

8. Add high temperature alarm, TAH- 126 --

fails CV PV- 106 open or bypass left open

- ---

;

, 5.2. Loss of products

1

!

/

! 5.3. Off-spec products

i - - -- -

-

I

--

I -

6.1. Loss of products

--

-

,

--

-

9. Add independent Tom Volke pressure monitor on column overheads with high and low pressure switches and alar~ns

15.1. PAL-106 I / (provided 1

1w

6. Control ,valve or controller falls CV LV- 107 open or bypass left open

I

5 . 1 Column will i depressure to flare

-"

To111Volke

i

of c o l ~ ~ m n 5 Control valve or controller

Tom Volke

room monitored as opposed to local device and add high temperature switch and

'

,

1

Responsible

----

-- --

-

1 6.1. LAL- 12 1

I 6.2. Off-spec prodi~cts 1 I

-

, 6.2. LSL- I2 1 trips P-1021s

-

6.3. Low level in column c o ~ ~ l d

I

-

I

CV FV-116 open or bypass left open

I

4 $.

9 =,'

I

I

bottoms pumps P, 1021s -

7.1. Excess reflux to column

16.1. $ 1 Safegilards f ' Ir.< are 9 [adequate

II

I

7. Control valve or controller

I

7.1. None

i

I

4

2

iSi

k 1%

";: 1

.:'

1 10. Add high flow

I

6

alarm to FRC- I 16

I

I 7.2. Unecononiical

performance

; '

8. PSV-105 8.1. Column will fails open depressure to flare ' due to 8.2. Loss of products spring . . . failure ,8.3. Off-spec products

1 1. Consider monitoring steam flow to column by adding i flow indicator on 3"-S10 1 as check on energy ! , consumption

1

11

i\ sE

3

!i

I

I 8.1. Block and bypass I valves PSV

1 nl~lrPI- 1 04

15

II

i

d

18.1, 9 Safeguards 2 are adequate '

I

1 ?

k,A

A

,4&

&<

,

W . 2 h &a

R

A,

-

. . .&

i! *

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6-2 1

Basics of HAZOP

r

Causes

Consequences

Safeguards

8.4. [Failure too infrequent to raise significant concerns]

for manual pressure relief when PSV-105 is removed for repair

I

RR Recommendations

Responsible Remarks

I

I

9.1. Steam wastage 9. Steam trap on 3"9.2. Uneconomical S-102 sticks open

9.1. None

1

1 1. Consider Tom Volke monitoring steam flow & Carl Hanks to column by adding flow indicator on 3"-S10 1 as check on energy consumption

I

1.2. Low/No Flow

I -

maintenance inspection of flow devices, such as FE-

system to make up in the event of sudden loss of steam to reboiler

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Basics of HAZOP

--- -

.-

Causes

6-22

-Conseq~~ences "

Ii

"-I

I

1 1

.

I

-

5. I . Column will 15. I. PAH-106 overpressure (provided P1C- 106 is 5.2. Loss of fi~nctional) products

/ 6. Control valve or controller fails CV LV- 107 closed

1I

I

5. Control valve or ' controller fails CV PVz 106 closed

-

Safeguards

4.2. Poss~ble 1 vacuum in column causing tray damage

closed

- -

1

/ 6. I . LAH- 107 1 (provided

1

- . 7.1. Loss of 17.1.FAL-I16 reflux to column (provided - 'FIC-116is 7 . 2 Off-spec fLlnctional) products

--

"

1

"

.-

-

1 7.3. Overf

/ column --

-

closed I

-

-

/1 8.2, Lois of products '-

__"

-

Tom Volke

15. Add high level switch and alarm on reflux drum v - 102

Tom Volke

- -

..

-

1 1. Consider monitoring steam flow to c o l ~ ~ mbyn adding flow indicator on 3"-S- 10 1 as check on

Tom Volke & Carl Hanks

16. Add independent pressure monitor on co1~11nn overheads with high and low pressure switches and alarms

Tom Volke

1 1. Consider monitoring steam flow to column by adding flow indicator on 3"-S-101 as check on energy consumption

Tom Volke & Carl Hanks

"-

...

19.2. Loss of products

- - "

!

'9.1. / Safegnards l are

I

9.2. Spare

9.3. High level in base of column

9.3. LG- 1 18

10.1. Loss of reflux to column

10.1. FAL116

products

10.2. PSV105

"

9. Add independent pressure monitor on column overheads with high and low pressure switches and alarms

- " -

9. Bottoms 19.1. No pump Pwithdrawal of 1021s stops j bottoms product

10. Reflux pump P1o 1IS stops

--

Tom Volke

--

8.1. None 102 sticks

- - - --

14. Interlock reflux return, FV- 1 16, and feed, FV- I0 I to close when PAL- 106 is act uated

1 .

- "-

16.1. Loss of products

7. Control valve or ' controller fails CV FV1 16 closed

Responsible , Remarks

L 'RR

.

-E-had

t$asP&42-

j 10.1.

1 Safeguards ; are

:,34&w&

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6-23

Basics of HAZOP

1

Responsible Remarks

Causes 1

10.3. Spare pressuring of column t

11. MOV122 fails closed

12. Temporary strainers on P-1011s plugged r 13. Loss of overhead condenser.

would cavitateldamage bottoms pumps P1021s

11.1. Interlock on MOV-122 positioner stops bottoms pumps when valve closes

strainers on P- 101IS are cleaned and removed when no longer required

would cavitateldamage bottoms pumps P1021s 13.1. Overpressuring of column to relief condition.

MOV- 122 positioner ZC122 to stop bottoms pumps when MOV- 122 valve closes

13.1. Low 2 3 flow alarm FAL-116 on loss of reflux.

I

13.2. Pressure relief valve

106 & PV106 opening to flare.

1

1.3. ReverseIMisdirected Flow

19. Check PSV-105 for controlling case for sizing valve. Must handle fire case, tube rupture in reboiler,total loss of reflux, loss of cooling medium, instrument or controller failure, instrument air failure, power failure, etc. 20. PV-106 to be checked for maximum discharge flow in event of cooling water failure to EX-102.

Phillip Smith

I

1.1. Possible explosive

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Basics of HAZOP

6-24

Table 6-4: List of Recommendations

- . - ... .. ,

-

-.

-

...

-. .-. .. . - - ..-

nsider monitoring stea 3"-S-101 as check on

/

_._

17. ' Provide interlock on M

num discharge flow in event of

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6-25

Basics of HAZOP

1

7

Recommendation

#

Resp

Pri Place(s Used

Status

I

I

7

22. Check on flow regime in 6"-P-113 to slugging Phillip ~

'23. Check that line 3"-P-104 is both self-venting and is not pocketed r

m

i

24. Evaluate need for emergency depressuring to prevent BLEVE in event of fire

Phillip Smith Study

25. Provide sample point on inlet feed. Also consider need for on-line analyzer for column feed.

Tom Volke

t

h

1.4.1

F

I

'

26.1 Provide quality control check on feed stream to column Add high temperature alarm on overheads to indicate trend towards off-spec distillate TR- 103 only 29.1 Add low temperature alarm to TR- 103.

' r

Incomplete

/ Carl Hanks 1 Study

I

Tom Volke

I

Incomplete

Phillip Smith Incomplete

ITom Volke / Incomplete

i

30. Consider adding independent high high level switch and alarm on reflux drum

Tom Volke

31. Check sizing of control valve TV-126 so that CV is not oversized and could cause column flooding when fully open. If necessary consider adding upper limit stop on control valve.

Phillip Smith Study & Tom Volke

Study

Check as to whether upstream water separation Reconvene meeting if not met. 33. Provide bolt torquing procedure as part of I

34. Consider need for environmental monitors.

Mary Patterson

Incomplete

Add isolation valve immediately upstream of stripper on reflux line 2"-P- 110. t

I

36. Make valve on 3"-P-102 feed to column car seal open.

Phillip Smith Incomplete

37. Add check valve to 3"-P-102, close to stripper feed inlet.

Allen Brown Incomplete

rm that C-101 a

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SUGGESTED READING (URLs current at time of publication)

"Guidelines for Hazard Evaluation Procedures" by AIChE, CCPS, 2"d edition, 1992 plus "Guidelines for Hazard Evaluation Procedures" by AlChE, CCPS, I st edition, 1985 www.aiche.orr/puL>cat/seadtl.as~'?Ac~k4in=2i~

"HAZOP and HAZAN" by T.Kletz, published by IChemE, 1992

www.icl1e1ne.or~/framesetsiia1~o~1ti1~S~iiii~est. htn~ "Size up plant hazards tllis way" by H.G.Lawley, Hydrocarbon Processing, April 1976, pages 247 to 258

www.livdrocarbon~~rocessin~.co~n~conte~~ts/pul~l icatiotis,:'hp/ "Eliminating Potential Process Hazards" by T.Kletz, Chemical Engineering, April 1, 1985, pages 48 to 59 www.che.com/ "An Introduction to Hazard and Operability Studies - The Guide Word Approach" by R.E.Knowlton, published by Chemetics International, 198 1

www.kvaerner.co~n/co~i~pa~~ics!co~iipan iesdctai I.asp'?id::.::.79(i "A Manual of Hazard & Operability Studies - The Creative Identification of Deviations and Disturbances", published by Chemetics International, 1992 w~vw.kvae1ner.con1/co11i~.~atii~s/~o1ii~~ai1iesdetai.asp~!id=796

"Some Features of and Activities in Hazard and Operability (Hazop) Studies", by J.R.Roach and F.P.Lees, The Chemical Engineer,October, 198 1, pages 456 to 462 \vwcv.ichenie.or~/fr~1111esets/i1bo~1 tusfra~nesethtln

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