JRHS 2008; 8(2): 13-20

Copyright © Journal of Research in Health Sciences

Application of a Hazard and Operability Study Method to Hazard Evaluation of a Chemical Unit of the Power Station

Habibi E (PhD)a, Zare M (MSc)b, Barkhordari A (PhD)b, Mirmohammadi SJ (MD)b               Halvani GhH (MSc)b

a Department of Occupational Health, Faculty of Health, University of Esfehan Medical Sciences, Iran

b Department of Occupational Health, Faculty of Health, Shaheed Sadoughi University of Medical S *Corresponding author:  Dr Ehsan Habibi, E-mail: Habibi@htlh.mui.ac.ir

Received: 19 September 2008; Accepted: 3 November 2008


Background: The aim of this study was to identify the hazards, evaluate their risk factors and deter­mine the measure for promotion of the process and reduction of ac­cidents in the chemical unit of the power station. 

Methods: In this case and qualitative study, HAZOP technique was used to recognize the hazards and problems of operations on the chemical section at power station. To­tally, 126 deviations were docu­mented with various causes and consequences.

Results: Ranking and evaluation of identified risks indicate that the majority of de­viations were cate­gorized as "acceptable" and less than half of that were "unaccept­able". The highest calculated risk level (1B) related to both the interruption of acid entry to the discharge pumps and an increased density of the acid. About 27% of the deviations had the lowest risk level (4B).

Conclusion: The identification of hazards by HAZOP indicates that it could, sys­temically, assess and criticize the process of consumption or production of acid and alkali in the chemical unit of power plant. 

Keywords: Hazard and operability, HAZOP, Risk assessment, Chemical unit


The incidence of major industrial accidents around the world led to the innovation of va­rious hazard identification techniques in­clu­d­ing Fault Mode Effective Analysis (FMEA), Fault Tree Analysis (FTA), Haz­ards and Op­erability Study (HAZOP) and Energy Trace and Barrier Analysis (ETBA) (1-4). Poten­tial hazards and accidents for personnel, equip­ments and the environment can be recogni­zed and prevented by special process (5). HAZOP, as a multidisciplinary team effort, was initially developed since 1960 for analy sis of risk factors and safety measures and can be considered as one of the most accurate methods for identifying hazards in the vari­ous

industries, especially chemical plants (1, 6-10). In this approach, members use their in­novation and initiative according to the basis of simulation and brain storming for iden­tification of devia­tions from the main proc­ess design, their relative causes, effects and fi­nally to present the method of control (11, 12). To determine the deviation of pa­rameters from the main aim of process de­sign, guide words includ­ing temperature, pressure, conductiv­ity, wa­ter flow, services failure, ut­ilization of in­struments and others are also applied (2, 7, 13). The qualitative matrix meth­ods, as an instru­ment for adopting a logical deci­sion, have also been used for determin­ing of any relative risk level (14, 15). There have been few scientific studies in the field of hazard identification ei­ther in Iran or other countries using HAZOP, but nothing found on chemical unit of power station (10, 12, 16-20).

This study therefore, was conducted to: 1) identify the hazards; 2) evaluate their risk factors; 3) determine the priority and esti­mation of qualitative hazards and 4) to pro­pose the measure for promotion of the proc­ess and reduction of accidents in the chemi­cal unit of the power station.


The present study as a case and qualitative study were conducted at the chemical unit of the power station in northwest of Yazd Prov­ince. Briefly, very pure and non ionized water are produced for high pressure boilers of the power station. Different processes in­cluding chlorination, removal of suspended particles larger than 5 microns by sand filters and creasy cartridges, removal of chlorine, re­verse osmosis and exchange of ions were taken place to decrease water conductivity. In addition, to restore saturated resins during the process, acidic and alkaline sections have been designed in which Ca, Mg and SO42- ions are replaced with H+ and OH- ions in the resins, respectively (Figure 1). 

Figure 1: System for Production of Ultra Pure Water in High Pressure Boilers of the Power Station

Initially, all the required documents including maps, de­tails of operations and systems, piping and instruments diagrams, technical details and di­rections for implementation of systems were gathered by main team members who were familiar with the design of Chemical Unit. The nodes of the processes including entry of raw water till the entrance of the sand fil­ters, tanks of the sand filters, car­tridge fil­ters, high pressure pumps, reverse osmosis system, permeate tank, caution resin tank, re­sin catcher, degasser, acid discharge, acid tank and mixing T shaped were recog­nized. Some of the nodes were altered dur­ing the study. The scenarios of deviation from the main process were also recognized by team members using guiding words (Ta­ble 1) and process parameters (Table 2).

Table 1: Some of key word used in HAZOP meth­odology

Key Words



Negation of the design intent

More than

Quantitative increase

Less than

Quantitative decrease

As Well As

Qualitative increase

Part of

Qualitative decrease


Logical opposite of the intent

Other than

Other than Complete substitution or the operational procedures occur abnormally

Table 2: Some of process parameters






No flow, reversed flow, more & low flow


More or low or no pressure


More & low opacity


More or low Temperature


Route blockage


More Conductivity


More corrosion

Abnormal operation

Valve, pump and etc failure

Pressure difference

P Δ More


Increase or decrease CL or oil in water


Low Recovery


More PH


More or low concentration


More or low  Level

Despite of ini­tial aim of the study, some of the operational pathways were not included due to similarity in the procedures. The pri­ority and estima­tion of the qualitative haz­ards, risk manage­ment in the form of risk assessment matrix were also determined. The risk factors were also classified in three following stages and then results were en­tered in the HAZOP work documents:

Determination  of  the probability of the con­sequences of deviation in 5 groups from fre­quent to rare.

Severity  of accident in 4 groups from catas­trophic to marginal.

Combination of severity and probability of each risk to determine the danger levels and the priority of control measures, qualita­tively.


A total of 14 nodes were recognized, evalu­ated and then documented in a 45-page which summarized in Table 3, 4. The opera­tional problems were mainly focused by team mem­bers and more attention paid on the devia­tions with negative impact on the operations of the system resulting in finan­cial losses and personal injuries.

Table 3: Sample of the HAZOP Results Summary (For Chemical Plant


Possible causes


Action Required

No Pressure

1- Blocked inlet and outlet of the pump

2- Ingress of air in pump

3-Pump Failure

4- Blockage at route and clogging before pump

5- Fouling of check valves

6- Manual valve failure

Line Trip, and no produce permeate water

1- Install switch on manual valve as safety lock

2- Install auto vent on line

3- Set up a filter with tiny mesh on the entrance of water

4- Periodical inspection & maintenance

Low  Pressure

1- Decrease in level of  Raw Water Vessel regard to inlet suction

2- Strainer blockage

3- Pipe or Vessel Rupture

4- Flange failure

5- Pump Corrosion

Line Trip, and no produce permeate water

1- Placed start and stop key of  raw water pump in PLC

2- Installing moisture sensor near sand filter pump

3- Controlling of manual valves

4- Periodical inspection & maintenance

Low Flow

1- No regular manual valve in outlet of Raw  Water Pump, Concentrate of Reverse Osmosis, High Pressure Pump

2- Failure in pump suction

3- Check valve blockage

4- Finishing using time of sand filter

Line Trip, and no produce permeate water

1- Controlling of manual valves

2- Regular backwash of sand filter

3-  Periodical inspection & maintenance

High Flow

1- No regularize manual valve

2- Mechanical failure in flange

3- Start accidental pump

Line Trip, and no produce permeate water and cracking pipe

Programming on PLC that no start additional pump when one pump is running

Water without  chlorine

1- Electrical or mechanical failure on chlorine pump

2- Unloading of chlorine vessel

3-Bblockage on chlorine injection rout

4- No standard percentage chlorine solution

Growing micro organisms & algae on raw water  & sand filter vessel & RO

1- Weekly inspection of  chlorine vessel

2- Maintenance & repairing of  chlorine pump

3- Full dissolving of  hypochlorite solution

Water with additional  chlorine

1-Increase at injection dose of chlorine pump

2- Decrease in raw water flow

3- Percentage of chlorine solution is more than standards

Increase corrosion, affect on RO membrane & more consuming of SMBS

1- Install chlorine sensor on the entrance of water that measure chlorine rate and regulate  injection dose of chlorine pump automatically

2- Install flow meter

3- Chlorine pump start automatically of PLC

Service failures

1- Tow phase in Acid pump in cause electrical failure

2- Acid Pump corrosion

3- Mechanical failure in Acid instrument

No unloading of  Acid, diffusion of  Acid  on place

Maintenance & periodical repairing

Generally, in this study, 126 deviations were identified in which 15% were related to nodes from the entry of raw water to entry of the sand filters, 15% filter tanks, 12% car­tridge filters, 5% high pressure pumps, 10% reverse osmosis, 3% permeate tanks to the DEMIN PLANT FEED PUMP, 9% entry of the cation exchange chamber to entry of the resin catcher, 7% resin catcher to degasser, 8% acid injec­tion pathway from the exit of the daily stor­age to entry of the mixing T shaped and 4%  were related to the mixing T.

Table 4: Sample of the HAZOP Results Summary (For Chemical Plant)


Possible causes


Action Required

Pump reversed

1-Enter error electrical phase into pump

2- Decrease on vessel level

3- No getting air on apparatus followed by repairing

Line Trip, and no produce permeate water

Install auto vent on routing

Increase in water hardness

1- Reduce in quality of raw water

2- Raising dust on vessel

Increase corrosion, decrease in Permeate water quality, decline in recovery, saturated of dmin vessel

Periodical water examination, regulate recovery & pressure in RO

Abnormal Operation (failure valve)

1- Fault at PLC programming

2- Mechanical failure in valves

3- Interruption air of pneumatic valves

Damage to valves

Maintenance & periodical repairing

Increase opacity

1- Deficit silica & anthracite height in sand filter

2- Creation flood channel among sand filter

3- Inappropriate granular in sand filter

4- Running away nozzle

5- Inefficient rinse after backwash

6- Use more extra of sand filter

Quick blockage of filters

1- Install opacity tester

2- Set up diffusion water

Increase Δ P

Pressure difference

1-Finish normal using time of cartridge

2- corrosion in routing

3- Running away washer among piping & stick on cartridge

Change in recovery, passing suspended particles

Install resin catcher before cartridge

Increase recovery

1-Decrease draining at RO concentrate

2- RO membrane torn

3- Running away RO oaring

4- Blockage rout and concentrate check valve

Increase Conductivity and decrease water quality also reduce in advantage using time

1- Regular flow meter calibration

2- Install recovery representative in PLC

3- install automatic regulated valve instead manual valve

Flow interruption to unloading acid pump

1-Running away stride

2-Blockage in stride

Diffuse acid in the place

Maintenance & periodical repairing

Entrance acid to alkaline line

1- Operator error in connecting routes together


1- Full isolation acid and base route

2- labeling acid and base route

3- using different flange for any route

Puncturing acid vessel

1- Chemical reaction

2- Physical failure ( knocking)

Sprinkling acid and environment pollution

Install plastic curtain surrounding vessel to prevent leakage

Decrease acid concentration to below 98%

In attendance impurities like water and iron

Equipment corrosion, reduce acid potency

Periodical inspection, catching humidity in air route

Strengthen acid concentration more than 4%

1- Increase acid injection pump course

2- Low water flow in entry of  Mixing Tee

Damage to resin, Equipment corrosion

1- Maintenance & periodical repairing of acid pump, valve control,

2- install flow switch high

Raised calcium and magnesium and sediment calcium sulfate    

Utilizing  water with more than 90 micro siemens conductivity

Affect on resin

1- Install on-line conduct meter

2- Line trip if water conduct is more than 90 micro siemens

As the results show, one deviation can have several causes and effects. In the study, 293 causes of deviations were identified of which the main causes included; failures in the level measurement instruments, simulta­neous start­ing of the pumps, non regulation of valves, presence of air in the water flow pathway, performance of production proc­esses manu­ally, torn reverse osmosis mem­brane, corro­sion of the acid pathway, defec­tive check valves, failures in course of acid pump, block­age of pathways, mechanical problems of va­lves, increased or decreased the amount of in­jection of pumps and in­creased corrosion.  The causes were mainly related to the equip­me­nts (43.5%), manual or operator (35.8%), logic control panel (9.2%) and 12% to other causes.

A total of 175 suggestions were proposed and, there were no any proposals for some of de­viations. Suggestions were mainly related to the modification and improvement of equip­ments or processes (42%), regular mainte­nance of equipment (35%), and the use of correct operational methods (23%).

According to the results, 10.4% of devia­tions were not acceptable, 35.7% were un­de­sirable, 24.6% were acceptable but needed reconsideration and 29.3% were acceptable and no need any correction act. The maxi­mum calculated risk level, however low per­cent, (1.5% of the deviations) was related to in­terruption of acid entry to the discharge pump and increased density of acid (1B). The low­est risk levels of deviations was re­lated to 4D(27%) followed by 3D(2%), 4C (9.5%), 4B (4%), 2D(12%), 3C (8%), 3B(6.5%), 2B(5%), and 7% of the deviations had other risk levels.


HAZOP was first developed and used for analysis of risk factors and safety measures by several researchers, working independ­ently, in the 1960s. There is now a body of work describing the identification of hazards using HAZOP methodology. In spite of sev­eral studies, nothing found on chemical unit of power station. This study, therefore, has characterized and determined the hazards for the first time, as comprehensive analysis. The investigation was supplemented, as ap­propriate, by more conventional qualitative matrix methods. All this builds to a profile of analysis and must be considered as a whole to achieve a full interpretation of the data.

With respect to the aims of the study, a large number of risks and hazards identified in which deviations were mainly related to nodes from the entry of raw water to entry of the sand filters (14, 21). In consistent with the findings of Angela Pulley in America, more than half of the identified causes were related to equipment defects followed by operator errors (17). The manual control sys­tem may also increases the number of devia­tions and, therefore, the automatic control panels should be applied (22).

For prevention of deviations, attention should be, therefore, focused on the applica­tion of instructions for regular inspections and main­tenance of systems. It is worth to mention that, the finding of this study is con­sistent to the observations of Shafaghi who worked on Ab­sorption Heat Pump in the US (19). Some of the suggestions in various nodes were simi­lar, so that it was impossible to propose any idea or there were no specific offer. The pat­tern of this study is similar to the model dem­onstrated by previous investi­gations (18). An explanation is that a num­ber of deviations in other nodes were not used due to the similarity in their causes, ef­fects and results. This point is also common in HAZOP procedure and have previously mentioned in other studies (18).

Ranking and evaluation of identified risks indicate that the majority of deviations were categorized as "acceptable" and less than half of that were "unacceptable". According to the results, the highest level of risk was related to deviation of acid and alkali which is con­sidered as "unacceptable". Discharge of acid and alkali, in spite of low percent of devia­tion (1.5%), would increase the chance of accidents and the process, therefore, need to be modified immediately (23). Most of the deviations in the present study were clas­sified as undesirable and need to be assessed by the top management. It is necessary to point out that, the most important problems during the study were the gathering of team members, inconvenience and difficulties for organizing of meetings and the lack of incli­nation during meetings (18).

The prominent suggestions were the modifi­cation and improvement of process and eq­uip­ments which play an important role in the reduction of hazards. The use of acid and al­kali in the workplace may increase the chance of accidents in the process and need to be modified immediately.

In conclusion, the identification of hazards by HAZOP indicates that it could, systemi­ca­lly, assess and criticize the process of con­sumption or production of acid and alkali. This technique can be, therefore, considered as an effective method for recognition and prediction of hazards and it may increase the safety levels, prevent accidents and enhance the reliability of systems via the reduction of operational problems.  


The author would like to thank Esfahan Uni­versity of Medical Sciences for their finan­cial support. We also offer appreciation to management, staff of the Power Station and members of the team, especially the leader of the chemical unit for their co-operation.


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