Building and operating a chemical plant without accident-preventive measures can cause an unquantifiable magnitude of hazards. Over the years, a good number of hazards have been recorded in the chemical industry around the world  and most of the recorded accidents were largely due to human error. The 1974 Flixborough disaster that was initiated by a reactor leak killed 28 persons, injured other 36 and destroyed properties . The 1976 Seveso disaster led to the pollution of a vast area of land and water bodies. Thousands of people were killed and properties worth billions of dollars destroyed in the 1984 Bhopal disaster which was a direct consequence of human error . Facilities worth billions of dollars were also destroyed in the 2005 Buncefield disaster while the 2010 BPL refinery disaster caused much pollution to water bodies . The accidents that have occurred in the process industry have prompted owners and operators of modern day’s chemical plants to incorporate safety measures to prevent accidents. The major industrial accidents recorded between 1956 and 1998 showed a decline in accidents in the chemical and process industries. This decline is traceable to the recent attention given to the study of accident forecasting and loss prevention in the chemical and process industries  . Preventive-mechanism entails recognizing possible hazardous scenes within the plant which may vary in size and which may be noticeable or not noticeable . Accident in the chemical and process industries can arise from the process itself, properties of the chemicals and their handling such as fire, explosion and exposure to toxic substances. For example, over temperature can lead to over pressure which can cause fire, explosion or toxic release that can cause accident. The predictions and prevention of accidents can be done by recognizing the hazards and the corresponding actions to be taken . The use of certain process schemes or materials in production in order to maximize profit may result in operability problems and increase accident risks in the plant. For example, steam stripping is very effective in the vapourisation of more products in refineries  but due to cost of steam, some refiners have employed the use of water steam which is less expensive   but with its associated hazards and operational problems.
Over the years, experts in process plant safety have developed risk-assessment procedures to enhance safety levels of process plants. Some of the many risk-assessment procedures developed so far include the Preliminary Hazard Assessment (PHA), the Fault Tree Analysis (FTA), the Energy Tree and Barrier Analysis (ETBA), the Failure Mode and Effects Analysis (FMEA) as well as the Hazard and Operability (HAZOP) . Each of these mechanisms has strengths and weaknesses and is specialized in handling a particular type of risk. Although HAZOP is time-consuming as it requires a considerable amount of time of preparation, it gives a proper, organized and critical examination of the process of new or existing facilities to evaluate the potential for equipment malfunctioning in terms of the resultant impacts . HAZOP performs a structural investigation of each unit in a process to depict what kind of deviation from the ideal operation can occur and what harm may be caused by such deviation. It is adopted in HAZOP study that a system is safe when key operability parameters such as temperature, pressure, flow or levels are in their normal conditions. Operability problems if not identified in HAZOP can result in production losses due to inferior product quality or process inefficiency. This means properly conducted HAZOP can help not just in plant and personnel safety but also prevents loss of continuity or loss of the product specification . Initial HAZOP study helps identify suitable protection on measures that may be implemented to avoid impending accidents  . HAZOP involves a study on how a plant might deviate from the intents while taking notes of the resulting appropriate solutions to these deviations. Since it is a group study, it creates a brainstorming environment that brings creativity and generates ideas. In HAZOP, a flow sheet on piping and instrumentation diagram (PID) for the plant is obtained. Each node on the PID is numbered where a series of disturbances are proposed. For each disturbance, potential causes and consequences are described and noted. Guide words are used to ensure that the design is explored in every conceivable way. It is paramount that a HAZOP team focuses only on consequences with serious effects since numerous consequences can be obtained. In this paper, the concept of HAZOP was applied to study the safety level of a crude distillation unit of a petroleum refinery.
2. Description of the Facility
Figure 1 shows the P & ID of the desalter while Figure 2 shows the P & ID of the preflash column of the refinery under study. The main units in the plant considered in this work are the desalter, the preflash furnace that uses fuel gas and fuel oil with medium steam and the preflash column as well as its associated pumps, controls and piping.
Crude from storage tank is pumped at 30˚C and 30 kg/sqcm through a valve into the first preheat train where its temperature is raised to about 125˚C and pressure reduced to 11 kg/sqcm. Water is injected into the crude both at upstream and downstream of the first preheating train to dissolve salts contained in the crude. Water injection upstream of preheating is manually controlled at
Figure 1. P & ID of the Desalter and its associated pipe work.
Figure 2. P & ID of the Preflash distillation column and its associated pipe work.
1% - 1.5% volume of feed while downstream injection is controlled by a valve and kept at 3.5% - 4% volume of feed. If the desalter water is acidic, it enhances corrosion hence a 0.06% of NaOH is injected into the desalter water to keep its pH at 7.5 - 8.0. Demulsifier chemicals are injected at 3 - 5 ppm of feed upstream of preheating to break oil/water emulsion and promote oil/water separation in the desalter.
Due to low velocity and long residence time, water can settle in the bottom of the desalter. Electrodes and electric grid are installed to generate an electric field in which water droplets too small to settle can electrically attract each other, coalesce in bigger drops and separate. Oil/water separation is also helped by demulsifier injection and significantly basic pH (caustic injection).
Downstream the first preheating train, crude flows through the mixing valve where mixing is promoted due to the pressure drop in the valve. Then the crude and water mixture enters the desalter and the salty water from the desalter bottom goes to waste water treatment unit while the desalted crude enters the second preheat before it proceeds to the preflash heater to raise its temperature up to the temperature required in the preflash column. The preflash heater uses fuel gas and fuel oil while atomizing steam is used to break up the fuel oil. The crude enters the preflash column and light fractions like LPG and light naphtha vapourized and are separated as overhead products, water and some hydrocarbons are withdrawn as side cuts while the crude heavier fractions remain in the bottom. Liquid fraction mainly heavy naphtha is also withdrawn as side cut and sent into the atmospheric column and the bottom sent to the atmospheric heater to raise its temperature to 350˚C before it enters the atmospheric column flash zone.
Selected lines and plant units in the P & ID in Figure 1 and Figure 2 were examined one after the other. For clarity, not all lines and units were considered in the study but only units like the desalter, the furnace and the preflash column as well as their associated pipe work that pose significant risks. Fouling and corrosion of equipment, increasing electrical conductivity of the crude oil, material losses, reduction in the efficiency of furnace and chocking of furnace tubes and flow lines are some of the major consequences that can arise from the malfunctioning of these selected units which can pose severe operability problems. Guide words were applied to each study node. In each node, a process parameter was identified and an intention was created for the node. For example, if the process parameter being considered was temperature, the first guide word like “low” was applied and a meaningful deviation like “low temperature” was developed. All the possible causes of low temperature as well as the likely consequences were determined. The study also identified existing operational safeguards but when a consequence is likely to pose a hazardous situation, recommendations were given for possible changes to be made to the system to eliminate or minimize hazard. The same process was carried out repeatedly for all the guide words on the same node. The next node was selected and the same activity was repeated on it.
3.1. Guide Words
The guide words used in the HAZOP study were as follows:
FLOW—high, low, no, reverse;
3.2. HAZOP Study of the Desalter
3.3. HAZOP Study of the Preflash Column
4. Results and Discussions
The main equipment of the crude distillation unit of the refinery considered in this work was the desalter, the preflash furnace, the preflash column and their associated pipe works and equipment like pumps. Using the method of HAZOP to evaluate the operability and safety level of the unit, 4 study nodes were identified. In the study of possible deviations that can occur in the nodes, 25 guide words were suitably applied on the nodes and 89 causes were identified. Most of the causes were due to equipment malfunctioning and a few may be classified as
Table 1. HAZOP minute sheet for the Crude Feed line and associated pipe work (Node 1).
Table 2. HAZOP Minute sheet for the Desalter and associated pipe work (Node 2).
Table 3. HAZOP Minute sheet for the Preflash furnace (Node 3).
Table 4. HAZOP Minute sheet for the Preflash column (Node 4)
human error. These causes gave rise to 46 consequences. All the consequences can only pose operability problems that may lead to shut down but none of the consequences can really be termed as very hazardous and life-threatening. The plant can, therefore, be said to have a high safety level. This may be due to the existing safeguards designed into the plant. 61 recommendations were given in the study to further improve the operability and safety level of the plant.
Based on the nature of recommendations given in the HAZOP study, in order to prevent operability problems and hazardous conditions in the plant, there should be regular inspection; regular maintenance of flow lines and equipment and possibly replace faulty equipment. It is highly recommended that more safeguards be incorporated into the design to further improve the safety level of the plant. However, it is highly recommended that a thorough HAZOP study should preferably be carried out during the design phase of a plant so as to have much influence on the design.
10-C-07 Preflash column
10-dLRC-3 Desalter level controller
10-E-01 Heat exchanger 01
10-FR-B1 Flow regulator in crude feed line to desalter
10-FRC-1051 Flow regulator controlling preflash furnace crude feed
10-FRC-1075 Flow regulating flow of preflash column bottom steam
10-FRC-11 Desalter water flow controller regulator
10-FRC-1181 Preflash column reflux flow controller
10-H-02 Preflash furnace
10-IFS-1051 Flow controller to shut down
10-LPA-15 Low pressure alarm in crude feed line to desalter
10-LRC-1188 Level regulator in preflash column
10-P-01AB Crude storage pump
10-P-02AB Discharge pump from preflash bottom
10-P-12AB Pump supplying desalter water
10-P-25AB Desalted crude discharge pump
10-P-26AB Preflash column pumparound pump
10-PRC-1 Desalter pressure controller
10-PRC-IV Pressure controller in crude feed line to desalter
10-TRC-1137 Preflash coil out temperature regulator
10-TRC-1175 Preflash column top temperature controller
FR-1146 Preflash furnace fuel oil flow regulator
FR-1149 Preflash furnace fuel gas flow regulator
L1 Desalted crude flow line from desalter
MPA Middle pumparound
PH1 First preheat train
PH2 Second preheat train
PV10 Emergency valve in preflash bottom
PV2 Control valve in preflash furnace medium steam supply line
PV3 Control valve in preflash furnace fuel oil supply line
PV4 Control valve in preflash furnace fuel gas supply line
PV5 Preflash column reflux control valve
PV7 Control valve regulating flow from preflash pump around into preflash column
PV8 Control valve regulating temperature in preflash pump around
PVI Control valve in preflash furnace crude feed line
SS Stripping steam
SW Sore water
V1 Control valve in crude feed line to desalter
V2 Flow valve in upstream desalter water feed line
V3 Control valve regulating downstream desalter water
V4 Desalter mix valve
V5 Valve in sore water flow line