Hazard Identification in a Combined Cycle Power Plant

Fire and Explosion Hazard Identification in a Combined Cycle Power Plant

ABSTRACT

INTRODUCTION

Fire and Explosion are the most prevalent accidents at chemical and process industries which can cause serious damage to properties and loss of productions. Fire and explosion hazards are considered as the first and second major hazards in chemical industries [1]. Besides that, release of toxic materials are prevalent accidents in process industries too. Among these three, fire is the most common but explosion is more significant in terms of its damage potential, often leading to fatalities and damage to property [2]. Also, fire can cause human fatalities, serious injuries, financial losses due to damage of equipment and disruption of productive activity, loss of employment and sometimes irreparable damage to the environment and also other costs such as insurance premiums would increase. Hence, identification of danger factors and the ways of controlling fire and explosion accidents in such these industries are very important [3]. In this paper, the hazard of fire and explosion accidents at processing sections of a combined cycle power plant using one of the well-known hazard index which is called Dow fire and explosion index, has been estimated. The under studying power plant is comprised process unites and facilities such as gas units, vapor units and hydrocarbon storage site. Natural gas and Gasoline are the main chemical materials that are used and stored in these facilities which consume in Turbine units as fuel to produce electrical energy. The Dow Fire and Explosion Index (hereafter called the DOW Index) is a common hazard index [4]. Hazard indices using the numerical values to classify the various sections of process industries in the terms of fire and explosion and identify process areas with a high risk and estimate the losses due to fire and explosion. However quantify risks in different sectors of the industry make it easy to interpret the results [5-7]. The Dow index has been used in many researches across the world. Among those are the studies of Gupta et al. (1997), Roy et al. (2003), Bernatik and Libisova (2004), and Suardin et al. (2007) [8-12]. These researches showed that this index has been used for different purposes such as rating and classifying the danger, determining the economic impacts, and designing safe processing industries too. Suardin et al. concluded that by applying the (F&EI) index, it is possible to design safer and more economical reactor and distillation system [13]. This index has been also used in a number of studies in Iran, especially in the chemical industries. The research of Atrkar Roshan et al. (2013), Jafari et al. (2012) and also Ahmadi et al. (2008 & 2012) are some examples [13-16]. In this study, the fire and explosion hazards of some process units at a combined cycle power plant using Dow index has been estimated.

MATERIALS AND METHODS

Process Unit Selection

The fire and explosion risk analysis system is a step-by-step objective evaluation of the realistic fire, explosion and reactivity potential of process equipment and its contents. The quantitative measurements used in the analysis are based on historic loss data, the energy potential of the material under study and the extent to which loss prevention practices are currently applied [5]. Dow index was developed by the Dow Chemical Company in the 1960s as a tool for plant engineers to give relative value to the risk of individual process unit losses due to fires and explosions and to communicate these risk to management in terms easily understood, i.e., potential of financial losses due to lost production and damage to plant facilities [17]. In fact, Dow index rates the potential occurrence of fire and explosion hazards in a process unit and estimates the costs in money due to fire and explosion accidents in chemical and/or process units. The latest version of Dow fire and explosion index guideline published in 1994 was applied to calculate the fire and explosion index at Turbine processes and Gasoline storage site. The general procedure of Dow index calculation is shown in Figure.1

Figure.1: Dow Index Procedure (Dow’s Guideline, 1994)

Process Unit Hazards Factor

The Dow F&EI is calculated from equation (1):

Equation (1): F&EI = MF × F₁ × F₂ = MF × F₃

Where MF (Material Factor) is a measure of the potential energy released from the fire or explosion produced by combustion or chemical reactions. It is determined by considering the flammability and reactivity of the materials that are exist at process unit and has a range of 1±40 [4, 5]. F₁ (General process hazard factor) is a measure of reaction and process unit characteristics such as exothermic or endothermic reactions, handling or transfer of chemical materials, outdoor or indoor units, access condition in emergency situations, drainage and spill control at process unit. F₂ (Special process hazard) is a measure of chemical material and operations specifications such as toxicity, amount of flammable materials in process or storage unit, use and distance to fired equipment, dust explosion, extreme pressure or sub-atmospheric pressure, equipment’s corrosion and erosion, leakage-joints and packing, rotating equipment and etc. Each item is represented in terms of “penalties” and “credit factors” [14]. F₃ (Process unit hazard factor) is derived from the multiplying the F₁ and F₂ values. According to the value of the calculated index, the fire and explosion hazard of a pertinent process unit is rated as light, moderate, intermediate, heavy or severe which are shown in Table.1 [5].

Table 1: Degree of Hazard for F&EI (Dow’s Guideline, 1994)

After the calculation of Dow index, F&EI will be able to determine the radius and area of exposure to fire and explosion incidents using equations (2) and (3):

Equation (2): Radius of Exposure = 0.84 × Dow F&EI

Equation (3): Area of Exposure

Where, R is the radius of exposure [5].

Loss Control Credit Factors

The preventive and protective measures that have been incorporated in the process design to reduce the fire and explosion hazard are taken into account in the form of Loss Control Credit Factors (LCCF). There are three categories of loss control features including; C₁ (process control) which is derived from the multiplying by factors such as emergency power, cooling, explosion control, emergency shutdown, computer control, inert gas, operation instructions and procedures, reactive chemical review and other process hazard analysis. C₂ (material isolation) is comprised from remote control valves, dump / blowdown, drainage and interlock items and C₃ (fire protection) which accounts for leak detection (alarm and shutdown), fireproofing for structural steel, fire water supply, special systems, sprinkler systems, water curtains, foam, portable fire extinguishers / fire monitors and cable fire protection (instrumentation and electrical cables) [5, 17, 18]. Loss control credit factor is calculated using equation (4):

Equation (4): LCCF: C₁×C₂×C₃

Loss Control features should be selected for the contribution they will actually make to reducing or controlling the unit hazards being evaluated [5]. As well as the Damage Factor is determined from the Process Unit Hazards Factor (F₃) and the Material Factor (MF) and referring to Figure 2. Damage Factor represents the overall effect of fire and blast damage resulting from a release of fuel or reactive energy from a Process Unit [5].

MPPD and BI Calculations

The replacement value of the equipment within the exposed area in combination with damage factor can be used to derive the Base maximum probable property damage (Base MPPD) [4]. The actual maximum probable property damage (Actual MPPD) is then calculated by multiplying the Base MPPD by loss credit control factor which is shown in equation (5). The Actual MPPD is used to predict the maximum number of days which is the time required to rebuild the plant to its original capacity, the Maximum probable days outage (MPDO). The MPDO is used to estimate the financial loss due to the lost production: the Business interruption (BI) [18]. BI is the lost profit to the company due to an incident and is calculated by the equation (6):

Equation (5): Actual MPPD = Base MPPD × loss control credit factor

Equation (6): BI ($US) = × VPM × 0.7

Where VPM is the value of production per month.

Figure 2: Damage Factor Determination (Dow’s Guideline, 1994)

RESULTS

The results of Dow index calculations for under studying units are illustrated in Table 2. For all under studying units, radius of exposure, area of exposure, value of area of exposure, damage factor, Base maximum probable property damage (MPPD), loss control credit factor, Actual MPPD, Days outage and BI loss in terms of US dollar has been calculated which are shown in Table 2.

Turbine Unit (Methane)

Methane as fuel in Turbine unit with the material factor of 21 had a highest material factor among chemical materials that were presented in process units and subsequently based on the result of Dow index value of 321, it can be indicated that Turbine unit with Methane as fuel, had the highest degree of fire and explosion risk (as mentioned in Table 3 which is considered severe). For Turbine unit with Methane fuel, radius exposure and area of exposure were calculated which are 82.2 m and 21227 m², respectively. Also for this unit, value of area of exposure was estimated 13.8 US million dollars and damage factor is gained 0.83. After that, Base MPPD by multiplying the value of area of exposure and damage factor is derived which is 11.45 US million dollars. Loss control credit factor is estimated 0.36 which by multiplying it into Base MPPD, Actual MMPD is derived 4.12 US million dollars. Maximum probable days outage for this unit is estimated 50 days and finally the loss due to unit pauses (BI) is calculated 3.03 US million dollars.

Turbine Unit (Gasoline)

After that, when Turbine unit uses gasoline as fuel has the second risk ranking with Dow index value of 236 and

Gasoline Storage Site

Gasoline storage site with Dow index value of 56.8 was the least which is ranked as light fire and explosion risk.

Table 2: Results of Dow Fire and Explosion Index Calculation

Table 3: The Fire and Explosion Index Ranking at Under Studying Units

 Discussion

According to the results of this study, Turbine unit with Methane fuel has the highest degree of fire and explosion risk. Therefore appropriate control and protective measures should be establish to reduce the fire and explosion risks in this unit. In the other hand, according to the gotten results, Turbine units have the sever ranking of fire and explosion risk and in spite of gasoline storage site is considered as lowest risk of fire and explosion, this unit constrains most losses in money due to business interruption. The reason of this matter is related to the great amount of gasoline fuel which is deposited in 4 vessels and it is about 17 million liters.

Conclusion

In the present study, the Dow F&EI in process units of a combined cycle power plant were calculated. Based on the results, Turbine unit that uses Methane as fuel with Dow index value of 321 has the highest degree of fire and explosion risk. Another Turbine unit with gasoline fuel is ranked second with Dow index value of 236 and severe fire and explosion risk and finally, gasoline storage unit is recognized the least unit in consideration of fire and explosion risk. The findings of this study can be used to estimate the loss due to fire and explosion and also can be used as insurance premium.

References

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[1] Methane is the major component by more than 96 % Concentration of Natural Gas which is consumed as fuel at Turbine Unit in hot seasons of year, alternatively. Hence the MF of natural gas was determined from Methane which has the highest MF value.