Practice Problems: Hazard Analysis
Question 1
A process safety engineer is conducting a Layer of Protection Analysis (LOPA) for a reactor system. The initiating event frequency is 1 × 10-2 events per year. The system has the following independent protection layers: a basic process control system (PFD = 0.1), a high-pressure alarm (PFD = 0.1), a safety instrumented function (SIF) rated SIL 2 (PFD = 0.01), and a pressure relief valve (PFD = 0.01). The tolerable risk frequency is 1 × 10-6 events per year. What is the mitigated event frequency, and does it meet the tolerable risk criteria?
(a) 1 × 10-7 events/year; meets criteria
(b) 1 × 10-8 events/year; meets criteria
(c) 1 × 10-5 events/year; does not meet criteria
(d) 1 × 10-6 events/year; marginally meets criteria
Question 2
A chemical plant safety manager is evaluating a dust collection system handling cornstarch. Laboratory testing indicates the minimum ignition energy (MIE) is 10 mJ, the minimum explosible concentration (MEC) is 50 g/m³, and the Kst value is 185 bar-m/s. Based on the Kst value, what is the dust explosion class, and what protection measures are most appropriate?
(a) St-0; no special protection required
(b) St-1; explosion venting or suppression recommended
(c) St-2; explosion venting or suppression required
(d) St-3; explosion isolation and suppression mandatory
Question 3
A process engineer is designing a relief system for a vessel containing liquid propane at 40°C. The vessel volume is 50 m³, and the required relief rate is calculated as 25,000 kg/hr. The relieving pressure is 1,800 kPa absolute, and the back pressure is 105 kPa absolute. The relief valve coefficient of discharge is 0.975, and propane properties at relief conditions show a specific gravity of 0.50. What is the required effective discharge area of the relief valve using the API 520 liquid relief equation?
(a) 1,250 mm²
(b) 1,840 mm²
(c) 2,100 mm²
(d) 2,450 mm²
Question 4
A safety engineer is evaluating the consequences of a toxic gas release. Chlorine gas is released from a ruptured 2-inch pipe at a rate of 150 kg/min for 10 minutes. Wind speed is 3 m/s, atmospheric stability class is D (neutral), and the release is at ground level. Using Pasquill-Gifford dispersion coefficients at 1 km downwind, σy = 68 m and σz = 32 m. What is the centerline ground-level concentration at 1 km downwind?
(a) 35 mg/m³
(b) 68 mg/m³
(c) 92 mg/m³
(d) 115 mg/m³
Question 5
A process hazard analysis team is conducting a HAZOP study on a distillation column. The design intent is "Transfer solvent from storage to column at 50 L/min and 25°C." Using the guideword "MORE" applied to the parameter "FLOW," what is the most critical deviation and its most likely consequence?
(a) More flow causes column flooding and potential overflow
(b) More flow causes decreased residence time and incomplete separation
(c) More flow causes pump cavitation and mechanical failure
(d) More flow causes increased pressure drop and energy consumption
Question 6
A safety analyst is calculating the probability of failure on demand (PFD) for a 1-out-of-2 (1oo2) voting configuration safety instrumented system. Each sensor has a dangerous failure rate of λ = 2 × 10-6 failures per hour, and the proof test interval is 1 year (8,760 hours). Assuming no common cause failures and using the simplified PFD equation for low demand mode, what is the average PFD for the 1oo2 configuration?
(a) 3.8 × 10-4
(b) 1.7 × 10-4
(c) 7.6 × 10-5
(d) 3.4 × 10-5
Question 7
A fire protection engineer is sizing a deluge system for a process area containing flammable liquids. The protected area is 400 m², and the design density is 12 L/min/m² for a 30-minute duration. The water supply pressure at the deluge valve is 8 bar, and the friction loss in piping is estimated at 1.5 bar. The elevation difference from the deluge valve to the highest nozzle is 6 meters. What is the minimum required water supply capacity in cubic meters?
(a) 120 m³
(b) 144 m³
(c) 168 m³
(d) 192 m³
Question 8
A process safety engineer is evaluating the flammability hazard of a mixture in a closed vessel at 100°C and atmospheric pressure. The vessel contains 30% methane (LEL = 5.0%, UEL = 15.0%), 20% propane (LEL = 2.1%, UEL = 9.5%), and 50% nitrogen by volume. Using Le Chatelier's rule, what is the lower explosive limit (LEL) of this fuel mixture (excluding nitrogen)?
(a) 2.9%
(b) 3.5%
(c) 4.1%
(d) 4.7%
Question 9
A safety consultant is performing a Fault Tree Analysis (FTA) for a reactor overpressure scenario. The top event "Reactor Overpressure" occurs if cooling fails AND temperature control fails. Cooling system has a failure probability of 0.02, and temperature control has a failure probability of 0.05. There is also a common cause factor of 0.15 affecting both systems. What is the probability of the top event occurring using the beta factor method?
(a) 1.0 × 10-3
(b) 1.15 × 10-3
(c) 1.5 × 10-3
(d) 2.0 × 10-3
Question 10
A plant engineer is evaluating explosion venting for a dust collector handling aluminum powder. The collector volume is 50 m³, the maximum explosion pressure (Pmax) is 9.0 bar absolute, the maximum rate of pressure rise (Kst) is 550 bar-m/s, and the reduced explosion pressure (Pred) is limited to 2.0 bar gauge. Using the VDI 3673 correlation, what is the approximate required vent area?
(a) 2.8 m²
(b) 4.2 m²
(c) 5.6 m²
(d) 7.0 m²
Question 11
A process safety manager is assessing the thermal stability of a batch reactor containing an exothermic reaction. The heat of reaction is 850 kJ/kg, the specific heat capacity is 2.5 kJ/kg·K, the reactor mass is 5,000 kg, and the cooling capacity is 150 kW. The Time to Maximum Rate (TMR) is calculated as 4 hours. Using the Stoessel criticality classes, what is the criticality class of this reaction?
(a) Class 1 - Not critical
(b) Class 2 - Low criticality
(c) Class 3 - Medium criticality
(d) Class 4 - High criticality
Question 12
A safety engineer is evaluating a pressure relief valve for a vessel experiencing external fire exposure. The vessel contains propane with a wetted surface area of 85 m². The vessel is uninsulated. Using API 521 guidelines for fire relief sizing, what is the required heat input rate for sizing the relief device?
(a) 1,850 kW
(b) 2,120 kW
(c) 2,450 kW
(d) 2,780 kW
Question 13
A chemical plant is conducting a consequence analysis for an ammonia release. A storage tank ruptures, releasing 10,000 kg of anhydrous ammonia as a two-phase jet. The ambient temperature is 25°C, atmospheric pressure is 101.3 kPa, and wind speed is 4 m/s. The liquid fraction that flashes is calculated as 0.18. What mass of ammonia forms an aerosol cloud?
(a) 1,800 kg
(b) 4,600 kg
(c) 6,400 kg
(d) 8,200 kg
Question 14
A process safety engineer is designing an inert gas blanketing system for a storage tank containing toluene. The tank volume is 200 m³, with a working capacity of 75%. The breathing rate due to thermal effects is 15 m³/hr. The oxygen concentration must be maintained below 8% by volume. The inert gas (nitrogen) supply contains 2% oxygen. What is the minimum nitrogen purge rate required using the continuous purge method?
(a) 1.5 m³/hr
(b) 2.5 m³/hr
(c) 3.5 m³/hr
(d) 4.5 m³/hr
Question 15
A safety analyst is calculating the Safe Siting distance for a control room from a potential vapor cloud explosion (VCE) source. The estimated TNT equivalent mass is 2,500 kg. The control room is designed to withstand an overpressure of 7 kPa. Using scaled distance approach where overpressure of 7 kPa corresponds to a scaled distance of 10 m/kg1/3, what is the minimum safe distance?
(a) 95 m
(b) 135 m
(c) 175 m
(d) 215 m
Question 16
A process engineer is evaluating an emergency depressuring system for a reactor containing 15,000 kg of propylene at 40 bar and 80°C. The target is to reduce pressure to 10 bar within 15 minutes to prevent runaway reaction. The depressuring line is 4 inches Schedule 40 with an effective length of 25 meters. The discharge coefficient is 0.85. What is the approximate required orifice diameter for the depressuring valve?
(a) 45 mm
(b) 65 mm
(c) 85 mm
(d) 105 mm
Question 17
A hazard assessment team is evaluating the consequences of a BLEVE (Boiling Liquid Expanding Vapor Explosion) involving a propane transport truck. The truck contains 18,000 kg of propane at 90% fill level. The heat of combustion of propane is 46,000 kJ/kg. Assuming 40% of the released propane participates in the fireball, what is the fireball duration using the Crocker and Napier correlation?
(a) 8.5 seconds
(b) 11.2 seconds
(c) 14.8 seconds
(d) 18.3 seconds
Question 18
A safety engineer is designing a flame arrestor for a storage tank vent handling hexane vapors. The tank breathing rate is 120 m³/hr at standard conditions. The MESG (Maximum Experimental Safe Gap) for hexane is 0.90 mm, and the required flame arrestor type must prevent deflagration propagation. The operating temperature range is -10°C to 50°C. What flame arrestor element spacing should be specified?
(a) 0.40 mm
(b) 0.60 mm
(c) 0.75 mm
(d) 0.90 mm
Question 19
A process safety analyst is calculating the Safety Integrity Level (SIL) required for a safety instrumented function. The initiating event frequency is 0.1 per year, the consequence severity requires a risk reduction of 1,000, and the existing mechanical protection layers provide a risk reduction factor of 10. What SIL rating is required for the safety instrumented function?
(a) SIL 1 (RRF: 10-100)
(b) SIL 2 (RRF: 100-1,000)
(c) SIL 3 (RRF: 1,000-10,000)
(d) SIL 4 (RRF: 10,000-100,000)
Question 20
A chemical facility is assessing the indoor release of methylene chloride in a confined space. The room volume is 500 m³ with a ventilation rate of 6 air changes per hour. A spill releases methylene chloride at a rate of 2.5 kg/min for 5 minutes. The molecular weight of methylene chloride is 84.93 g/mol. Assuming complete evaporation and well-mixed conditions, what is the steady-state concentration in ppm?
(a) 850 ppm
(b) 1,420 ppm
(c) 2,180 ppm
(d) 2,950 ppm
