CH3121 Chemical, Biological & Plant Safety Individual Continuous Assessment 2 2025, Singapore

Quiz: Source, Dispersion and LOPA Analysis- Individual CA2                        

1. Perform Analysis using Source Model
2. Deploy ALOHA software and generate the toxic and dispersion contours.
3. Summarize the results and do a comparison through your observation.
4. Perform LOPA to develop extra independent protection layer(s)

Problem:

An ammonia storage tank leaks at its bottom piping through an effective opening with a diameter of 50 mm. NH₃ is stored under saturated liquid conditions at 11.58 bar and 30^oC. Location of leak in piping is at 10-meter elevation above the ground level. Typical details of the NH₃ storage tank are shown in the following diagram.

a. Estimate NH₃ liquid spill (in kg) collected in the bottom dike in one hour. (Assume (i) steady state between spill rate and evaporation rate. (ii) Evaporation occurs over entire cross section of the dike). Estimate the total NH₃ vapor release to air by flashing and evaporation.

NH₃ liquid temperature after depressurizing through 50 mm opening to atmospheric pressure (atmospheric boiling point of NH₃) = – 33^oC

Ammonia liquid density at -33^oC = 673.1 kg/m³

Heat of vaporization of NH₃ at atmospheric pressure= 1170.87kJ/kg

Difference in specific volume between vapor and liquid NH₃= 1.38115 m³/kg

C_p for NH₃= 2.103 kJ/kg.K

Dike dimensions: 10m(L) x 10m(B) x 0.6 m(H).                                       (8 Marks)

b. Perform toxic dispersion analysis for released NH₃ using ALOHA software. Assume open country conditions and atmospheric stability class B. Wind speed is 2 m/s. Assume a humidity of 80%. Site’s ambient temperature is 30^oC. Use ALOHA Gaussian and Heavy gas dispersion models to develop ERPG1, ERPG2 and IDLH risk contours. State any assumptions used in the model.  (6 Marks)

c) Perform LOPA and develop an appropriate solution to reduce the NH₃ storage tank overpressure frequency to well within the acceptable frequency. Due to changes in NH₃ make-up rate (much higher than the original design rate), PSV on top of the NH₃ storage tank is under sized. Operator mistake during NH₃ make-up can result in overpressure in the tank. State any assumptions used. Use typical PFD (probability of failure on demand) values of protection layers from lecture notes/ textbook or any other reference (If external source of reference is used, please cite it clearly). If SIS (safety instrumented system) solutions are required, develop an SRS (Safety requirement specifications) document.  (6 Marks)

 Initiating event frequencies for overpressure are:

Acceptable overpressure frequency for the site is 10^-5 incidents/year.

–  submit via “Assignments” portal on Blackboard by 2359 hrs. 7^th Nov 202