Mini-QRA Now Fully Automated

Spreadsheet Mini-QRA For Systems Containing Flammable Chemicals – Part 1

Spreadsheet Mini-QRA For Systems Containing Toxic Chemicals – Part 2 of 2

Introduction

A two part series on Quantitative Risk Analysis, QRA, is presented for systems containing either flammable or toxic chemicals. Part 1 describes a Spreadsheet Mini- QRA For Systems Containing Flammable Chemicals (Flammable Mini-QRA) and Part 2 describes a Spreadsheet Mini-QRA For Systems Containing Toxic Chemicals (Toxic Mini-QRA). Both spreadsheets are in Microsoft  Excel 97 Workbook format (PC or Mac) that can be used as clones for other case studies. The risk methodology is based on an intermediate level QRA found in the 1989 CCPS / AIChE text book Guidelines For Chemical Process Quantitative Risk Analysis (CPQRA).

Flammable Mini-QRA is particularly useful in locating new plant sites so that negligible risk (1E-08 fatalities per year) is found at nearby hospitals or schools. The program can also be used to determine Individual Risk, IR, at non-hardened buildings within the plant site or at company fence lines. Utilizing only vapor cloud explosion results, the program can also be used to locate hardened control rooms for either of two blast criteria: 1 Ton of TNT at 200 feet or 1 Ton of TNT at 100 feet.

Nineteen individual spreadsheets in the author’s QRA library were combined into one Microsoft Workbook with data exchanged between tabs as required. The Workbook includes two heavy gas dispersion models (continuous and instantaneous by Britter and McQuaid), flammable consequence models (flash fire, vapor cloud explosion, jet fire, BLEVE), a generic failure rate file and a hydrocarbon physical property file. Once scenario options and input values have been selected, dispersion and consequence calculations converge automatically to user specified end points. IR results are sorted automatically on descending distances and plotted in the CPQRA staircase format. A far-field “best fit” trend line allows prediction of distances to particular fatality levels (1E- 04, 1E-06 and 1E-08).

Automation of the Mini-QRA in the EXCEL Workbook format provides a more transparent training instruction than the examples in CPQRA. After a short training period, a user should be able to clone a new intermediate level QRA in about four hours. This makes the Mini-QRA a valuable engineering tool during early stages of a project. If mitigation alternatives from Mini-QRA case studies are not acceptable, a more time consuming, detailed QRA can be authorized. Results from the Mini-QRA will expedite the more costly, detailed QRA process.

The methodology used in both Flammable Mini-QRA and Toxic Mini-QRA follow MIACC QRA Guidelines, Version 2, established by consensus at the Workshop held in Cornwall, Ontario, in June of 1993. As currently presented both spreadsheet programs would assist greatly in directing further MIACC work on Version 2. Mini-QRA Workbook Content

The Workbook now includes the following workbook sections (tabs) with automatic convergence to user specified LC50 endpoints:

(1) Databases tab for storage of case studies and for directing the user through all the documentation, option and numeric input values.

(2) Input tab for data entry and for summary of all the calculations.

(3) An example of plot plan requirements in the QRA study. Scanning the plot plan is not necessary.

(4) An example of PID requirements in the QRA study with a listing of credible scenarios. Scanning the P&ID is not necessary.

(5) A BLEVE tab where the user can call on 16 different radiation methods.

(6) U.S. Army TNT Surface Blast routine that can be used for VCEs.

(7) TNO Flash Fire Model that can be used for Instantaneous releases.

(8) TNO Multi-Energy Model that can be used in place of the TNT Model.

(9) The Britter / McQuaid Heavy Gas, Continuous Dispersion Model.

(10) The Britter / McQuaid Heavy Gas, Instantaneous Dispersion Model.

(11) The staircase Individual Risk Plot - Fatalities / Year versus Distance.

(12) The Individual Risk Plot with Trend Line of far-field results.

(13) The Individual Risk Plot with only VCE results as an example.

(14) A section for eight point Wind Rose data entry and storage.

(15) A section for retrieval of Equipment and Pipeline Failure Rate data.

(16) A convenient summary of the Event Trees used in the QRA study.

(17) An example of how to use a Fault Tree Analysis in the QRA study.

(18) A Physical Property File for over 60 flammable hydrocarbons. (19) An automatic sorting routine that sorts the results in the required order.

Validation

The 1974 Flixborough Incident is presented as a demonstration example. The Mini-QRA can be performed as an emergency preplanning exercise to determine the safe separation distances for the non-hardened control room or non-hardened administration offices, the corporate fence line and the distance to negligible risk where hospitals, schools and other public housing can safely coexist with the plant site. The results of the Mini-QRA can also be compared to the actual results to determine if the preplanning exercise was sufficiently conservative to protect the public.

In Flammable Mini-QRA, the following Consequence and Risk Criteria are used:

VCE LC50 set at 5 psig peak incident overpressure by a TNT blast model with 10% blast efficiency. If confined volume dimensions and location are known for a section of the plant, the TNO Multi-Energy model can be substituted for the TNT model.

Flash fire LC50 set at the maximum of the LEL distance or the LC50 distance from the Eisenberg thermal radiation model. As an alternative to this standard flash fire model, the more accurate TNO Flash Fire model can be used to predict LC50 distances for third degree burns to people who are wearing non-protective clothing, not being shielded by equipment and without the benefits of escape. Escape can be studied.

BLEVE LC50 distance determined by the Eisenberg thermal radiation model.

Negligible Risk distance is assumed at 1E-08 fatalities per year. This criterion is based on recent risk developments in the Netherlands where law has mandated detailed QRAs for some time now.

Corporate fence line distances should be greater than 1E-06 Fatalities per year distances.

Non-hardened buildings should be located outside the 1E-04 Fatalities per year distance. The program can be used to locate two classes of hardened buildings, 1 Ton of TNT at 200 feet or 1 Ton of TNT at 100 feet, within the ISBL.

Tryout

Arrangements can be made with the developer to test drive the program. For more information, contact:

Dick Hawrelak 1732 Notre Dame Place Sarnia, Ontario, N7S 3S7 519-542-9567 or 519-542-8832 (phone & fax) E-mail address: [email protected]