Question 5 of 6: Process Hazards

Question 5: Process Hazards (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

The two lists are the General Process Hazards and Special Process Hazards of the Dow Fire & Explosion Index (F&EI), the standard method for ranking the loss potential of a process unit. (The paper as printed contains two slips, reproduced verbatim in the box above: the stem reads “the magnitude of the following an incident,” and part (b) again asks for “twelve general process hazard factors.” The paragraph immediately above (b) defines special process hazards, and the Dow index has exactly six general and twelve special factors, so (b) is answered as the Special Process Hazards — stated here as an assumption under NOTE 1 of the paper.)

(a) Six General Process Hazards

These set the base factor — the general energy character of the process:

1. Exothermic chemical reactions — the reaction itself releases energy (e.g. oxidation, polymerisation, hydrogenation).
2. Endothermic processes — units that must be supplied energy (calcination, cracking, pyrolysis), where loss of heat control is a hazard.
3. Material handling and transfer — loading, unloading, conveying and warehousing of flammable/reactive material.
4. Enclosed or indoor process units — confinement that prevents dispersion of a leak.
5. Access — adequacy of emergency access to the unit for fire-fighting and escape.
6. Drainage and spill control — the ability to contain and divert spills and fire-fighting water away from the unit.

(b) Twelve Special Process Hazards

These raise the probability and severity of an incident for the specific unit:

1. Toxic materials — they complicate and delay emergency response.
2. Sub-atmospheric pressure — risk of drawing air into a system that must exclude it.
3. Operation in or near the flammable range — process or storage that can enter the explosive envelope on upset.
4. Dust explosion — finely divided solids that can deflagrate.
5. Relief pressure (elevated operating pressure) — higher pressure increases leak rate and stored energy.
6. Low temperature — risk of brittle fracture of carbon steel.
7. Quantity of flammable/unstable material — the inventory in process and in storage that could feed a fire.
8. Corrosion and erosion — wall thinning that leads to loss of containment.
9. Leakage — joints and packing — flanges, seals and glands as leak sources.
10. Use of fired equipment — furnaces and heaters providing a ready ignition source.
11. Hot-oil heat-exchange system — large inventories of combustible heat-transfer fluid above their flash point.
12. Rotating equipment — large pumps, compressors and agitators as mechanical failure and leak points.