Structured Approach to Fire Damage Assessment Reduces Costs - Part 1 of 2

By Vince Carucci

Major fires sometimes occur in process plants and cause significant losses. Unless a structured approach is used for fire damage assessment, the tendency could be to overestimate the amount of equipment to replace and possibly prolong unit downtime.

This article highlights the methodology used for a fire damage assessment that was done, focusing primarily on pressure vessels, heat exchangers, and piping systems. A second article will provide several practical application tips that might be useful to others.

Assessment Methodology

API RP 579, Recommended Practice for Fitness-For-Service, provides guidance for conducting a fire damage assessment. The most important aspect of the assessment is to identify the temperatures or “Heat Exposure Zones” within the fire area. This is the most critical activity of the assessment since all inspection plans and fitness-for-service evaluations follow from this. A Heat Exposure Zone is established based on the temperature that occurred in that area during the fire.

Table 1 summarizes the six Heat Exposure Zones defined by API RP 579.

Heat Exposure Zone
Temperature Range, °F
Potential Damage
(Partial summary. Refer to API RP 579 for details.)
I
Ambient
None; acceptable to operate.
II
Ambient to 150°F
None to major equipment. May have water or smoke damage to insulation, delicate mechanisms, and electronics.
III
>150°F to
400°F
None to major equipment. Some damage to nonmetallic materials possible (e.g., gaskets, valve packing). Damage to wiring, electronics, machinery drive belts.
IV
> 400°F to 800°F
Damage to ancillary equipment (e.g., wiring, circuit boards, motors). Replace nonmetallic gaskets and valve packing, springs in supports and valves, bearings. Check heat exchanger tube rolls and instrument tubing joints for tightness.
V
> 800°F to 1350°F
Replace ancillary equipment, small piping, tubing, copper materials, gaskets, packing, B7 bolting. Clean, inspect, and pressure test all major equipment (i.e., pressure vessels, heat exchangers, rotating equipment). Remove oxide scale to determine underlying damage. May require metallurgical sampling to determine effects of high-temperature. Repair or replace distorted structural steel and piping.
VI
> 1350°F
Almost everything will be scrapped if it was not protected by insulation, water spray, or fireproofing. Thorough inspection and testing required. Check areas of severe oxidation. Check piping and equipment in low temperature service for loss of fracture toughness.

Table 1 Heat Exposure Zones

Remember that the Heat Exposure Zone temperatures refer to the external temperature in the fire regions, not necessarily the equipment or piping metal temperature. The metal temperatures that the equipment and pipe actually reached determine the extent of metallurgical damage and the need for replacement. Equipment that was protected from the fire temperature by external insulation, or cooled by internal liquid, can often be retained for use with nothing more than a confirming inspection and hydrotest.

Clues to Assigning Heat Exposure Zones

Heat Exposure Zones are established by identifying clues to estimate the exposure temperatures for locations within the unit. These clues are based on the expected visual condition of various materials after fire exposure and include:

API RP 579 contains temperature exposure information for most common materials found in a process plant. The temperature clues are then used to define the Heat Exposure Zones. This mapping is conveniently done on unit plot layout drawings in both plan and elevation views. The Heat Exposure Zones plus visual observations are then used to determine what follow-up inspections might be necessary (e.g., wall thickness measurements, hardness checks, or in situ metallography), and if more detailed engineering evaluations are required.

Conclusion

Once the shock of a major process plant fire has subsided, the pieces must be put back together. The structured fire damage assessment approach contained in API RP 579, plus additional practical application methods, can help minimize reconstruction costs and allow safe resumption of unit operation as fast as possible. The next article in this series will provide some tips for doing this assessment.

Read Structured Approach to Fire Damage Assessment Reduces Costs - Part 2 of 2