ML20203G117
| ML20203G117 | |
| Person / Time | |
|---|---|
| Site: | Perry  |
| Issue date: | 07/29/1986 |
| From: | CLEVELAND ELECTRIC ILLUMINATING CO. |
| To: | |
| Shared Package | |
| ML20203G077 | List:
|
| References | |
| NUDOCS 8607310336 | |
| Download: ML20203G117 (21) | |
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O ANALYSIS OF THE MODE OF IGNITICN AND COMBUSTION PROPAGATION FOR BOTH CHARCOAL ADSORBER BED COMBUSTION EVENTS O
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f ANALYSIS OF THE MODE OF IGNITION AND COMBUSTION PROPAGATION FOR BOTH CilARCOAL ,
ADSORBER BED COMBUSTION EVENTS i
An engineering review and analysis was performed to determine the most ,r probable cause of ignition for each combustion event. Additional evaluations were performed to determine the potential for future ignitions, review the adequacy of the methods for combustion detection and extinguishment and to determine the adequacy of existing protection measures for the Off-Gas carbon fire hazard. Corrective actions have been implemented as required to prevent recurrence of combustion in the adsorber beds during remaining testing activities. These corrective actions in conjunction with normal system '
operation will preclude Off-Gas System carbon ignition during plant t
operations.
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We have determined the most probable causes of ignition for the two events to ,
be as follows:
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- 1. First Event - Localized heating of the vessels with the radiant heaters in conjunction with low process flow velocities resulted '
in exceeding the threshold for ignition.
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- 2. Second Event - Residual hot spots within the adsorber beds reignited upon reintroduction of instrument air.
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Factors Affecting Carbon Ignition As indicated in the section of this report on the physical and chemical properties of activated carbon, the key parameters of activated carbon which were examined include:
- 1. Process stream flow velocity
- 2. Moisture content of the carbon
- 3. Rate of heat input to the carbon
- 4. Presence of volatile contaminants on the carbon O
- 5. Presence of carbon fines (particle size and distribution)
At lower process flow velocities, the rate at which heat is removed is reduced and lower ignition temperatures are observed. The desorption of excessive moisture from carbon will raise the ignition temperature by carrying away heat. High rates of heat input, which reduce heat removal rates during oxidation, will result in lower ignition temperatures. The presence of sufficient quantities of volatile contaminants and carbon fines can also cause significantly lowered ignition temperatures.
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t' The carbon samples taken from the adsorber beds were tested for the above mentioned physical and chemical properties and were found to meet the requiremantsforashcontent,volatilecontent,apparentdensity,ha[dness, moisture content, particle size distribution and ignition temperature.
Process flow during the period prior to securing the temporary radiant heaters (instrument air purge) was determined to be 100 scfm. This purge flow rate yields an approximate four (4) foot per minute face velocity. Heat input rates from the radiant heater were also analyzed (Attachment 1). They were determined to be in excess of 500'F.
Evaluation of Potential Ignition Sources for the Initial Event Adsorber bed thermocouple temperature data were received and analyzed.
Indicated thermocouple temperatures in adsorber beds 12A, 12B, 13A and 13B remained below 250'F throughout the entire event. The thermocouples responded to condition changes such as heater de-energization, changes in process flow rates and nitrogen (N2) purge rates by indicating corresponding decreases in temperature, traceable along the process flow path. There was no indication of combustion in adsorber beds 12A, 128, 13A, or 13B. The temperature l
excursions experienced in adsorber beds 14A and 14B indicate definite carbon ignition and combustion. The thermocouple temperature readings in the 15A and ISB adsorber beds indicate possible limited combustion of the carbon in these beds.
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Evaluations were performed to determine if the carbon could have ignited at a temperature less than its certified minimum ignition temperature due to the presence of volatiles and/or excessive moisture. Both of these factors were eliminated as contributing to ignition based upon the previously mentioned physical and chemical testing results.
The most probable cause for ignition of the adsorber bed carbon was localized heating of the vessels with the radiant heaters in conjunction with low process flow velocities. The low process flow velocities lowered the ignition temperature below its ASIM certified minimum ignition temperature. The location of the larger 13.5 KW heaters provided high rates of heat input to the vessels in a localized area. Calculations, (included in Attachment 1),
indicate that the 13.5 KW space heaters placed within a few feet of the n
f i external vessel surface could have raised the adsorber vessel steel
%J temperatures above 500*F. This is evidenced by the darkened color of the vessel coating at the bottom of vessel 14A and 14B. Sufficient heat was applied to a relatively small area of the vessel surface and this resulted in local oxidation of the carbon within the vessels. The heat generated by this oxidation, in the presence of continued external application of heat from the space heaters and the continued supply of oxygen, led to ignition. This localized ignition of the carbon then progressed vertically through the adsorber bed, in the direction of the oxygen source, with a flow velocity insufficient to remove the heat being generated. As the burning carbon
( reached the upper portion of the bed, the level of free oxygen increased and resulted in higher combustion temperatures which blistered the vessel coating.
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Photographs of the blistered vessel coating are included in the last section of this report. The position of the 13.5 KW space heaters and the lower heating capacity of the 6 KN heaters did not supply an equivalent level of heat input to the other adsorber vessels as evidenced by minimal coating coloration changes on vessels other than 14A and 14B.
Evaluation of Potential Ignition Sources for the Recurrence of Combustion The adsorber bed thermocouples indicate only localized temperature conditions due to the low thermal conductivity of the carbon. Therefore, localized carbon temperatures in areas remote from a thermocouple may be significantly higher than indicated by the thermocouples. The present thermocouple configuration has, however, proven satisfactory for quickly identifying O cemeestioe ectivitv in the chercee1 edsorber beds under nerma1 f1ew conditions.
When the nitrogen (N2 ) Purge was secured and a nitrogen (N2 ) blanket established following the initial event, heat removal from the areas in the adsorber beds where combustion occurred slowed significantly. Heat transfer from the carbon through the adsorber bed vessel walls was the only remaining heat removal process. This process is slow due to the low thermal conductivity properties of activated carbon. It is apparent that the cooldown slowed to such an extent that hot spots remained within the beds.
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( Based upon the rapid rise in thermocouple temperature indication shortly after the reintroduction of the process air flow, the ignition source for the combustion event recurrence was most probably residual " hot spots" remaining within the adsorber beds. These " hot spots" were undetectable by installed thermocouple temperature readings. The " hot spots" were localized areas of carbon at temperatures above the minimum ignition temperature for carbon at low process flow velocities.
The phenomenon of " clean carbon oxidation" mentioned in our previous letter which involves the rapid oxidation of carbon following its chemical reduction may have also been a factor, but probably not a significant one based upon the localized reignition that occurred (i.e. if clean carbon oxidation had been a prime contributor, it would possibly have been seen in the DISA&B vessels as O
V well as the D14A&B vessels).
Evaluation of Extinguishment Methods The use of nitrogen (N 2) as an extinguishing agent for these events was based upon availability, effectiveness, the desire to minimize deleterious effects on the carbon and associated equipment and to enable determination of combustion product activity within the effluent stream to allow monitoring of the extinguishment. The N2 purge was effective in controlling combustion and conditions in the adsorber beds improved over time although extinguishment was slow.
O Similar carbon combustion events at other facilities and NFPA standards on similar hazards were reviewed as to extinguishing agents, application times and restart conditions. The total volume of nitrogen flow used at Perry for extinguishment and cooling was greater than at the Brown's Ferry Nuclear Plant, although the duration was slightly shorter.
Based on this information, the use of Ny for controlling the adsorber bed combustion events was appropriate for combustion control, for limiting equipment and carbon damage and for ease of monitoring combustion product activity.
Evaluation of the Adequacy of Existing Protection Measures for Off-Gas Carbon Fire Hazards b
J Based upon the evaluation of the ignition source and the consequences of combustion in the charcoal adsorber beds, additional physical fire protection features for the off-Gas System carbon fire hazard are not warranted. The combustion events were not the result of conditions inherent in the system or of conditions normally found during plant operations. Control of the use of space heaters has been addressed and PAP-0508 - PNPP Operating Rules and j Practices has been revised to include a safety review of the use of temporary l
heaters in the plant.
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ATTACHMDTT 1 Radiant Heater Temperature Analysis Calculations O
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P WP No. 6077 CDv. 2/83 CALCULATION COVER SHEET syg,7 I ,, g bN ASSIGNMENT NO. CALC.NO.
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