Samuel S. Dannaway, PE, President, S.S. Dannaway Associates, Inc., Honolulu
FPE Corner Improving sprinkler system reliability
I
n my January article, I noted that sprinkler system reli- ability is an important issue for performance-based fire safety engineering. Of course, it is important in any
design. However, in prescriptive design, sprinkler reliabil- ity is implied or assumed to be reliable, while, in a proba- bilistic fire risk analysis, the engineer must be able to quantify the reliability. There are two components to total fire protection or
sprinkler system reliability, operational reliability and per- formance reliability1. Operational reliability is the proba- bility that the sprinkler system will operate as intended when it is needed. Performance reliability is the probabil- ity that the sprinkler system, once operated, effectively does the job by controlling or suppressing the fire. Total sprinkler system reliability is equal to operational reliabil- ity and performance reliability (i.e. total system reliability = operational 5 performance). There is a very comprehensive U.S. sprinkler reliability
analysis based on National Fire Incident Reporting System (NFIRS) data in the 2006 NFPA report by Dr. John Hall2. Improvements regarding data collection for sprin- kler systems in the 1999 edition of NFIRS has made analysis of both operational and performance reliability possible. From the report it is concluded that operational reliability is 93% and performance reliability is 96%. The overall sprinkler system reliability is thus 89% (0.93 5 0.96). In a review of several studies of sprinkler reliabili- ty Koffel3 cites figures ranging from 81.3% to 99.5%. Some in the fire protection industry decry the “mere
89%” overall reliability as a reason to limit sprinkler trade-offs in codes in favor of more passive protection, i.e. more compartmentation, more concrete, gypsum board, fire doors, etc). In my view, an 89% overall effectiveness is pretty darn good. The passive fire protection industry would do well to provide reliability data for their products (If it exists, I could not find it). Though 89% reliability is good, there is substantial
room for improvement. In looking at operational reliabili- ty, Dr. Hall reports the following reasons that sprinkler systems failed to operate:
Cause System Shut Off
Manual System Intervention Defeated System
Lack of Maintenance Wrong Type of System
System Component Damaged
Percentage to Total Failures 65%
16% 11% 5% 3%
The obvious thing designers can do to minimize the
chance of a closed valve is to provide proper supervision for all valves controlling water supplies to sprinkler sys- tems. The best method is to electrically supervise the con- dition of the valve with a tamper switch and to have the signal report to a constantly attended location. One way to improve operational reliability by reducing
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failures due to lack of maintenance is to keep the design sim- ple. For example, do not use preaction sprinklers where wet pipe sprinklers would be acceptable. Wet pipe sprinklers do not rely on a properly functioning detection subsystem for operation. There are also many failures related to dry pipe systems, particularly in storage occupancies. Again, where conditions permit, wet pipe systems should be used. The Hall report cites the following reasons for ineffec- tive sprinkler performance:
Cause
Agent Did Not Reach Fire Not Enough Agent Released
Inappropriate System for Type of Fire System Component Damaged
Manual Intervention Defeated System
Percentage to Total Failures 50% 32% 11% 4% 3%
From this information, it is apparent that obstructions to
sprinkler discharge are a major problem and that perfor- mance will be improved by ensuring that systems are designed and installed to avoid obstructions. In addition to avoiding obstructions, ensuring that 100% sprinkler cov- erage is provided for the entire building will serve to improve system performance. The issue with “not enough agent released” points to
two factors under the control of the designer. Assuring an adequate water supply to sprinkler systems as part of the design and providing sprinkler design criteria suitable to the hazard will improve performance reliability. I suppose that I can be accused of stating the obvious in
regard to improving sprinkler system reliability through design. But if it is so obvious, why are we having any fail- ures at all? On a side note, though sprinkler reliability is not specif-
ically addressed in the analysis of residential sprinkler performance in Prince George’s County, Maryland4, the report contains several interesting facts. PG County has required sprinkler systems in all new townhouses and sin- gle family residences since 1992. From 1992 to 2007, there were 101 fire deaths in non-sprinklered townhouses and single family dwellings, while there was not a single fire death in sprinklered townhouses and single family homes. Granted that only approximately 10% of these res- idences in PG County are sprinklered, it is still an impres- sive testament to the reliability of sprinkler systems. n 1. Richard W. Bukowski, P.E., Estimates of the Operational Reliability
of Fire Protection Systems, NIST, 2002. 2. John R. Hall Jr., Ph.D., An Analysis of Automatic Sprinkler System
Reliability Using Current Data, National Fire Protection Association, February 2, 2006. 3. William E. Koffel, P.E., Reliability of Automatic Sprinkler System,
Revised September 2005. 4. Steve Weatherby, Benefits of Residential Fire Sprinklers: Prince
George’s County 15-Year History with its Single-Family Residential Dwelling Fire Sprinkler Ordinance, Home Fire Sprinkler Coalition, August 2009.
February 2011
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