WATER MIST FIRE PROTECTION RELIABILITY ANALYSIS Shuzhen Xu Research Risk and Reliabiliy Area FM Global Norwood, Massachuses 262, USA David Fuller Engineering Sandards FM Global Norwood, Massachuses 262, USA Absrac This paper presens resuls from a reliabiliy sudy of waer mis fire proecion sysems conduced a FM Global. The sudy includes a deailed Failure Modes and Effecs Analysis (FMEA) o idenify all he major poenial failure modes, which include demand, quiescen and operaional failures. Faul rees are hen consruced for ypical waer mis sysem configuraions o evaluae heir failure probabiliies. Due o he shor hisory of indusrial applicaion of waer mis sysems, no specific reliabiliy daa are available. Therefore, in he calculaion of sysem failure probabiliies, he componen failure daa are obained from oher applicaions. The failure probabiliies and heir confidence bounds of ypical waer mis sysem arrangemens lised in Sandard 75 of he Naional Fire Proecion Associaion are compared o auomaic sprinkler sysems in he paper. The major failure modes are idenified hrough an imporance analysis. Nomenclaure F() Cumulaive disribuion funcion, cdf (---) P(...) Probabiliy (---) R() Reliabiliy funcion (---) Time (ime unis depend on he applicaion, hours, days, years). λ Failure rae for he exponenial disribuion, assumed consan (ime -1 ) Inroducion Auomaic sprinkler proecion has proven o be a highly reliable and effecive means of reducing propery loss and business inerrupion due o fire. To dae, waer mis sysems have been classified by he loss prevenion indusry as special fire proecion sysems and have been limied o very specific hazard applicaions. Auomaic sprinklers coninues o be recommended for general building proecion even when specific hazards in he building are proeced by waer mis sysems due o concerns abou heir reliabiliy. If heir reliabiliy was equivalen o auomaic sprinkler sysems, he poenial would exis o reevaluae he requiremen for general building sprinkler proecion and o furher expand waer mis applicaions. In order o make his evaluaion possible, a mehodology ha allows for he direc comparison of he relaive reliabiliy of waer mis and sprinkler sysems was needed. This paper presens he mehodology and some resuls from a reliabiliy analysis of waer mis fire proecion sysems. In his analysis, sysem failure is defined as insufficien waer supply in a fire even, i.e., insufficien waer flow or amoun, delayed waer supply or no waer supply on demand. The analysis includes a deailed Failure Modes and Effecs Analysis (FMEA) o idenify all he major poenial failure modes, which include demand, quiescen and operaional failures. Based on he FMEA, various faul rees are hen consruced for ypical waer mis sysem configuraions idenified in he Naional Fire Proecion Associaion Sandard 75, Waer
Mis Fire Proecion Sysems. The reliabiliy assessmen is carried ou on hese ypical waer mis sysems. However, due o he shor hisory of indusrial applicaion of waer mis sysems, no specific componen reliabiliy daa is available. Therefore, in he calculaion of sysem failure probabiliy, he componen failure daa is obained from relaed applicaions. The calculaed failure probabiliies resuls and he confidence bounds of he seleced waer mis sysems should be considered as preliminary esimaes because he daa used for he componen failure modes are mainly from sprinkler sysems. Therefore, he resuls are presened as relaive failure probabiliies compared o hose of auomaic sprinklers. The relaive failure probabiliy for each ype of sysem is defined as he raio beween he failure probabiliy of he waer mis sysem and he lowes of he auomaic sprinkler sysems. The major failure modes are idenified hrough an imporance analysis and are discussed in he paper. Waer Mis Sysem Descripion Waer mis sysems are fire proecion sysems which use waer in small droples o exinguish fire. The mechanisms of exinguishmen include flame cooling by drople heaing and evaporaion, oxygen depleion by seam expansion and combusion producs, and weing of surfaces. While he waer mis fire suppression sysem has been sudied for a leas 5 years, is pracical applicaion is relaively new. The designs of commercially available sysems are disincly differen. Generally hey can be caegorized as follows[1]: 1. Type of aomizaion mehod: single fluid, win fluid. 2. Delivery ype: we pipe, dry pipe, pre-acion and deluge sysem. 3. Pressure supply mehod: gas propellan, pumps. 4. Operaion pressure level: low pressure ( 12.5 bar), medium pressure (>12.5 bar and <34.5 bar), high pressure ( 34.5 bar). 5. Waer source: self-conained waer ank/cylinders, privae waer source, public waer source. 6. Mis discharge ype: coninuous discharge, cyclical discharge. Alhough differences exis in he various producs, here are eigh ypical configuraions which are commonly used and lised in he Naional Fire Proecion Associaion Sandard 75 [1]. The characerisics of hese eigh sysems are simplified as follows: Sysem A: high pressure and gas driven sysem wih sored waer. Sysem B: high pressure and gas driven wih muliple accumulaor unis. Sysem C: low pressure win fluid waer mis sysem. Sysem D: single fluid mis sysem. Sysem E: pump driven waer mis sysem. Sysem F: posiive displacemen pump assembly wih unloader valves on each pump and pressure relief valve on discharge manifold. Sysem G: gas pump uni for machinery spaces and gas urbine enclosure. Sysem H: gas pump for ligh hazard applicaions. Failure Modes and Effecs Analysis Failure modes and effecs analysis is a qualiaive inducive mehod o sysemaically analyze all conribuing componens failure modes and idenify he resuling effecs on he performance of he sysem [2]. I is frequenly used o assis developmen of a faul ree analysis. In his sudy, he following general assumpions are made o faciliae he analysis: 1. All sysems and componens have been adequaely designed and manufacured. For
example, his means ha pipes can wihsand anicipaed inernal and exernal forces and manufacuring defecs are no a cause of failure. 2. The sysems under analysis have been operaing and/or esed over a sufficienly long period of ime such ha hey have passed he early failure period bu have no reached he wear-ou failure period. 3. No posiive inervenion is assumed o occur afer he waer mis sysem is demanded o funcion. For example, manual inervenion in he form of opening an improperly closed valve during he fire is no considered. The FMEA used in his sudy is illusraed in Table 1. Mos failure modes idenified in he FMEA can be caegorized ino hree ypes: operaion failures, demand failures and quiescen failures. The caegorizaion is moivaed by he way daa is presened in he lieraures. Operaion failures apply o componens in coninuous service such as he mode of rupure of waer conrol valve shown in Table 1. Quiescen failures modes are hose of componens which are normally dorman or in sandby unil esed or demanded (such as flow ransmier). They are characerized by a ime varian funcion. Probabiliy of demand failure is ime invarian and denoes he probabiliy of he sandby sysem o operae when demanded. Table 1 Examples of FMEA of waer mis sysems Iem Failure Modes Main Effecs Main Causes Conrol Elecric Insufficien waer pump drive supply when required No waer supply when required Waer Leak conrol valve Inappropriaely Pressure/ flow ransmier closed No signal Insufficien waer discharge Failure o operae during fire even No waer discharge Failure o sar No power supply Insufficien waer discharge Rupure during operaion No waer discharge Human error No waer discharge on demand Wrong seing Device failure Inspecion Inspecion Discharge es Inspecion Inspecion Reliabiliy Daa The quaniaive analysis of waer mis sysems requires knowledge of he reliabiliy daa associaed wih he failure modes for each componen considered. Due o he relaively shor use of commercial waer mis fire proecion sysems, no specific reliabiliy daa were found during his sudy. Therefore, he reliabiliy daa from ouside sources and oher field applicaions [2-4] are used by assuming ha he failure daa of he componens in waer mis sysems componens are equal o hose of he same componens when used in oher applicaions. Componen Reliabiliy
The esimaion of sysem failure probabiliy is based on he basic evens failure probabiliy calculaions. For he operaional failure modes, he failure probabiliy calculaion is given by: λ F( ) = e (1) 1 Where : duraion of he fire even. For quiescen failure modes, he relaed failure probabiliy depends on how long he componens have no been inspeced when he sysem is demanded. So he failure probabiliy of his ype is represened by is mean value, F ( i ), in one inspecion inerval calculaed using he following equaion: λ s i 1 i 1 i λ e 1 s s F ( i ) = F( s ) d s (1 e ) d s 1 = i = + (2) i λsi Where: i : inspecion inerval, s : he ime ha he componens has no been inspeced when he sysem is demanded, F( s ): failure probabiliy a he ime s The daa associaed wih demand failure modes are probabiliies, herefore here is no need for furher calculaion. However, for he failure modes of no power in uiliies, a differen formula is used o incorporae he redundancy and repair characerisics as follows: F = 1 P( A) (3) P ( A) = P( A B) P( B) (4) P( B) =1 λst r (5) λ P( A B) = e so (6) Where: Even A: he power or fire service main is available during he ime of he fire even. Even B: he power or fire service main is available o he sysem a he ime when he fire sared. λ s : failure rae of power uiliy or fire service main. T r : repair ime. : he ime fire even lass. Sysem Failure Probabiliy Once he failure probabiliy of every componen is calculaed, he failure probabiliies of he sysems can be esimaed by using faul rees analysis (FTA) based on he FMEA. Failure in his sudy is defined as insufficien waer discharge in a fire even, i.e., insufficien waer flow or amoun, delayed waer discharge or no waer discharge on demand. The calculaion of he sysems failure probabiliies is conduced on eigh ypical sysems configuraions lised in he Naional Fire Proecion Associaion Sandard 75 [1] assuming a one year inspecion inerval. The resuls obained are preliminary due o he fac ha he componen reliabiliy daa used are from oher applicaions. Therefore he resuls are presened as he relaive failure probabiliies compared o he auomaic sprinkler sysems failure probabiliies obained from an unpublished prior sudy. The relaive failure probabiliy for each sysem is defined as he raio beween he
failure probabiliy of waer mis sysems and he lowes of he auomaic sprinkler sysems. Figure 1 shows he 9% confidence bounds and mean value of he relaive failure probabiliy for each sysem assuming one year inspecion inerval. As can be seen in Fig. 1, he failure probabiliies of waer mis sysems vary depending on he design. Sysems E and F have higher failure probabiliy han he res of he sysems sudied. However, he values in Fig. 1 can vary depending on he design of componens. For example, if he waer conrol valve seleced for he calculaion in Fig. 1 doesn have a posiion indicaor of open/close, hen is failure probabiliy will be higher han if i has such an indicaor. Relaive failure probabiliy 12 1 8 6 4 2 A B C D E F G H NFPA-75 sysems Figure 1 9% confidence bounds of relaive failure probabiliies of ypical waer mis sysems mean value; 9% confidence bound; he range of relaive auomaic sprinkler sysems probabiliies relaive o he lowes level. Imporance Analysis Fussell-Vesely (FV) imporance is uilized o perform he imporance analysis on he above waer mis sysems. The FV imporance measures he probabiliy conribuion of a basic even o he sysem failure, given ha he sysem has failed. Therefore, he imporance measure is an indicaor of he failure modes which are mos likely o resul in he waer mis sysems failing o provide adequae waer discharge on demand. SAPHIRE sofware [5] was used for he imporance analysis. I is found ha he significance of each failure mode depends on he design of he sysem, however he more common failure conribuors are empy waer ank, low gas propellan pressure, improper conrol seings, conrol panel or wiring failure, shu waer conrol valve and fire deecor failure. Human error, such as empy gas or waer cylinders, or wrong seings is a common facor resuling in sysem failures. The conrol panel and wire connecion (cable), if hey are used, always appear in he lis of he firs five conribuors o he sysem failure. When he waer mis sysem shares he waer supply wih he sprinkler sysem, he imporance of mainaining he reliabiliy of he common waer source is highlighed. Conclusions The reliabiliy of waer mis sysems is assessed by using FMEA and FTA echniques and compared wih ha of auomaic sprinkler sysems. This assessmen is conduced on eigh ypical waer mis sysems lised in NFPA-75. In he calculaion of failure probabiliies, he
primary source of he daa used is from oher applicaions since no specific reliabiliy daa are available due o he shor hisory of indusrial applicaion of waer mis sysems. Therefore, he relaive failure probabiliy for each sysem sudied is presened. The imporance analysis shows ha problems wih human error, conrol panels, cables, and fire deecors are he mos significan conribuors o he waer mis sysem failure o perform adequaely in a fire even. Reference [1] NFPA-75, 23, Naional Fire Proecion Associaion Sandard on Waer Mis Fire Proecion Sysems, Quincy, MA. [2] McCormick, N.J., 1981, Reliabiliy and Risk Analysis, Academic Press, San Diego, CA. [3] NPRD, 1995, Nonelecronical Pars Reliabiliy Daa, Reliabiliy Analysis Cener, Rome, NY. [4] PERD, 1989, Process Equipmen Reliabiliy Daa wih Daa Tables, Cener for Chemical Process Safey of he American Insiue of Chemical Engineers, New York, NY. [5] Smih, C., Knudsen, J., Wood, T., 26, Advanced SAPHIRE, Idaho Naional Lab, ID.