본 연구에서는 금속플렉시블호스(CSST)에서 발생하는 사고의 소손 패턴을 객관적으로 제시하는데 있다. CSST의 국내·외 기준, 저항, 절연저항 및 소손 패턴 등을 분석하였다. 그리고 실험 결과에 대한 신뢰성 검증은 미니탭 프로그램(Minitab PGM)을 이용하였다. 또한, 실험실의 온도는 20~22[℃], 습도는 18±2[%]를 일정하게 유지하였다.
CSST의 적용범위는 KS D 3625에서 3.24[kPa] 이하, KGS AA 535에서 3.3[kPa] 이하, JIA F 002-98에서 4.2[kPa] 이하로 확인되었다. 튜브의 재료는 동합금, 스테인리스강을 사용하였으며, 이음쇠의 재료는 강, 주철, 동합금을 사용한 것으로 해석되었다. 기밀시험은 60[s] 동안 실시하며, KS D 3625는 19.6[kPa], KGS AA 535는 0.02[MPa], JIA F 002-98은 0.35[MPa] 이상을 유지해야 한다. 연소기용 CSST는 연결부의 중간에 금속성의 클램프링 또는 개스킷이 삽입되어 있었고, 배관용은 절연링 및 패킹이 삽입된 것을 알 수 있었다.
연소기용으로 사용되는 황동이음쇠형(brass fittings type) 및 강이음쇠형(iron fittings type) CSST의 저항 측정은 저항측정기(Resister Hitester)를 사용하였다. 황동이음쇠형의 산술적 평균(average) 저항은 7.36[mΩ]이었으며, 강이음쇠형 CSST의 산술적 평균 저항은 11.4[mΩ]으로 분석되었다. 배관용으로 사용되는 흰색절연링형(white insulation ring type) 및 황색절연링형(yellow insulation ring type)의 절연저항 측정은 절연저항측정기(Digital Megger Hitester)을 사용하였다. 흰색절연링형 및 황색절연링형의 산술적 평균 절연저항은 각각 49.5[MΩ], 1.72[kΩ]으로 분석되었다.
휴대용 가스 토치를 이용하여 CSST에 화염을 공급하기 위한 장치는 KS C IEC 60811-1-1, 60332-1-2에 근거하여 구성하였다. 화염은 CSST의 좌측 하단에서 60°로 공급하였으며, 토치의 노즐 끝과 CSST의 거리는 180[mm]이다. 화염의 길이는 약 80[mm]로 유지시켰으며, 화염을 제거한 후 CSST는 자연 냉각을 실시하였다.
연소기용 및 배관용 CSST에 휴대용 가스 토치로 화염을 60[s], 120[s], 180[s] 동안 각각 공급한 후에 자연 냉각을 실시하였다. 황동이음쇠형의 산술적 평균 저항은 6.67[mΩ], 6.70[mΩ], 5.56[mΩ] 등으로 분석되었다. 강이음쇠형의 산술적 평균 저항은 8.46[mΩ], 10.4[mΩ], 10.9[mΩ] 등으로 분석되었다. 흰색절연링형의 산술적 평균 절연저항은 9.21[MΩ], 0.24[MΩ], 0.29[MΩ] 등으로 분석되었다. 황색절연링형의 산술적 평균 절연저항은 0.72[kΩ], 10.1[kΩ], 16.7[kΩ] 등으로 분석되었다.
KS C IEC 60811-1-1, 60332-1-2에 근거하여 화염 공급 장치를 구성하고, 휴대용 가스 토치로 CSST에 화염을 일정하게 공급한 후 소손 패턴을 해석하기 위해 디지털카메라(Digital Camera)를 이용하여 촬영하였다.
연소기용으로 사용되는 황동이음쇠형 및 강이음쇠형 CSST에 화염을 공급하는 실험을 실시하였다. 60[s] 동안 화염을 받은 황동이음쇠형은 테플론이 소손되었으나 그 밖의 금속 부분은 특이 사항을 확인할 수 없었다. 그리고 180[s] 동안 화염을 받은 볼밸브와 너트의 표면은 변색되었다. 또한, 60[s] 동안 화염을 받은 강이음쇠형은 테플론이 소손되었고, 이음쇠 내부의 구리 개스킷이 회색으로 변색되는 것을 알 수 있었다. 그리고 180[s] 동안 화염을 공격 받은 볼밸브와 너트의 표면이 회색으로 변색되었으며, 구리 개스킷은 황색으로 변색되는 것을 알 수 있었다.
배관용으로 사용되는 흰색절연링형 및 황색절연링형 CSST에 화염을 공급하는 실험을 실시하였다. 60[s] 동안 화염을 받은 흰색절연링형은 테플론이 소손되었으며, 이음쇠는 적색으로 변색되었다. 그리고 180[s] 동안 화염을 받은 고무 패킹은 용융되었고, 보호피막은 너트에 용착된 것을 확인하였다. 60[s] 동안 화염을 받은 황색절연링형은 테플론이 소손되었으며, 180[s] 동안 화염을 받으면 이음쇠가 적색으로 변색되었고, 고무패킹과 절연링은 용융되었다.
CSST에 대전류를 흘렸을 때의 소손 패턴을 해석하기 위해 대전류공급장치(PCITS; Primary Current Injection Test System)를 이용하였다. 대전류는 100[A], 130[A], 160[A], 190[A]를 각각 120[s] 동안 CSST에 공급하였다.
연소기용으로 사용되는 CSST에 대전류를 흘리면 초기에는 튜브 표면에 있는 보호피막의 연소에 따른 검정색의 연기가 간헐적으로 발생하는 것을 알 수 있었다. CSST에 대전류의 공급이 지속되면 튜브에 적열부(hot zone)의 발생과 성장이 확인되었다. 그리고 어떤 순간에 불꽃을 수반하는 연소가 확산되는 특징을 나타내는 것으로 확인되었다. 대전류에 의해 소손된 CSST의 실체 사진의 분석에서 튜브는 청색으로 변색되는 것을 확인할 수 있었고, 그 밖의 부분은 정상제품과 차이를 거의 없다는 것을 알 수 있었다.
배관용으로 사용되는 흰색절연링 CSST에 PCITS로 대전류를 공급하는 실험은 진행할 수 없었다. 왜냐하면 CSST의 연결부에 절연링 및 패킹이 삽입되어 있어서 통전이 불가능했기 때문으로 판단된다. 그러나 배관용으로 사용되는 황색절연링 CSST에 대전류를 흘리면 수초 이내에 격렬한 불꽃 방전이 발생하면서 용단되는 것을 확인할 수 있었으며, 소손된 CSST를 분해하여 실체 사진을 분석한 결과 튜브가 용융되어 구멍(hole)이 생성되는 것을 알 수 있었다.
가스설비에 사용되는 CSST의 통계적 저항, 절연저항, 확률도, 표준 편차(SD), AD, P값 등의 해석은 미니탭 프로그램을 이용하였다.
연소기용으로 사용되는 황동이음쇠형 및 강이음쇠형 CSST에 휴대용 가스 토치로 화염을 180[s] 동안 공급한 후 자연 냉각을 실시하였다. 그리고 소손된 CSST의 특성을 분석한 결과 통계적 평균(mean) 저항은 5.555[mΩ], 표준 편차는 0.102, AD는 1.013, P값은 0.010으로 해석되어 신뢰성이 우수한 것을 판단된다. 강 이음쇠형의 통계적 평균은 10.950[mΩ], 표준 편차는 0.452, AD는 0.526, P값은 0.166으로 95[%] 신뢰구간(CI)에 대한 신뢰성이 우수한 것을 판단된다.
배관용으로 사용되는 흰색절연링형의 산술적 평균(average) 절연저항은 0.29[MΩ]이고, 180[s] 동안 화염을 공급한 통계적 평균(mean)은 0.287[MΩ], 표준편차는 0.004, AD는 6.818, P값은 0.005 이하로 해석되었다. 따라서 P값을 제외한 나머지 값들은 신뢰성이 우수한 것으로 판단된다. 배관용으로 사용되는 황색절연링형의 산술적 평균 절연저항은 16.71[kΩ]이고, 180[s] 동안 화염을 받은 후의 통계적 평균은 16.713[kΩ], 표준 편차는 1.733, AD는 0.945 및 P값은 0.015으로 95[%] 신뢰구간에 대한 신뢰성이 우수한 것을 판단된다.
이상의 연구 결과에서 알 수 있듯이 CSST의 저항, 절연저항, 구조 및 연결 방법 등에 따라 특성 차이가 있음을 알 수 있었다. 또한, CSST에 열에너지 및 전기에너지 등을 공급하였을 때 저항, 절연저항 및 소손 패턴 등의 차이가 발생한다는 것이 객관적으로 제시되었다. 즉, 사고 현장에서 수거된 CSST의 저항, 절연저항, 소손 패턴 및 용융 흔적 등을 분석하면 화염의 공격 방향, 소손 원인, 공급된 에너지원의 종류 등을 과학적으로 입증할 수 있을 것이다.

핵심어 ; 금속플렉시블호스, 저항, 절연저항, 소손 패턴, 황동이음쇠형, 강이음쇠형, 절연링, 튜브, 개스킷, 대전류공급장치, Minitab PGM, 평균, 확률도, 표준편차, AD, P값, 신뢰구간

The purpose of this study is to objectively present damage patterns by burning that occur to a CSST. This study analyzed the related domestic and overseas standards, resistance and insulation resistance of the CSST as well as the patterns of its damage by burning, etc. The reliability of the test results was verified using the Minitab program (Minitab PGM). The temperature and humidity of the test room were maintained to be constant at 20~22[℃] and 18±2[%], respectively.
It was found that the application range of the CSST was less than 3.24[kPa], 3.3[kPa] and 4.2[kPa] for KS D 3625, KGS AA 535 and JIA, respectively. It was analyzed that copper alloy and stainless steel were used as the tube materials, and steel, case iron and copper alloy were used for fittings. It is required that the leaking test be performed for 60 seconds by maintaining pressure above 19.6[kPa], 0.02[MPa] and 0.35[MPa] for KS D 3625, KGS AA 535 and JIA F 002-98, respectively. It was found that a metal clamp ring or gasket was inserted at the middle of the connecting section in the case of the CSST used for the combustor, and insulation ring and packing were inserted in the case of tubes used for piping.
The resistance of the CSST with brass and iron fittings used for the combustor was measured using a resistance measuring device (Register Hitester). It was analyzed that the arithmetic mean resistance of the CSST with the brass fitting was 7.36[mΩ] and that of the CSST with the iron fitting was 11.4[mΩ]. The insulation resistance of the white and yellow insulation rings was measured using an insulation resistance measuring device (Digital Megger Hitester). The arithmetic mean insulation resistance of the CSSTs with white and yellow insulation rings was analyzed to be 49.5[MΩ] and 1.72[kΩ], respectively.
The device to supply a flame to the CSST using a portable gas torch was configured based on the KS C IEC 60811-1-1, 60332-1-2. The flame was applied at 60° from the left bottom of the CSST and the distance between the torch nozzle tip and CSST was 180[mm]. The flame length was maintained at approximately 80[mm]. Natural cooling of the CSST was performed after removing the flame.
After supplying a flame to the CSST used for the combustor and piping for 60[s], 120[s] and 180[s], respectively, using the portable gas torch, natural cooling was performed. It was analyzed that the arithmetic mean resistance of the CSST with the brass fitting was 6.67[mΩ], 6.70[mΩ] and 5.56[mΩ], respectively, and that the arithmetic mean resistance of the CSST with the iron fitting was 8.46[mΩ], 10.4[mΩ] and 10.9[mΩ], respectively. In addition, it was analyzed that the arithmetic mean resistance of the CSST with the white insulation ring was 9.21[MΩ], 0.24[MΩ] and 0.29[MΩ], respectively and that the arithmetic mean resistance of the CSST with the yellow insulation ring was 0.72[kΩ], 10.1[kΩ] and 16.7[kΩ].
The flame supply device was configured based on the KS C IEC 60811-1-1, 60332-1-2. After supplying a flame to the CSST constantly using the portable gas torch, its picture was taken using a digital camera in order to analyze the patterns of the damage by burning.
This study performed tests of supplying a flame to the CSST with brass and iron fittings. In the case of the CSST with the brass fitting used for a combustor which was exposed to the flame for 60 seconds, its Teflon was damaged by burning. However, anything particular could not be found from other metallic parts. The ball valve and nut surface which were exposed to the flame for 180 seconds were discolored. In addition, it was found that in the case of the CSST with the iron fitting which was exposed to a flame for 60 seconds, its Teflon was damaged by burning and the copper gasket inside the fitting was discolored to yellow. Moreover, it was found that the ball valve and nut surface which were exposed to the flame for 180 seconds were discolored to grey, and the copper gasket was discolored to yellow.
This study performed the test for supplying a flame to the CSSTs with white and yellow insulation rings used for piping. It was found that in the case of the CSST with the white insulation ring which was exposed to a flame for 60 seconds, its Teflon was damaged by burning and the fitting was discolored to red and that the rubber packing exposed to the flame for 180 seconds was melted and the protective film was deposited on the nut. In the case of the CSST with the yellow insulation ring which was exposed to a flame for 60 seconds, its Teflon was damaged by burning and the fitting was discolored to red, and when exposed to the flame for 180 seconds, the rubber packing and insulation ring were melted.
This study used a Primary Current Injection Test System (PCITS) in order to analyze the patterns of damages by burning when a high current was applied to the CSST. High currents of 100[A], 130[A], 160[A] and 190[A] were supplied to the CSST for 120 seconds, respectively.
It could be seen that when a high current was supplied to the CSST used for a combustor, black smoke occurs intermittently at the beginning due to the combustion of the protective film on the tube surface, and that if a high current was continuously supplied to the CSST, a hot zone appeared and grew at the tube, showing characteristics that combustion accompanying a flame had spread at a certain point in time. Analysis of the photos of the CSST damaged by burning due to the high current showed that the tube had turned grey and that other parts had almost no difference from those of normal products.
This study could not perform the test of supplying a high current to the CSST with white insulation ring used for piping through the PCITS since an insulation ring and packing inserted into the connecting section of the CSST blocked the application of high current. However, it could be seen that when a high current was applied to the CSST with the yellow insulation ring used for piping, it was fused and cut in a few seconds with the occurrence of an intense spark discharge. The analysis result of the photos of the disassembled CSST damaged by burning showed that the tube was melted and holes appeared on it.
This study analyzed the statistical resistance, insulation resistance, possibility plot, standard deviation (SD), AD, P-value of the CSST used for a gas facility using the Minitab Program.
After supplying a flame to the CSSTs with brass and iron fittings used for a combustor for 180 seconds using a portable gas torch, natural cooling was performed. The analysis result of the characteristics of the damaged CSST revealed the statistical mean resistance of 5.555[mΩ], standard deviation of 0.102, AD of 1.013 and P-value of 0.010, showing outstanding reliability of the CSSTs. The analysis result of the CSST with an iron fitting revealed the statistical mean resistance of 10.950[mΩ], standard deviation 0.452, AD of 0.526 and P-value of 0.166, showing outstanding reliability for the 95[%] confidence interval (CI).
The analysis result showed that the arithmetic mean insulation resistance of the CSST with the white insulation ring used for piping was 0.29[MΩ] and that when a flame was supplied for 180 seconds, the statistic mean was 0.287[MΩ], standard deviation was 0.004, AD is 6.818, and the P-value was 0.005. Therefore, it is thought that excluding the P-value, other values had outstanding reliability. The arithmetic mean insulation resistance of the CSST with the yellow insulation ring used for piping was 16.7[kΩ]. After the CSST was exposed to the flame for 180 seconds the statistic mean insulation resistance was 16.713[kΩ], standard deviation was 1.733, AD was 0.945 and P-value was 0.015, which showed outstanding reliability for the 95[%] confidence interval (CI).
As can be seen from the above study results, characteristics differ depending on the CSST’s resistance, insulation resistance, structure, and connection method. In addition, the study presented objectively that when thermal energy and electrical energy were supplied to the CSSTs, there was a difference in the resistance, insulation resistance and patterns of damages by burning. That is, it will be possible to scientifically prove the attack direction of a flame, cause of damages by burning, types of sources of supplied energy by analyzing the resistance, insulation resistance, damage patterns and traces of melting of the CSSTs collected from the scenes of accidents.

Key Words: Corrugated Stainless Steel Tube (CSST), Resistance, Insulation Resistance, Pattern of Damage by Burning, Brass Fitting, Iron Fitting, Insulation Ring, Tube, Gasket, Primary Current Injection Test System (PCITS), Minitab PGM, Mean, Possibility Plot, Standard Deviation, AD, P-value, Confidence Interval