# Application of FBG Based Sensor in Pipeline Safety Monitoring
## 5 Experimental Results  
### Leakage Monitoring  
Due to limitations of the water pump, the actual pipeline model cannot simulate the leakage under the flow state. Prior to starting the leakage experiment, the pipeline was filled with water and pressurized to 1.2 MPa. During the leakage experiment, the inner pressure dropped from 1.2 MPa to 0 MPa by opening the valve suddenly. The Micron Optics SM130 (Micron Optics, Inc., Atlanta, GA, USA) was used to demodulate the wavelength of an FBG strain hoop sensor. The sampling rate in this leakage experiment is 1000 Hz. Figure 10 presents the wavelength variation of sensor L1 when the leakage was simulated by opening valve S1. It can be seen that the wavelength remains constant before the leakage occurrence. However, when the leakage happened, the wavelength detected by the FBG strain hoop sensor suddenly declined. This is due to the NPW propagating along the pipeline model, causing the inner pressure to suddenly drop, simultaneously leading to the hoop strain decrease. The time when the turning point (displayed in Figure 10) occurs can be considered as the arrival time of NPW. The experiment results demonstrate that the wavelength detected by the FBG strain hoop sensor can be taken as an effective indicator of leakage occurrence and the FBG strain hoop sensor is suitable for the pipeline leakage monitoring.  
When leakage occurred at S1, the hoop strain variation of six sensors (L2, L4, L6, L8, L10 and L13) with the same interval of 40 m is displayed in Figure 11. Obviously, the last sensor to detect the turning point was L13, which is located the farthest from the leakage point among the six sensors. However, for sensor L2, which is located closest to the leakage point S1 among the six sensors, is the first sensor to detect the turning point. In conclusion, the closer the distance between the leakage point and the sensor, the earlier the turning point occurs. The experiment results demonstrate that NPW propagates along the pipeline model at a certain speed. The time when a turning point occurs can be considered as the arrival time of NPW. Based on the difference in time it takes for the NPW to reach the FBG strain hoop sensors, the leakage location can be calculated. Additionally, it can be seen from Figure 11 that hoop strain variation of different sensors under the same leakage is different, the amplitude of hoop strain variation decreases with the increase of the distance between the leakage point and the FBG strain hoop sensor. This phenomenon illustrates that the NPW energy attenuates gradually when NPW propagates along the pipeline, and perhaps there is a certain relationship between the NPW energy attenuation and its propagation distance. The experimental results suggest that a leakage  
Figure 10: Wavelength responses induced by leakage point S1.  
localization method according to the NPW energy attenuation is worthy of being studied. It can also be concluded from Figure 11 that the method of leakage localization based on the NPW is limited by distance. Therefore, the principle of NPW energy attenuation will be studied comprehensively in the next stage.