# Development and Application of Unesaled borehole Leakage Detection Device Based on Flow Method
## 2 Research and Development of Unsealed Borehole Leakage Detection Technology Based on Flow Method
### Detection Principle of Leakage Detection Device  
When the location of the expandable capsule is located at measurement point 1 in Figure 1, the first enclosed space is formed between the expandable capsule and the grout-sealed section, and the length of the first enclosed space is L\({}_{1}\). The flow measured at this time is called Q\({}_{11}\), and Q\({}_{11}\) = Q\({}_{1}\) + Q\({}_{2}\), since Q\({}_{1}\) is the pure seam gas flow extracted by the first enclosed space; Q\({}_{2}\) is the amount of air leakage from the grout seal section. If the seal is intact, the flow measured in the first enclosed space is the pure seam gas flow Q\({}_{1}\).  
When the location of the expandable capsule is located at measurement point 2 in Figure 1, the second enclosed space is formed between the expandable capsule and the grout-sealed section, and the length of the second enclosed space is L\({}_{2}\). The flow measured at this time is called Q\({}_{22}\), and Q\({}_{22}\) = Q\({}^{\prime}\)1 + Q\({}_{2}\). This is since Q\({}^{\prime}\)1 is the seam gas flow extracted by the second enclosed space. If the seal is intact, the flow rate measured in the second enclosed space is the pure seam gas flow Q\({}^{\prime}\)1.  
The two measurements were subtracted to obtain  
\[\Delta\text{Q}=\text{Q}_{22}-\text{Q}_{11}=\left(\text{Q}^{\prime} _{1}+\text{Q}_{2}\right)-\left(\text{Q}_{1}+\text{Q}_{2}\right)=\text{Q}^{ \prime}_{1}-\text{Q}_{1} \tag{1}\]  
where \(\Delta\text{Q}\) is the flow corresponding to the length of the L\({}_{2}\)-L\({}_{1}\), and thus the flow rate per unit length of borehole Q\({}_{\text{E}}\) is obtained:  
\[\text{Q}_{\text{E}}=\left(\text{Q}^{\prime}_{1}-\text{Q}_{1}\right)/(\text{L} _{2}-\text{L}_{1}) \tag{2}\]  
Figure 4: Installation position of pressure limiting valve.  
Then, \(Q_{1}=Q_{E}L_{1}\), and then obtain  
\[Q_{2}=Q_{11}-Q_{1} =Q_{11}-Q_{E}L_{1} \tag{3}\]  
Based on the magnitude of \(Q_{2}\), it is possible to determine not only whether there are air leaks in the sealed section of the borehole, but also to obtain a specific value for the amount of leakage.  
According to the theory of air leakage around boreholes, if there is no leakage in the detected L\({}_{1}\) and L\({}_{2}\) sections, the increase in the flow rate between the first and second measurement points is only the pure seam gas extraction in the tested section \(\Delta\)L (the length of L\({}_{2}\)-L\({}_{1}\)); the increment of \(\Delta\)Q (the flow rate corresponding to the length of L\({}_{2}\)-L\({}_{1}\)) will be very small, as shown in Figure 5a. If there is a leakage in the test section \(\Delta\)L, the increment of \(\Delta\)Q will be larger when the expandable capsule is tested at measurement point 2, as shown in Figure 5b. Due to the complex and variable extraction conditions in coal mines, a complete absence of air leakage rarely exists. Generally, when the ratio of the \(\Delta\)Q to the flow measured at the previous point is less than 5%, consider the measured borehole to be in a non-leakage state, and when this ratio is greater than 30%, the borehole is leaking.