Setting Method Of Pressure Drop Rate Threshold Of Block Valve In Gas Pipeline

In order to isolate the leaking pipeline section in time after the leakage of gas transmission pipeline and avoid the continuous increase of natural gas leakage,block valves that can be automatically closed must be set along the pipeline.For the block valve that can be closed automatically,the control logic shall be set to judge under what conditions it performs the closing action.Therefore,it is of great significance to study the control logic and threshold setting method of block valve in gas transmission pipeline for the normal operation of gas transmission pipeline and reducing methane emission in case of leakage accident.The control logic commonly used for the block valve of domestic gas transmission pipeline trunk line is to set a pair of [pressure drop rate,duration]thresholds,which has small detection range and long response time for leakage accidents,and may be closed by mistake under non-leakage conditions.In view of the shortcomings of the current control logic,a control logic for setting multiple pairs of thresholds,supporting threshold setting method and specific control flow are proposed.The improved control logic speeds up the response speed of the block valve to large leakage accidents and increases the detection range of small leakage accidents.In order to obtain the variation of pipeline pressure with time under various transient conditions,a general mathematical model coupled with gas flow simulation in the pipeline and natural gas leakage conditions is established.The variation laws of gas pressure,temperature,flow and leakage rate with time are solved by the combination of characteristic line method and large hole model,and the calculation program is written based on Python,as a calculation tool for case analysis.Taking the second Shaanxi Beijing gas pipeline as an example,57 transient conditions are simulated for 8 block valves on the Yulin Xingxian section of the pipeline.Using the proposed threshold setting method,five successively increasing pressure drop rate thresholds are set for each block valve according to the duration of 180 seconds,150 seconds,120 seconds,90 seconds and 60 seconds,the leakage aperture of 0.25 D,0.33 D,0.5D,0.75 D and D(D is the inner diameter of the pipeline).According to the 5 pairs of thresholds set for each block valve,the performance of each block valve under leakage and non-leakage conditions when using the improved control logic is simulated,and the performance is compared with that when using the existing control logic.Compared with the existing control logic,the improved control logic can detect all leakage conditions with a leakage aperture of 250 mm,while the latter can only detect part;When the leakage aperture is greater than or equal to 500 mm,the reaction time of the former is 65 seconds shorter at least than that of the latter;In all simulated non-leakage conditions,the former has no false shutdown,while the latter has false shutdown in a valve closing condition and compressor station startup condition.The results show that the improved control logic has wider detection range and faster response speed than the existing control logic,and can identify non-leakage conditions more effectively,which can provide a useful reference for the practical application of block valve in domestic gas transmission pipeline.