Design of irrigation pipeline emptying anti-freezing mode in seasonal frozen soil region

Abstract: In order to meet the anti-freeze requirements, traditional irrigation pipelines generally are laid below the ground frozen layer in seasonal frozen region. Although the deep-buried mode of the pipeline solved the freezing problem of pipelines, it has increased investment in irrigation projects and prolonged construction schedules, affecting the development of water-saving irrigation projects relying on ground water. Since irrigation was suspended during the winter, which provided the possibility that irrigation pipelines could be buried in the frozen ground. In order to solve the problem of pipelines damage, we proposed an anti-freeze mode for irrigation pipelines based on evacuation. First the emptying mode of pipelines and version of air compressors were recommended through analysis and comparisons. Because of the limited volume of irrigation pipelines, micro reciprocating piston compressor was recommended. Working pressure, rated discharge capacity of air compressors were 0.4 MPa and 2.5 m3/min, respectively. The laying out of pipelines was obtained according to similar designing rules. The pipeline distribution, length, diameter, as well as the volume of typical plots with different length to width ratios and areas were estimated through a series of design and calculation. The pipeline capacity increased along with the length to width ratio as well as the plot area. The dimensionless pipeline volume formula was obtained based on calculations and regression analysis. A mode of working-suspending-working was introduced into water drainage of pipelines. We estimated the working time of an air compressor by the pipeline volume and discharge capacity of air compressor, and the suspending time was set according to experiences. After that draining time of all typical plots was calculated and the time per area for the plots of 6.67 hm2 was longer than any others. The covering area of the recommended air compressor was suggested to be 266 hm2 under the conditions of 8 hours per day in a weekly working period. We conducted field experiment in Heilongjiang province in order to verify the effect of the emptying mode of pipelines. With the impact of carrying machinery, reducing the potential investment and the need for pipe insulation, the burial depth of pipelines was selected to be 80 cm. Frost heaving and thaw settlement is another key factor in the seasonal frozen earth region. When the pipe was buried in the ground frozen layer, it was necessary to carry out the pipe trench filling. Replacement by non-frost materials was a common and effective measure. We set up 3 types of pipeline ditches backfilling conditions including original soil, medium sand, slag. We dynamic monitored the temperature in pipelines from November 2017 to March 2018. The result showed that the calculated drainage time could meet the need of pipeline emptying. The temperatures fell first and then increased slowly and the lowest temperature was -3.66, -2.62 and -3.22 ℃ in the ditches with backfilling with original soil, slag and medium sand. When the burying depth of pipelines was 80cm, the temperature differences in the pipelines were not obvious, which varies from 0.8 to 1.23 ℃, under the three condition. Using original soil or non-frost materials (such as sand and slag, etc.) for replacement and backfilling, the pipelines would be safe if the forced draining and evacuation mode is used through an air compressor.