After the welding of the outer protective casing at the joints of prefabricated direct-buried insulated pipes is completed, an inspection-the airtightness test-must be performed. Many people wonder: why is an inflation check necessary when the pipes are already welded?
The purpose of the airtightness test is to verify the sealing integrity of the joint's outer protective casing. The joint is the weakest point in the entire piping network; it is where the outer casing welding, heat-shrinkable sleeve sealing, and on-site foam insulation application take place. If a problem arises at any of these stages, groundwater could seep into the insulation layer through the joint. The airtightness test involves inflating the joint's outer casing after fabrication to check if the seal meets standards.
The specific procedure is as follows: a hole is drilled into the surface of the joint's outer casing to install an inflation fitting. Once the casing temperature drops below 40°C, air or nitrogen is pumped in through the fitting to a pressure of 0.02 MPa. After stabilizing the pressure for two minutes, soapy water is applied to the joint connection area to observe for any bubble formation; the absence of leaks indicates a pass. Upon completion of the test, the drilled hole is promptly plugged and strictly sealed.
A pressure of 0.02 MPa may seem low, but it is sufficient to detect minute leaks. Soapy water forms bubbles at leak points, allowing operators to visually identify areas with poor sealing and perform immediate repairs. Without this test, sealing defects might not be exposed until the pipeline has been in operation for years-by which time the pipe is already buried, making repair costs significantly higher.
The airtightness test is typically used in conjunction with spark testing (holiday detection). Spark testing checks the outer casing surface for pinholes or cracks, while the airtightness test verifies the integrity of the joint seal; the two methods complement each other. A failure in the airtightness test indicates defects in the outer casing welding or the heat-shrinkable sleeve seal, necessitating rework.
Although this step is simple, it has a significant impact on the long-term operation of the piping network. Once water enters the joint area, it is only a matter of time before the insulation layer fails and the steel pipe corrodes. The airtightness test takes little time but can prevent the need for major repairs years down the line.