Hydraulic pressure test first. First, there may be construction after the pressure test, the need to blow again, the second is the expansion of the equipment pressure test, creep, immersion is conducive to the attached foreign matter off, three is sometimes the need to test pressure in sections, the last unified blowing, four is after the blowing is conducive to the drainage of drying.
GB 50235-2010 "industrial metal pipeline engineering construction specification" Section 9.1.1: "Pipeline in the pressure test after passing, should be blown and cleaned. And prepare pipeline blowing and cleaning program."