I’m asking experts to explain the principles and purposes of pressure relief before and after a ball valve

Why release pressure? Do you mean to remove the residual medium in the valve chamber?

Liquefied gases are sensitive to pressure buildup; therefore, a hole is made on one side of the ball in the ball valve to prevent vaporization and pressure accumulation. As for pressure relief before the valve and pressure relief after the valve, I’m hearing about it for the first time. However, if there is a concern that the pressure in the pipeline might build up due to the vaporization of liquefied gas when all the valves are closed, safety valves or burst disks can be installed directly on those valves.

This post was last edited by Carson on 2023-1-31 09:49. This problem does not occur when the ball valve is open; however, when it is closed, the flow channel inside the valve ball gets filled with fluid, which remains there (Figure A). The coefficients of expansion of the fluid and those of the valve core and seat differ significantly, which makes problems likely to arise. It’s fine for small-diameter valves with small temperature differences, but for large-diameter valves (such as those used for fuel here, DN50 and above), it’s necessary to create openings to relieve pressure. As for which side to drill, the common approach is shown in Figure B (blue represents the fluid, red represents leakage); the hole is made on the high-pressure side (upstream of the fluid), with sealing being the main consideration (in this scenario, the seal on the high-pressure side fails, while only the seal on the low-pressure side works). In Figure C, sealing is not possible and leakage occurs continuously. Additionally, there is another method using small balls to relieve pressure and empty the downstream pipeline after cutting.