Is it necessary to use a spring washer on the flange of a metal wound gasket when tightening the bolts?

I use ASME flanges for pipes, and I’ve never used spring washers.

I’ve never seen bolts and spring washers used on pipe flanges.

It can loosen easily; after all, flange connections are very important

Pipe flange connections generally do not use bolt washers; using them instead can lead to loosening over time, resulting in leaks. Mechanical structural connections generally require it.

Spring washers are used to prevent loosening, while flat washers do not have such a function; they merely increase the contact area. Spring washers can prevent loosening – for example, spring washers are usually used on the bolts that connect the motor to its housing, as vibrations from the motor could cause the nuts to loosen if no spring washers are used. Fasteners on equipment that experiences vibration are usually equipped with spring washers, while washers are generally not required on flanges. Whether to use a spring washer on the flange depends on the medium flowing through the pipe; if pulses are likely to occur, it is advisable to use a spring washer. Additionally, for high-speed fluids or those with frequent changes in diameter, there is no one-size-fits-all solution. On some valves, spring washers are required for both the stuffing box gland flange and the other flanges. In short, flat washers increase the contact area for fastening ； Spring washers are used in areas subject to vibration to prevent loosening ； No spring washers are required to fasten PVC flanges.

“How to explain that adding it actually leads to loosening during use, resulting in leaks?

Refer to Appendix A of HG/T20613 \"Fasteners for Steel Pipe Flanges (PN Series)\"; flat washers are generally used for pipe flange fasteners.