The VB21 is a compact vacuum breaker (破真空器) designed for steam systems. It features a high-precision ball valve core with line-contact sealing and a hexagonal body for easy installation. Both the body and cap are available in stainless steel or brass. The valve automatically opens to admit air and break the vacuum when system pressure drops below atmospheric, with an opening differential of 4.6 mmHg. Its maximum working medium includes steam and air.