This might happen for example, after steam sterilisation, when the vessel is refilled again with a cold medium. The temperature drop causes the pressure inside the vessel to drop very quickly and significantly below the atmospheric pressure: a vacuum may build up inside the vessel.
A similar situation can occur in heat exchangers and process plants during start up and shut down. So they have to be secured against not only a high operating pressure, but also a high vacuum.
With the new series 1940 and 1960, a vent valve is available in both high quality stainless steel 1.4404 and in gunmetal CW617N that fulfils exactly this task. The versions 1945 and 1965 are loaded with a stainless steel spring and can easily be set at the desired low pressure within the range of -100mbar to -800mbar. A particular highlight is the high performance at relatively small nominal sizes.
> Data sheet: Type 1940/1945
> Data sheet: Type 1960/1965