No idea what a vacuum rating would be, but tank car vacuum relief valves are set to open at a very low threshold - something like 1psi vacuum or even less. Here's a great video of what happens when a car is not vented properly.
http://www.youtube.com/watch?v=E_hci9vrvfw" onclick="window.open(this.href);return false;
If you're interested, Midland Manufacturing in Skokie, IL has a lot of valve info/specs on their website.
http://www.youtube.com/watch?v=E_hci9vrvfw" onclick="window.open(this.href);return false;
If you're interested, Midland Manufacturing in Skokie, IL has a lot of valve info/specs on their website.
