A lot of smaller rotary vane vacuum pumps come equipped with an anti-suck back valve and this is built into the pump to stop migration of vacuum pump oil into the process chamber and vacuum lines. But many larger rotary vane pumps and rotary piston pumps do not have an anti-suck back valve. It’s just not feasible to have it built into the pump the way they are designed. So these pumps need to be equipped with a vacuum vent valve. It is a very simple mechanism that is very easy to install, but is critical in keeping your vacuum process from being contaminated by the vacuum pump oil.
Here’s an example. Let’s say you have a Univac 200 series vacuum pump connected through piping to a 6’ x 4’ vacuum chamber. Once the vacuum pump pulls the chamber down to its ultimate base pressure (say 50 microns) what we have is a 6’ x 4’ chamber and attached piping at 50 microns. Now the vacuum pump, as long as it’s running, is OK because it can maintain the level of vacuum equal to or less than the level of vacuum in the chamber. Therefore, the oil will stay in the vacuum pump. But if there’s an interruption in the power supply or if the vacuum pump is shut off, the levels of vacuum in the system would change rapidly. Here’s how they would change. The vacuum chamber actually becomes a large reserve of vacuum – its volume is many times greater than that of the vacuum pump on the other end, and it’s also sealed so it maintains its vacuum. Meanwhile the vacuum pump only has a set of exhaust valves separating the vacuum integrity of the pump and atmosphere, and these valves are in no way even close to being airtight. So what will happen is that air will pass from the atmosphere, through the exhaust valves into the pump through the piping into the vacuum chamber. When this migration takes place from the higher to lower pressure, the oil will also follow and migrate into the chamber. (It’s like when you drink soda through a straw – it goes from a higher pressure in the glass to a lower pressure in your mouth through the straw.) The oil has now contaminated the piping, the vacuum chamber, and all of the product in the chamber. This is now a considerable clean up with lengthy down time for the system. Not to mention that when you start up your vacuum pump again it won’t have oil in it, and when it sucks the oil from the chamber back in, it will bring with it all of the contaminants from the chamber and piping.