Do you have a DCI Dry vacuum system that you are having regular foam sensor alarms with?
One commonly overlooked cause can be the check valve on the drain line.  If the check valve is not sealing completely when the vacuum is running it will pull air into the bottom of the tank.  This steady stream of air bubbling up through any liquid in the tank can create a huge amount of foam in the tank and rise up to trip the foam sensor. 
A quick way to test for this is simply to turn the ball valve on the drain line off and run the vacuum like that.  If the foaming stops then you know that the check valve was the issue.
Have you found any other common causes for the foam sensor to be activated?