Knowledge Base


Anti-Syphon Valve Selection and Installation

FILED UNDER: Anti-Syphon Valve



An Anti-Syphon Valve is needed when the centerline of the pumps are below the top of the tank. If the centerline of the pump is above the top of the tank an Anti-Syphon Valve is not needed. The Anti-Syphon Valve is installed in a vertical position at the highest point in the suction line with no part of the line between the valve and the tank below the maximum oil storage level. The direction of oil flow through the valve should be in the side and exiting the bottom with the cap pointing straight up. Although suitable for outdoor installation, consideration should be given to the possibility that moisture entrained in the oil might freeze in the valve and immobilize the poppet. Complete instructions are furnished with each valve.

Steps to Troubleshoot


In the non-flow mode, spring pressure holds the valve poppet closed against the hydrostatic head. When flow is required, an additional vacuum is created by the oil pump, lifting the poppet and opening the valve. Since the total downstream pressure of the hydrostatic head plus the pump suction is required to lift and hold open the poppet, an opening in the suction line will dissipate the vacuum, close the valve and prevent an oil siphoning effect. Valve closure will be positive regardless of whether or not the pump is in operation.


Due to restriction in the valve it is important to properly size the valve based on the max flow rate of the system rather than by pipe size alone. To select the proper spring range the head is measured from the top of the tank to the centerline of the inlet of the pump.



Some common troubleshooting tips are:

1      Make sure the valve is piped properly (Vertically with the cap facing up and the inlet in the side of the body and the outlet at the bottom)

2      Always make sure the suction leg is primed prior to running the system to save life on the pumps.

3      If you are unable to pull oil through the valve, 1st verify the flow rate of the system to the valve and then verify the spring range is correct.

4      To verify the spring size there will be a hand stamped number on the side of the valve underneath “Type A” indicating the head size (5, 10, 15, or 20).

5      If seeing dangerously high vacuum numbers or you still can’t pull oil try removing the spring (for testing purposes only) to see if the pumps are able to draw the oil. This will allow you to see if the system is capable of pulling oil at all and also prove there are no restrictions such as a rag being stuck in the piping.

6      In the case of losing prime chances are air is getting into the system somehow. To prove this pull prime and notice the vacuum reading, then shut of the pumps and isolation valves. It should hold that same vacuum reading if there are no air leaks. If the vacuum reading starts to drop off then air is getting into the system somehow. Check the cap and gasket to the Anti-Syphon Valve if those are good then look down stream (ex. Strainers) for places air could get in the line.