Anti Siphon Valve Operating Principle

[RobsVette]

Hey guys,

I am having a hard time grasping how an anti-siphon valve works on the common #2 fuel oil system. I have used the google search but can't seem to wrap my mind around it.

Could someone please explain how the valve actually operates? I know it is used to prevent oil from spilling out of the piping if there is a break between the valve and the pump suction. But as stated I'm having a tough time grasping the fundamentals of whats going on here, during operation of the pump. This would include first start up and priming and then continuous operation.

Thanks in advance for any help.
Rob

 

[BigInch]

Fortunately it's nothing to scratch your head about. It's essentially just a "reversed check valve" pointing from atmosphere into the pipe and located at the top of the gooseneck. Line is full of liquid and the line breaks. That would normally create a potential siphon, if the break was lower than the liquid level of the tank. With a siphon, the gooseneck would be an area of negative gauge pressure, atmospheric pressure higher than internal pressure. That opens the check valve, admitting air into the piping, breaking the negative pressure and stopping the flow of diesel. The check valve is typically held closed by a float on the downcomer side of the valve. When some initial volume of diesel is evacuated from the downcomer, the float is no longer held against the opening to atmosphere, so it drops and opens the check valve filling the downcomer with air.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[Pumpsonly]

It actually is like an automatic air release or vent valve. Google Air release valve or vaccum breaker and you will find it.

 

[BigInch]

Upside-down maybe. As I recall an air release valve lets air out of the pipe.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[Pumpsonly]

air release valve lets air out of the pipe when the pipe is being fill and lets air into the pipe when the pipe is being drain.

 

[zdas04]

It seems like the key to understanding an anti-siphon valve is to understand a siphon. Think of a manometer with multiple legs. The pressure at any point in the manometer is the same as the pressure at that elevation on every leg of the manometer. If you cut the manometer pipe at the bottom of a trough, the fluid will flow out the break. This will create a low pressure area at the top of manometer. The low pressure area will facilitate the flow on the opposite leg from the break towards the low pressure. That will lower the pressure in the near side leg of the next loop. This will continue until the manometer and any reservoir is empty or enough air backflows into the process to create a gap that disrupts the communication between one leg and the next. The vacuum breaker provides that disruption by allowing air to enter the process in sufficient quantities to fill the area above two legs with air, breaking the siphon. In short the vacuum breaker disrupts communication between adjacent portions of the pipeline.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[RobsVette]

hey guys,

Thanks for the responses, this definately helps me understand how the valves work. I do have a further question.

When applied to a traditional fuel oil system (#2 diesel) with a tank and pumpset located next to it. What is the difference in the line break versus the pump flowing oil. Wouldn't either case cause a low pressure area on the top of the gooseneck? The pump operation should draw oil our off this area and send it through the pump, thus creating a pressure slighthly lower atmospheric and allowing the atmospheric pressure on the tank surface to push the oil up the gooseneck and then have it fall to the suction of the pump.

I assume the answer here is that is has to do with varying degrees of pressure differential, because the anti-siphone valve we usually see do have a spring in them. Here is the one I am most familiar with. This seems to indicate that the opening in the line downstream removes the slight vacuum from the valve and allows it to close.

I guess stately simply it allows the slight vacuum from the oil pump to pull the valve open, but any break in the line returns the pressure to normal and the valve closes. Is this correct?

http://www.preferredutilities.com/assets/documents/Anti-Syphon Valve - Datasheet.pdf

 

[BigInch]

The valve port sizes x pressure can be made to balance, raise, or lower the ball float.
For example, 15 psia atmospheric pressure acting through a 0.25 in2 air inlet port on a ball = 3.75 lbs pushing the ball down, but 5 psia pipe internal pressure (-10 gauge pressure, allowing plenty of siphon flow) acting upwards on 1 in2 of ball surface = 5 lbs up, holding the ball against the air inlet port. If the line breaks, the liquid runs out, pressure decreases to 3 psia acting on 1 in2 of ball = net 0.75 lbs down, ball drops, allowing air in.
pressure x port size or pressure x area on which the pressure acts = force.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[Pumpsonly]

RobsVette

You understanding of the anti-siphon valve working principle is correct.

See the OPW ASV catalogue.