atmospheric above ground storage tanks syphon

[timbones]

I am looking at someone elses design of several API 650 flat bottom oil storage tanks that have a syphon in them that is confusing.

As an example, on tank is 5 m in diameter and 7 m in height. The syphon tube runs from an elevation of 0.5 m from the bottom of the tank to a height at the high liquid level (where the "u" bend in the syphon is) and then back down inside the tank to an exit nozzle that is also at 0.5 m. The exit nozzle is marked on the data sheet as an overflow nozzle. The syphon includes a syphon breaker connected to a nozzle on the roof of the tank. What the heck does this syphon do?

What I figure is that as the tank fills up, so does one side of the syphon. When the level in the tank gets to the high liquid level at the top of the syphon "U" bend, the syphon will start to work and will start to drain down the tank until the level reaches 0.5 m and the syphon is broken (or a level in a downstream tank equalises the system). Is this right? Why would I want to drain the tank so completely under an overflow condition? Wouldn't just overflowing a enough off the top to keep the tank below the high liquid level be sufficient?

 

[JStephen]

If the syphon breaker is always open, you would not get any syphon action. You could fill the tank until it first started running through the syphon tube (at the inside of the inverted U). But it would only drain out until it got back down to that point- not to the bottom of the tank.

The arrangement as described doesn't really make sense to me. Could it possibly be an overflow from something else that flows into the tank, and the syphon breaker is to prevent it from syphoning back out? I think one of the fire codes has a requirement that pipes discharge near the floor. Or maybe it's intended to act as a conventional overflow, only the tank has a lot of water at the bottom, and they want that or heavier product to overflow first.

And it is possible that the arrangement simply isn't workable for the intended function- that has been known to happen.

 

[rmw]

Picture in your mind what would happen if you went to the return bend at the top and sawed off the siphon leg (inlet side) of the pipe. The tank would then drain from the highest point where you made the saw cut when that level was reached.

Are you storing anything in the tank that would tend to stratify so that someone wanted to take the drains from the bottom?

This literally was someone's pipe dream. Is it per drawing or did some idealistic pipefitter come up with his version of the ford escort?

rmw

 

[rzrbk]

This arrangement would be used if the tank held oil and water both. In this case, the U-tube would not be a siphon, but in an overflow, would minimize the oil leaving the tank, and maximize the water. This tank may have originally been expected to have significant amounts of water in it.

Other posts are correct in that this should only drain to the top of the u-bend and quit, so that no more liquid is overflowed than necessary.

 

[timbones]

I'm pretty sure the syphon breaker is usually closed. It probably is there so that the overflow can be stopped before the whole tank is drained.

This style of syphon is on several tanks in the plant. Most of them will have a stratified oil/water mixture. But some just have oil and some have just water. It makes sense to overflow some of the water near the bottom. But someone would have to climb to the top of the tank quickly to break the syphon or the whole tank would get drained down, including the oil.

The only other possibility I can think of is that the syphon breaker has some sort of float type control valve associated with it. I haven't got the drawings for the syphon, so I can't be sure. If this was the case, the syphon would activate once the level in the tank reached the "U" bend in the syphon and deactivate when the syphon breaker is opened by whatever float control valve device is present. With this arrangement, any overflow would be the heavier stuff in the tank which would hopefully be water and not oil. Anyone familiar with this kind of arrangement?