minimum seal leg height to prevent siphoning

[mikejoey]

Does any one have an article that shows how to determine the minimum seal leg height to prevent siphoning?

In our system the vessel pressure is about 200 psig. There is an overhead line that goes through a condenser and we return the cold condensate to the vessel. At times there is siphoning such that the vessel pressure swings. We believe our seal leg height may not be correct.

It will help me out if anyone knows of an article that might give some insight.

Thank you

 

[katmar]

The seal leg height only has to be high enough so that the static pressure is greater than the pressure difference between the vessel and condenser. It's as simple as that. But I doubt whether that is your problem. If it were the seal leg would just be blown out and you would not see pressure swings. Unless you have a very high pressure drop in the vapor line from the vessel to the condenser this is unlikely to be a problem.

When I have seen this happen it has usually been because of a bad condensate return piping layout, or because the return piping is undersized. These situations lead to vapor blocking in the return piping. The condensate builds up into the condenser until there is enough head to drive the condensate into the vessel and then you can get a siphon forming. The line drains and then starts the cycle again. But it is not because of an inadequate seal leg height.

If you can give a dimensioned sketch with some flow rates and pressures, and pipe sizes you will get more relevant advice.

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