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Sizing an Anti-Syphon Vent

Sizing an Anti-Syphon Vent

Sizing an Anti-Syphon Vent

(OP)
Gents,

I'd like your thoughts and comments on the following sizing methodology for an Anti-Syphon Vent.  

A swan-neck was installed on the liquid piping downstream of a vessel to ensure that a liquid level was retained within the vessel at all times.  The hydrocarbon laden gas from the vessel was routed into the vent system.  I suspect that siphons are regularly forming thereby pulling the liquid out of the vessel.  Using available tappings I propose to run a pipe from the top of the swan-neck into the gas line exiting the previously mentioned vessel.

Taking Froude No.   Fr = (V^2)/(9.81*d)  

Where Fr >0.31 gives full bore liquid flow in vertical pipes (Equation 5-158 from Perry’s 6th Ed.).

Setting Fr = 0.31 I established the minimum flowrate at which I expected a vacuum to be pulled.  

The swan-neck was 300mm therefore any vacuum would have to form an equivalent change in pressure to empty the vessel.  Therefore I sized the vent on the basis that it had to provide a flow of gas from the vent system into the swan neck at a rate equivalent to that of the highest expected liquid flowrate (240m3/hr) into the vessel AND at a pressure drop of less than 300mm H2O.

Consequently I came up with a 2” NB anti-syphon vent serving a 4” NB liquid line.  

I’ve discussed this extensively with my colleagues and the only recommendation that came up was to make sure that the vent was at least 0.33 x NB of the liquid line.

What are your thoughts?

(I have also posted this as Thread378-56552 in the Piping & Fluid Flow forum)

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