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)
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)
