Hi,
We have a 2" open drain line that will be used to drain a main seawater header. I believe this will require a high point vent connection in order to allow draining of the main header (this drain line will be used after the header is disconnected from the system and depressurized).
What are the criteria for sizing this vent connection? Any resources I can look at?
Thanks
Sherif
Hi Sherif,
In one project which I was involved, free draining velocity criteria was considered as 0.9 m/s. For 2" drain valve, maximum allowable flowrate would be around 6.6 m3/hr.
If I were you, I'd check hydrostatic from header whether flowrate could go higher than 6.6 m3/hr or not. In case I could have higher flowrate, thus some limitation is required. Let say, additional globe valve (by certain opening) or consider smaller drain valve to limit maximum flowrate as 6.6 m3/hr only.
Related to vent line, my understanding is that this vent line would act like inbreathing (air would be sucked due to volumetric displacement of drained liquid). If vent line is too small, actual volume air entering seawater header would be slower than liquid draiing rate. This would cause vacuum on the seawater header.
So, vent line (or inbreathing line) should be designed by actual volumetric displacement of the liquid draining rate. An 3/4" vent valve should be okay in this case (for 6.6 m3/hr volume displacement).
Alex