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# Free Vent Calculation : Velocity

## Free Vent Calculation : Velocity

(OP)
Hi,

When it comes to venting calculation I still cannot figure out how it should be sized. For instance, using API 2000 we can calculate the inbreathing, outbreathing and fire relief case flow. I hope this time someone can figure me out on this;

1) For inbreathing case: What is the velocity used to determine the minimum area required? Is it air velocity (sonic speed) or do we use design wind speed to determine the size of the free vent (i.e. gooseneck)

2) For outbreathing case : What is the velocity used? Definitely not design wind speed so how? Do we use the area of the tank (i.e. cross section area of the tank from plan view)?

I'm not an expert when it comes to venting and I wanted to learn more on this topic. Thank you in advance.

### RE: Free Vent Calculation : Velocity

Oh dear, you really are a long way from getting it right.

The only velocity you need is the velocity of the gas (air), which is entering or leaving the tank, i.e. the same volume flow rate as the fluid entering or leaving.

The aim is not to ovr pressurise the tank or subject it to any sort of vacuum pressure.

Sew this post and search on this site a bit more.

If in doubt make it at least the same size as the biggest nozzle which lets liquid in or out.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Free Vent Calculation : Velocity

Sizing equal to the largest pipe entering the tank is usually OK, as LittleInch has suggested, and is a good place to start.

I think API 2000 requires that the vent be certified and have a pressure / flow curve or set of curves. Your best bet is to select the proper size vent from a reputable vendor's offerings, using their performance curves, taking into account any uncertainties in your required flow calculations and environmental conditions ( snow / ice, freezing fog, corrosion, etc ).

Don't forget your emergency venting needs.

### RE: Free Vent Calculation : Velocity

(OP)
Hi,

Finally I have found what I have been looking for all this time. The sizing was actually already been there in Appendix D of API 2000. I guess I'm just too blind (and lazy..) to go through all the pages in API 2000.

Thanks to all that helped me with my doubt.

Thank you LittleInch and IFRs

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