How can I calculate the line size of a vent for an atmospheric tank? 2

Size it large enough that an mud-dobber/inscect/animal can't feasibly built a nest and plug your vent. I rarely see vents on tanks that are smaller than 4".

Oh, and where there's flow, there's always pressure differential (or head difference).

what are the dimensions of the tank and what product is to be stored in it?

I doubt filling the tank will force air out at 141 fps, and if it does, that's far too fast anyway.

Size the vent for twice the maximum fill rate and/or 30 fps, but take DLite's comment into consideration and don't make it any less than 4".

What's the motivation for a 3/4" vent. Are you trying to set the Guiness record for the smallest tank vent ever installed?

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

Don't overlook drainout & pumpout the induced vacuum may determine the vent. Rapid cooling is notorious for crimping a shell.

What's the maximum mud-dauber rate of fill for a vent line in south Texas/LA/AL/FL during the mud-dauber season, assuming no global warming during the life-of-structure .....

MAN! Real engineering. 8<)

BTW the answer to the second question is whatever flowrate you have in the vent is driven by a pressure drop from the vent's inlet to the outlet. The air inside the tank must be at that pressure (at least) to sustain the flow. Without a compressor or other pressure source keeping that pressure up, the tank would have to be at a very high initial pressure in order to sustain a constant flowrate for any time long enough to vent it. Remember that your idea is to VENT the tank, not to keep the pressure inside high.

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

Have a look at the tables in API 2000 to get your worst case flow rate (thermal breathing + inflow or outflow) then use the Darcy–Weisbach equation to solve for the diameter.

Darcy won't give you a large enough vent, first because there are better equations for low pressure IMO, and especially if you use 145 fps as the velocity criteria.

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

See tables 8.1 and 8.2 in UL 142.

Select the 4" vent as suggested and validate it, rather than trying to calculate the absolute minimum size you could use.

-Captain Obvious

You are not going to put a 100NB vent on a 5 or 10kL tank, even a 20 or 30kL.. On smaller sized tanks there is nothing wrong with using 50NB or 80NB vents. Obviously on the larger vertical storage tanks vents will be 100NB or larger and incorporate emergency venting.

I have found good correlation with the Darcy-Weisbach equation for low pressure systems provided the relative pressure drop does not exceed say 10-15%. Of course in compressible flow with high line speeds and large pressure drops I would not use this equation, but we are talking about free venting. Clearly his vent is undersized with those velocities.

I do as 1gibson suggested select a pipe size and schedule, then verify it meets the calculated minimum pipe size. I am not suggesting using the Darcy-Weisbach equation will suit all design conditions but am open to suggestions that would work better. BigInch, can you supply some reference links that show this method provides large error over my stated operating range? And what other equations would you recommend using over D-W?

Cheers

There are good practical suggestions in the above posts. Getting back to the fundamental of your questions and aside from practical points, I see the case as follows:
The more filling flow rate, the more air volume needed to be vented outside. so by increasing the filling rate, the flow rate in the vent pipe increases as well till you get chocked flow at the vent pipe. This is what you should avoid. chocked flow for a pipe with a know diameter and atmospheric pressure on discharge can be calculated. Compressible adiabatic flow equation for air flow through an orifice can be used. However, we know that sonic flow will occur when discharging air at a pressure exceeding 1.9 times the outlet pressure. So the maximum capacity of your vent pipe will be achieved if the tank pressure gets to 28 psia (13 psi gage). if you consider a discharge coeff of 0.7, then you can use the following equation:



This is how you can calculate the max capacity of your vent with a know diameter. You will see that a smaller sized vent have the capacity you are looking for, but do not overlook the practicalities.

Storage tanks are much stronger against internal pressure than external pressure, so you should evaluate the pump out situation as well. Note that in 2009 API 2000 was revised to bring it in line with the more conservative ISO 28300. Make sure you are using the current versions of the standards.

A useful article, with a spreadsheet to do the calculations, is available at
Katmar Software - Engineering & Risk Analysis Software

"An undefined problem has an infinite number of solutions"

Katmar, I don't see any sonic flow there. In fact it doesn't even look like Darcy, so I think it will probably give good answers.


"An undefined problem has an infinite number of solutions" Can I add ... "None of them good."

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

BigInch - I agree that this is not a good application for sonic velocities. In my experience, when the gas going in or coming out of an atmospheric storage tank reaches sonic velocity the tank is history.

Although I don't generally subscribe to the notion "if some is good then more is better" when it comes to having big vents on tanks it might actually be true.

Katmar Software - Engineering & Risk Analysis Software

"An undefined problem has an infinite number of solutions"

Thank you all of you for your useful tips. Actually, I am very new to this website and I am grateful that you made my first experince in asking for engineering tip ehis much successful and enjoyable.

Special thanks to WATERPIPE, BIGINCH and ENGADDICT for their great tips. Also thanks to KATMAR, DLITE30 and 1gibson.

Mech29,

If you really want to thank them, click the little purple star under their posts where it says "thank ___ for this valuable post". Make their day.

Hello All,

2nd question:

Is weep hole in the reinforcement pad of the nozzle, lugs or attachments, an ASME SEC VIII mandatory requirement? or they are better to have by ASME? or not at ASME at all? I could not find a requirement in UG-36 or UW-16. If you know where to find it could you please let me know?

Thanks