Redundant configuration help, sizing Open Vent, etc

[lpfaustini]

Dear community, maybe someone could help me:

For a 75,000 barrels diesel oil storage tank with frangible roof, I'd like to clear my doubts:

1. If the roof is frangible, do I need an emergency valve (EV)? (API 2000 says "no")
2. If the tank is open to atmosphere, should I consider internal pressure 0 Kpa and vacuum 0 Kpa? (*API-650 mention 0.25kPa and 0KPa only if the tank meets appendix H)
3. Using API 2000 5th, I calculated the inbreathing and outbreathing, where can I find the OPEN VENT sizing calculation? Or how can I size it?

*4. We have other tank for gasoline storage, with IFR and supported cone roof. Is there any problem with this configuration? (IFR and supported cone roof together)

Thanks in advance,

 

[IFRs]

1) You do not need an emergency vent if you have a frangible joint
2) If you have Appendix H circulation vents, then yes otherwise no
3) from vendors - each has their own set of curves for their devices but first check the latest API 2000, there may be an appendix with values

Is this a floating roof tank?

 

[TD2K]

Emergency vents are cheap in my opinion compared to replacing a roof.

I don't like designing tanks for 0 internal pressure or vacuum if that is your second question. To estimate inbreathing or outbreathing means you need a dP and that's sort of difficult when the tank is designed for 0 pressure or vacuum.

If you mean by open vent something like a gooseneck, I do it allowing for an entrance and exit loss, pipe and fitting head loss coefficients. Bird screens are common which is another pressure drop you need to allow for.

 

[IFRs]

For your question number 4, IFRs are by definition under a fixed roof and many of these are column supported, so this is an acceptable and quite common configuration.

 

[lpfaustini]

TD2K (Chemical)

Emergency vents are cheap in my opinion compared to replacing a roof.

That's the very good point. Wouldn be better installing an EV instead of replacing the entire roof?


IFR

I can not understand how an IFR tank with venting meeting appendix H will eliminate vacuum inside the tank and a tank without IFR open to atmosphere won't.

 

[IFRs]

IFR tanks in accordance with API 650 Appendix H have two square feet of open vent per 10 feet of tank diameter. It is not possible to have pressure or vacuum inside these tanks. A fixed roof tank without an IFR typically has conservation vents with pressure settings on both vacuum and pressure relief in accordance with API 2000 and is subject to pressure or vacuum loads. If you are talking about an open-top tank without a fixed roof, then obviously it is not subject to vacuum or pressure loading.

 

[JStephen]

If I remember the wording in API-2000, vent flow rates for regular vents are supposed to be determined by testing. You can get "free vents" and gooseneck vents with tested flow rates from several vendors.

 

[lpfaustini]

Ok IFR's, I can get your point. But can't we have appendix H vent on a non-appendix H tank?

Another thing: API-650 mentions Pfe as "internal floating roof external pressure". I'm assuming this pressure exists only if the tank doesn't have an open vent, and its value will be equal to the tank internal pressure.

 

[IFRs]

You could put Appendix H vents on a non floating roof tank and in that case I would suggest that the tank would not be subject to pressure or vacuum.

API 650 Pfe is a load related to floating roof supports and connections only and does not affect nor is it related to the tank shell design. It is applied regardless of the tank vent situation. It is not really "pressure" but a uniform live load.

 

[lpfaustini]

IFR, hope you had happy new year...

About Pfe,
But isn't Lf1 already specified for "Internal floating roof uniform live load" (according to 5.2.1(e))? So what's the difference between Pfe and Lf1?

Regards,

 

[IFRs]

Numerically Pfe and Lf1 are equal for a roof with automatic drains. Pfe represents the pressure generated as the tank is emptied when the roof is not floating and the minimum value is 5 psf even if roof manufacturer's may have sufficient vacuum breakers to limit this load to a lower value. Lf1 is a uniform load representing liquid on top of the roof and is 5 psf if there are automatic drains. The values are API minimums and a rof manufacturer may design for higher loads. Note that these loads only apply to roof supports and connections.

 

[lpfaustini]

"Lf1 is a uniform load representing liquid on top of the roof"

I see now, I would never imagine liquid on top of the roof. Thank you IFR. I wonder why API-650 can't be clearer about this.

Regards.

Thank you all for the responses.

 

[IFRs]

I am drawing my own conclusions based on the fact that Lf1 changes value depending on if if the floating roof has drains. Drains are presumably for liquid. In my opinion, drains are overrated, probably are not going to be where the liquid is, spend their whole life leaking vapors and are weak points on the IFR, especially on non-contact designs. I don't use them on my IFR designs.