API 650 And API 2000 Venting requirements (with emergency vent)

[Legoman92]

Hi all,

I'm designing a diesel tank 14m Diameter, 15m high tank with a fixed roof (rafters and roof plate).
Max inlet flow rate = 870 m^3/h DN250
Max outlet flow rate = 440 m^3/h DN250
Tank is at atmospheric pressure

I'm a Structural Engineer and haven't had any exposure to the Venting requirements set out in API 650. I've calculated the in-breathing and out-breathing flow rates (I think)which are:
Outbreathing rate = 870 m^3/h
Inbreathing rate = 1470 m^3/h (thermal governs)

I've also calculated the emergency pressure vent flow capacity = 18362 Nm^3/h

On the client supplied data sheet they've nominated 3 x DN200 free vents on the roof (for us to confirm) and I'll need to nominate a diameter on the emergency pressure vent (which their standard drawing nominates a Protectoseal 53300.

2 questions:
a) what am I missing in order to calculate the diameter of the free vents required?
b) what am I missing in order to calculate the diameter of the emergency pressure vent? Is this diameter typically specified by the manufacturer or by the designer?

The company wasn't going to engage a mechanical engineer for this project, but I'm hesitant to nominate sizes for these as I feel it isn't really my domain.

Any help would be appreciated,

Thanks

 

[IFRs]

Look up API 2000 "Venting Atmospheric and Low-Pressure Storage Tanks" - that should have everything you need.

 

[JStephen]

Some miscellaneous notes:
-API-2000 includes the "old method" (read stuff off tables, assuming all fuels are the same, basically) and the "new method" (calculate everything based on actual product properties). The "old method" is far simpler. Usually, nobody bothers to furnish the actual properties that are needed for the "new method".
-API-650-5.8.5.2 and 5.8.5.3 define maximum venting pressures. It is common to see vent set points confused with the design pressure, but the way these paragraphs are worded, design pressure has to be high enough so that the set point plus any overpressure required for flow does not exceed design pressures.
-If you have free vents for standard venting, it may be most economical to increase the size of them (if needed) to allow emergency venting, rather than adding a dedicated emergency vent. Note that both emergency and normal vents are considered for emergency venting.
-Confirm with the customer if flame arrestors are required, which will affect vent flow rates.

 

[Legoman92]

The client wants 100% redundancy on the venting plus originally wanted a frangible roof, which I’m not sure we can do with this tank so it was suggested to provide emergency venting instead. I’ve looked through API 2000 and am a bit confused how you calculate nozzle sizes from the flow rate ? Is there an equation somewhere ?

 

[IFRs]

Emergency venting flow rate requirements can be determined from API 2000 and the maximum design pressure can be calcualted from API 650, but to select a vent size you'll need the manufacturer's performance data (see section 5).

 

[JStephen]

Some manufacturers (Varec, for example) have flow curves in their online brochures. Chose the lowest set point that will work, go up to the highest allowable pressure allowed for the tank, and see which size that corresponds to.

 

[IFRs]

Note: there are "normal" vents and "emergency" vents. You will need both. Emergency vents will be needed because you don't have a frangile roof-to-shell joint. Both are coverred in API 2000. As mentioned by JStephen you can use the annex in API 2000 for a quick view of what's needed.

 

[Legoman92]

Yes I’m aware I need both. I was hoping to specify an emergency vent using manufacturer info. Can “free vents” be sized via numerical methods? Or do you need manufacturer information to do this ?

 

[Legoman92]

In regards to 5.8.5.3, what is meant by "For self­-anchored tanks, the pressure relief devices shall be adequate to prevent internal pressure from exceeding
the tank design pressure as determined in F.4.1 (subject to the limitations in F.4.2 and F.4.3, as applicable). In
calculating limitations per F.4.2, use MW or MWS = 0. "

Does this mean that that you change Mw and Mws to = 0 and re-calculate to design pressure? Which in my case doubles to 1.62 kPa ? This clause is confusing to me

Thanks guys

 

[JStephen]

Emergency venting is intended for a situation where a tank is engulfed by fire, so a very rare occurrence. So it is not considered necessary to combine that with wind or seismic loading. So for emergency venting only, you can calculate a higher allowable pressure by omitting wind and seismic from that equation.

I don't think there is a requirement that normal vents and emergency vents be physically separate items- just that the normal venting capacity and the emergency venting capacity be adequate in each case.

 

[Legoman92]

Oh that makes sense. Thanks JStephen!