API 620 Conical Roof Design (Newbie) 1

[EngFax]

Hello,

I'm trying to design a tank roof per API 620 Std. I'm trying to calculate the thickness based on external pressure. When I compute T1 and T2 at the roof shell juncture, both forces are negative (compressive), I need to compute the thickness based on section 5.10.3.5, in equation 18 and 19 we need to compute R´ and R″ (R´ and R″ shall be equal to R1 and R2 ...). But R1 is equal infinity since the roof is a cone, which means the thickness is equal to infinity (Confusing!!!).

Can anybody explain to process of calculating equations 18 & 19 when R1 is equal to infinity?

A sample example would be also appreciated.

Thanks!

 

[fegenbush]

You should be calculating T1 and T2 using equations (8) and (9) in 5.10.2.5(b). Neither of those equations use R1, but they do use R3, which is defined in 5.10.2.5(b).

 

[EngFax]

Thanks for your response.

I am calculation T1 and T2 using equations (8) and (9) in 5.10.2.5(b), but the problem is in calculating the required thickness when both T1 and T2 are negative of unequal magnitude, based on section section 5.10.3.5 we need to compute equations 18 & 19 which use R1 and R2

Do you have a calculations example of a conical roof design based on external pressure per API620 Std?

 

[fegenbush]

Please read 5.10.5.1. I was pretty sure that you weren't going to have luck with a basic cone roof on an API 620 tank with vacuum, but that pretty much nails it. I believe you will require a roof with double curvature (ellipsoidal or dome, not umbrella) in order to resist the vacuum.

 

[EngFax]

Great thanks

 

[EngFax]

>> "require a roof with double curvature (ellipsoidal or dome, not umbrella) in order to resist the vacuum"


Why not umbrella? I thought umbrella or dome is the same.

 

[JStephen]

A cone roof can be used for external pressure, see API-650, App. V.

An umbrella is built from plates curved in one direction only, so the overall shape is similar to a dome, but the plates are not spherically dished to a dome shape. Per one of the older "Interpretations" of API-620, it is not applicable to umbrella roofs.

 

[fegenbush]

JStephen,

I was tempted to refer EngFax to API 650 for the design of the roof, but (s)he specifically said 620. I assumed that the internal pressure was higher than 2.5 psig or the tank was refrigerated.

You are correct that if the tank could be a 650 design, that would solve the problem of vacuum.

 

[EngFax]

I'm actually working on a tank with 11 psi internal pressure and .4 psi external pressure.

Can I mix between standards? i.e cone roof internal pressure design per API 620 and external pressure design per API 650 App. V?

is the limit of external pressure in API 620 1psi?

 

[fegenbush]

No, you cannot mix standards unless specifically stated (nozzle calcs in API 650, e.g.).

API 620 does not place a limit on the external pressure to which you can design.