API 620: 1

[lucie]

Hi, I am designing a low pressure tank with 9 m of diameter and 13 m of heigth. It is a cylindrical tank with flat bottom and coned roof.

My question is about the application of section 5.10.3.5 when calculating the shell thikness.

In this case, since we have a cylindrical shell, R1=infinity (5.10.2.5c). For this reason, R' (o R'' depending on the sign of unit forces) will be infinity.
Is section 5.10.3.5 applicable in this case? and how?

I hope someone could help me as I can´t find a solution. Thanks.

 

[JStephen]

Read Sections 5.10.5 and 5.10.5.2.

Basically, that part of the standard is not set up for cylinders with external pressure (which is the only way you can get both T1 and T2 negative). They should tell you this before you get to that section rather than afterwards! It looks like it will also not work with cones.

You might also refer to ASME Section VIII if the vacuum is considerable, or use the methods from API-650, Addendum 4 (one of the appendices) if the vacuum is not too high.

 

[lucie]

Hi JStephen,

I have no vacuum in the tank.

The reason why T1 and T2 are both negative is the hydrostatic pressure of the tank. In all the courses of the shell, except in the top shell course, the hydrostatic pressure (P1, negative sign) is higher than the gas pressure (Pg, positive sign), so P (P=P1+Pg) is negative and, as a result, T2=P*Rc is negative as well.

T1 is also negative, as P and W are both negative.

In the top shell course, the hydrostatic pressure is smaller than the gas pressure, and for this reason P has a positive sign, and also T2. T1 is negative due to W, which is negative and higher than P.

In the case of the top shell course, I apply 5.10.3.3 without problems, but in the rest of the courses, where both T1 and T2 are negative, I don't know how to aply section 5.10.3.5, as for a cylindrical shell R1=infinity.

Am I mistaking the signs of the forces?, I don`t understand why you think that I have external pressure in the tank.

Thanks a lot.












JStephen

 

[JStephen]

If you have a pressurized tank with liquid, then P1, Pg, and total P will all be positive. T2 will be positive in a cylinder. T1 will be positive if the pressure uplift is greater than the weight of the roof.

 

[lucie]

Thanks a lot for your help. I had mistaken the signs of the forces.

Then, my equations should be:

T2 = (P1+Pg)*Rc (P1, Pg and T2 all positive).

T1 = Rc/2*(P1+Pg+W/At) (P1 and Pg positive and W negative)

One last doubt,

"At" is the area of the circunference = pi*(Dtank/2)^2 or the area resisting the force = PI*Dtank*thickness?

Thanks again.

 

[JStephen]

At is the area of the whole cross section, pi*(Dtank/2)^2.