Vertical Atmospheric Tower API,.... 3

[mancini]

May anyone give me a recommendation to the Code for design / fabrication /... to be used for a vertical tower (dia 7 m lower cylinder x with 7m height / transition from dia 7 m to dia 14 m / upper cylinder with dia 14 m and 40m height), atmospheric equipment (only liquid up to the top), design temperature 70 C, Minimal temperature 5C, material Carbon Steel A516 Gr 60 and no internal or external design pressure.

Additional info: This kind of equipment is typical in the pulp industry, the client suggests to use the API 650, but it is not possible to use API 650 to this equipment because there is the cone transition at lower part ( 7m x 14 m diameter). Anyway if the API 650 is used, at lower cylinder the thickness will be more than 50 mm thickness, so additional problems will apper because the limit to use API 650 is 40 mm and probably the heat treatment would be necessary,…

 

[JStephen]

I'm not aware of a code applicable to this. Typical approach would be to specify API-650 or other industry standards "to the extent applicable".

API-650 has thickness limitations and is not set up for this geometry.
AWWA D100 covers municipal water storage tanks, including single pedestal elevated tanks, and might come closer to covering the geometry involved. However, the hoop stress design for elevated structures is very conservative.
API-620 or ASME Section VIII could be used as well. They are both fairly conservative due to the assumed pressurized applications.

 

[dig1]

I suggest API 620 as most pulp storage tanks (which I assume this is) are designed to this code. The lower (dilution) section is needs to be designed for combined axial compression and tensile hoop.

Be careful of the "no internal pressure and no external pressure". Often loop seals are added which can create either an internal pressure or vacuum.

I assume this is not a digester blow tank due to the temperature; if it is then there is a concern about flashing steam from the hot pulp. There are cases where blow tanks have collapsed in cold weather environments because the vacuum protection/loop seals froze and "cold" filtrate was dumped in the hot tank.

Patrick

 

[JoeTank]

This sounds like a high density stock tank. I have never seen one with a thickness in excess of 50mm. API-650 is the most used code, but you need a storage tank engineer to sort thru the non-code aspects. Any experienced tank engineer can provide the design requirements. Since the high thicknesses are typically due to compressive stresses, you should be able to solve the excess thickness issues fairly easily.

Joe Tank

 

[mancini]

Hi,

Thanks by the posts,

This is a low density tank (1,1), in fact the problem which we have faced is the total load at lower shell (added to moment from wind load), these loads (mainly due to wind moment and stress concentrations) have increased the thickness at lower shell, so with the liquid column + wind loads and using the ASME Sec VIII Div 1 & 2 + FEM to the definition of the lower shell thickness, we faced with thickness more than 50 mm at the lower shell. In fact with this thickness of 50 mm, this will demand heat treatment if we consider the ASME.
Regarding the API 650 we also understand that it should not be used for the lower shell and cone transition, this is the reason to use ASME and FEM, so now we have two "problems":
1) Increased thickness (if we compare only with the thickness calculation with the "pressure" from liquid column)
2) With the thickness 50 mm, from ASME it is necessary to have the PWHT.

By the way, as this equipment has considerable dimensions (is not a pressure vessel), do you have seen a similar thickness and no PWHT? Do you have any suggestion about some Welding Procedure to avoid PWHT? Any other recommendation?

 

[JoeTank]

What is govering the thickness? Longitudinal Compression? Local bending stresses at the intersection of the lower shell and cone? I've designed about a dozen stock tanks per API-650 and never had to resort to FEM. The rules of API-620 are used to supplement missing info in API-650 for such tanks.

Joe Tank

 

[dig1]

Hi Mancini,

I've seen these handled several ways in the past but most projects I have worked on in the past 8-10 years have been exclusively API 620 except for recent cases which I will discuss below. Before that time, we designed the upper cylinder to API 650 and the cone and dilution zone to API 620. I also worked on one where the customer asked that it be designed to ASME Section VIII, Div. 1. I have also seen older ones (prior to early 90s) designed to API 650 only.

You are right that the lower portion carries the pulp above the dilution zone which is in the annular space outside of the 7 m diameter bottom plus the wind/seismic loads. This can result in some fairly high compressive loads. One project I worked on had a bottom thickness of 100 mm due to very high seismic loads. The sections over 38 mm were PWHT (this tank was of similar dimensions to the one you have).

I agree with JoeTank that thicknesses greater than 50 mm is not normal for these tanks but given the dimensions I am not surprised based on what I have seen. This must be close to 10000 m^3 gross capacity (the cone height isn't given to I can't calculate for sure)

If you decide on API 620, look at the requirements of API 620 section 5.25 for PWHT. If you use ASME then those PWHT rules apply.

Depending on where you are located (basically for engineering abilities and material availability)and where the tank is being installed (if any local regulations apply) there may be another option. I know of a tank manufacturer in Europe that uses basically an EN 13445 analysis for combined stresses and a higher strength steel (P355NH). This analysis allows the higher yield strength to be used for compression. There are more stringent fabrication tolerances and NDT for this but it definitely save money.

In these types of applications (6000 m^3 to 10000 m^3), there is often a dramatic reduction in the required amount of steel compared to API 620 w/ A516 materials. I have designed a few tanks in this manner and for the larger sizes there really is a significant reduction in weight.

In any event, I again agree with JoeTank that you need to find someone experienced with this type of tank design to review this with you.

I hope this helps somewhat.

BR,

Patrick

 

[mancini]

Hello Joe and Patrik,

For additional information the stress which is governing the thickness at the lower shell is the compressive stress. As you had mention, the big differences are probably because we have used ASME Code, so now we will try the API 620.

Regarding the other options to make the calculation, we have also seen some manufacturers which have used the European code EN 13445 and shell material P355NH EN10028-3, but we also believe that this will impact on other problems of: availability of base materials, WPS qualifications, inspections procedures qualifications, inspectors qualifications, other controls,... so may be it is not so easy to follow all the rules from Europe.

Your posts were very useful for us. As you suggest, after the calculations ready we send them to an experienced tank reviewer.

Best regards,

Mancini

 

[JoeTank]

Just an FYI... API-650 permits shell plates up to 1.75 inches thick without pwht of the shell welds.

Joe Tank