Buckling design of Stack
Today I want to talk about the Buckling calculation of Stacks. In this discussion, two methods outlined in Annex H of API 560 will be compared and the difference of two major calculating approaches, Allowable-Stress and Limit-stress approach will be discussed.
As per Annex H of API 560, for the detailed design of stacks, two methods are proposed. The first is the API method, which is based on an allowable-stress approach for stability and vulnerability to wind-induced vibration and is determined by limiting the stack’s critical wind velocity within a specified range.
The second method is the ISO method, which is based on the limit-state principles from EN 1991 (Eurocode 1) and EN 1993 (Eurocode 3) and the CICIND model code for steel chimneys. It is also analogous to the method given in ASME STS-1. Stability is based on the critical buckling strength and susceptibility to wind-induced vibration.
Before we delve into the specifics, it's important to distinguish between the two fundamental design approaches:
1. Allowable-Stress Approach:
In Mechanical engineering design such as the design of pressure vessels, this method uses the actual loads predicted to be experienced in practice to calculate Stress and deflection. Such Stresses will be compared with Allowable values that are factored with Safety Factor, against failure mechanisms such as Plastic failure, Yield, Buckling, Fatigue, Leakage, and Brittle Fracture. However, the predicted Stresses almost always assume that the material is linear Elastic.
In structural engineering, the Permissible Stress design approach has generally been replaced by Limit State design, except for some isolated cases. ?Please refer to Eurocodes and ANSI/AISC 360 (Specification for Structural Steel Buildings) for more information.
2. Limit-State Principle:
Limit-state design is used in structural engineering. As per EN 1990, Limit states means the states beyond which the structure no longer fulfills the relevant design criteria. Limit-State design requires the structure to satisfy two principle criteria: Ultimate Limit States (ULS) and the Serviceability Limit State (SLS).
In Europe, the limit-state design is enforced by the Eurocodes while the United States has been particularly slow to adopt the Limit-State design. The method can be found in ANSI/AISC 360 which is the Specification for Structural Steel Buildings. ?
In addition, As per Clause 6.4.1 of EN 1990, the following Ultimate Limit States (ULS) shall be verified:
a. EQU: Loss of Static Equilibrium of the structure
b. STR: Internal failure or excess deformation of the structure
c. GEO: Failure or excessive deformation of the ground (Soil or Rock)
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d. FAT: Fatigue Failure of the structure
Now, let's explore the methods outlined in API 560's Annex H:
1. Stability of Steel Shell (API Allowable-stress Method) as per section H.2:
This method is based on an allowable-stress approach which has been discussed above. The maximum longitudinal Stress in the stack shall not exceed the smaller of the results of Equation (H.1) and Equation (H.2). Fy in the below equation is the minimum Yield Strength. This method is quite easy and most of the time is used to calculate the Buckling of Stack.
2. Stability of the Steel Shell (ISO Limit-state Method) as per section H.3:
This method is based on the limit-state principles from EN 1991 (Eurocode 1) and EN 1993 (Eurocode 3) which have been discussed previously. The proof of the stability of the shell is provided by satisfying Equation (H.3). The ultimate limit value in this method is the design buckling stress factored with the partial factor for resistance of the shell to stability.
The calculation of Stack Buckling necessitates consideration of factors ranging from material properties (Modulus of Elasticity and Yield Strength) to geometric configurations (Diameter and thickness), with methodologies like the API and ISO approaches providing essential frameworks for ensuring structural reliability and safety.
In conclusion, both methods are integral to ensuring the structural integrity and stability of stacks, each offering unique perspectives rooted in established engineering principles. Have you ever used these methods? Which one do you prefer? Let us know in the comments.
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Lead Static / Package Equipment Engineer
9 个月Hi Do you have any report about buckling of stack? There are difference diameters of one specific stack due to flow analysis. In this case we consider the conical sections. What is your idea about the buckling design based on API 560? Can we model correctly with two or three difference diameter? Thanks
CEO at FurnicoHeat - High Temperature & Fired Equipment | Pollution Control Services
10 个月Quite informative. Thank you! Never seen these two methods give different results in assaluye conditions up to 51 m of stack height. In other words their results are so close that the choice of plate thickness is not altered as per my experience. I normally trust structural engineers with high rise structures, but also would prefer to check for ASME/API criteria for heights above 50 m due to effects of wind, although a personal preference and no code mandates it. All in all, the final result depends on who designs the stack! Structural engineers use ISO & mechanical engineers mainly use STS-1 normally due to software availability. I check both in high elevations.
Head of Mechanical Department in Dorriz Company
10 个月Fired Heater Woman ??????