Design standard for flues and ducts? 2

[MrStohler]

I would appreciate it if you could help a structural engineer looking for some background information.

What are the design standards that govern the design of flues and ducts that connect to boilers? Particularly for the power generation industry (primary air ducts, forced draft duct, induced draft ducts, secondary draft ducts, economizer outlet flues etc.) I am not seeking design information about boilers unless these components are considered to be part of the boiler. Am I correct in assuming that they are not part of the boiler?

Are minimum (and maximum) design pressures set by any particular standard or code? Or must pressures by calculated by CFD?

Are the stresses in the plates and stiffeners governed by AISC or is there an alternate standard for these components?

Any insight or references you might be able to call out would be appreciated.

 

[Jonkatz]

MrStrohler,
In most cases I am familiar with, the air/flue gas ducts you are interested in are within the boiler vendor's scope up to some agreed connection point. For example, the economizer outlet may be the cut-off and then the stack vendor may take over - or the EPC company.
So ducts are usually part of the boiler, but not a "Pressure Part" (like drums, tubes, etc.)
In my recollection, the BPV does not dictate the ductwork design specifically, (but I could be wrong about that). Client specification may govern, or manufacturer's standards otherwise. Design pressures will usually be dictated by the boiler and burner design. For example,
the primary air pressure will be dependent on the burner pressure requirement (maybe 25-30" WC for example) and flue gas ducts may have near ambient pressure behind the Induced draft fan and slightly positive pressure downstream.

Hope this helps.
JK

 

[twjag]

Is your inquiry for a power boiler, or for a heat recovery steam generator?

 

[MrStohler]

I am inquiring about flues and ducts connected to coal fired boilers (usually, although some are natural gas) the are used to provide steam to turbines in the range of 100MW to 650MW.

I understand that when the Unit is initially designed the boiler supplier, ESP supplier, or stack supplier might be responsible for the design of portions of the flues and ducts. But when modifications are made (an SCR for example) is there one or more codes or standards that set the design requirements for the flues and ducts. I am most interested in any code or standard that sets allowable stresses (or strength requirements) for the plates and stiffeners (AISC? but AISC does not appear to address high temperature conditions). But I am also interested in pressure and temperature requirements.

Thanks

 

[Jonkatz]

MrStrohler,
Now I think I understand your question better.
Allowable stresses for all boiler materials are tabulated
in the BPV -section 2 (materials) I beleive. The stress tables in the BPV utilize ASTM designations, so if you have a steel plate material, you can look up the allowable stress at the design temperature. The design temperature will obviously vary depending on location, insulation thickness, etc.
Hope this helps.
JK

 

[twjag]

The flues & ducts external to the boiler, on both inlet & flue gas sides are beyond the scope of ASME Section I. The design pressures & temperatures for these flues & ducts are as specified by the boiler manufacturer. For inlet ducts on forced draft (FD) boilers, the minimum design pressure will typically be equal to the static pressure rating of the FD fan. Good practice is to design using ASME allowable stresses. Do not use AISC allowables. Large ducting in normally internally braced & externally stiffened.

 

[MrStohler]

"Steam, Its Generation and Use, 40th Ed. p.7-16" by Babcock & Wilcox indicates that buckstays for boilers (not exactly the same components I am interested in but similar) should be designed in accordance with the latest AISC ASD specification BUT modified for use at elevated temperatures and using safety factors consistent with ASME Code Sections I and VIII.

Do these sections of ASME BPV provide safety factors that are applicable to "structural" components of flues and ducts?

Jonkatz: Thanks for your feedback. ASME BPV Sect II D contains appendices which contain criteria for establishing allowable stress, the bases for establishing external pressure charts, and information required for approval of new materials. Is this section applicable to "structural" components of flues and ducts?

Thanks for everyone's feedback. I am trying to narrow down the number of code section I need to buy.

 

[pipewelder1999]

I worked for about 1-1/2 years as an inspector for a power company that was having SCR's built for coal burning power plants from 600-1300 MW. I became very familar with the project specifications and do not recall any reference made to and codes/specifications other than AWS D1.1 and AISC. I worked fairly close with the engineering department since they were not here to see the fabrication and I may be able to get in touch with one of them an ask. Clean air recently became less valuable I think.


You are correct that the ductwork is not part of the boiler proper. There is an ASME Standard for stacks and there may be one for related ductwork but I haven't seen it referenced for rectangular ductwork. Gerald Austin
Iuka, Mississippi

http://www.weldinginspectionsvcs.com

 

[DSB123]

MrStohler,
I would suggest you get hold of a copy of "The Structural Design of Air and Gas Ducts for Power Stations and Industrial Boiler Applications" by the Air and Gas Duct Structural Design Committee of the Energy Division of the American Society of Civil Engineers (ISBN No 0-7844-0112-8). This is an American Society of Civil Engineers Special Publication and I find it to be invaluable reference for Ductwork design. The Chapters are :-
1. Introduction
2. Ductwork Arrangement and Behaviour
3. Structural Material - Selection, Applications and Properties
4. Service Conditions and Design Loads
5. Loading Combinations and Associated Design Stresses
6. Plate Design and Stiffener Location Considerations
7. Ductwork Global Structural Analysis
8. Structural Element Behaviour
9. Structural Design of Flow Distribution Devices
10. Drawing, Fabrication and Construction Techniques and Considerations
11. Insulation and Lagging
12. Maintenance Examination of Existing Duct Systems


Hope this Helps

DSB123

 

[MrStohler]

DSB123

Thanks for the reference. I ordered the book today.