ASME I boiler: Life expectance

[PressParts]

First: Hello to all
I´m a new member in this forum.
I would like to know if there are any reference to boiler life expectance under ASME I CODE.

Thanks in advance

 

[metengr]

PressParts;
The answer is no, not directly mentioned. However, to answer your question a minimum of ~ 20 years (100,000 hrs). This is not mentioned in words but is really derived from the basis for establishing allowable stress values in ASME Section II, Part D, Appendix 1 to afford the designer proper selection of materials.

The key for establishing long design life of pressure-containing components (boiler or pressure vessel) is consideration of time dependent damage mechanisms in service. For most applications these damage mechanisms are fatigue, creep, combination of each, or corrosion.

Since ASME Section I design does not deal with cycling of boilers or boiler components directly, the only real time dependent damage mechanism that limits useful life of components is creep (corrosion is lumped under this mechanism because thinning results in wall loss which can increase susceptibility to creep failure in service).

So, when you review the basis for determination of minimum allowable stress values for use of materials, one of the time dependent parameters that is evaluated is creep deformation over 100,000 hrs at service temperature.

 

[davefitz]

ASME sect I specifically addressed this question about 10 years ago, and I believe they revised the preamble to state that section I offers NO IMPLIED LIFE Warranty- it is up to the combined efforts of the designer and operator to deal with issues of erosion, corrosion , fatigue , etc.

As a practical matter, the boiler designer needs to ensure the unit lasts as least as long as it takes to recieve the final payment or past the extended warranty period, which may be as long as 3 years. The next 27+ yrs is up to the operator.

It as often been the source of endless amusement to watch as the boiler OEM will blame nearly any boiler failure on operator error or water quality.The response is as automatic as is the common response to a street beggar asking for money.

 

[InTheField]

Most boiler OEMs design for an average life of 250,000 hrs for north america- however as mentioned, this is not mentioned in the code. Outside of north america boiler life is usually part of the bid specifications.

 

[davefitz]

InTheField,

Yes, as Meteng stated, if the only damage mechanism is creep AND the actual in situ metal temperatures of the heated parts meet the OEM predictions, then the use of ASME I allowable stresses will yield a life greater than 250K hrs.

Unfortunately, there are other damage mechanisms at work. Some very much shorter life boiler events I have witnessed included:

3 MSW fired boilers had catastrophic corrosion failures of the furnace waterwall due corrosion ( lead related fluxes from batteries and paint in the MSW )within 1 year of initial operation. In the 1980's there were at least 10 such sets of rapid failur of stoker fired MSW boilers.Typical solution is to plate waterwall with inlconel weld overlay, and use a lower pressure steam cycle so that the waterwall metal temp is below 575 F.

2 MSW boilers had widespread failures of superheater tubes due to hi temp choride attack- the sootblowers would entrain ash that removed the protective oxide scale from the superheater tubes and the unprotected surface layer quickly eroded in the hi choride atmosphere. Typical solution is to replace sootblowers with rapper hammers.

3 combined cycle plants had near-catastrophic cracking of the P91 main steam pipe to 1.25 Cr 1 Mo V turbine stop valve within 1 yr of service - incorrect weld geometry combined with hi pipeline loading and hi thermal stress during very fast startups. While the code appears to deal with the weld geometry ( and the designers disagree with this), the ASME code does not adequately yet deal with the lack of creep relief of shakedown stresses when using P91 piping.

Various fluid bed boilers with rapid erosion failures of furnace waterwall tubes assocaited with high velocity erosive bed solids.

Various accelerated creep failures of heated tubes ( usually ferritic reheater tubes) due to incorrect prediction of operating metal temperatures.