Time dependant and time independent properties as per ASME

[coolniks37]

Could anyone explain these terms and their significance in ASME Boiler Section I construction?
Also, does ASME Section I requires a fatigue analysis?
At what conditions, a fatigue analysis is required?

 

[DSB123]

Time dependant properties are associated with creep properties.
Do not have ASME I so you will need to read ASME I and there will be a section regarding whether fatigue assessment is required or not or it may refer to ASME VIII Div 2 wherein there is a Screening procedure to determine if fatigue is necessary or not.

 

[metengr]

Fatigue analysis is not required for Section I. Section I is a design by rule code book and does leave the door open for design by analysis, looking at creep fatigue, mostly.

 

[coolniks37]

What are the real conditions at which fatigue occurs in boilers? When should I say that fatigue exists in a boiler? Any standard definition for this? BS EN 12952 says, if boiler is designed for more than 500 cold starts, then it needs to be examined for fatigue? Does ASME Sec 8 says something similar?

 

[TGS4]

The real fatigue issue in boilers is, as metengr stated, related to creep-fatigue interaction. What rise to problems is usually a rapid heat-up or cool-down which will initiate high stresses. These stresses will relax with creep, but have significantly accelerated creep damage because the thermal stresses can be much higher than the pressure stresses. If the thermal stresses exceed yield, as is often the case, the cumulative creep life of components can be substantially reduced. The primary driver for this is how hard you drive your heat-up and cool-down. Slow is much better than fast in this regard.

If the new boiler is for electrical power generation, I would assume frequent starts and stops to adjust for demand and variable generation (think wind and solar) on the order of once per day. If the boiler is for some other continuous service, it may not be an issue

 

[dineshakabari]

Hi TGS4, In line with your explanation of start and stops. Does it make any difference if starting/stopping of boiler is at slower or too slower speed? I mean if speed is made slower so is it possible to extend fatigue life of the equipment? I request to share if you have some definition of the speed/value.

 

[TGS4]

If the start-up and shut-down speed is slow, then you can avoid creep-fatigue interaction. The papers that I have seen (you'll have to do your own searching for "power boiler creep-fatigue") show a step-change type of behaviour: once the start-up speed is "too" fast, the behaviour switches to creep-fatigue and the life can drop from 100+ fatigue years to 2-4 fatigue years.

 

[racookpe1978]

The problem is not so as "deciding" to implement creep-control into the design by "requiring" heat up and cool-down rates be controlled and slow and steady and not-interupted and infrequent be continuously implemented ..

But, you see, once those very reasonable and practical "operational" requirements are sewn up inside the design, you find that the boiler HAS TO startup immediately from a below-freezing 20 degree F day, or that the controlled heatup suddenly got aborted due a motor failure or controller error or site blackout or regional storm or fire damage ... Or the "controlled" cooldown that was required suddenly became a uncontrolled abort because of a burst pipe or dust explosion or cooling water interuption or lube oil fire.

You need to plan for a gradual failure or a series of failures because they will happen sooner or later. Allow for some unexpected event. (The events you do plan for will, of course, not happen. )