"HK-40, heater tube"

Question:

"If we have data of a equipment ( material HK-40) : stress and design temperature, by using figure 4s ( page 48-49) can we use this formula to determine design life of this equipment (Ld)? Could I use that Ld to estimate remaining Life of Equipment? Could I apply that formula to estimate remaining life of Heater support, because in API 560 there is no formula to estimate remaining Life. If we couldn't, what is the correct formula to calculate remaining life of tube support?"

Answer:

Yes you can. The Larson Miller parameter is a parametric formula for plotting time and temperature variables against stress. API 530 has already performed the calculations and presented the data in the form of a graph of minimum and average rupture life. If you know two variables, you can determine the third one. Only if you know the stress history of the component accuractely, and most people don't. If you do, then you can use the Linear Damage Rule for estimating remaining life. Theoretically, if you know the metal temperature and stress accurately, you can use the curves to estimate life, if the component is operating in the creep range. In reality, as S-R life is very sensitive to temperature, most people do not know the stress/metal temperature sufficiently well to make an accurate prediction of life. Also, creep properties vary widely, as suggested by the spread between the average and minimum rupture curves. This material property uncertaintly adds and additional variable to accurate life prediction.

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