Definition of Knockdown Factor (KDF)

[Jason8zhu]

I believe many of you guys has seen this term Knockdown Factor. I met this reduction factor in case of buckling of plates, corrosion.
Can anybody give me some guidance on the actual meaning of Knockdown Factor (KDF)? Or any book/doc I can refer to?

Thanks.

 

[Crannog]

Never used one but some quick googling shows:

http://scholar.lib.vt.edu/theses/available/etd-101198-161441/unrestricted/ch1.pdf

http://www.sciencedirect.com/science/article/pii/S002076831000051X

It sounds like an empirical method for reducing 'best-case' calculated values to match experimental/observed values.

-Drew

 

[ahmadhasan]

The term knock-down factor is commonly used to describe the reduction in fatigue life in a corrosive environment (e.g. sour service) compared to performance in air. However, the mere concept of such a reduction factor is potentially misleading, particularly when comparing different welding procedures that demonstrate different in-air performance.

To demonstrate the performance of girth welds in a corrosive environment, strip fatigue tests are conducted in air and in a simulated service environment, to determine an appropriate knock-down factor, which is then applied to the base design curve. However, there are a number of ways that such knock-down factors can be calculated, with different degrees of conservatism. For example, two different welding procedures may exhibit a different fatigue performance in air, but a similar performance when tested in a sour environment. The better performing weld (in air) is therefore assigned a greater knock-down factor, and possibly a more stringent sour design curve. In other instances, fatigue performance in air may significantly exceed that required. The determined knock-down factor, between strip tests in air and in a sour environment, can then be very large. Applying this reduction factor to the design curve results in a very stringent sour design curve, and may penalize the use of a girth welding procedure that results in good in-air fatigue performance.

There are no explicit, published guidelines for calculating corrosion fatigue knock-down factors.[/tt]

Ahmad Hajeer, PMP, MBA