Fitness for Service (FFS) Assessment

The failing old narrow-gauge railway
The failing old narrow-gauge railway

What is Fitness for Service (FFS) and why is it Important?

Component Materials will come under increasing degradation during service caused by a range of factors.

FFS is important in engineering installation as it can be important in demonstrating the integrity of these engineering components during their service life thus ensuring their continued safe and reliable operation. Non-Destructive Examination (NDE) can be a quick way to provide information regarding the presence and development of crack-like-flaws. By using a FSS assessment, we can judge as to the severity of such flaws and their consequence on component integrity.

These factors could be

  • The Material of construction
  • Operating temperatures
  • Operating environments
  • Primary or secondary stresses

These can lead to degradation through various mechanisms such as

Metallurgical degradation –

This is seen in Graphitisation where we see for example a microstructural change in carbon steels when operating for extended periods (400 to 600OC). in these situations, we see decomposition of carbide phases leading to graphite nodules.

We can also see Temper embrittlement in in low alloy steels operating at certain temperatures. Sigma phases can also develop in post weld het treatments in stainless steels that lead to loss in fracture toughness. Radiation can also lead to degradation where neutron irradiation is seen to cause matrix damage as well as changes in local Chemistry to stainless and ferretic steels.

Mechanical degradation

Is seen in fatigue situations that could be caused by thermal or mechanical cyclic conditions leading to incremental crack growth. Corrosion Fatigue can also lead to crack growth enhanced by corrosion processes. On the other hand, creep cavitation and rupture can cause formation of inter-granular cavities and subsequent cracks in materials under creep conditions. Other mechanical degradations are brittle fracture and ductile tearing. In the former, we can see rapid fracture in under stress with limited plasticity while in the latter, progressive fracture as a result if development of interlinking voids and imperfections generated by plastic flow.

Environmental/electrochemical degradation

This is perhaps the most widely understood cause of degradation in which we see this in Galvanic corrosion that occurs at junctions of dissimilar metals, Atmospheric corrosion that occurs in contaminated air, stress corrosion – cracks are initiated at the surface under a combination of stress, temperature and chemical environment.

Common Standards:

  • BS7910:2005, “Guide to Methods for assessing the acceptability of flaws in metallic structures” – British Standards Institution, 2006
  • R6 Revision 4, “Assessment of the integrity of structures containing defects”, British Energy Generation Ltd, 2006
  • API 579, “Fitness for Service”, American Petroleum Institute Recommended Practice, 2000