Wall loss piping Fitness For Service
  • In: Training | On: Sep 17, 2020

Are you interested in Fitness for Service assessments? Try this question to test your knowledge.

The Question

Note, there’s only one answer.

Fitness-for-Service assessments are carried out on pressure equipment or structural components that are corroded or damaged in some way. Looking at it from a code-related rather than practical viewpoint, what exactly is an integrity assessor trying to do during such an assessment?

 

a) To prove the item still meets its design code

b) To achieve a concession to the original design code

c) To re-certify it to another design code

d) To do an out-of-code assessment

Answer

We’d say the answer is (d)

A Fitness for Service (e.g. API 579) assessment is done primarily when a component is found to be non-compliant with its original design code i.e. it is out of code, or is likely to breach code requirements before the next ‘Event’. This ‘Event’ could be, but is not limited to;

 

  1. The next inspection
  2. The interval until an intervention can be made, such as a repair or some other form of remediation
  3. The period of operation until a replacement can be made

 

Now, the question above relates purely to the code related aspect of Fitness For Service. In our experience, nearly always, an API 579 assessment is completed when an area of degradation or damage has been found and is either not permitted by the original design code (i.e. cracking) or has exceeded the minimum requirements of the design code (i.e. the wall thickness is below tmin).

 

When an assessment is done like this, we would could this a REactive assessment. You are reacting to an issue which may have negative consequences. Nothing wrong with that at all. That’s what the intent of these Fitness for Service assessments are.

 

However, the methodologies in API 579 can be a potent tool to use in a PROactive way. Many inspection intervals across the world are established by the rate in which a material is corroding. (considering wall loss in this instance). One of the fundamental principles of the API in-service inspection codes is the ‘Half-Life’ concept. In effect, you would inspect the equipment at a period calculated by half the remaining life.

 

What is the remaining life I hear you ask… Simply put, the remaining life is the time period until the wall loss reaches code calculated minimum thickness based on a corrosion rate.  So then, if we can reduce the minimum code thickness required, we can, in effect, extend the life of the equipment and therefore increase the inspection intervals, therefore reducing inspection costs and increasing uptime. Phew!

 

But you can’t reduce design code minimum thickness… they are fixed equations. Well, that’s correct, but you can adjust the variables that are used in these equations. There are many ways to do this legitimately, and we teach you these in our highly popular API 579 Fitness for Service Practical Training course. No death by PowerPoint here, just useful real-life worked examples that you are likely to do in the real world. Differential calculus – no thanks…

 

Here is one way to do this (this one is on the house!)

Let’s think back to the basic Barlow minimum thickness calculation, where the minimum thickness is determined by t=(PD)/(2SE).

P – is the internal design pressure (usually)

D – is the Outside Diameter (for piping at least)

S – is the Allowable Stress

E – is the joint efficiency or quality factor

 

Taking a closer look at S, we know this to be the ‘Allowable Stress’ of the material. In other words, it’s a factor of safety incorporated into the design calculation to prevent unintentional yielding, or will still, plastic collapse. It provides some safety margin in case of over pressurisation or high-temperature excursions, for example.

 

This is usually taken from the relevant design standard and is calculated by using a fraction of the Specified Minimum Yield Strength (SMYS) (or proof strength in some cases) and the Specified Minimum Tensile strength (SMTS).

 

For this to make sense, we must clarify what the SMYS and SMTS mean.

 

 

Click Here to open a section of a very common piping material specification A106 (this is an old one); we can see that there is a value for SMYS and SMTS. This value is the minimum value that must be achieved during destructive tensile tests for the material to fulfil the requirements to be stamped as A106.

 

 

The grade represents the strength of the material.

 

Now we’ve established that is a MINIMUM requirement, in reality, what you will most likely find is that the steel has a higher actual strength than that required. So, why does all this matter anyway?

 

Remember; the allowable stress listed in the standards, such as ASME B31.3 is calculated using the SMYS/SMTS and then adjusted for temperature. Think of it like this; you are using a ‘specified minimum allowable stress’ although that terminology doesn’t exist. Therefore, if you can locate the original material certificate (easier said than done) and you are happy it’s genuine (unfortunately, many are not) then by using the original factors of safety and adjusting for temperature, you can use allowable stress based on the materials ACTUAL strength and not merely the minimum required to comply with the specification.

 

To complete the example, let’s see how this works.

 

From Para 302.2.2 ASME B31.3 2016 edition, we know that the allowable stress (referred to as design stress) for carbon steel shall not exceed the lesser of 2/3rds of yield or 1/3rd of ultimate tensile strength.

 

If we were to use table A-1 in B31.3 to obtain the allowable stress for A106 Grade B at 300 degrees F, the value that would be used in the minimum thickness calculation would be 20,000 psi (20ksi).

 

Let’s suppose we were fortunate enough to have the material certificate for the pipe which is subject to wall loss and we found that the tensile and yield strength established using the destructive tensile test was 64,000 psi and 38,500 psi respectively, we could calculate the actual allowable stress of the material by;

 

66000/3 = 22000 psi

38500/1.5 = 25666 psi

 

The lesser of these is 22000 psi which, providing there is no temperature correction required, becomes the new allowable stress for the thickness calculation. How would that affect our new required thickness? Download our example sheet here.

 

Want a further challenge? Try our FFP brain teaser https://www.linkedin.com/pulse/fitness-service-brain-teaser-paul-wilkinson/

 

Our classroom-based Fitness For Service Training Course using API 579 has been hugely popular with many of the large oil and gas operators and UK  refiners. The feedback has always been positive and better still. what they like most is that the content is aligned with real-world problems; it’s not all about scientific theory.

 

Can’t manage the classroom course? Not to worry, our Fitness For Service Training Course using API 579 eLearning course is launching on the 1st October 20202!