API 687: Chapter 0, Section 11 Inspections Methods and Testing
- Fernando E. Romero, P.E.
- Aug 29, 2024
- 6 min read
The word method is an awesome word. It has Greek origins, composed of two terms, meta: “in pursuit” and hodos: “a way”.
Method= “in pursuit of a way”. So, like we say about life, the method is the journey, not the destination.
This section is all about nondestructive testing (NDT), and just like the word method, this is an awesome section of the API RP 687. It’s direct, concise, and to the point.
This section is extremely valuable because it recommends and establishes references for:
Guidelines for personnel qualification for nondestructive testing
Which components should be nondestructively tested
Requirements for the main inspection methods used to assess rotating equipment
Radiography (RT)
Ultrasonic Inspections (UT)
Magnetic Particle Inspections (MT)
Liquid Penetrant Inspections (PT)
Acceptance criteria
Personnel qualification for nondestructive testing
The normative reference used by API for the requirements for qualifying personnel is: SNT-TC-1A. This is a document published by the American Society of Nondestructive Testing.
This and other ASNT relevant documents can be found through this link: https://asnt.org/MajorSiteSections/Standards
The SNT-TC-1A will cost you about $89 but is money well spent if you wish to understand the world of qualifying personnel that works in NDT.
The document includes an “Ethics Statement” which to me is impactful, because it shows that NDT and those who perform these evaluations have a great responsibility.
It states: The NDT professional has a responsibility to act ethically in all activities, including the proper collection, interpretation, and reporting of NDT data.
The same way doctors or trustworthy people should act. A dishonest or lazy NDT professional could let a flaw go by and that, then, is the initiation point of a failure.
The document then explains the levels of qualification for NDT professionals:
NDT Level I
This individual should have sufficient knowledge and be qualified to:
Properly perform specific NDT
Perform Specific Evaluations for the acceptance or rejection of determinations, according to written instructions, and record results.
The Level I should be supervised and instructed by a certified Level II or Level III.
NDT Level II
This individual should have sufficient knowledge and be qualified to:
Set up and standardize equipment.
Interpret and evaluate results with respect to applicable codes, standards, and specifications.
Organize and report the results of NDT tests.
Level II must be familiar with the scope and limitations of the methods.
Level IIs are usually assigned the responsibility to train Level Is.
NDT Level III
This individual should have sufficient knowledge and be qualified to:
Develop, qualify, and approve procedures and techniques.
Interpret codes, standards, specifications, and procedures.
Designate methods and techniques to be used.
A Level III should have sufficient practical background in:
Applicable materials
Fabrication
Product technology
Level IIIs also establish techniques and assist in establishing acceptance criteria for the inspections that need to be performed.
A Level III should be capable of training Level I, II, and III.
And that’s it. Those are the three levels. And the next time you read an NDT report, the name and the level of the inspector should be included in the documentation.
Also, the next time you are in a repair facility or an NDT facility, make some time to meet and get to know the NDT inspectors working on your stuff.
Which components should we test?
RP 687 states one should NDT all the components mentioned within the document. So that covers everything.
And in practice this is true. In a repair facility during Phase I, Phase II and repairs we will NDT all components, rotating and non-rotating alike.
Having said this, special attention should be paid to components that have undergone a repair process, especially welding.
Pressure boundary components that have been weld-repaired or machined.
In a steam turbine, those would be steam chests or casings that have been welded, internally or externally.
And cases that have been inserted or that were machined to cleanup erosion or re-establish fresh fits.
This is important because pressure casings are designed to withstand a specific design pressure, and welding, as we know, introduces heat and can alter the mechanical properties of materials.
In addition, machining removes material, which in turn modifies the pressure boundary, and, therefore, triggers the NDT of these components.
Rotating components that have been weld repaired.
Rotating components are highly stressed parts, and, once again, weld introduces heat which may alter the microstructure and heat treat of the base metals. Therefore, NDT is required to assess the condition of the parts post-repair.
Requirements for the main inspection methods used to assess rotating equipment
API RT 687 states that only techniques that provide quantitative results should be used.
This is a powerful statement because it assures that the tests will provide accuracy, reliability, an objective assessment, and adhere to an industry standard.
Quantitative techniques provide measurable AND repeatable data. Quantitative results eliminate subjective interpretations that can vary between inspectors and ensure that a consistent assessment is performed.
Below I will expand a bit on the four main techniques listed in 687.
Radiography (RT)
If you are using X-rays for inspection, all those inspections should conform to ASTM E94: Standard Guide for Radiographic Examination Using Industrial Radiographic Film.
I will be frank. I don’t know much about radiography. I think we are all familiar with this method; at the very least, if we’ve gone to the dentist for a cleanup, we’ve been shot in the jaw with x-rays so a doctor can glance at a black-and-white picture of our skeleton.
I do know that if you will be shooting X-Rays you have to have a Radiation Officer and ensure that a strict safety protocol is followed.
RT is more often used when manufacturing new cast components like cast cases, steam chests, or cast pressure vessels.
In a repair shop, you don’t see radiography applied every day. I would say that less than maybe 2% of equipment repair projects will require radiography.
Ultrasonic Inspections (UT)
The UT method is utilized very often. It works like sonar on a submarine, the sonar sports fishermen use to see fish underwater, or the ultrasound doctors use to check on babies inside their mothers’ tummies.
A technician won’t be looking for enemy submarines or fish; instead they are looking for interruptions or discontinuities in the metal, otherwise known as cracks or voids.
UT is often used to inspect parts:
Cases and Stationary Components
Cases
Diaphragms
Rotating Components
Blade tenons
Impellers
Disks
Inspecting raw material forgings:
Shafts
Blades
Impellers
Inspect welded components
UT can also be used for other useful things like:
Measuring the thickness of the casing walls
Detecting coatings and their thickness.
What is important to consider with UT is that there are some restrictions on when and where it is effective. The bottom line is that the geometry and proportions of the components being inspected via UT matter.
Imagine going to the mirror room at the county fair and flashing a flashlight. You will get reflections everywhere and will most definitely get confused.
To mitigate these challenges, astute NDT departments will manufacture calibrated blocks with “known indications” that simulate the different geometries or components that are often inspected to use during the training of inspectors or to ensure the equipment is working properly.
Magnetic Particle Inspection (MT)
This is my personal favorite NDT method because it uses electromagnetic fields and black lights!
We use this method on ALL ferro-magnetic parts. Ferro-magnetic is a just a fancy way to say magnetic.
Every self-proclaimed fan of NDT owns a copy of ASTM E 709: Standard Guide for Magnetic Particle Testing.
Every repair shop certainly should own this reference, since it is the standard to follow for the technique that must be used.
This document describes the method, and practices related to wet magnetic particle inspections, from part preparation and cleaning to the recommended intervals for maintenance and calibration of the equipment.
Liquid Penetrant Inspections (PT)
This is the last technique listed in the document.
It is performed in a very similar fashion to MT, where you spray a dye solution on the part, and the dye will wick into any crack.
The main difference is that the part being tested does not have to be magnetic.
PT is useful when inspecting Chrome Carbide or Tungsten Carbide coatings or parts made from Superalloys or Austenitic Chrome Nickel Type alloys like:
Superalloys:
A-286 - used often in hot gas expanders and high temperature applications
Discaloy - in hot gas expanders
Stellite - used to clad surfaces or installed in the leading edge of steam turbines
Nimonic - used in corrosive environments
Haynes 25, 188 - used in blading of hot gas expanders
Inconnel - used in hot temperature applications
And alloys that end in “oloy”
Austenitic Chrome Nickel and Manganese Type Alloys:
200 series steels
300 series steels
UT Inspections should all conform to ASTM E165: Standard Practice for Liquid Penetrant Testing for General Industry.
There are many other ASTM references listed in API RP 687 that have specific practices and criteria for the type of component being inspected. I’ve only listed the main ones that specify the general methodology requirements.
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