API 687: Chapter 0, Section 8 thru Section 9: Guidelines for Disassembly and a bonus! The Eddy Probe
- Fernando E. Romero, P.E.
- Jul 19, 2024
- 9 min read
Remember that Chapter 0 will issue recommendations that apply to complete machines (case + rotor).
Section 8: Receiving
This short section highlights a couple of important things.
The first has to do with safety, and it is that the repair partner should understand and plan all lifts and ensure that all equipment used for lifting is certified.
This is important because lifting operations, whether they involve cranes, hoists, forklifts, or other lifting equipment, are considered high-risk activities due to the potential hazards and serious consequences associated with them.
There are several regional and global organizations, including OSHA (USA), ANSI/ASME (USA), ISO (International), LEEA (International), BSI (UK), and CEN (Europe) that have published standards that regulate the safety of industrial lifts.
I’ll expand a bit on both OSHA and ANSI since those are the ones I have experience with.
If you have never done so before, I encourage you to explore the world of OSHA publications.
All OSHA standards are in the public domain, published by the U.S. Department of Labor.
Reading them can be a bit exhausting, but, at the same time, admirable for each individual sentence is preceded by “section numbers” or “paragraph numbers”.
OSHA docs are a good example of the application of hierarchical numbering systems.
OSHA will reference in its content several ANSI/ASME B30 documents which must be purchased.
Within the ANSI website, there is a page that provides a great introduction and history of these documents and can be found following this link:
The next important take-away from Section 8, I summarize as: Identify, Verify, Segregate, and Store.
Basically, the call is to apply good housekeeping to ensure no part gets lost or misplaced.
One more point is the emphasis on taking pictures. And these days when everyone has a great camera in his pocket, it is highly encouraged to take as many pictures as possible every step of the way during the inspection and repair process.
Just make sure they are properly tagged, so they can be properly segregated and stored.
To give you a real-life reference, where I work, we will accumulate approximately 650 photos through the lifespan of a single inspection/repair project.
Section 9: Disassembly
This section also prudently offers a safety related warning, and it is prior to disassembling that one must ensure there are no substances trapped inside the equipment. On a compressor this is important since some of the working fluids can be hazardous, and it may not be possible to ensure a case is completely clean and free of said fluids when it is shipped to a shop. The best way to mitigate any exposure risks is to get a hold of SDS sheets and be prepared to apply the “hierarchy of controls”.
If you want to learn more about the hierarchy of controls, follow this link to this cool OSHA published 5-page pamphlet.
After ensuring cleanliness, the next steps are to systematically take the equipment apart and tag and “match-mark” everything as you go.
Basically, deconstruct everything carefully so you can build it together later.
Steam turbines and compressors ship with operations and maintenance manuals, which have cross sectional drawings, bills of materials, some exploded assembly drawings, but no step-by-step assembly instructions.
Maybe OEMs (or maybe not) have step-by-step assembly manuals like you would expect in a LEGO set, but independent service providers do not have that privilege. One must then rely on having good mechanics and a staff that is mechanically- and engineering-minded, understanding how things come apart or are put together, and which tools and processes to use.
Chapter 0, Section 9.2.2: Prior to Disassembly
In section 9.2.2, there are three statements that begin with “For complete machines…” that I think are great ideas to be expanded on.
The first statement says…verify the equipment rotates freely, if applicable. More specifically, the intent is to verify that the shaft or rotor rotates freely.If a rotor can spin freely inside a case, it means there are no internal conditions that would obstruct its movement. It means the rotor is not heavily bowed or rubbing, it means there are no broken pieces or no “foreign” materials that would get in the way of allowing the rotor to spin. If you had a forced outage and your machine came to a sudden and abrupt stop, it cannot be re-started, and it does not spin when uncoupled, that is a very strong sign there is something “major” going on inside. Now, what hides behind the “if applicable”:
The next consideration is, “what type of shutdown was it?”. I suggest you attempt to check that the shaft rotates freely. Determine whether the shutdown was carried out under normal circumstances, meaning the machine was properly shut down and there were no abnormal behaviors observed. Determine whether the machinery protection or the operators triggered an emergency trip because the machine was involved in an incident such as high vibration, some surge event or over speed event, or suffered lube oil system failure. The reason is to preserve the evidence, the same way you would want to preserve a crime scene. If there was any indication of a possible failure it is best to preserve it intact until the equipment can be deconstructed in a controlled environment.
But how hard must you try? If you need to use tools or a crane, like having to wrap a strap around the coupling and using a crane to pull as if you were trying to start a gigantic lawn mower, be extremely careful. Once again, think safety. You do not want to overstress a sling or rigging; you don’t want to risk entanglement or cause any collateral damage by pulling in the wrong direction or getting any parts in a bind. If you must use a giant pipe wrench or strap wrench, you may bruise or scar the shaft.
The next statement says…record the coupling flange radial runout.Now remember, this is supposed to apply to complete machines that are still assembled. And once again, it depends on how clean of a shutdown it was. Recording runouts on worn out bearings may not be ideal. And consider, the rotor or shaft will undergo a thorough inspection once it is removed and supported by v-blocks. Those readings will be much more accurate and reliable.
The last “for complete machines” statement suggests performing two checks:
a “bearing lift check”, by placing indicators positioned on an exposed area of the shaft and lifting the shaft end to measure how much shaft vertical movement can be accomplished. Not to be considered a measure of bearing clearance, since this will depend on the bearing design and condition; but an indication that the shaft is free to move.
a “float check”. This is where you place an indicator on the end of the shaft and measure the total amount of axial movement. Once again, not necessarily an accurate indication of actual thrust bearing clearance. An indication the shaft is unconstrained.
Once more, if tools are required to lift or push, be extremely careful when using cranes or hydraulic jacks. Either one can pull or deliver much more force than required.
Chapter 0, Section 9.2.3: Disassembly
This section contains seven short paragraphs describing general considerations.
It starts by, once again, emphasizing taking as many pictures as possible. Pictures of everything. General shots and close-ups of details and features. It is important that the pictures are clear, in focus, and that if you take closeups there is a clear reference point, so you know which way is up. In other words, the general orientation of the subject of the picture is clear.
I’ve looked at job pictures many times and cannot tell which shaft end is pictured, which seal area, which stage. So, pictures are imperative, as long as you know exactly what they are supposed to be of.
Another recommendation is to take samples of residues or deposits. Remember, treat the equipment as if it were a crime scene, especially if there is suggestion of a failure. Those deposits could have clues about the root cause or failure mechanism.
The last notable recommendation is to match-mark components as the machine comes apart. This means to physically mark, engrave, or leave a durable sign that shows how parts are oriented or fit together. Many times, parts look very similar to each other or can fit in multiple orientations. More than once, I have seen issues when seals have been installed backwards.
Basically, bag and tag anything suspicious, any broken bits and pieces, any loose material, and save them for the equipment owner.
Nothing should be discarded without the consent of the owner.
BONUS: The Eddy Probe
Let’s dive deep into this, because it is incredibly interesting and important, and the 687 really does not explain the magic of eddy-current sensors or probes.
Let’s start with a little background.
To monitor the condition of rotating equipment, an owner or operator will rely on a dashboard of information, the same way we would monitor the condition of our vehicle while we drive it.
We may have temperature sensors, a tachometer, a speed display, and lights that turn on to communicate a warning or an alarm.
A sensor is a device that responds to some sort of input from a physical environment and converts that response or measurement to a signal that can then be transmitted to a display. Most modern sensors used in industry convert their measurements into electrical signals.
One of the most common conditions one must monitor when running turbines or compressors is the shaft vibrations. If things vibrate excessively, they will eventually break.
Remember that in turbomachinery, shafts are supported by hydrodynamic bearings.
Shafts essentially float on a very thin layer of oil and live inside bearings with clearances that are measured in thousandths of an inch.
So, the challenge was, how to detect extremely small movement of a shaft or rotor without touching it? The answer was to use the magic of magnetic fields and eddy currents.
At the tip of the eddy probe, there is a coil. A current flowing through that coil induces an electromagnetic field around that probe.
If that probe tip comes close to a conductive material, such as the shaft surface of a rotor, the probe’s electromagnetic field will produce eddy currents to flow on the shaft surface.
Those eddy currents on the shaft surface will induce their own electromagnetic fields, that in turn will alter the probe’s field.
Like a never-ending game of tag, each field will influence the other, and the intensity of that influence can be used as a measure of the distance between the probe and the shaft surface.
And for low allow steels like 4140, that influence, or variations, can be translated to linear measurements of distance over a range that is useful to detect shaft movements inside a bearing.
This technology and solution are very common now-a-days in rotating equipment, but this was not always the case. Before 1960 the world did not have a reliable solution; the eddy probe was not commercially available.
I had the great luxury of working for and with Bob Eisenmann, Sr., who not only wrote THE book on machinery diagnosis but worked with THE Mr. Don Bently when the eddy probe technology was invented and implemented to better understand the rotordynamic behavior of rotors and shafts as they operate inside machines. Mr. Bently founded a company that essentially has become a household name for machinery protection systems and condition monitoring.
Working with Bob was like drinking from a fire hydrant, he was an exceptional engineer and teacher.
So much so that people often refer to eddy current probes as “Bently probes” or “proximitors” a term coined by Mr. Bently himself. Of course, there are other brands, but the ones that are most widely used in industry are the ones made first by Mr. Bently.
Now, to set record clear, the “proximitor” is not the “probe”. A Transducer System, has three main components.
A “proximity probe” or “probe”, the thing where the coil is housed and has to be near the target surface.
An extension cable. (not shown)
A “proximitor sensor” or “proximitor”, a little metal blue box that connects to the probe and receives the power and outputs the measurement signal.
Inside this impenetrable little proximitor box, the geniuses from Bently Nevada hid the magic that makes this system work. And so zealous they were to hide their secret, the whole box is filled with epoxy.
Driven by my own fervent desire to learn how things work, I’ve destroyed a few of these boxes trying to see what is inside of them.
We will dive deeper into the usefulness of systems on later posts.
So, eddy probes can measure very small changes in distance. Essentially it is like using a microscope. Any imperfection or variation on the shaft surface will be detected; therefore, the shaft surface needs to be extremely smooth, and the way to accomplish this is by burnishing.
Burnishing is different than grinding or polishing because it does not remove material. Grinding and polishing use an abrasive material to remove material from a surface. Instead, burnishing is accomplished by pressing down very hard and smoothing the surface without removal of material.
It is the same way you would take a spatula to smooth icing on a cake. You would press and smooth down the surface until it shines like a mirror.
Another consideration to keep in mind is that since eddy probes work with magnetic fields, the shafts should not be magnetized. A service provider must measure the residual magnetism in a shaft or rotor and demagnetize it to ensure it is at a level below 2 Gauss. Otherwise, one could get false readings of displacement.
The conclusion so far is that probe areas need to be extremely smooth and, in order to preserve them, they often get wrapped in protective tape.
The danger is that every human being is born with a desire to never want to unwrap or unravel tape but to cut it. Therefore, the need for the “DO NOT CUT” warning must be applied to all shafts.
This warning may seem silly or unnecessary but consider that a rotor will be shipped across borders, handled by logistics companies, and may be “inspected” at a Customs checkpoint or unwrapped to be displayed at a trade show or something like that.
And if the warning is not understood, someone is likely to cut right through that tape and destroy not just the probe areas but damage journal or seal areas as well.
And I say this not as a hypothetical warning, but as something that in a 20-year career I have witnessed plenty of times.
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