(AI does not how to spell! )

 Today we tackle two terms: Indications and Maximum Continuous Speed.

Indications

Non-destructive testing (NDT) is crucial in assessing rotating equipment due to its ability to detect defects without causing damage to the equipment.

Rotating equipment, such as turbines, pumps, compressors, and generators, operate under high stress and in demanding environments. Ensuring their integrity and reliability is essential to prevent costly failures, unplanned downtime, and safety hazards.

There are two organizations, the American Society for Nondestructive Testing (ASNT) and the American Society for Testing and Materials (ASTM), that contribute with authority over the methods, procedures, and interpretation of things found during non-destructive tests.

One of the documents created by the ASTM is a normative reference of API 687. This is the ASTM E1316, Standard Terminology for Nondestructive Testing, and as the name suggests it contains nearly 600 terms, establishing the language used across NDT practices.

Things found during non-destructive tests are called indications.

More formally, an indication is: a response or evidence obtained from a nondestructive examination.

An indication suggests the presence of a discontinuity or imperfection in the material being tested.

An indication represents the initial detection of a potential flaw that needs to be further analyzed to determine its relevance and severity.

I can hear kids saying that an indication is anything sus that comes up during NDT.

Let’s look at an example. Take for instance an impeller that was fabricated by having vanes milled to the cover plate and then fillet welded to the backplate.

Let’s focus on the outer diameter of the impeller discharge.

Often, one may visually detect faint lines, discontinuities, cracks, and indications.

If the impeller was magnetic, we could perform wet magnetic particle inspection (wet mag).We will delve deep into the beauty and science of Wet Mag when we get to that chapter.

But for now, you should know that during a wet mag inspection, fluorescent particles deposit themselves anywhere there is a surface discontinuity.

Under ultraviolet light, it becomes very easy to see those particles.

So, now we have found an indication! An indication of a possible imperfection and a potential flaw that must be further evaluated.

ASTM E1316, along with its hundreds of terms, also describes steps that must be followed during nondestructive testing.

First, the test must be carefully carried out by a trained and certified inspector.

Secondly, during the tests, observations will be made and indications may be detected.

Thirdly, the trained and certified inspector will determine if the indications are real. Meaning, sometimes tests can return false positives, things that look like indications that turn out to be the influence of other factors but not actual defects.

Lastly, an engineer will evaluate the impact of the indication. For a thorough evaluation, this information is needed:

• Location, size, and depth of the indication

• Part material and heat treat condition (e.g. what hardness, how ductile, what tensile strength does it have)

• Operating conditions (e.g. running speed, working fluid, etc.)

• Repair and Operational history (e.g. were these indications detected before? was there an upset in the operating conditions?)

The takeaway is that detecting an indication is only the beginning. It must be then determined if it is relevant or real, as well as the impact it has on the form and function of the part.

• Indications do not always mean there are cracks.

• Relevant indications do not always mean the part is scrap.

• Engineers must evaluate a relevant indication to determine if it is acceptable or not.

Another way to put it, if you were stranded on an island and had to survive by performing NDT on a steam turbine, remember this acronym: “I don’t want to DIE”.

Detect

Interpret

Evaluate

On to our next term.

Maximum Continuous Speed

This is one of the first terms I ever learned as it is used very often in the context of rotor repairs.

Maximum Continuous Speed (MCS) refers to the highest speed at which a machine, such as a pump, compressor, or turbine, can operate continuously without exceeding its design limits in terms of mechanical integrity, vibration, temperature, and wear.

I sometimes hear this term defined as Maximum Continuous Operating Speed (MCOS). It sort of rolls better off the tongue when you add that vowel.

Not all machines operate at the MCS or MCOS all the time. Process equipment such as mechanical drive steam turbines, compressors, or pumps will often run within a range of speeds, but technically should never exceed the MCS.

Operating beyond the MCS can lead to increased risk of failure, reduced lifespan, and potential safety hazards.

In contrast, power generation steam turbines, single shaft gas turbines, and power turbines all run at a fixed speed determined by the line frequency of the generator they are driving.

In the context of repairs the MCS is relevant because it is used in many instances:

• Determining the balance tolerance

• Assessing the interference fit of any stacked-on component, such as impellers, disks, sleeves, etc.

• Determining the spin test speed for testing impellers that have undergone repairs

• Determining the “at speed” balance speed used for high-speed balancing

• Analyzing or performing structural simulations of components under centrifugal load, for example:

  • Assessing the stress distribution or redesigning a blade root attachment

  • Assessing the geometry of steam turbine blade tenons when replacing blades

  • Assessing the means to locking blades on a rotor

  • Evaluating the impact of any detected indication on a rotating component

  • Calculating the stresses on an impeller bore

As you can see, you cannot really assess the fitness of a rotor without knowing the maximum continuous speed.

This speed needs to be communicated by the equipment owners to the repair facility performing the inspections and repair work.

It needs to be confirmed to be the latest or actual speed, since machines could have been re-rated.

And one can never assume that the MCS of a similar equipment, make or model may have an identical MCS as another.

The impact of using the incorrect MCS could be catastrophic. If a lower speed is used, decisions such as size and material selection could result in inferior performance. If a higher speed is used, the parts may be over-sped during the repair process.

What is important to know is that in rotating components or parts, the stresses induced by centrifugal forces are proportional to the square of the speed.

This relationship is due to the fact that the centrifugal force (Fc) acting on a mass (m) at a radius (r) from the axis of rotation is given by

Where 𝜔 is the angular velocity. Since stress is directly related to the force, it follows that stress increases with the square of the speed (𝜔2).

We have our first serious equation! And, also, a dilemma! Which units to use?

Since I am writing articles in both English and Spanish, and since I have spent my life equally in countries that use US customary units or the metric system (SI), I will explain things in both. (US Units in blue, SI Units in green)

In US customary units:

Fc is the centrifugal force (measured in pounds-force, lbf)

m is the mass (measured in slugs)

r is the radius (measured in feet, ft)

𝜔 is the angular velocity (measured in radians per second, rad/s)

In SI units:

Fc is the centrifugal force (measured in newtons, N)

m is the mass (measured in kilograms, kg)

r is the radius (measured in meters, m)

𝜔 is the angular velocity (measured in radians per second, rad/s)

Between you and me, I wish we all used SI units. Then we wouldn't have to deal with slugs or remember the difference between pound and pound-force. OR someone needs to make scales that measure mass in slugs instead of pounds!

We shall dive into aspects of unit systems and the tumultuous relationship between mass, weight, and gravity some other time.

Back to the example describing the sensitivity of working with an accurate MCS.

Imagine you had to replace blades on a steam turbine stage and now must fit a new lock to close the stage and hold the blades in place.

Your scale says the lock piece is 100 gms.

The loading radius is 16 inches.

The MCS is 8,000 RPM.

Carefully performing our calculations and taking care of our units, the resulting centrifugal force pulling on the lock is:

If we were to be given the incorrect MCS by a factor, let’s say 500RPM higher or lower, the results are illustrated in the graph below.

We could be making decisions considering a force lower by 782 lbf (3,400N) or higher by 832 lbf (3,619N).

If the speed was incorrectly given by a factor of approximately 1.414 higher, the force would be double (1.414 or the square root of 2, one of my favorite numbers of all time).

To finish this definition, I leave you with a picture of real life situation so you can fully appreciate the impact of what can happen if you do not use the correct MCS.

There is something missing in this picture.