API RP 687, Chapter 1, Section 10: PHASE I Rotor Inspections. Dimensional inspections (PART 2) - ALL ABOUT RUNOUTS

Let’s talk about runouts!

What we measure, why we measure them.

Rotating equipment needs to run smoothly. This statement can be qualified in different ways.

The rotor inside a steam turbine or compressor must run very smoothly. This means the assembly needs to be dimensionally assembled correctly, straight, concentric, smooth, well balanced.

One way to ascertain how round and concentric things are is by measuring runouts.

When you design a rotor, the expectation is that features like journal bearings, seal areas, and land fits are perfectly round and smooth. We expect assemblies to come together perfectly, with no misalignment.

This is impossible to achieve, because our manufacturing tools are not perfect. At every step of the manufacturing and assembly process, we introduce small deviations or imperfections.

So one way to check for how “perfect” our shaft surfaces are is by measuring runouts. Doing this we can check for two things.

• We can determine how round something is (or determine how much “out of round” it is)

• We can determine if components on a shaft are eccentric to the journal

In the image below we have a simple single stage compressor rotor.

Let’s look at the journals first.

I mentioned that despite our best efforts, we cannot always manufacture things that are perfectly round or smooth because of microscopic imperfections in our tools and processes.

A shaft or journal to the naked eye may look perfectly round, but if we looked very closely and measured with instruments with a very small precision we would be able to measure almost imperceptible deviations.

This is very similar to listening to Neil Degrasse Tyson talk about how smooth the earth is, despite it has mountains and deep ocean trenches. It’s all a matter of scale.

https://youtube.com/shorts/OMP5dNsZ-6k?si=CUBvfJ9hzqagYM1a

Anyways, the setup for measuring runouts is by supporting the rotor on two pedestals and cradle the journals in “vee blocks”. Vee blocks will allow the journal to spin and glide so we can measure the runout with a dial indicator that measures with a resolution of 0.0001 inches.

The journal will contact the vee blocks on two points.We must set a dial indicator opposite to one of the contact points to measure the true diameter of the journal.

If we do not put the dial indicator directly opposite the contact point, we will not measure the true diameter of the journal as it spins. Instead, we will measure a smaller quantity.

Let’s say you position the indicator exactly at the noon or 12’o clock position.

Another way to get the wrong measurement is by not placing the indicator perfectly perpendicular to the surface of the journal.

Once you are set up correctly, let’s assume the journal is in fact perfect. If you spin the shaft, your indicator will not move at all.

Now let’s assume the journal is not round, but elliptical or “egg shaped”.

The journal will still have two points of contact, and the indicator would measure the width of the journal as it spins.

The needle will wobble from the largest width to the smallest width.This measure is called Total Indicator Reading (TIR) and lately in some circles it is known as Full Indicator Movement (FIM).

Look at the shape of the journal, it is like an egg, and we are measuring the distance from one side to the other. We are measuring the total deviation of the egg to a perfect circle.

This may indicate there was a problem when the shaft was being manufactured and the journal was not ground perfectly round.

This means that if we were measuring eccentricity, the value of the eccentricity would be ½ of the TIR.

Now let’s imagine the surface is sort of round, but very rough.

We call this condition “wavy”. And in the context of an inspection, if this was a journal or a seal area, it means the surface may not be smooth enough for proper function.

This may show there has been erosion, or some damage due to rubbing.

When we are inspecting a rotor, the expectation is that the journals need to run perfectly. They need to be perfectly smooth and perfectly round.

The journal surface is what will support the rotor within the bearings.

The next step is to measure the runout of the other areas of the rotor or components relative to the journal.

This means we move the indicator to all the other surfaces we need to measure.

I will expand on what this looks like in my next post.

I’m burned out on journal runouts. It took me a while to figure out how to make these gif animations in PowerPoint. Yes, you read correctly, PowerPoint.

I am sadly and somewhat proudly admit I am a PowerPoint Power User!