Why we use displacement probes in rotating equipment and why we care about AC/DC voltages.

I have been thinking about this for months now, and I finally had time to sit at my desk this morning and type it out.

Why do we measure displacements in rotating equipment?

It is all part of the idea of “condition monitoring” and “machinery protection”.

Like when someone goes for a stress test during a physical. Or when a patient is monitored during surgery.

A patient may have several probes connected to them for different reasons. Temperature, heartbeat, blood pressure, oxygen blood content, etc.

In the same way, we monitor different vital signs of machinery when they operate. The more critical the process is, the more surveillance is required.

The intent is to be able to diagnose and predict the condition of the equipment before failure occurs.

We measure process temperatures and pressures; we monitor shaft speed, bearing temperatures, and vibrations; and we can also measure shaft displacement.

Shaft displacement is extremely important, because in a steam turbine, compressor, gas turbine, or pump we are going to have a rotor or shaft spinning at very high speed, under extreme temperature and pressure conditions. Spinning against non-stationary components.

Inside rotating equipment, it is always a tight fit.

Loose fitting components, in a sense, mean lower efficiency.

Have you ever driven down the highway and the lanes merge into one, or you get closed in between two semi-trucks while driving at 70 mph!? I have seen it in movies, and it looks scary.

Well, enough of the imagery. The point is that inside rotating machines, space is tight.

Clearances are measured down to several thousandths of an inch.

And this is why we measure displacement, shaft displacement to be precise.

To understand if things are working as intended inside a turbine or compressor, we monitor the position and behavior of the shaft inside the bearings.

From that we can infer if the internal alignment of the rest of the rotor is maintained.

We can get an idea of how the shaft is “behaving” inside the case.

We call this the “rotordynamic” behavior. The MS Word dictionary does not seem to know this word because it is always showing red squiggly lines under it.

What I see every time I type this word.

But I know from other literary references that this is a real word. It represents the field of study of the dynamic behavior of rotating structures.

To measure shaft displacement, we use proximity probes.I have described these before in earlier posts:

API 687: Chapter 0, Section 8 thru Section 9: Guidelines for Disassembly and a bonus! The Eddy Probe

What is runout? (Part 1)

Proximity probe systems allow us to measure a voltage that is directly proportional to the distance between the shaft and the probe.

Let’s start imagining we only have one probe installed in a bearing housing.

Shaft sitting at the bottom of a bearing.

When there is no rotation, the shaft sits at the bottom of the bearing. If we measure the voltage from the proximity transducer, we will see a steady DC Voltage.

We call this the gap voltage, and when we install a probe in a machine we will set this to show a voltage of 10 or 12 VCD. This represents the middle of the range of the probe.

Shaft lifting as it spins.

When the shaft begins to spin, it is like an airplane taking off.

The shaft rises from the bottom of the journal and when the rotor reaches running speed, the shaft centerline should be at “cruising altitude”.

We call this measuring the “shaft centerline” and we display this on a “centerline plot”. In this plot we map out how the centerline rises from 0 RPM to running speed, and it usually looks like this:

Centerline Plot.

The blue line is the “altitude” of the shaft as it climbs up the oil wedge and reaches cruising altitude.

The blue numbers represent the shaft speed in RPM.

Spinning shaft in bearings experiencing vibrations.

Once we reach cruising altitude and speed, our machine, our steam turbine or compressor is “in operation” or working.

We all should expect a smooth ride, but, inevitably, as things wear out or just like the weather changes, “process conditions” or the “mechanical condition” of the machine can change causing turbulence.

We will dive more into vibration monitoring and what these signals look like in the next post.

For now, I want you to remember this:

• The position and behavior of a shaft inside a bearing can be measured with a displacement probe.
  

• One of these probes is called a proximity probe and it outputs a voltage directly proportional to the gap between the probe and the target (shaft)
  

• When we monitor equipment and measure shaft displacement we can measure two things:

  
  

  • The position of the shaft within the bearing.This is like measuring the “cruising altitude” of an airplane with an altimeter.This is represented by the gap voltage, which is the DC component of the signal from the proximitor.

Centerline Plot.

• We can measure the vibrations of the shaft.This is like feeling turbulence when we fly on the airplane.This is represented by the AC component of the voltage measured by the proximitor.

Waveforms or "squigglies"" like Mr. Bob Eisenmann Sr. would call them.