Back to Basics - Rubs I

The following is a continuation of our previous discussion on rubs. We will begin with a brief review of some basic concepts and then give an example of a rub with supporting data plots. In upcoming blog articles which will continue this discussion, additional rub examples will be depicted.

To begin, let us remind ourselves that it is not sufficient to only evaluate whether or not a rub is present. One must also make every effort to try and gain insight as to the initiating mechanism that has caused the rub to occur in the first place. A rub is not a machinery malfunction in and of itself. A rub always results from some other primary malfunction source such as:

• high vibration

• tight or incorrect clearance

• thermal growth

• rotor bowing

• distorted/twisted turbine casing or bearing housing

There are several symptoms that may be indicators that a rub is present:

·         Thermal bowing of the rotor

·         Abnormal changes in shaft centerline position

·         Changes in 1X vibration behavior at constant speed

·         Abnormally high 1X vibration amplitude while trying to pass through a critical speed

·         "Modified" critical speed frequency response region

·         Abnormal (elliptical or highly elliptical) orbit shape

·         Significant reverse precession vibration components Sub and / or super harmonic vibration components

·         Wear, damage, loss of efficiency

·         Noise leaks

The presentation will discuss the analysis of the necessary vibration data plots in order to properly evaluate and determine whether or not a rub mechanism is present within the critical1 rotating machinery asset. Vibration data plots from multiple field projects will be used to help illustrate the diagnostic concepts.

Before proceeding, let's review a few definitions and concepts.

Single plane transducers only display motion along the longitudinal axis of the transducer.

Orthogonal transducers not only display motion along the longitudinal axis of each transducer, but when combined as a pair or XY coordinates in a Cartesian coordinate system form the shaft orbit, displaying the full dynamic motion in the XY radial plane. Refer to Figure 1.

Vibration signal characteristics

1. Amplitude:

- Overall / Direct

- nX Filtered

2. Phase

3. Frequency

4. Form / Shape (XY pair) 5. Radial / Axial Position

6. Precession

The 1st Law of Machinery Diagnostics reminds us that much about the nature of machinery behavior can be determined via transient data analysis as rotative speed changes and the speed dependent terms in Equation 1 vary accordingly.

Orbit / Time Base Data Plots are used to primarily examine the shape or form of the vibration; i.e. the two dimensional centerline motion of the shaft as it vibrates in the xy plane perpendicular to the axis of rotation. The Orbit / Time Base Data plot is often used to analyze constant speed vibration response data as a function of time, load, thermal changes, or any other relevant process variable. Amplitude information is simultaneously plotted from a pair of orthogonal (90°) transducers. The amplitude information, coupled with a phase angle reference, is essential for analyzing the dynamic motion of the rotor. Interpretation of Orbit / Time Base Data plots provides insight into the nature (shape and form) of the vibration which changes as the forces which act on the rotor system change; i.e. changes in steady state preloads and support stiffness asymmetry.

An orbit can be used to determine the direction of precession of the rotor. The Phase Trigger mark on the orbit, the blank/dot sequence on the orbit plots, shows the direction of increasing time, which is the direction that the shaft is moving (the direction of precession).

Once the Phase Trigger mark sequence is determined, it will show the direction of increasing time. This is the direction that the rotor moves in the orbit, regardless of the direction of rotation. The direction of precession can be compared to the direction of rotation to determine whether the precession is forward (vibration precession direction same as rotational direction) or reverse (vibration precession direction opposite rotation direction).

On complex orbits, the rotor may undergo forward precession over part of the orbit and reverse precession over another part of the same orbit. Refer to Figure 2.

Starting at the phase trigger marker and moving around the orbit in the "blank to dot" direction (CCW in this example), count the number of positive peaks that occur in the X direction and in the Y direction. In this example, the X transducer sees two positive peaks in (one is smaller than the other), and the Y transducer sees only one positive peak. The X:Y frequency ratio is 2:1, with vibration components of 2X and 1X respectively. As seen in this example, the shape of an unfiltered orbit can provide important information about the frequency content of machinery vibration.

Shape can also provide clues to the presence of excessive radial loads that may exist. High radial loads tend to push the rotor to high eccentricity ratios in a fluid-film hydrodynamic bearing. The oil film that supports the rotor produces very high spring stiffness at high eccentricity ratios. This stiffness is highest in the radial direction and lowest in the tangential direction.

Figure 3 shows two unfiltered orbits with high radial loads. Note that the two machines turn in opposite directions (black arrows). Both orbits display remarkably similar shapes; in fact they are nearly identical when corrected for rotation direction. The arcs indicate the location of the bearing wall, and the red arrows indicate the likely direction of the applied radial load.

The first item to be realized is that radial or axial rubbing is not a machinery malfunction. A rub is secondary indicator that occurs when there is contact between rotating and non-rotating components. Some of the primary causes that can lead to a rub are:

• high vibration

• tight or incorrect clearance

• thermal growth

• rotor bowing, etc.

• distorted / twisted turbine casing or bearing housing

A rub can be radial, axial or a combination of the two. When the actual rub/stator contact occurs over a small fraction of the vibration cycle, it is called partial rub. When it occurs over a majority or all of the vibration cycle, maintaining continuous contact, it is called full annular rub. A partial rub is the most common manifestation.

Axial rubs can result from a mismatch in the thermal growth rates between the rotor and casing. During a cold startup, the turbine rotor expands faster than the more massive casing. A related problem can occur when a slide key or trunion on the machine casing hangs up and prevents free movement of the casing during startup or shutdown. The constrained machine casing can deform, displacing internal parts, resulting in an internal rub.

(In the next article, the author discusses the mechanism of a partial rub, and dynamic stiffness.)

(Richard Thomas is Principal Engineer, Metrix SETPOINT Condition Monitoring Group)