API 687 Chapter 0: Definitions (continued) – Phase I Inspections, Phase II Inspections, and Phase-Referenced

This week we take on three compound terms that begin with the word Phase.

The first two, Phase I Inspection and Phase II Inspection, will surface again in a later chapter since they are part of the rotor inspection sequence. They are important because they are part of the daily lingo spoken in repair shops, so the taskforce thought they were worthy of including them in the definition section of the document.

Before we dive into the terms, we must make an important distinction between two types of repair projects one will encounter at a repair facility.

We can differentiate between performing inspections and repairs on a complete unit, meaning a complete machine, case, and rotor; OR performing inspections and repair only on a rotor that has been removed from service.

I make this distinction now, because the terms Phase I and Phase II inspections apply only to rotors and not cases.

Without further ado:

Phase I: Inspection and Cleaning of an Assembled Rotor

A Phase I constitutes a preliminary assessment and inspection of a rotor before it is taken apart. The only parts that API will prescribe can or may be removed are coupling hubs or thrust collars.

Depending on rotor configuration and design, if one must remove a thrust collar, it is also likely or necessary to remove a thrust collar nut, possibly a sleeve, maybe a tach gear, and/or other parts that in the industry we lovingly call “trim parts”.

Trim parts is an important term that somehow escaped the taskforce’s radar and was not included in the Definition section of API 687.

Trim parts, however, is defined in Chapter 1, Section 13.5.1 as “field-removable parts”.

Trim parts are components at the very end of either shaft end. Usually anything outboard and inclusive of the coupling hub or thrust bearing assembly, and even seal sleeves. These are often removed in the field while conducting field maintenance.

The unwritten agreement is that these components are removable by design. They are meant to be removed in the field and are not part of the main structural body part of the rotor.

In conclusion, Phase I, an inspection of an “assembled rotor” may exclude the trim parts.

Having said this, API 687 also insists that the equipment owner should be the one deciding what type of inspections to be performed first, and that nothing should be done without the consent of both the owner and the service provider.

A Phase I will contain at a minimum the following activities:

1.      Visual Inspection

2.      Initial Cleaning

3.      Dimensional Checks

4.      Non-destructive Testing

5.      Balance Check

6.      Reporting and Recommendations

The order and feasibility of performing these activities depends on the condition of the rotor and is determined as the inspection is carried out.

Every Phase I should conclude with a report which includes inspection results and technical recommendations on the repairs required to return the rotor to service, if this is feasible.

If during Phase I, there is evidence that further, more in-depth inspections are necessary a Phase II may be recommended.

Phase II: Disassembly and Detailed Inspection of Rotor Components

This applies to assembled rotors, which will be most centrifugal compressor rotors and some small-medium steam turbine rotors.

During a Phase II Inspection, the rotor assemblies are fully taken apart, and the same activities performed during a Phase I are performed on each individual component.

This results in a larger report, that will include all the data collected during the inspections, results, observations, and recommendations.

API 687 does suggest that a Phase II Inspection could be performed as a partial disassembly of a rotor. The extent of a Phase II Inspection must always be mutually agreed upon by owner and service provider.

In the end, all recommendations of what to inspect and how should be part of a conversation between owner and service provider, based on facts that may include the operational and service history of the equipment and the condition of the equipment as it is systematically assessed through the inspection process.

Now we move on to the next term.

Phased-Referenced

The term phase adopted in science is used to describe a position in a cycle of a repeating event.

Rotating equipment components, such as rotors, disks, impellers, sleeves, couplings, are all “cyclic symmetric”. They essentially have a cross section or geometry that remains the same or repeats as it is rotated around an axis.

I basically just gave you a convoluted description of a circle.

To make things simple, imagine a cross-section of a rotor or a shaft. It looks like a circle.

In engineering drawings, one would draw it as shown below; there would be a crosshair that defines the geometric center of the shape.

We can describe anything and everything in this shape or circle if we use a polar coordinate system (AKA the flat cousin of cylindrical coordinates).

With a polar coordinate system, you can specify any location by magnitude or radial distance and an angle!

Angle… finally said it! Angle!

If we use a polar coordinate system, any point on that shaft could be represented as below.

This is important because during an inspection, or when balancing a rotor or diagnosing a vibration issue, it will be useful to describe observed features of events in terms of these polar positions.

In rotating equipment, it is common to define a “zero” reference on a consistent and known location, such as a kay phasor notch, or thrust collar keyway.

Imagine there is a deep scratch on a rotor, and it is noticeable while you are measuring runouts.

You would write that in an inspection report as a phased-reference measurement. Based on that “zero” point you would say “deep scratch 225°”.

In conclusion, phased-referenced is a measurement or mention of something considering the angle between the angular position of the observation and the zero reference point.

The term phased-reference is also used in the context of rotor balancing and machinery diagnostics. To help describe the perceived location of an imbalance or shaft deflection. And I won’t lie to you, it’s going to be tough when we get to dynamics, vibration, and balancing. Things are much easier to understand when they are not moving.