API RP 687 Chapter 1: Rotors – General Sections 1 through 9

There is no get out of Phase I free card!

We have finally made it through Chapter 0, or as I call it, “the Introduction.”

Today we begin our trek through Chapter 1, which begins to focus specifically on rotors.

Before we dive into the technical content, I would like to review the structure of the document.

Chapter 1 is structured as follows:

It contains 80 pages, sixteen sections, many of them directly making a reference to a section from Chapter 0, that is why the number of pages in the rightmost column in the table above says zero pages.

From these sections, the most content is in the topic of Rotor Assembly and Balancing.

Chapter 1 also contains five informative annexes. The most content is in Annex C in the topic of drive couplings. The next largest annex is Annex D and contains a great template for a quality plan of ITP for rotor manufacturing.

But in my opinion, the most interesting content is on the smaller annexes. There is one that describes the procedure for verification of residual unbalance, and just the thought of explaining this one day is making me giddy right now.

There is also an annex that describes in detail the finesse of making total indicator readings and finally an annex that describes rotor restoration methods.

In all, this is going to be a great chapter to cover.

Let us begin with:

Section 4: Process for Overhauling and Refurbishing a Rotor

This is a very short section, but for me personally relevant because it contains the answer to a question, I was asked last week.

The question was:

Is it OK to skip a Phase I and go straight into a Phase II inspection?

Before I give the answer, let’s review what API suggest is the approach to Phase I and Phase II.

A.    Phase I inspection

This is a very simple sequence. Arrival of an assembled rotor, cleaning, inspection, and reporting results.

B.    Review of inspection information and determine if a Phase II is required

In this step, the repair facility and the equipment owners should meet and discuss the findings and recommendations.

The big decision is, is there evidence the rotor should be disassembled, and a Phase II inspection performed?

C.    Phase II inspection

If after Phase I, it is concluded there is evidence of trouble, for example, the rotor is bowed and it seems the origin is a heavy rub, or there are indications on the backplate starting at the bore of an impeller, or it looks like the impellers have migrated axially. It will be recommended that a Phase II inspection is performed.

This is the phase where we unstack or disassemble the rotor into its subcomponents and repeat the same process: clean, inspect, report.

So, to the answer to the question.

 Yes, you can go directly to Phase II, but technically you do not skip Phase I.

You still HAVE TO do all the inspections. Afterall, at some point the rotor will have to be re-stacked or reassembled and the Phase I inspections and numbers will come in handy then.

If you already have enough information when you take your machine out of service, or even before sending the rotor to a shop, you already can easily see problems with wear, tear, or other conditions, it is perfectly fine to request you perform a Phase I + Phase II inspection.

I would say, that only if a rotor was badly damaged, been in a catastrophic event, with parts missing, it would be a waste of time trying to measure runouts, or getting dimensions from it.

In other words, if its safe to do a Phase I, perform a Phase I.

Section 7: Shipment of Equipment

The next section with a useful warning is section 7, particularly paragraphs 7.2 and 7.3.

The first indicates the rotor being shipped to the repair facility should be adequately preserved. The reason is to protect the rotor from damage, or environmental deterioration.

This is important especially because if the rotor had a condition such as a crack, or if there was a failed part, like a failed blade on a steam turbine. The failed surfaces can fall under the attack of “environmental deterioration” like corrosion.

The same way a crime scene can get dirty if the police do not stop outsiders from entering. Once the crime scene or fracture surface gets dirty or corrodes, the work of the investigator, in this case the metallurgist, may be inconclusive and not get to the real root cause of a failure.

So, when a rotor is removed from operation, please preserve it as soon and as well as possible.

Paragraph 7.3 recommends protecting the shaft areas where the vibration probe areas are located. We have touched on these many times. Probe areas need to be protected because they are fragile, so the best practice is to wrap it with bright colored nylon tape.

At the same time, this wrapping makes it tempting for people to want to use a knife when it comes time to take it off. For which API recommends labeling the area with signs that say, “PROBE AREA, DO NOT CUT.”

Section 9: Disassembly

The main focal point of this section surrounds what to do with balance weights.

Before we dig in, let us start by laying the foundation on what balance weights are used for.

In the context of rotating equipment, rotors are meant to rotate and run smoothly, meaning without vibrations.

The same way the tires in our cars need to be balanced, or the ceiling fans in our homes need to be balanced. If they are not balanced, as these devices rotate and pick up speed, they will begin to vibrate or wobble to the point that we may be afraid they will break or fall apart.

No one wants to be hit on their head by a falling set of fan blades spinning at 380RPM!

(If 380 RPM does not mean much to you, consider that on average, ceiling fan blades are 24 inches long. At that rotational speed, it would be the equivalent of being hit in the head with a blunt wooden sword at 55 miles per hour. Needless to say it would hurt a little.)

Unbalance exists or occurs, when a rotating structure has an uneven distribution of mass around its rotational axis. This is true for all rotating structures.

Imagine our ceiling fan, and one of its blades has a big hole in it.

The hole results in an uneven distribution, causing a “heavy spot” on the side opposite to the hole. The mass on the heavy side will cause a centrifugal force that makes the fan wobble.

The same happens in steam turbine and compressor rotors. Despite our best efforts, we cannot manufacture things with 100% perfection or dimensional accuracy.

Also, the bigger the part, the larger the diameter, and the faster the speed, the higher the influence of an unbalance force will be.

Now, most rotors are long slender bodies or structures. Take for example the three-stage compressor rotor I used to illustrate the Phase I inspection process.

Three stage compressor rotor.

This rotor is made up of several components.

Its main component is the shaft, and mounted on the shaft will be three impellers, a balance piston, and some sleeves among other trim parts.

Each one of these components needs to be balanced, or in other words, in all these components we could have an uneven distribution of weight, potentially resulting in unbalance.

There are nine components, and if each component were not individually balanced, there would be nine opportunities to have an unbalance.

Therefore, during the manufacturing process, one of the final steps is to balance each component by itself.

There are two methods for balancing or correcting the weight distribution of parts:

• Material can be added, by fastening screws or loading weights into grooves.

  
  

• Material can be removed, by drilling or blending (grinding/sanding) material off.

The main difference between these two methods is that removing material cannot be undone.

Therefore, material removal is mostly done when permanent corrections can be done. For instance, when a new compressor impeller or new steam turbine disk is being manufactured.

Here are a few examples of how material is removed or added from rotors or rotating components.

Most of the recommendations from Section 9, have to do with removal balance weights.

Let us do a quick review of when would weights be added or removed to either balance individual components or an assembled rotor:

• Weight can be added or removed during manufacture/component balance.

  Component balance is when a component itself is being balanced as part of the manufacturing process or right before it is installed onto a shaft.

  
  

• Weight can be added or removed during the assembly balance.

  This is when a shaft or rotor is being put together, and progressively, one piece at a time is added, and the resulting balance after assembly is checked and corrected.

  That way you can say, “you correct as you go” step by step as you add more components on the rotor.

  
  

• Weight can be added or removed in the field.

  This happens when despite best efforts in the shop, once a rotor is installed in the case in the field it behaves showing vibrations. This happens because when a rotor is in a shop, it is not coupled to other rotors as it will be in the field.

  So it is not uncommon that some rotors have to be “tweaked” a little bit once they are installed in the field.

Another thing to consider is that when you inspect a rotor, you may not know its entire history.

The rotor may have been repaired by someone else, and you don’t really know If the balance weighs you find on the rotor are from when the components were balanced or when the rotor assembly was balanced.

This is why Section 9 prescribes a few considerations about marking and removing these weights.

It is always a good practice during a Phase I inspection to document the location and the magnitude of weight that is found on a rotor.

One may not be able to say WHY exactly the weights were added, but a good record may be useful when discussing the repair history with the owner.

There are locations on a rotor that should be reserved for field balance weights. One could assume with a bit of risk that weights in these locations were added in the field and not in a shop. But there is no 100% guarantee this will always be true.

If the work scope of repairs calls for a Phase II, the full disassembly of a rotor, a good practice is to document the weight locations and then remove them before starting over by attempting to balance the individual components before stacking.

This will ensure each component is in its best balance condition, and that when they are stacked the possible unbalance from the assembly will be minimized.

We will end with these recommendations for now. The main takeaway is to document thoroughly how things are found!

We will get into deeper aspects of balancing in future posts.