API 687: Chapter 0, Section 12 - Repair Processes and New Component Manufacture

This week, we tackle Chapter 0 Section12, which covers requirements for Repair Processes.

Section 12: Repair Processes and New Component Manufacture

This chapter opens with 4 main concepts that sort of get repeated often throughout the section. I shall paraphrase what they are below:

The first one is more of a warning that says:

Previous upgrades or mods may have changed the dimensions of the parts!

So always be careful during an overhaul and check all the dimensions, assembly clearances, and fits for any new and old parts to ensure the final assemblies will fit together and maintain the required clearances.

The second is also a warning:

The equipment may have not been originally built to or maintained to API standards.

Once again, one must be careful during an overhaul to check for fit and function of all parts and the final assemblies. This warning also is to remind owners that if the equipment being repaired was not originally built or maintained to API standards, during a repair, it may not be feasible to comply with all the API recommendations.

The third concept is a good suggestion:

Upgrade when you can, make sure upgrades are validated, especially material changes.

If improvements and benefits can be introduced, they should be considered. Care needs to be exercised when making material upgrades to ensure that the new materials are adequate and will bring the expected benefits.

In engineering, we say there should be a design review, and one step of that review is to “validate”.

Validation is the process of ensuring a design or product meets the needs and requirements of its intended use. In the context of repairs and upgrades, one must ensure the upgrades introduced in an existing design have been validated through testing.

The fourth and last opening concept says:

Using previously manufactured components, surplus parts and their acceptance needs to be agreed upon by ALL parties.

This statement serves to set realistic expectations for both parties (owner and repair facility). As repair guys, we want to help everyone fix their equipment. At our shop, we can control our processes; we can verify the validity of our materials; we can speak about the quality and workmanship of the work done by our technicians and mechanics.

But if an owner brings a box of pre-existing spares or ships us brand new parts that were manufactured by someone else, we may have to spend time certifying that we can use those parts.

Imagine running a 5-star restaurant, but patrons bring their own ingredients. What happens when someone finds a hair or a fly in the soup?

There is a fifth concept that appears briefly but is connected to the previous 4 and it is about maintaining “interchangeability”.

Interchangeability

When owning and operating critical rotating equipment such as steam turbines or compressors, an owner is likely to own multiple machines and would be managing a stock of spare parts to maintain such machines.

Maintaining interchangeability is key, because it allows the owner to be efficient and effective in the following areas:

• Repairs: Interchangeable parts allow for quick and efficient maintenance. Failed or worn parts can be replaced with identical parts without need of custom fitting or adjustments. This minimizes downtime.

• Inventory Management: Standardized, interchangeable parts; simplify inventory management. You stock fewer parts, knowing that the parts fit multiple machines. This reduces inventory costs and storage space.

• Quality Assurance: Interchangeable parts are manufactured to a defined set of drawings and specifications. There is always a risk of making a mistake when multiple sets of parts are being manufactured, if they are all different.

After these 4 or 5 concepts, in Section 12, there are some conceptual land mines disguised as notes that for me were hard to walk by without exploding in a stream of theory and explanations.

These notes come right after line 12.1.10 and say:

Small rotor to stator clearance changes can move critical speeds Small bearing clearance changes can move rotor critical speeds Changes in shrink fits can adversely affect rotor dynamics

The first reason why I want to explode is not out of joy or love of engineering, but because these statements are technically accurate but are not quantitatively accurate enough.

How small is small?

The second reason I want to explode, is because there is sooooo much we could talk about! And at the same time, I am sad I don’t have the time to burst into these subjects in one post.

We will avoid stepping on these landmines for now, but I leave you with this:

Changes in dimensions, be it clearances or fits, will influence critical speeds and rotor dynamics. But it is important to understand how much change is required to cause a significant effect; it is also important to understand how significant the effect needs to be to affect the function and performance of the machine.

Chapter 0, Section 12 then covers another set of recommendations under the paragraph:

12.1.11 Corrosion, erosion, pitting, and rubs:

I call these: three wear mechanisms and one byproduct.

What I mean is, Corrosion, Erosion and Rubs are three mechanisms of wear and tear that are often encountered when operating rotating equipment.

Pitting is really a byproduct of other wear mechanisms, particularly corrosive wear. This is where chemical reactions between the rotor material and the environment can lead to localized corrosion, resulting in pits or tiny craters.

This paragraph is brief, but it prescribes what type of repair method should be applied when facing these wear mechanisms or byproducts.

Minor rubs, pits, corrosion damage, erosion can be repaired by polishing. Basically, buff the part and remove the damage.

But how minor is minor? How small is small?

For example, on a later Chapter and Section of API RP 867, Chapter 1, Annex E the document suggests that scratches or dents less than 0.005 inches deep on a journal surface do not need to be removed, only the raised edges need to be blended or removed.

This gives us an idea of how “small” is, in this example, 0.005in.

To get an idea of how small this is, the average sheet of paper from a notebook or printer at home is 0.005 in thickness.

The document then states:

Major erosion or corrosion wear should be repaired by welding.

What about major rubs? How do we fix major rubs? Why is welding only good for major erosion or corrosion wear?

The answer has several parts:

1. Erosion and corrosion wear, as wear mechanisms, consist of the fluid or environment acting on the part and removing material or promoting the loss of material.
   

2. Rubbing is caused by contact, and, when metals come in contact, it can lead to a lot of localized heat being generated.

   Therefore, rubbing damage needs to be carefully assessed. Rubbing damage is often accompanied by indications of surface cracks caused by the heat. High concentrations of heat can also alter the mechanical properties of the metal. So API RP 687 is saying that it is not guaranteed that ALL major rub damage can be repaired by welding. First, the extent of the damage must be determined.

   In addition, welding is a wonderful tool, but it comes with some risks, since it involves applying heat to a part.

This is a perfect segway to paragraph: 12.2.2 which talks about Welding Repairs.

API recommends that all welding should be per ASME BPVC Section VIII.

If you want to know more about ASME BPVC and how it is relevant here, check out my earlier blog post:

In this section, API issues useful recommendations such as:

During an inspection or repair project, the proper method for identifying or determining the chemical composition of a material is through Positive Material Identification process or PMI. I have written about this before as well, here:

This is reinforced by another statement that specifically disqualifies using bills of material, material test reports, or equipment data sheets as acceptable sources of material composition.

Another useful recommendation is:

12.2.2.11 Weld map of all weld repair locations should be a part of the job documentation.

This is useful for monitoring the condition of a previous repair on a future inspection or repair project.

A final useful recommendation is that one should not weld on pressure containing walls made of cast, ductile, or nodular iron.

This is because despite the “ductile” in the name, cast iron tends to be brittle. The high carbon content can lead to the formation of brittle phases. The brittleness increases the risk of cracking under stress.

Cast iron also often contains porosity and inclusions which can be the source of problems during welding. Inclusions can expand or new voids can be created from the heat from welding.

These are two of a handful of reasons why it is not recommended on a pressure-containing structure made from cast iron.

The final sub-topic in Section 12 is:

12.3 New Component Manufacture

This section occupies less than half a page, but it contains some of the fundamental rules for determining the source technical or design information for manufacturing new components.

It states:

When OEM drawings are not available, reverse engineer.

This means, one must have access to parts to use as samples.

A part may have to be sacrificed or destroyed in order to properly reverse engineer it.

For instance, if mechanical testing is required to determine precise material properties, a steam turbine blade or disk may be cut so a test sample can be machined and sent to a lab.

Another example is an impeller with a very narrow gas path, a section may have to be cut to measure a vane profile.

A second stanza states:

When you reverse engineer, hold a design review between owner and repair shop.

Since reverse engineering is a process where creativity and versatility are applied, it is wise to have design review sessions between the equipment owners and the repair shop.

In the first session, start with understanding any challenges from the reverse engineering process, how dimensions or material properties are determined, or what manufacturing processes and in what order they are going to be applied.

I would suggest in this exercise to also throw in the application of an ITP, or Inspection and Test Plan, where a repair shop describes all the steps involved and the owner specifies if they want to be present or check a step.

Basically, owners should be aware of the processes used and a repair facility should be open enough to ensure the equipment owners understand the process.

Owners should get an experience as if they were watching “How it’s Made” but with their own parts.

I would say that the only areas that are off limits would be:

• IP, like manufacturing drawings, can be reviewed, but copies should not be issued for just anyone to take.

  
  

• Owners should respect the fact that repair facilities may want to protect trade secrets or IP that they don’t want leaked to their competitors.

Another recommendation about reverse engineering says:

During reverse engineering it may be necessary to get field measurements.

This means that if we are working and only have a rotor as a sample, or a stationary diaphragm, we may need to have access to the case to ensure we get the case dimensions to guarantee a proper fit and location.