CA-15 versus CA6NM

[JJPellin]

We are currently evaluating proposals for the repair of our warehouse spare pump for FCC Main Column Bottoms service in an oil refinery. We have two options that I need to compare.

The existing case is made of CA-15 (12% Chrome - ASTM A743 GR CA-15). It is badly damaged from erosion. This case can be repaired with massive welding required. The repaired case would be treated with a Boron Diffusion process and then coated with HVOF Tungsten Carbide.

As an alternative, the manufacturer is proposing that we purchase a complete new case of CA6NM (13-4 Ch-Ni - ASTM A487 GR CA6NM CL A). The new case would receive the same Boron Diffusion treatment and the same HVOF Tungsten Carbide coating.

The new case would cost about 25% more (US$150,000 versus US$120,000). I expect to be asked to justify my recommendation to pay the additional amount to get the new CA6NM case. I need to be able to explain the differences and benefits. A few thoughts occur to me, but I am hoping that you can provide additional details for the benefits of the new case option.

These are the items that I feel are relevant to this decision:

CA-15 is basically obsolete for pump case metallurgy. All of the API pump manufactures offer CA6NM for these (12 Chrome) applications.
CA6NM has better properties when corrosion is the problem. Specifically in Naphthenic acid service, CA6NM is our material of choice.
I have seen CA-15 cases develop cracks. I have never seen a cracking problem in a CA6NM case.

The service we are looking at (FCC bottoms) does not have a corrosion problem. Since we are dealing with erosion as the limit to service life, I have less experience to base my recommendation on. But, these are the items that I would think might be relevant:

Effectiveness of the Boron Diffusion treatment. Is there any advantage to CA6NM over CA-15 when this treatment is involved?
Coating bond strength. Is there is any advantage when HVOF Tungsten Carbide coating over CA6NM as compared to CA-15?
Repairability. Both materials can be weld repaired with PWHT. But, is there is any advantage in terms or repair options or repair success with one versus the other?
Cracking potential. As I noted, I have never seen a cracking problem with CA6NM. But, is there any real difference in terms of cracking potential?

I would appreciate any information you can provide to help me better understand the difference between these materials for this service.

Johnny Pellin

 

[EdStainless]

I don't see why there would be any difference in the Boronizing or HVOF. Those will be the same.

I can't say that for weld repair on a high carbon case. It sounds like extensive weld repair on the case would be very problematic. At least a lot of NDT as well as post weld annealing. The risk of 'chasing cracks' in trying the repair is probably high.

For the same reason that new pumps have switched materials you should. You used the old one, now it is time to move to something that will last longer and be more reliable.

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Plymouth Tube

 

[metengr]

Based on my experience with weld repair of CA-15 materials in Power Generation, this is certainly weldable material with proper precautions (preheat maintenance and PWHT). The build-up of eroded regions is straight forward. The downside risk will be PWHT after extensive weld repairs followed by line boring to account for distortion from welding. This will be costly.

I believe you answered your question based on the statements below;

All of the API pump manufactures offer CA6NM for these (12 Chrome) applications.
CA6NM has better properties when corrosion is the problem. Specifically in Naphthenic acid service, CA6NM is our material of choice.