ASME VIII note 35 + Corrosion Allowance 0 + API 510 Inspection Code = Instant re-rate? 3

[Diego N. Bruzzone]

Dear all:

My question is not related to the design of the equipment, it is related to the subsequent inspection, applying API 510. How are periodic inspections planned for equipment with 0 excess corrosion thickness? As I understand it, strictly applying the code, an FFS should be done as soon as the equipment reaches over corrosion thickness 0. On day 1 of entry into service.
This is really worrying in equipment where note 35 of the ASME VIII code has been applied, equating the MAWP with the design pressure. With the subsequent pressure reduction of the hydraulic test. There is no room left to make a rerate. This situation leads to doing an FFS, rerate and retest on day 1 of commissioning.

Thanks in advance

 

[Andrew ONeill]

I'm not super familiar with those codes and clauses but deal with FFS all the time. I suggest designing any vessel or tank with 0mm corrosion allowance is just plain dumb, as it results in the situation you are describing. The end-user end ups spending much more money and resources doing FFS assessments, than just adding 0.5-1mmm in material thickness in the original supply.

Even worse, is the end user normally only find this out when they do the inspection (several years after commissioning) at which point what should be a standard outage time, gets grossly extended as a formal FFS assessment is now required.

TLDR - don't design vessels and tanks with 0 corrosion allowance,


Andrew O'Neill
Specialist Mechanical Engineer
Australia

 

[r6155]

Please, read again API 510.
Corrosion is not the only problem that may exist.

Regards

 

[mk3223]

IMO, the perfect equipment with zero C.A. in the operation isn't existed at all.
The typical corrosion allowance (C.A.) is required as a design criteria on the Client and Engineering practices for either CS or CRA material of the production equipment, similar to the process piping application.

 

[EdStainless]

While a lot of CRA equipment is designed with 0CA, it isn't built to the absolute minimum thickness allowed.
There are always question of tolerance and fabrication that must be allowed for.
And with CRAs it isn't thinning that you are concerned about, it is localized corrosion.
A lot of this comes down to knowing what to look for and where it is most likely.
I know of many SS vessels that got their first detailed inspection at 10years.
But they knew the process.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[r6155]

The adopted thickness minus the required thickness acts as corrosion tolerance.

Regards

 

[r6155]

I suggest an inspection the first year of service. After that you can define the next one.

Regards

 

[GD2]

It's normal to built pressure equipment/piping with 0 CA.
Think of a pressure vessel ASME Sec VIII Div-1, SA-516 Gr 70 Plate:
When the plate is ordered, it is ordered to the "Minimum Required Design Thickness" which is essentially the 'design Thickness" with no CA.
You will also note the following Code requirement (Say ASME Sec VIII-1 UG-16(c)):
1) Mill Under tolerance.
Plate material shall be ordered not thinner than the design thickness. Vessels made of plate furnished with an undertolerance of not more than the smaller value of 0.01 in. (0.25 mm) or 6% of the ordered thickness may be used at the full design pressure for the thickness ordered.

2) If the specification to which the plate )is ordered (SA-516) allows a greater undertolerance (given in SA-20), the ordered thickness of the materials shall be sufficiently greater than the design thickness so that the thickness of the material furnished satisfies UG-16(c).

The above two conditions will end up in getting a plate with higher ordered thickness than the "Required Design Thickness". This plus UG-16(c) that allows a negative margin of (smaller value of 0.01 in. (0.25 mm) or 6% of the ordered thickness) will give an extra thickness that can be used for the full design pressure.

When you do the FFS, you can set the UG-16(c) wall thickness as the minimum thickness for MAWP. Because the ordered thickness will be higher than this, the FFS will be assessing the extra thickness that comes by virtue of ordering, even with no CA.

Hope I could clarify.


GDD
Canada

 

[KevinNZ]

It does not matter that the plate used for construction is thicker than the minimum required for the design. If the drawings say use 12.7mm (1/2") plate and 0mm CA and inspector finds 12.6mm then the vessel is not compliant with the drawings/design and FFS kick in. Inspectors do not know what the minimum required for the design was nor whether the extra thickness has been used for nozzle load, EQ strength etc.

 

[Andrew ONeill]

@KevinNZ, completely agree this is what happens in real life. Then the equipment remains out of service for a longer time while a bunch of engineers go and do FFS work. The original designer doesn't care, they've moved onto the next project and continue to "save money" by eliminating all excess material from their next design.

By the customer now incurs a business cost (lost production) that is orders of magnitude more than the added cost of 0.5mm of steel in the first place.

If vessels are designed with 0 CA I highly suggest the design minimum thickness is clearly shown on the drawings for each component, alongside the specified fabrication thickness. That way an inspector can assess against this, instead of the corrosion allowance number (I have seen this in practice for some vessels and it makes life a lot easier for all involved).


Andrew O'Neill
Specialist Mechanical Engineer
Australia

 

[FoxRox]

Extra shell thickness above the minimum required is often (almost always) used as credit for opening reinforcement, so it would not be valid to consider that as after-the-fact corrosion allowance.

Not to mention the effect CA has on weld sizes.

CA must be included in the original calculations, or there is none.

I agree a vessel should always have CA, but I don't argue if a customer specifically requests zero CA.

 

[r6155]

@ KevinNZ
Your example is wrong: 12.6 mm is within tolerance.
The inspector knows the tolerances, he also understands the calculations.

Regards

 

[r6155]

1) There is PV that CA = 0, it depends on the product.
Example C= 0 for NH3 service in carbon steel.
2) the plates are purchased according to the commercial thickness: 12.7 mm 25.4mm......etc

Regards

 

[Diego N. Bruzzone]

Thanks for your answer. I work for the end user and you described the situation exactly as it occurred. I realized this not years after purchasing the equipment but a couple of months later. This occurred on equipment purchased through larger equipment integrators. Small pressure vessels incorporated into lubrication systems of large rotating equipment, for example.
This has not happened to us on a single occasion, I have found several pieces of equipment provided in these conditions. And they are ASME stamped pressure vessels. From different suppliers, from different countries.
This should be a wake-up call for all end users.
I am very aware of the damage mechanisms described in API 571, which is what applies to the industry where I work. I am aware that the remaining life of equipment may not be defined solely by the corrosion rate. But, unlike other damage mechanisms, corrosion can never be completely neutralized. Therefore, in the absence of other damage mechanisms that shorten the life of a piece of equipment more quickly, the reduction in thickness due to corrosion will be what defines the remaining life of a piece of equipment. Do you agree?
I agree that in corrosion resistant alloy equipment the corrosion rate can be very low for very friendly services. However, my question is about how to manage the integrity of this equipment in compliance with a specific standard. API 510.
The adopted thickness cannot be used as CA without reducing the MAWP, which is by definition a re-rate.
Let's remember the definition of MAWP. It is the maximum pressure that corroded equipment can withstand, with the thickness with which it was built.
An inspection after one year of service is the API recommendation when the corrosion rate is not known. Another thing is to explain to plant management that we must inspect all new equipment after one year of service, because we do not know if it had at least one year of useful life.
Inspectors only know how much material is being taken to reinforce a connection or for a change in shell section, or to resist external loads, when they do an FFS. Let's remember that this is only applicable for individual parts. On a day-to-day basis, the first alarm sounds when the AC runs out, this is not an opinion, it is the management proposed by API 510.
In my case, the application of API 510 as an integrity management system adopted by the company. Insurance premiums are calculated based on compliance with this standard. Only API 580 can be applied instead of API 510 because API 510 stipulates it. Individual opinions have no place.
So, in that context, I renew the query, how are devices with CA=0 managed according to API 510?

Regards

 

[r6155]

@ Diego N. Bruzzone
i suggest a reading of:

(API 510) Acoustic emission examination for detecting structurally significant defects. ASME Code, Section V, Article 12
provides guidance on performing acoustic emission examination.

Regards

 

[GD2]

Diego,
I went through your first post again and realize that you are trying to decide on an inspection plan (API 510/API 580) on a vessel made with 0 CA.
Here is my say:
1. The vessel is not supposed to corrode. This was what was established by the designer with the given process conditions.
2. As field Inspection, you may not have requirement to insect this vessel because there is no corrosion.
3. You may not list this vessel with other vessels in your next inspection schedule because this is non-corroding vessel.
4. In actual operation, it may not go perfectly the way it was designed. there could be unforeseen corrosion/errosion or other damage in service.
5. I would suggest installing corrosion monitoring device or actual thickness measurement until you are satisfied that there is no corrosion.
6. The concept of FFS is different. You don't call out until there a defect/damage is found. To say, FFS is the next step at the end of your regular API 510.
7. On the question on MAWP/Design Pressure - The Owner has to ask for calculation of MAWP in their P.O. MAWP is calculated after the vessel build up. If this is not done, the Design Pressure equals MAWP.

Hope it helps.

GDD
Canada