Lethal Service PWHT Requirements of a Carbon Steel Vessel with Clad / Weld Overlay

[AWDMIKE]

Our client has a pressure vessel to be used in lethal service. They have explicitly stated that the vessel may be constructed using one of two options:

1. Carbon steel in its entirety (which would require PWHT per UW-2), or,
2. Carbon steel with a chromium-nickel based cladding or weld overlay “to avoid PWHT”.

The client is suggesting that the PWHT requirement in UW-2(a) may be avoided by preventing contact of the “lethal” substance with the carbon steel material.
Upon reading UW-2(a), it states with reference to pressure vessels containing lethal substances that:
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When fabricated of carbon or low alloy steel, such vessels shall be postweld heat treated.
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UW-2(a) seems to be clear; that any pressure vessel in lethal service constructed of carbon or low alloy steel shall be PWHT. There aren’t any exceptions or footnotes within that statement.

The first argument that has been used is that Table UCS-56-1 general note (b)(3)(e) may be used to avoid PWHT, which reads as follows:
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(b) Postweld heat treatment is mandatory under the following conditions:
(3) for welded joints of all thicknesses if required by UW-2,except postweld heat treatment is not mandatory under the conditions specified below:
(e) for corrosion resistant weld metal overlay cladding or for welds attaching corrosion resistant applied lining (see UCL-34) provided preheat to a minimum temperature of 200°F (95°C) is maintained during application of the first layer when the thickness of the pressure part exceeds 1-1/4 in. (32 mm).
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This first argument suggests to some people that a corrosion resistant weld metal overlay cladding may be applied to carbon steel and that no PWHT need be performed for the vessel in lethal service. I believe the intent of the Code here is to allow the fabricator to attach a corrosion resistant barrier to a lethal service vessel after it has undergone PWHT without the requirement to re-PWHT the area. Notes (a) – (d) appear to be consistent with this intent.

The second argument uses the reference to UCL-34 which states:
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(a) Vessels or parts of vessels constructed of base material with corrosion resistant integral or weld metal overlay cladding or applied corrosion resistant lining material shall be postweld heat treated when the base material is required to be postweld heat treated.
When the thickness of the base material requires postweld heat treatment, it shall be performed after the application of corrosion resistant weld metal overlay cladding or applied corrosion resistant lining unless exempted by the Notes of Tables UCS-56-1 through UCS-56-11.
(b) Vessels or parts of vessels constructed of chromium stainless steel integral or weld metal overlay cladding and those lined with chromium stainless steel applied linings shall be postweld heat treated in all thicknesses, except vessels that are integrally clad or lined with Type 405 or Type 410S and welded with an austenitic electrode or non‐air‐hardening nickel–chromium–iron electrode need not be postweld heat treated unless required by (a) above.
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The second argument suggests to some people that a cladding or lining may be used and need not be PWHT if the carbon steel portion need not be PWHT (and they suggest UW-2(a) does not apply since the fluid does not contact the carbon or low alloy steel). In our case the thickness of the carbon steel portion would not require PWHT by UCS-56. This argument implies that the requirements of UW-2(a) do not apply if the liner is in place, and in other words, the liner produces a barrier from the fluid whereby the barrier contains no carbon or low alloy steel.

That brings us to some ASME Code Interpretations. The first one is:
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VIII-1-83-207
Question: Would a vessel fabricated of carbon steel plate material be required to be postweld heat treated per UW-2(a) of Section VIII, Division 1, when the inside of the vessel is lined with a corrosion resisting chromium-nickel stainless steel lining?
Reply: Yes. See UCL-34.
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This reads that no internal lining (at least one comprised of chromium-nickel stainless steel) would exempt the vessel from the PWHT requirement of UW-2(a). The reply, however, refers the reader to UCL-34, perhaps to warn them of the potential dangers of PWHT on unstabilized austenitic chromium–nickel steels.

The second interpretation is:
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VIII-1-95-137
Question (1): Are the requirements in UW-2(a) of Section VIII, Division 1 for performing full radiography and postweld heat treatment on vessels which are to contain lethal substances applicable regardless of the calculated pressure and thickness for the vessel?
Reply (1): Yes.

Question (2): Are the requirements of UW-2(a) superseded by the provisions of UCL-34 and UCL-35?
Reply (2): No.
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So while the first question does not address lined/cladded/overlayed vessels, the second question indicates that UCL-34 (for PWHT) cannot supersede the requirements of UW-2(a).

Is it so obvious to everyone who has read this long post that our lethal service pressure vessel, fabricated with carbon steel of any thickness and lined/cladded/overlayed with a corrosion resistant chromium-nickel stainless steel or any material deemed to be corrosion resistant, must be PWHT after the lining/cladding/overlay is applied?

Thank you to everyone who has read this lengthy post and for your replies.

 

[metengr]

My reply is this, if a carbon steel vessel is to be placed in lethal service, UW-2(a) applies meaning PWHT is mandatory. What you do with the vessel internals and application of a corrosion barrier coating does not apply in UW-2(a).

If you read Note 2(c) below

for welded joints of all thicknesses if required by UW-2, except postweld heat treatment is not mandatory under the conditions specified below:

The first part of Note 2(c) states PWHT is applicable if required by UW-2, otherwise PWHT for vessels not fabricated to UW-2(a) the following exemptions below apply.

 

[marty007]

AWDMIKE,

I would agree with your interpretation.

My personal way of thinking about cladding is that the pressure boundary is the base layer, and all requirements for NDE/heat treatment/etc apply to the base layer. Whatever you choose to line it with doesn't affect the base layer with respect to the above mentioned details.

Aside from that, the code adds a number of provisions on how the layer is applied and as you discussed PWHT requirements with respect to the base layer.

(I'm sure there are other details, but this covers about 99% of cladding applications)

As a side note, I appreciate your thorough post. Clearly you've put a lot of thought into this already.

Cheers,
Marty

 

[AWDMIKE]

Thank you both for your replies. If it wasn't clear from my first post, I also believe it must be postweld heat treated, with the understanding of the warnings coming from UCL-34.

For whatever reason, this seems to come up every so often and it seems that there is always someone with the "new" idea that if they can keep the fluid away from the carbon steel then they can avoid the lethal service requirement.