PetroStreet

In order to save the material cost and to have desired corrosion resistance three techniques are usually applied in pressure vessels over pressure containing but lesser corrosion resistant base metal:
1. Lining
2. Weld Overlay
3. Cladding

Lining
Corrosion resistant plates are stuck on intermittently inside the vessel just by attachment welds

Weld Overlay
Welding is performed on the vessel base metal directly with corrosion resistant welding material. This how corrosion resistant material is fused over the base metal.

Cladding
Pressure vessel plates are bonded with clad plates (corrosion resistant plate) using a specified cladding process like explosion bonding / friction bonding. The clad plate has 100% metallurgically bonded to the base plate.

For detailed requirements on all these three, see ASME Sec. VIII Div.1 UCL.

Where the cost impact in applying any of the above three is not justified, vessels are then directly fabricated from alloy steels having sufficient required corrosion resistance.

Ashfaq good information.... Can you specify the conditions based on which one decides either to go for lining /cladding / or weld overlay for a particular vessel?

You have to consider each and every aspect before to choose among lining or cladding or weld overlay option.
First consideration is the attachments of supports to the vessel internal. If the supporting system for internals is to apply a significant load onto the vessel wall, lining may not be the right option. And even if you choose weld overlay or cladding, be careful that the clad plate is bond shear tested or weld overlay is 100% ultrasonically tested for any dis-bonding. Cladding or weld overlay dis-bonding in vessels with high temperature, high pressure hydrogen service can result in hydrogen diffusion into clad dis-bond area which may lead to bulged area in cladding.
Then for those cases, where vessel may come under vacuum during operating conditions, lining may get detached. Designer has to consider the effect of vacuum on lining in that case.

Now we discuss the most important and concerned factor that is economy, often cladding or weld overlay is not economical for lower thickness vessels. For vessel thickness lesser than 3/4", it is not recommended to go for cladding or weld overlay as these processes are very expensive and time consuming especially cladding. So in that case, you may decide between lining option or to directly fabricate the vessel in high alloy corrosion resistant steel plate.

Hope this would help.

Ashfaq,
If there is a requirement of PWHT in cladded/lined/overlaid vessels then what measures shall be exercised to avoid disbondment as PWHT is done upto austanizing temperatures?

Get your answer here:
http://www.thepetrostreet.com/forums/vi ... ?f=11&t=51

Hope this would help.