5.2 Plant maintenance: Pressure Vessel Repair

Operation stage

• Operator: Ensure these vessels operates within their specified design limits and measures shall be taken to ensure no overpressure beyond the specified allowable times. Not more than 500h/yr for 20% overpressure and not more than 100h/yr for 33% overpressure. 

• Mechanical Integrity: Ensure these vessels are constantly monitored on-stream visually, check all vessel body paint, nozzles, stud bolts, supports and anchors are in good condition without distortion or corrosion, inspect under insulation if any suspected damage to insulation are observed and apply suitable non-destructive testing over time time of operation to observe any signs of material failure, especially in processes susceptible to such failure modes. Read API 570 for the recommended inspection intervals and API 571 damage mechanisms for some examples of damages with photos. 

Repair stage

• What constitutes to a Repair? 

A method to restore the equipment condition to where it can operate safely within the Maximum Allowable Operating Pressure (MAWP) and Allowable operating temperatures.

Adding of nozzles can also constitute to a repair, if
- It does not require a reinforcement pad
- It is smaller than any existing nozzles on the vessel

Adding of any other nozzles are termed an "Alteration", which usually comes with rerating as the vessel strength could be significantly reduced.

• Avoiding Repairs 

First thing when problems are identified, management point of view may be safety, cost, operational we don't know. But these are all the potential problems to avoid a repair, can we actually avoid?
The answer is yes, and how do we do it?

•  Rerating
• Rerating to a lower MAWP and operating pressure for thinned vessel walls for continued operations until minimum thickness is hit again. This is usually the easiest way out.

• Rerating shall be performed by an Engineer or Manufacturer.

• Fit For Service Assessments (FFS)

• In reference to API 510, Chapter 7, it shows how to assess the pressure vessel in damaged condition to extend its life. Not all scenarios are covered, they cover only the usual problems for example pitting, localized wall thinning and wall thinning near by a weld seam.

• If the problem cannot be found in API 510 Chapter 7, move on to API 579-1 or ASME FFS-1 for the full assessment. They come in level 1 to 3, for 1 is simplest and 3 is detailed. 

• Level 1 are usually performed by End User Engineers, and level 2 and 3 usually outsourced to software or consultants. 

Nevertheless, basic Risk Assessments should at least be carried out to evaluate the Consequence or Probability of a leak before putting the damaged equipment back to service.

• Is it hazardous to working persons?
• Is it environmentally harmful ?
• Would it affect operations and customers adversely? 

• When are Repairs required?

The number one reason for a repair would be corrective maintenance. This is usually carried out as follows:

• [Unplanned] As leakage or damage has been initiated.  
• [Unplanned] Thinning beyond acceptable thickness, damage is unsafe for continued operation, rerating is not possible and FFS assessment fails,
• [Planned] Equipment suffers from damages which can last till the next outage 

• Repairs in USA and Repairs elsewhere

National Board U-stamp vessels: In the USA, the repair shall be performed by a certified NB R-Stamp repair shop under approval of Authorized Inspector in compliance to NB-23.

• If outside of USA, check local jurisdiction or company best practice for R stamp requirements. If required is usually for Quality Assurance or Company requirements 

• Generally pressure vessel repairs without R stamp are commonly done. Alternative codes and guidelines can also be used for repairs such as ASME PCC-2 or API 510. Check Company guidelines, if none to consult a pressure vessel engineer.

• API 510 Inspection, Repair, Rerating & Alterations: 
• Alternatively an API 510 repair procedure can be used also under the approval of the Authorized Inspector. Refer to Appendix D of API 510 document for the check list

• Typical repairs based on API 510 includes Temporary and Permanent Repairs.

• Ensure that all minor repairs are authorized by the pressure vessel inspector before commencement. Approval for major repair may not be given for an ASME Section VIII designed vessel until approved by an experienced Pressure Vessel Engineer.
• Crack repairs shall be consulted with the pressure vessel engineer before proceeding, as cracks may propagate even after the repair. 
• Pressure tests may not be required after the repair. Unless it is believed to be necessary by the inspector. Pressure test if required, may be waived if suitable Volumetric NDE are in place to ensure integrity of the repair.

• Temporary Repairs includes:
• Lap patch - This is also commonly known as doubler, sometimes could be done on-stream as emergency repairs, if the process fluid is not hazardous or flammable, and welding process permits so.  A plate of usually similar material, properties and profile is welded to the external wall of the pressure vessel over the leaking spot. There are size/thickness restrictions where you need to calculate and avoid welding over to or near existing weld seams, the edges needs to be rounded to minimum 1" radius.

• Pipe Cap/Nozzle - Welding a pipe cap or nozzle to seal in the leaking spot. These are non-penetrating.

• Leak sealing - Possible for very small pressure vessels, usually below 8" diameter. The leaking spot could be box in or wrap sealed by epoxy-metal binders.
• Lap band repair - A huge band is attached around the vessel body, this is not recommended unless the repair needs to remain in place for a longer period of time.

• Permanent repairs includes:
• Shell course replacement - cutting out the damaged section for replacement. This is usually an expensive job, especially if done on sections of a huge vertical column, high costs largely attributed to lifting and manpower.
  
  
  

• Strip lining - a thin layer of sheet metal, usually of a superior material is lined internally. Cheaper than the other methods.

• Insert Plate - A piece of the pressure vessel is cut out and replaced with a new piece of metal plate of the same profile over the spot of leakage. The edges should always be grounded unless the side falls on an existing weld seam. In contrary to the sketch, the corners of the insert plates shall be rounded to eliminate residual stresses.
  

• Weld overlay - Adding a layer of weld filler metal over the thinned down sections of the vessel to restore the wall thickness. Very tedious and time consuming. Watch the manhours.  Take note that the repair thickness shall not be more than 50% of the minimum required thickness of the vessel (exclude Corrosion Allowance)
  

• Groove Repair - This method usually used for cracks and pinhole leakages. The section where leakage is present is grind down to a U or V groove. Weld metal is then deposited to restore the surface.

• Additional Considerations
• Materials 
• SS Cladding/Plate lining to P3, P4, P5 materials tends to crack when excessive heats are applied during welding. Check for delayed cracking with UT 24h after job completion.
• Hydrogen service vessels shall be outgassed and checked for its hardness after welding

• Post Weld Heat Treatment
• Check whether if vessel to be repaired require PWHT during fabrication. If yes, PWHT shall also be applied during repair in compliance to latest edition of ASME VIII Div 1 requirements (or refer to MDR). PWHT by oven is unlikely for in-service vessels however the following are usually used
• Local banding PWHT
• Preheat in place of PWHT
• Controlled Deposition Welding in place of PWHT

Check API 510 Section 8 for the full requirements of PWHT by alternative methods, as there are certain material tests which needs to be performed, certain requirements to welding procedures on top of ASME IX WPS/PQR requirements, certain requirements for temperature, certain requirements for welding electrode type and not forgetting material considerations.

For additional repair procedures and guidelines on top of API 510, refer to ASME PCC-2.