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How They Work

A degraded pressure boundary is typically identified by NDE inspection or obvious signs of leakage. The PMCap repair method can be applied in either case.  The owner can identify the required size of the area that needs to be encapsulated or have PMC Engineering determine the size by review of the inspection data. 

The size of the area to be encapsulated is based on the required remaining service life specified by the owner.  Using the original Design requirements, inspection data and the total service hours to date, a conservative metal loss rate is determined to predict the size of the area that will be below Code minimum thickness requirements at the end of service.  The PMCap is sized to encapsulate the projected unacceptable area to ensure Code compliance is maintained for the duration of the required service life.  The PMCap dimensions conservatively bound the area that is projected to be below minimum thickness.

Various approaches are used to determine the optimum PMCap thickness based on application of corrosion allowances, corrosion resistant materials or linings.  Typically, PMCaps are designed so that they can be installed without dissimilar metal welds or post weld heat treatment.

TYPICAL PMCap Section View
Typical PMCap

The following information is typical information required to complete a PMCap design for a power plant application:

1. Component  Drawing showing dimensional data and shell thickness
2. Manufacturer’s Data Report
3. If Manufacturer’s Data Report is not available then the following information is required:
  a. Original Code of Construction including Edition and applicable Addenda
  b. Internal Design Pressure and Temperature
  c. External Design Pressure and Temperature
  d. Material - shell, head, nozzles, nozzle reinforcement pads
  e. Corrosion Allowance
  f. Joint efficiency factor (E) / radiography requirements
4. If PMCap encapsulation component is in the vicinity of a nozzle, or for a nozzle encapsulation, then provide Nozzle Loadings
5. Required PMCap material of construction if other than existing shell material.
6. Additional material requirements such as impact testing
7. Optional liner material -  A stainless steel (weld Number P8) impingement barrier liner is generally used but other materials such as low alloy steel (CrMo) are available.
8. If lap plates are to be encapsulated, specify length, width, thickness, and attachment fillet weld size of lap plates.
9. Dimensions of area to be encapsulated by the PMCap  or 
Note: The following data is not required if area encapsulated by the PMCap is specified by owner.
  a. Latest NDE data of shell thickness
  b. Total Service Hours at time of NDE inspection data – include Service hours at original operating conditions and service hours at power uprate conditions if applicable.
  c. Power uprate (percentage increase from original power rating)
  d. Required years of service after repair is completed (specify if continued service is at original operating conditions or at power uprate conditions).
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