Repairing PVHO Windows


Scratches and gouges are always a present risk. All PVHO windows are designed with a minimum required thickness but typically fabricated thicker than minimum. Small blemishes and shallow scratches can be hand polished out using acrylic polishing compound. Anything more is best performed by an experienced PVHO fabricator. The allowable depth and size of scratches and gouges vary per geometry and location of the flaw as prescribed in the PVHO-2 Manual. Repaired windows are provided with a PVHO Form VP-6 and need to be pressure tested per PVHO-1.


Crazing is caused by either thermal or mechanical stresses imparted in the acrylic molecular structure during production, transport, manufacture or in use. Crazing is not cause to take a PVHO window out of service, unless visibility becomes inadequate. Crazing is a sign of material degradation caused by induced stresses. Over repeated cycling, crazing propagates and must be monitored as these areas are most likely to form cracks. Cracks will dictate when a window must be removed from service (per ASME PVHO-2 Guidelines).

More on Crazing

Crazing is a micro-feature that is associated with the fracture and failure of the polymers in an acrylic material. Crazing shows up as a haze on the surface of the window made up of a multitude of very fine, hair-like straight or randomly oriented cracks that become clearly visible if illuminated at an angle by a bright light. Crazing is an indication of surface degradation. This may be thermally, mechanically, radiation, or chemically induced. This includes weathering, UV exposure, thermal shock or thermal cycling, local heating (such as mechanical sanding or polishing) and inadvertent use of cleaning solvents on the acrylic.

How is Crazing Repaired?

It is possible to re-machine, sand, polish and anneal the window and hope that all the affected material is removed but this always makes the window thinner and offers no guarantees that the original residual stresses are all eliminated. It is usually safer and simpler to replace the window.

How to Determine if a Window is Repairable

  1. Determine depth and size of scratch/gouge on window
  2. Refer to ASME PVHO-2 Manual (above) APPENDIX I to determine severity of the flaw
  3. If the window must be repaired, determine if it is “repairable”.
  4. Verify from PVHO-1 FORM VP2 the minimum window thickness calculated.
  5. Compare against the “Actual” thickness from the fabrication drawing.
  6. With the information from the flaw size, determine if there is enough material that can be machined to maintain the minimum thickness.
  7. If it appears to be repairable, submit the following information and documentation for a price quotation:

a. Dimension and description of flaw (photo preferable)
b. Maximum depth of flaw
c. Copy of window drawing
d. Copy of ASME PVHO-1 Form VP2 (design details)
e. Copy of ASME PVHO-1 Form VP1 (Actual as built Fab Cert)

Our Repair Process

  1. Any window that has been in service must be annealed for initial stress relief.
  2. Measurements of actual thickness and flaw dimensions verified to establish success of repair.
  3. Machining, sanding and polishing of window.
  4. Final annealing process to stress relieve window per PVHO
  5. Final measurement of window dimensions and completion of ASME PVHO FORM VP6 to describe the repair
  6. Return original marking on the window and mark repair on the window according to PVHO-1 marking protocol.

Hand Polishing & Cleaning Compounds

Complete Window Cleaning Kit for Acrylic

Complete Window Cleaning Kit for Acrylic

NOVUS 1 & 2 Window Safe Cleaner

NOVUS 1 & 2 Window Safe Cleaner

NOVUS 3 Acrylic Polishing Compound

NOVUS 3 Acrylic Polishing Compound


ASME PVHO-2 Guideline

The ASME PVHO-2 In-Service Guideline Manual is the ultimate reference to determine repair options. Small scratches can often be hand polished while larger scratches, gouges and blemishes require the windows to be machined, sanded, polished and annealed according to PVHO schedules. This is best done by an experience acrylic window manufacturer. Conditions for a PVHO window to be taken out of service are described in the ASME PVHO-2 manual, available on-line through ASME by clicking on the cover image on the right.

PVHO Documents Needed for a Repair

Several documents are required to process a PVHO repair. These documents are needed to determine the maximum allowable depth of the flaw and compare to the actual size of the window to maintain the minimum thickness required by PVHO.

  • Window Engineering Drawing
  • PVHO Form VP1 – Fab Cert (Old ENCL 5)
  • PVHO Form VP2 – Design Cert (Old ENCL 1)
  • Description and Depth of Flaw