PVHO Window Facts

It’s All About Acrylic

Acrylic is a strong clear plastic material that is widely used in aircraft canopies, aquariums, medical systems and submersible vehicles. It is a polymer of methyl methacrylate (MMA) with additives to enhance strength and UV resistance.

There are many different grades of acrylic available. This includes castings, extrusions and injection molded materials. Only select castings meet minimum strength standards needed for Pressure Vessels for Human Occupancy (PVHO). The requirements and safety guidelines for design and fabrication of PVHO windows is given by the ASME PVHO-1 rules.

Important Things to Know:

  • Acrylic becomes stronger at lower temperatures.
  • Maximum working temperature under PVHO is 150°F (66°C).
  • Capable of pressures of 20,000psi
  • Coefficient of expansion is eight times (8x) that of steel.
  • Solvents (such as chlorinated solvents, methanol, benzyl alcohol …) can ruin an acrylic window. Use only recommended cleaners for acrylic.
  • Typical life span of a PVHO window is 10 years and 10,000 cycles.
  • Only a limited number of window geometries are accepted for PVHO.
  • ASME PVHO-1 rules describe design and fabrication of new windows.
  • ASME PVHO-2 rules describe in-service maintenance of windows which includes allowable dimensions for scratches, gouges and blemishes.
  • No mechanical sanding or buffing is allowed on a PVHO window in service.
  • Acrylic can be hand polished to obtain a perfectly clear surface.
  • Acrylic can be repaired but only through an experienced manufacturer.
  • A major difference between acrylic and polycarbonate is that small scratches in polycarbonate cannot be polished out.