1-ATM Flat Disc Viewport Design   


PRELUDE
"It is worth remembering, that since its inception in 1977, the Standard (ASME PVHO) has maintained a perfect safety record over -14.5 to 20,000 psi (-0.1 to 138 MPa) pressure range in hundreds of different manned and unmanned applications. It would, therefore, be foolhardy to ignore the guidelines for acrylic window installations in pressure vessels for human occupancy presented by the Standard."     Handbook of Acrylics, Stachiw, Page 1035



MATERIAL
Viewports for small submersibles are a critical component of the vessel.   If not designed and manufactured correctly, the viewport could fail at depth and cause serious injury or death.   Viewports are required to be fabricated out of Acrylic material that has been manufactured to meet or exceed ASME PVHO standards.   Some brand names that meet these standards include Plexiglas G, Plexiglas GP, and Acrylite GP.

Always ask your supplier for the specification sheet that applies to the material you want to obtain, and verify that the material meets or exceeds ASME PVHO standards.

Steel of appropriate grade and dimensions is the ONLY appropriate material for the viewport housing (seat and rim), and MUST be fabricated according to ABS or ASME standards.   Such details are dependent upon design criteria and beyond the scope of this basic knowledge article.



DESIGN ATTRIBUTES

  1. The acrylic lens for the viewport must be dimensionally sufficient to withstand the water pressure at maximum design depth.   The design parameters include water depth, lens diameter, and lens thickness. See the Acrylic Lens Calculator for an illustrative design based upon various parameters.

  2. Every acrylic plane disc viewport needs to rest in a seat support that penetrates the submersible hull and is sized appropriately to bear the stresses that will be placed upon the viewport when the vehicle is submerged. The seat support itself is a hull penetration and must be fabricated according to ABS and/or ASME PVHO standards.   See the Viewport Thru-Hull Calculator for an illustrative design based upon various parameters.

  3. For plane disc viewports, the bearing support (the flange that the viewport rests upon) must provide a specific ratio between the viewable segment inside the submersible and the overall diameter of the viewport.

    In the diagram below, you can see that the overall diameter of the viewport is marked as Do.   This is the high pressure side of the viewport that is exposed to water when submerged.   You can also see that the viewable diameter on the inside of the hull is smaller and marked as Di.   This is the low pressure side of the viewport that is exposed to the 1-ATM interior of the submersible.

    ASME PVHO standards require that the ratio of Do/Di must be 1.25 or greater.
    Do/Di = 1.25 (or greater)

  4. To calculate these dimensions, first decide what size viewing diameter you want to provide on the inside of the hull.   Next, multiply that dimension by 1.25 and you will have the required overall diameter of the viewport.

    Example: (inches)
    Di = 8
    Do = 8 * 1.25 = 10

    In this example, we want a viewport that will provide 8 inches of visible diameter as seen from inside the submersible cabin.   We multiplied 8 * 1.25 and calculated that the outside diameter must be 10 inches to comply with ASME PVHO standards.