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" A ship in port is safe, but that's not what ships are built for. " ~ Grace Murray Hopper Many of are standard PSUBS viewports are configured to have only one seal. So those that want a even tougher arrangement, and/or want to go by PVHO standards, you might want to consider having a primary and secondary sealing arrangement. "Mounting Arrangement The Standard specifies in great detail the acceptable mounting arrangements for the different window geometries to insure good sealing under design pressure and adequate support to the window’s bearing surfaces. The Standard provides great latitude in the sealing arrangements that the designer may choose. There are four mandatory requirements, however, that must be satisfied by the sealing arrangement chosen by the designer. 1. Two seals, a primary and secondary, located in tandem are required. They may take the shape of 0-rings or gaskets. The interface between conical bearing surfaces of the window and the seat may be considered as a secondary seal. 2. The two seals together must provide pressure integrity over the whole range of ambient temperatures to which the pressure vessel may be subjected in service. This means that for diving systems, the seals must keep the interior dry regardless of whether the system will be employed in the Arctic (-400F/-400C) or the tropics (1200F/480C). A few sealing arrangement and seal materials are competent to cover such wide temperature ranges. 3. At least one of the seals must keep the joint water-tight after a long-term sustained pressure loading causing the window to creep significantly. For the gasket seal under the window retaining ring acting as the primary seal for windows there must be sufficient precompression during assembly to insure that there is still enough compression of the gasket even at the end of the long-term pressurization that causes the window to displace significantly. 4. 0-ring grooves are not allowed on the surfaces of windows or on bearing surfaces in window seats as they act as crack initiators." EFFECT OF ASME PVHO-1 SAFETY STANDARD ON THE DESIGN OF VIEWPORTS IN PRESSURE VESSELS Dr. Jerry D. Stachiw PE, Fellow ASME http://www.hydroports.com/documents/New_Orleans_Meeting-Feb_2004.pdf Regards, Szybowski |