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Below Dr. Stachiw describes his professional opinion on what he has calculated and tested verses what the PVHO standard has adopted. When I see data from experts like this, I have further reason to not wish to follow every thing that ABS and PVHO currently considers to be best practice. Another case in point is when Paul Moorhouse was building Alicia, and ABS required him to use a scaled down tanker prop shaft. He found that to be massive over kill and it added a lot of weight in the stern he didn't need there. Plus the extra cost of the larger parts. EFFECT OF ASME PVHO-1 SAFETY STANDARD ON THE DESIGN OF VIEWPORTS IN PRESSURE VESSELS Dr. Jerry D. Stachiw PE, Fellow ASME STACHIW ASSOCIATES "PROJECTED PERFORMANCE OF THE WINDOW IN SERVICE The Standard specifies design life for Standard Geometry windows configured on the basis of the Standard. Unfortunately, the statutory authorities confuse the concept of design life with service life insisting on removal of windows from service upon expiration of the design life, regardless of the actual condition of the window. Experience has shown that frequently some of the window removed from pressure vessels at the expiration of their design life set by the Standard still meet the acceptance criteria for brand new window and, therefore, should have remained in service. In other words, the expiration of design life assigned by the Standard on the basis of expected typical service conditions does not necessarily signify the termination of service life. One needs to distinguish here between the design and service life of acrylic windows. The design life is the projected service life in years of a window under typical service conditions and severe ambient environment. The service life, on the other hand, is the actual life of a window under actual service parameters and ambient environments that affect the windows after installation in a given pressure vessel. The service life terminates when the safety margin between the maximum working pressure (i.e. design pressure) and the short-term critical pressure at peak ambient temperature decreases to 2 (i.e. 100 percent margin of safety). If a window designed on the basis of ASME PVHO-1 Safety Standard criteria is cycled less than 10,000 times to a working pressure of lesser magnitude than the design pressure, the sum of pressurization durations is less than 40,000 hours, the peak ambient temperature is less than the design temperature, and the window is not exposed to weathering, the service life of such a window will exceed its design life by 10, or more years, and its cyclic fatigue life by 104 or more cycles. On the other hand, a window subjected over 10,000 cycles to design pressure at peak design temperature under continuous weathering may have to be replaced prior to expiration of design life if its STCP has decreased below the safety margin of 2 required by ASME PVHO-2 Safety Standard. What this signifies is that relying totally on the length of design life assigned by the Safety Standard as the sole reliable indicator of service life is neither safe, nor economical. In one case, unsafe windows (i.e. with safety margins less than 2) are kept in service until design life expires, while in the other case a serviceable window (i.e. with safety margins in excess of 2) are removed from service. The only approach to prevent this is to rely on regularly scheduled inspections commencing immediately after installation in the vessel. The in service inspection procedures for PVHO vessels are covered by ASME PVHO-2 non mandatory In-Service Guidelines. With the help of these procedures, it is feasible to detect early deterioration of the window or good condition at the end of the design life. The underlying philosophy of the in service inspection procedures is that the absence of visible deterioration of acrylic (i.e. discoloration, crazing, or cracking) by itself does not guarantee absence of material deterioration that might make the window unsafe for continuing service. Only if that observation is additionally supported by test findings on windows of the same configuration in similar service environments can the absence of visible deterioration be considered to be a valid indication that significant deterioration of acrylic has not taken place and that the window may remain in service. SUMMARY The ASME PVHO-1 Safety Standard represents the minimum design requirements for a safe viewport with acrylic window for a given design pressure and temperature. The designer is encouraged to exceed those requirements within existing design constraints in order for the windows’ service life to extend significantly beyond the minimum design life assigned to it by the Standard. The minimum requirements mandated by the Standard that should be exceeded routinely during the selection of design criteria are: 1. Select the upper limit of the temperature range on the tables listing the CF factors (i.e. do no interpolate the CF factor on the basis of the selected design temperature) 2. Increase the calculated value of minimum window thickness by > 0.250 in (6 mm) to provide extra material for future removal of gouges and spalls on window surfaces without reduction of the window’s depth rating based on minimum thickness. 3. Increase the soaking time specified by the Standard for annealing of window after completion of all fabrication procedures." Page 6 http://www.hydroports.com/documents/New_Orleans_Meeting-Feb_2004.pdf Regards, Szybowski |