pressures of two window groups with the 0.001 -inch (19,060 psi and 19,270 psi) and 

 0.025-inch (18,490 psi and 19,190 psi) radial clearances. The difference in the 

 average critical pressure was approximately 10%, the windows with the 0.150-inch 

 radial clearance failing at the lower critical pressures. 



It would thus appear that it is to the designer's advantage to specify small 

 clearances between the flat acrylic window and its flange, since by doing this he 

 accomplishes two desirable objectives. His design not only results in a window that 

 has superior critical pressure, but also is easier to seal in the flange. The small 

 radial clearances are ideal for sealing the window in the flange with a radial O-ring 

 seal, or silicone rubber potting-type seal. Because of these findings, the main body 

 of window test program was conducted with windows that fit into the steel flanges 

 with a 0.005- to 0.010-inch radial clearance. 



FINDINGS 



The exploratory tests in the window mounting investigation seemed to indicate 

 that (1) varying the Dq/D; ratio from 1.33 to 2.67, (2) changing the radial clearance 

 between the window and the flange from 0.001 inch to 0.025 inch, and (3) substituting 

 a radial O-ring seal for a grease seal have no significant influence on the critical 

 pressures of flat acrylic windows with a 0.5 t/D| ratio. When the radial clearance is 

 increased to 0.150 inch, the critical pressure of the 0.5 t/D- ratio window is reduced. 



Whether these conclusions are applicable to flat acrylic windows with t/D- 

 ratios other than 0.5 is unknown. Some of the data generated in the main body of 

 the flat window program have raised serious doubts that the conclusions hold for the 

 whole t/D] range. For example, the critical pressure of windows with a nominal 

 0.167 t/D| ratio and a 1.5 Dq/D| ratio was discovered to be 723 psi for a radial 

 clearance of 0.005 inch and 2,100 psi for a radial clearance of 0.001 inch. 



Thus, it would appear that for t/D. ratios less than 0.5, any change in radial 

 clearance below 0.005 inch influences its critical pressure considerably. Future 

 studies will attempt to clarify this problem. 



26 



