Flange and Gasket Selection for External Pressure

The external pressure at 5m depth is only about 0.5 bar. Unless something else is going on here, Class 150 flanges will be more than adequate, whether flat or raised.

Thank you Tornado. Sorry i meant to say 50 meter depth. so external pressure is 5bar. if i use raised face type WN flange then the Blind Flange will also be raised face. What type of Gasket i should use.

Weld-neck flange is fine. 150 lbs configuration is the usually most readilly available and therefore least expensive. Face configuration of either types you stated. Almost any gasket compatible with air and the water is OK. Just remember to torque the flange bolts/nuts to the proper specified torque and with a "sdtar pattern.

"Torquing" has astutely been recognized by most of the world's offshore industry as being highly unreliable in subsea applications (and, for the same fundamental reasons, also being unreliable on many critical topside applications).

Yours seems like a non-critical application which will not have adverse effects to either the environment or to personnel safety if it leaks. However, if you are concerned about the integrity of other subsea joints, you would be well advised to join your contemporaries and consider "tensioning" as a much more controlled method of bolting.