Sealed CIP Arm on Hot Liq. Tank

[brewingnewbie]

just noticed on the back of my hot liq. tank that the cip arm was capped shut.

should this be left open to vent and avoid pressure build up?
its a stem jacketed tank.

[Jephro]

Is there an overflow on it? There has to be some way it's venting...otherwise you would have collapsed it or blown it up by now.