<< Up >>

OK, I got through eventually. But I was stuck for long enough, because every door in the game up to this point opens when you walk up to it. If it doesn't, it's either locked or barred from the other side, or has a really obvious button to 'use'.
The pipe hatch has no obvious button, and makes the "I'm locked" noise when you walk up to it. Every experience up to this point says that these cues mean that the door is locked and some event needs to trigger to open it.
Even setting prior experience aside for the moment, it is the only 'door' in the game which is totally featureless, and (in real life) would have to be opened simply by pushing against it. The only logical thing to do is walk up to it: there is no knob, lever or handle for Gordon to 'use'!
Valve really dropped the interface design / user expectation ball with this one.

(Admittedly this isn't nearly as bad as when I played original Half-Life; I had to redo practically the entirety of Surface Tension. The Barney you rescue just after the flaming pipe incident steadfastly refused to unlock the door for me, due to some glitch in where the AI chose to stand upon entering the security station... and I didn't have a save point between the cliff face and his rescue.)

<< Up >>