It is a good idea to maintain the correct order while destroying the tanks. Otherwise, you may have problems returning to some places without raising suspicion of your opponents. The extraction point is in the northern part of the base.
In the second hangar, jump down to the floor and follow the narrow corridor to the west. In front of you, you will soon notice an armored car and under the stairs that you are standing on there is the second valve (M8,8), which you have to use. Then, return upstairs, to the second level. It may be difficult to distract the opponents here, so it is better to snipe them down (there are two guards below and one in the connecting corridor opposite). You had better climb down the stairs to the North nd jump over the barrier to prevent being spotted by the sniper here.
Return to the hangar (M8,9) and climb up the stairs again. Then, head for your next objective, i.e. the largest tanks. It is on this level that you find the valve (M8,14) to the north. You will attract no attention while using it.