To find the secrets in Doom level Refinery:

When your first appear on this level, you'll be looking
south, down a wide hallway, towards a large rectangular
cage in the distance.
Walk to the end of the hallway and turn left to begin to
walk around the cage.
Behind the cage to the south is a long wall with 2 open
doorways in it: one to the southeast, one to the southwest.
Go through the left-hand (southeast) doorway, and you'll
step into a hallway.
Walk down the hallway, passing the blue-keyed door on your
left.
Continue down and around this hallway until you come to a
doorway on your left (west), but do not enter it yet.
Turn to face east (so your back is to the doorway), and
look at the wall in front of you.  At the far-left end of
this wall is a secret panel.  Walk up and Activate it like
a switch to open it.  When the panel opens, step through
and walk into the chamber get credit for a Secret found.

Step back out of that chamber, and turn west to face the
nearby doorway.
Take a step through that doorway, and turn left to face
south.  In the wall in front of you, at the southeast end,
is another secret panel.  Activate it like a switch, and it
rises up.
Step through, and you'll get credit for a Secret found.

You'll be in an egg-shaped room with a small pool of green
slime in the center.  Turn right (west) and go out through
the open doorway in the west wall.
Pick up the shotgun and continue west through the next open
doorway ahead of you.  You'll be standing in the south end
of a long north-south hallway, and you'll be facing west.
Look for a doorway ahead (west) and slightly to the left,
and step through.
Continue west several paces until you come to another open
doorway in the west wall.  Step up to this doorway, and
you'll see a pool of slime below you.  Jump down into the
slime and quickly turn right, and run into the stairway
you'll find there.
Walk slowly up the stairs to get credit for a Secret found.

At the top of the stairs, drop down into the small room
ahead.  Pick up the blue keycard you'll find in the north
end of this room, then exit the room through a door in the
east wall.
Walk east several paces and turn right at the end of the
corridor.  Go straight (south) until you come to an open
doorway on your left (east).  Turn and step through it.
Continue straight (east), back into the egg-shaped room
with the small slime pool in the floor.
Go around the pool to the right, and look for a vertical
section of the wall in the southeast corner of the room
that opens when you Activate it.  When that wall section
rises, step through to get credit for a Secret found.

Walk straight (east), and follow the right-hand wall around
north, until you come to the blue-keyed door on your right.
Walk through the blue-keyed door and turn right.  Follow
the corridor south and around to a small, open courtyard.
At the far end of the courtyard, turn left and look for an
open doorway in the northeast corner.
The doorway leads into a darkened area crowded with 4-sided
floor-to-ceiling pillars.  Step in, and look for a doorway,
far ahead and off to the right, at the far northwest end of
this darkened area.  That doorway leads to a wide platform
at the edge of a large rectangular pool of green slime.
Step to the right-hand edge of that platform, and turn to
face the slime pool.
The slime is corrosive, so you'll want a radiation suit...
There is one located in a small chamber on the other side
of the wall to your right: Step off the platform and make a
quick U-turn around to the right, to get into that small
chamber.  Pick up the radiation suit to get credit for a
Secret found.

Step back into the slime pool, and turn right (east).  Look
for a long, narrow tunnel that leads away from the pool.
Go east down that tunnel, and follow it as it turns right,
leading into another, smaller, enclosed slime pool.
Walk into the center of this pool, then turn around to face
the northeast corner.
To the right of the tunnel you just came from is a short,
narrow passage that contains a Soul Sphere.  Pick it up to
get credit for a Secret found.

To get to the level's exit area, go back through the short
tunnel, and into the large rectangular slime-floored area.
Look for a large opening to the right, in the north wall.
Step through the opening, turn left (west), and follow the
winding hallway around to your right, and you'll find the
door leading to the exit.

PC Doom players can click here to access a demo that shows
how to find all official Secrets on this level.