First I make a regular square brush with a dark looking metal brush. Then I select CURVE, then PRIMITIVES, then CYLINDER.
Now you have a nice looking tube. You can stretch the brush to the desired lenght and width. then hit CTRL+B to select the bend mode. now you can hit TAB to select various bend modes...
Just try and try it all over again untill you have the tube you want. Problem with primitives is that you CANNOT scale the textures nicely. Thats why I use a dark looking metal texture. I also made a func_barrel from the tubes, So when you shoot at them there's oil and water comming out....

You then have to make sure it's indestructable, otherwise with a grenade explosion the tube will blow up which can make strange results like the brushes who connect the tubes hanging in mid air...
hope this helps...