After testing different methods in hammer, I came up with this method for curving a pipe created by displacements.
This tutorial is about making the curve in a pipe created by using Subdivide.

The Vertex Tool
Using Displacements
Using Subdivide
And of course how the hammer editor works.
I'm going to try to make this understandable for any noobs, by explaining things, and using a lot of pictures.
Remember that the Questions are only for the step they are put in.
- Step1 , Creating the Displacement Brush -
First, create a basic block, I made mine 128*128*256. (Which is going to be a big pipe, big enough for a player to crouch inside <- Just used for you to get a clue how big it is, you're not going to make a hollow pipe in this tutorial)
This is the brush we will be making the pipe out of. I used the texture 'models/props_c17/metalladder002'

This creates a displacement, if you decrease the Power, you will get less moveable points on the displacement.

This removes the selected faces.

Copy the Displacement 2 times above eachother. (Copy: Drag and Drop while holding shift)
Copying it 2 times should leave you with 3 identical displacement brushes.

After a moment, you will have 3 cylinder shaped displacements but the upper and the lower one will have quadratic ends.


---- Q: What happends if I skip this section, and just Subdivide the first brush? ----

- Step3 , Connecting 2 pipes -
Remove the top and bottom displacements. (Use the delete button to delete them when they are selected)
You don't want any of the quadratic shaped ends.

This is how you want your pipe to curve, it doesn't matter what way it curves, because you can rotate it later.

Select the vertex in 2d mode, by dragging a box over it, just to make sure that you select the vertexes on BOTH sides (Look at the picture in 3D view, you'll notice that there are 2 red dots).


Now you should have a "connection" which looks like mine. Now, copy the displacement as shows in the picture (so it touches the original).
Just a step for making the pipe curve correct, this will be deleted later.

...

We want to do the same for both ofcourse.

This will create the curved pipe this tutorial was all about.


---- Q: What happends if I skip this section, and subdivides only the 2 displacements? ----

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---- Q: What happends if I skip this section, and just Sews the 2 displacements? ----

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---- Q: What happends if I skip this section, and use cylinder shaped displacements at each end (The ones we created in the beginning, Instead of the ones I used vertex tool on)? ----

- Step5 , Final Touch -
Copy/rotate around to make it fit in your level.
You now have a curved displacement pipe. In the picture below, I made some more pipes, with other dimensions.

- Example Map: Not Uploaded -
An example wouldn't help the creation of the pipe.