There are many ways to do this. Here’s one that’s very simple. This example took just over 5 mins. I’m using 2 addons - Bevel after Boolean and Quickpipe.
Here are the steps I used:
1 - start from a single vertex and extrude verts to create path of the pipes. Use vert beveling on corners depending on pipe bend required(or do this later if you prefer, at the mesh stage)
2 - create pipes from edges using Quickpipe
3- further adjust curves or convert to mesh when ready
4 - go through the process of using Bevel after Boolean tool to create intersections with smooth beveling. Note: you will need to understand how BAB works and what is required of input meshes before the operation, for best results. This is very easy. BAB has a long thread with all the info you need: [WIP]Bevel after Boolean
5 - delete pipe caps and add a Solidify modifier to achieve your tubular form. Note: you can also use a scaled down along normals duplicate of your BAB result to hollow out the mesh with a further boolean
6 - use bevel modifier or sub-div modifier or creasing or whatever method you want to smooth the final surface
7 - there may be cleanup of some edges around the intersections, due to unwanted surface shading and depending on your initial topology during the BAB, but this is easy and fast to do.
8 - you now have a watertight mesh ready for printing, with very little manual work.
Good luck.