Ok, so I tried applying the ellipses to the pipes I currently have. I am taking this in baby-steps if that's ok because I am still trying to get the hang of it.
So for now, two questions: 1. how do you know your ellipse is shaped properly? and 2. any advise on getting the shading on it right?
And thank you for the brilliant help.
Great, these curves are much better than the older ones. But keep in mind that if you mirror a half of one of your red lines, it must be
a complete ellipse and in some cases they arent ellipses.
1. how do you know your ellipse is shaped properly?
The easy way: Practice, practice, practice

Not only in pixel art, get your pen and paper and try it out on paper first. If its looking good than of course it is good but you need to learn to confess your failures to yourself. This is the only way to avoid the hard way.
The hard way:

Start from beginning with simple geometric objects, a vanishing point, help-boxes and so on.
1.-5. Keep in mind, that every (circular) pipe, duct, tube or whatever has a circle in a cut-away view. So if you want to draw it
in perspective, imagine how a ellipse would fit in a squared object. Every ellipse has a major diameter (green) which is almost the same direction of the squared diameter. Transform your circle to an ellipse with its major diameter leading to the squared one. Repeat this for your
backside of the pipe or some other segments to get your lengh.
6. These are some examples of pipes wich angles differs. Notice how the green diameter changes
But this is only a little help, I beg you NOT to do it like I did.
Its ok if you try this out once or twice but it is very time-consuming... You
better do it freehand and see how it fits the best.
2. any advise on getting the shading on it right?
In my opinion it is hard to tell right now but i try. The upper part of the upper pipe needs more contrast, it looks to plane right now.
The lower pipe needs more shadows or they must be bigger at the bottom.
So long,
Lorath