As a quick followup, I also tested and confirmed that two boilers connected directly to each other also have the same issue (you can walk between them despite the connected pipes). Adding a pipe between the boilers however causes the expected behavior (I am prevented from passing through the boilers ...

1. What did you do?

Create an offshore pump and connected a boiler to it (to the water connections)

2. What happened?

I discovered I could walk between the offshore pump and the boiler. The graphics overlay with my character's sprite as I pass through the space.

3. What did you expect to happen ...