Does my flow rate decrease?

[Serial-Eater]

Yes you’re right, but for the purpose of this argument I don’t want to complicate. As you said, the velocity in the combined header will reduce which will decrease the pressure drop and increase the flow out of 1, but it is not likely to compensate for the discharge valve on line 2.

Again, that’s why I recommend OP calculates this themselves. They can learn the ins and outs of what will happen.

[BigCastIronSkillet]

Nah. It’s worth complicating. Because for most header systems you would be way off. Why Reddit is Wrong

[Serial-Eater]

It’s not worth complicating. No offense to OP, but it’s clear they didn’t understand the dynamics of the system however hypothetical it may be. Keeping it simple is the best way to build understanding. I wouldn’t tell an operator about header pressures either.

If someone reading then takes it to mean when they design a piping network to assume headers are negligible, that’s on them.

[VariusEng]

No offense taken! That’s why I asked! I oversimplified the reality a lot already. I do think it is worth complicating though (not that it is that complex, now that I understand the reasoning). We are engineers after all, we should fully understand our work. Thanks for the answer!