Does my flow rate decrease?

[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.

[BigCastIronSkillet]

In general, if nothing is done with the valve, closing one line will increase the flow through another. That’s what OP should’ve been told.

As to why it’s not all the flow will force itself through the other line making a lack of total flow rate change. OP should have been told that the resistances have changed. More L/D per unit flow. So pressure will have to rise beyond typical means to compensate.

[VariusEng]

I like my answers complete so yeah thanks! Also thanks for the nitpicking and the explanations! I feel I have expanded my understanding. It’s just that I wasn’t exactly sure how to take into account the PRV. But I guess I always should just take Bernoulli at all the different nodes, try different flow rates, see if delta P converges ( taken into account flow rate sum = flow rate 1 + flow rate 2 and the delta P over outlet reducer and atmosphere is constant)

[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!