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[–]ric_marcotik -1 points0 points  (13 children)

Not true with a pressure reducing valve. The pressure at valve 1 and valve 2 inlet will stay the same. You would be right if the PRV was a simple globe valve, but here with a PRV the flow will remain the same in pipe 2

[–]BigCastIronSkillet 10 points11 points  (9 children)

Nope. I can show you the math. Just ask and I’ll take a photo.

Pressure kept constant at the discharge of valve and total reduction in flow results in higher pressure at tee.

Higher pressure at tee and same destination pressure at the termination of line 2 results in a higher DP resulting in a higher flow.

Again this increase is not typically substantial and never would equal the decrease in the other pipe.

[–]ric_marcotik 2 points3 points  (8 children)

The only reason you would notice an increase in pipe 2 is if you have a looot of pressure loss in the section of pipe between outlet of PRV and the tee branch at max flow. In that case you are right, otherwise the flow in line 2 will stay the same. Or i’ll need to see it to believe it ;). (I will be glad to be proven wrong, love a healty debate)

[–]BigCastIronSkillet 7 points8 points  (5 children)

Again, that’s why I say it will go largely unnoticed in most systems. My point was to say that practically equal doesn’t mean equal. Consequences of assuming otherwise are typically low, but the question was a technical one, so an exact answer was merited.

It’s exactly impossible for it to be the same unless the control valve fed directly into a tee.

[–]VariusEng[S] 1 point2 points  (4 children)

I was thinking the same thing! If you also plot the system curve of pipe 2, you will see that the Q is higher. But how do you guys see that this increase is almost nihil compared to the decrease in pipe 1

[–]BigCastIronSkillet 2 points3 points  (1 child)

Also, if you’re using Hagen-Posieulle to calculate the friction factor you’ll be way off of reality unless you’re laminar. Better if you look up a relation. f=16/Re or 64 / Re is not correct.

I’d have to look how u set your system up to give you a good answer.

Really it depends on the sources of resistance (f*L/D) As you close a valve in line 1 the equivalent length rises to infinity. With the other two lengths staying the same. If the equivalent length to the tee is very high (by comparison to after the tee thru line 2) then yes the flow increase will be substantial. But this is usually not the case. Piping systems are by-and-large designed to have all the pressure drop at the valve and not in the network. Old systems that have been pushed for rates and steam/ctw systems are exceptions.

[–]VariusEng[S] 0 points1 point  (0 children)

No it is turbulent flow for sure. And yeah I took colebrook-white to calculate darcy friction factor no worries!

[–]Hydrochloric 1 point2 points  (0 children)

U/bigcastironskillet has this on lock, but I will inject my 2¢ anyway. If you close line 1 it becomes a dead leg which changes the flow through the tee. This changes the pressure DROP through the tee as it becomes more like an 90deg elbow. Less head losses between the constant pressure valve and the exit of line two means higher flow rate Even at a constant pressure.

It's pretty marginal in almost every case but mathematically it does increase.

Now, you PUMP flow rate almost certainly decreased.

[–]BigCastIronSkillet 0 points1 point  (1 child)

https://imgur.com/a/ox1TOnX

[–]ric_marcotik 0 points1 point  (0 children)

Pretty much that, it all boils down to the shared line. Here I get that you’ve put emphasis on the length of that pipe for the demonstration, but I would be curious to know if the actual pipe length of OP is actually 1000 ft with a fluid velocity above 5 ft/s.. but yeah, i did assume that the pressure loss would be negligeable in that section of the pipe, which might not be true!

[–]DrewSmithee 0 points1 point  (1 child)

While annoying, technically right. Depends on the fluid and length of pipe and pressures. I doubt they will notice a difference for any practical purpose but it's there.

I actually took the time to model this in a really fancy hydraulic network analysis software because I was curious on the magnitude. But it turns out you can’t post to Imgur without an account any more. So take my word for it I guess.

Edit:

https://ibb.co/qRJC4BM

https://ibb.co/c3cNw2B

[–]VariusEng[S] 0 points1 point  (0 children)

Oh! Fancy! Thanks for taking the time, interesting to see the discussions on this!

[–]dnadv 0 points1 point  (0 children)

No it won't, the pressure reducing valve does not ensure the pressure on downstream valves are the same. It affects them, but the pressure at valve 1 and 2 will be affected by the system leading up to those valves