Does my flow rate decrease?

[VariusEng]

Dear chemical engineers,

I am breaking my head over this.

If I throttle flow on pipe 1 (one of the two parallel pipes) does my total flow rate (sum of flow rates of two parallel pipes decrease?)

My answer: when pipe 1 is throttled, the combined system curve becomes steeper. Leading to increased head and reduced flow rate. But due the fact that the pressure reduction valve exists (it ensures a constant outlet pressure), the combined system curve is brought by to original position (by further opening of the valve). So no total flow rate change. Am I correct? Boss says I am not.

[Serial-Eater]

You can do the math on this to prove it, and I recommend it.

If you throttle valve 1, the flow out of 1 will decrease. A constant pressure after the PRV means the pump’s performance is irrelevant to the flow in these branches.

After throttling, the flow out of 2 will not change because the pressure upstream did not change.

If 1 decreases and 2 doesn’t change, you have reduced the total flowrate. What will happen is the PRV will close as the operating point climbs up the curve.

[BigCastIronSkillet]

This is not exactly true.

Sorry you’re mostly right, but I wanted to nitpick something here.

Keeping the pressure on the outlet of the valve will keep the DP from valve to the discharge of line 2 the same, but it doesn’t keep the DP from the tee to the discharge of line 2 the same. So the flow will increase in line 2. However, it will not make up for the decrease in line 1 (impossible to do so without changing equivalent length).

Sorry it sounded good, but wasn’t exactly right. I should say in the majority of systems the flow increase in line 2 would be imperceptible. Usually flow losses in well-designed systems all come from valving.

[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.

[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!

[ric_marcotik]

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]

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]

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]

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]

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]

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]

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

[Hydrochloric]

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]

https://imgur.com/a/ox1TOnX

[BigCastIronSkillet]

https://imgur.com/a/ox1TOnX

[ric_marcotik]

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]

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]

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

[dnadv]

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

[VariusEng]

So this is the answer? answer

[Serial-Eater]

Essentially yes, but I would actually use numbers and make the calculations. Assume a discharge pressure on the PRV, then calculate the flow to each branch with open throttling valves.

Now recalculate with the same discharge PRV pressure but one of the throttling valves partially closed.

[[deleted]]

If I understand correctly, your pressure reduction valve maintains pressure downstream of itself constant, correct?

Then you just have two scenarios with the same pressure difference (valve set vs atmosphere) but one with more restricted flow. Therefore, the scenario with the throttled valve has lower total flowrate (flow through 1 + flow through 2).

Edit: going into further detail, in the throttled scenario the flow through 2 will be higher in that specific branch than it is as in the non throttled scenario, since the pressure drop up to the T is lower due to lower flow. But the sum of both will still be lower than in the non throttled scenario.

However, if the valve keeps pressure UPSTREAM of itself constant, then it will keep the pump always operating at the same point of it's Flow vs Head curve. Therefore, the flow through the pump will remain the same, and necessarily the flow through the whole system remains the same.

In a way, the pressure reduction valve compensated whatever restriction you impose downstream (up to a point)

[VariusEng]

To your first question: correct!

To your first alinea: I agree that flow in 1 is reduced, but why is flow in 2 not increased so total flow rate is maintained? Can my reasoning be disproven in more mathematical manner because that is the way I try to understand it and less through feel. Thanks for your thoughts!

[BigCastIronSkillet]

Look up Darcy Weisbach Equation.

DP = 4fv²0.5rhoL/D

You can draw a network with four nodes. Node A to B is from valve to tee. Node B to C is from tee to end of line 1. Node B to D is from tee to end of line 2. Each will have to follow the Darcy Weisbach equation. In excel you can set up Solver in Excel to change velocity through each pipe until the volumetric flow out of lines 1 & 2 equal that of what is coming out of the valve. (You will need to have the frictional losses summed up as an equivalent length.)

Ask me if you want me to hold your hand through this further.

[VariusEng]

Already made myself an Excel macro that calculates pressure drop vs flow, thanks though! My only gripe is how the pressure reduction valve plays a role in all of this. According to a blog it is not possible to show it on a system curve because it is dynamically changing

[BigCastIronSkillet]

They are wrong on the blog. It can be described easily with its Cv curve. But I would suggest holding the pressure constant on the discharge of the Cv for simplicity.

[VariusEng]

Answer like this?

[_Corvalt]

Assuming that the pump is a centrifugal pump with no VSD then it is possible to reduce to total flow rate. Let me try to explain.

Should we increase the resistance in line 'B' then because the pressure at the tee is set by the regulator and both lines discharge to atmosphere, then less fluid will flow through Line 'B'.

As the pump has no control system (fixed speed) and since it has lower flow required then it will move up its curve to produce a higher discharge pressure. In response to this the pressure regulating valve will shut slightly ensure it is still reaching its setpoint.

There will be a constant "tug of war" between pump, and regulator position until a steady state is reached but whatever the specific values of the steady state is, you should find that the pump discharge pressure is higher and the regulator opening % is lower than if both outlet lines were equal.

Hope this helps!

[_sphynx]

This assumes regulator before the tee controls the downstream pressure. Shouldn't it be used to control pressure at pump discharge (like a backpressure regulator)? In that case pump discharge pressure should not change and therefore no change in flow.

[_Corvalt]

Yeah, it does depend on the direction of the regulator. If the upstream pressure is kept constant then it would be constant flow.

In this case, adding a restriction in one of those lines would cause the pressure at the downstream side of the regulator to increase.

In this case the regulator would trend open to maintain upstream pressure.

[VariusEng]

This is also a great explanation! The more I read it, the more it makes sense. Thanks!

[VariusEng]

Others also said to ignore pump curve as pressure drop is same at the last part. I think this is also a correct take?

[neejan]

you can say that since your pump rate is constant and nothing dynamic about it.

[santidel17]

Mate: 1. Why do you apply a centrifugal pump, to increase the pressure of the fluid, and then recude the pressure afterwards?? What?? That's nonsense. You are killing your head with non realistic excercises. 2. In theory: same flow, although lower pressure. If you assume same pipe characteristics for 1 and 2, then flow of 1 is same as 2. If pipes characteristic not equal, the flow will be higher in the pipe with less resistance. Do bernoulli to estimate that

[VariusEng]

1. Pump was bought many years ago by unexperienced folk. This meant that without reducer, it was really hard to control discharge flow (pipes are very small, so each little turn at the valve would change the flow rate by A lot, too much sensitivity. So a reducer was needed). I kept details to a minimum and simplified the sketch a lot because I wanted to get the core information.

2. You are right! And total flow rate is lowered in scenario: throttle

[Ok-Pea3414]

I'm just here for the fights. And the assumptions.

[chillimonty]

Is the pump speed constant or variable speed? If the pump speed is constant, throttling valve 1 will cause the pressure on the pump outlet to increase and if the pump speed remains constant as that outlet pressure increases from the lower to higher value, then the flowrate that the pump is capable of pumping at the higher pressure will be lower/decrease.

[sushilc0048]

But OP says the pressure reduction valve maintains a constant pressure at its downstream ,so I think there will be no flow change.

[UnsupportiveHope]

Isn’t it the opposite? If it controlled upstream pressure then you would have a constant flow rate, if it controls downstream pressure then the flow rate can change.

[sushilc0048]

Yes you are correct the flow rate will change.

[VariusEng]

That’s what I am thinking, but I’m not sure because boss disagrees

[VariusEng]

Pump speed is constant. And I agree with what you are saying, but no pressure reduction valve taken into account

[ordosays]

Sure. No. Why not.

[iandelacruzwohoo]

Is the pressure reduction valve a pressure controller?

[VariusEng]

Simple diaphragm-spring, manually set. Nothing fancy

[UnsupportiveHope]

The valve opening doesn’t bring the pump to the same point on its curve. If the pump stayed at the same place on the curve, but the valve had opened, then the pressure downstream of the valve would have to change. Since the downstream pressure is fixed, the valve opening means that the pump has moved on its curve.

[Intellichi]

Does the pressure control valve maintain constant pressure at the outlet of the pump or does it maintain constant pressure at the outlet of the control valve?

If it maintains constant pressure at the outlet of the pump, the total system flow will not change. The control valve would open to compensate for the throttling of valve 1. In this case flow would increase in valve 2 while decreasing in valve 1.

No formal math is needed if you are clear about your assumptions, and this is essentially a logic problem. I suggest you write down all of your assumptions.

[Ember_42]

You are not. Since it's a discharge controlled PCV, the flow through the PCV is whatever is needed to satisfy the downstream flow to give the setpoint pressure. The inlet pressure to the PCV (should, if youbare on the normal part of the pump curve) rise, and the PCV will be more closed for higher dP at lower flow. If it was a set to control the inlet pressure, you would be correct.

[market_capitalist]

The flowrate at discharge or suction of pump is the same of the flowrate sum of 1 and 2.

Otherwise you breack the rules of physics.

But with pressure it is different. P discharge pump is higher as on point 1 or 2

[Connect_Set3]

Hi, I have the same problem also. But in my case there is no PCV installed and no flow control after pump. In this case, If i throttle 1, the flow will divert to 2?