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# Water Velocity Calc.

03/28/2017 11:44 AM

How would I calculate the velocity in individual .5" circuits exiting a water manifold with a 2" line supplying the manifold that has a GPM of 160?

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#1

### Re: Water Velocity Calc.

03/28/2017 12:09 PM

I suggest you use algebra. Carefully.

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#3

### Re: Water Velocity Calc.

03/28/2017 12:14 PM

Not even algebra--just arithmetic.

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#2

### Re: Water Velocity Calc.

03/28/2017 12:13 PM

Need to know how many 0.5" circuits there are (and, are they 0.5" inside diameter?).

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#4

### Re: Water Velocity Calc.

03/28/2017 12:18 PM

Qindividual = 160/n.

v = Qindividual/A.

A = (pi)(i.d.)2/4

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#5

### Re: Water Velocity Calc.

03/28/2017 12:35 PM

Wow! You're pretty good!

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#6

### Re: Water Velocity Calc.

03/28/2017 12:39 PM

That would be correct if the flow were divided equally across the <...n...> smaller lines. However, the real world isn't really like that and one must consider the possibility that the resistance to flow in some of the smaller lines may be greater than and in others smaller than others, the shape and configuration of the larger manifold, where the take-offs are on it, etc., etc.. It becomes a network calculation activity using much in the way of scrap paper and pocket calculator batteries, with many reiterations, brain-ache and grindings of teeth if done manually.

In the limit, the best way to assess it is to measure it. However, the original poster has withheld from the forum whether this is a real installation or a virtual one. Which doesn't help.

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#13

### Re: Water Velocity Calc.

03/28/2017 1:42 PM

That's true, but ya gotta start somewhere....

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#23

### Re: Water Velocity Calc.

03/29/2017 5:21 AM

"Dear Gurus, How do I work out how many calculator batteries.........?"

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#7

### Re: Water Velocity Calc.

03/28/2017 12:48 PM

A 2" id line feeding at 160 GPM would exceed the capability of the 4 ea 1/2" id lines which would max out at about 18 fps velocity and 21 gpm this due to friction pressure loss...

Assume Average Pressure. (20-100PSI) About 12f/s flow velocity the 2" line about 127 gpm and the 1/2" lines about 14 gpm ....

http://irrigation.wsu.edu/Content/Calculators/General/Pressure-Loss-With-Outlets.php

https://flexpvc.com/Reference/WaterFlowBasedOnPipeSize.shtml

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#8

### Re: Water Velocity Calc.

03/28/2017 1:15 PM

Where did the quantity 4 come from?

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#14

### Re: Water Velocity Calc.

03/28/2017 3:03 PM

Haha, I think somebody was teeing off and yelled fore!!

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#9

### Re: Water Velocity Calc.

03/28/2017 1:15 PM

Yes this is a real world situation. I am wanting to determine how switching from a 2" feed line to a 1.5" feed line will affect the flow rate through an injection mold. The manifolds feed 18 lines. The I.D. of the .5" fittings are .400".

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#11

### Re: Water Velocity Calc.

03/28/2017 1:26 PM

[18 (0.4)2.5]0.4 ≈ 1.27 < 1.5. Even if you reduce the main feed from 2" to 1.5", the branches collectively are still the dominant restriction. Unless your pressures are really borderline, this should still work.

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#15

### Re: Water Velocity Calc.

03/29/2017 12:55 AM

I am trying to figure out how you get this justification. I think its looking at linear pressure drop so it then depends on how long the sections of piping are for each segment but I would agree the branches have a higher linear pressure drop

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#27

### Re: Water Velocity Calc.

03/29/2017 11:27 AM

At 160 gpm, the velocity is at the high end, 15 ft/sec, for 2 inch pipe. Pushing the 160gpm through a 1.5" pipe, that's 25 ft/sec, increases the friction losses by 350% plus. So maybe not a slam dunk conversion, depends on how much upstream piping you have, and fittings.

Nothing changes on the downstream piping, so that pressure drop stays the same. The Branch Tee flow (1/2" pipe) pressure drop from your header, if the header changes to 1.5" from 2", will be much worse (lower supply pressure at the beginning of each line).

The 9 gpm nominal through your 1/2" lines is also on the high end, 9.5 fps, but you've been living with that already. But you may not be able to afford to lose more pressure in your supply header.

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#19

### Re: Water Velocity Calc.

03/29/2017 4:19 AM

<...how switching from a 2" feed line to a 1.5" feed line will affect the flow rate...>

It won't; if it did, there would be a "leak". It only affects pressure drop per unit length of pipe, which increases because in order to get the same amount of stuff though a smaller pipe, the velocity increases. The square of the velocity is in the Darcy-Weisbach equation and it also appears in the friction factor correlation, which also has to be figured into the equation.

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#29

### Re: Water Velocity Calc.

03/29/2017 11:34 AM

Indirectly, the higher pressure drop associated with a smaller pipe can push the flow lower, especially on a centrifugal pump...

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#10

### Re: Water Velocity Calc.

03/28/2017 1:24 PM

Water is incompressible, so volume in = volume out. 1 gal = 231 cu in so 160 GPM =616 cu in/sec. Divide 616 by output manifold area in square inches to get average velocity in inches per second. The flow through a pipe is faster through the center and slower near the outside due to friction between the water and the inside of the pipe.

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#12

### Re: Water Velocity Calc.

03/28/2017 1:37 PM

If you knew the supply pressure on the existing manifold (with your subsequently stated 18 ports with 0.4" I.D. mold cooling lines), along with target velocity and flow of each line, you could estimate what the required pressure would be at the connection of a smaller feeder line to the manifold would be.

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#16

### Re: Water Velocity Calc.

03/29/2017 1:00 AM

I came at this with Linear Pressure drop (psi/100ft) DP100hdr= k Qhdr^2/ dhdr^5

So for the leads DP100Lead = k (Qhdr /18)^2 / (dhdr / 5)^5

= DP100hdr * 5^5 / 18^2

= DP100hdr * 9.6

So unless the header is 10 times the length of the 0.5" sections the branches dominate as far as piping losses are concerned.

For the change to from 2" to 1.5" (2/1.5)^5 gives a 4.2 fold increase in pressure drop in your header. I don't know if this will prove significant but it depends on lengths how much spare head you have in your pump and motor!

However there are all sorts of other losses in circuits.

I am also concerned with the flow velocities. My old chart gives about 16fps for your flow which seems high. In oil and gas plants I would normally keep this to 10 fps. Going to 1.5" in this case will increase velocity as a square so 28fps which is high for a liquid. But for short high pressure systems this could be quite acceptable.

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#17

### Re: Water Velocity Calc.

03/29/2017 2:12 AM

http://www.pipeflowcalculations.com/pipe-valve-fitting-flow/flow-in-pipes.php

Bernoulli's principle states that the pressure of a fluid decreases when either the velocity of the fluid or the height of the fluid increases. Bernoulli's principles is integral to the design of airplane wings and ventilation systems. The actual equation itself resembles conservation of energy, however, in lieu of studying the motion of an individual particle, Bernoulli's principle generalizes for a collection of particles with a uniform density.

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#18

### Re: Water Velocity Calc.

03/29/2017 2:40 AM

Dear Mr.krk725,

You have given Flow as 160 GPM which means 160 x 4.54 = 726.4 LPM = 12.10 Litres/Sec.= 12.1/1000 = 0.0121 M^3/Sec.

Area of 2 Inch Pipe is (3.14/4) x (2 x25.4)^2/1000 = 0.0020 M^2 Hence in velocity in this pipe is 0.0121/0.0020 = 6.5 Metres/Sec.

But you have not mentioned how many (0.5) Inch branch or (0.5) x (25.4) = 12.7 MM branch is in the distribution.

Hence the flow of 160 GPM or 00121 M^3 to be devided by no. of branch to get flow in one branch. Area for one branch can be calculated. Since flow per branch is known, area per branch is known, the velocity can be calculated.

DHAYANANDHAN.S

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#20

### Re: Water Velocity Calc.

03/29/2017 4:21 AM

Excuse me. Why do you assume the poster is male?

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#21

### Re: Water Velocity Calc.

03/29/2017 4:38 AM

Because females wouldn't post so naively?

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#28

### Re: Water Velocity Calc.

03/29/2017 11:33 AM

You are right. The poster can be lady also.

Example - The moderator of CR4 - Savvy Exacta is a lady,

The I should have written Dear krk725,

You have posted as Anonymous Poster - I cannot guess you whether you are HE or SHE.

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#30

### Re: Water Velocity Calc.

03/31/2017 11:59 AM

All Anons are neutered upon entering the CR4 facility.

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#22

### Re: Water Velocity Calc.

03/29/2017 4:55 AM

Post #9

18 branches at id 0.4"

Although the 2" and 1.5" lines have not been specified as to precise id yet!

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#24

### Re: Water Velocity Calc.

03/29/2017 6:54 AM

From a background in injection moulding and currently working for a water supply authority, your "real world" claim seems flawed.

Your flow rate equates to 3 gallons per second, about the equivalent of a normal fire hydrant standpipe running direct from the main.

Your water velocity is beyond "reasonable" in both the 2" header and the 1/2" circuits, let alone any potential change in direction in the water galleries of the tool. Hoses and pipes would literally jump around when being turned on and off.

If you need that capacity of cooling, then you need larger diameter pipes all round.

I'll leave it to the thermo guys here to do the calcs, but even with a 5 degree temperature rise across the tool at 3 gallons per second, the energy flow is HUGE. I've seen 5 Ton tools cooled by significantly less. We used in-line flowmeters and I never saw more than 5L/minute in any 1/2" cooling circuit, so even with 18 lines runing that would be less than 100L/minute, or around 25 gallons.

I suspect that you have a pump rated at 160GPM, but really the cooling flow is pressure controlled and will never be 160GPM.

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#25

### Re: Water Velocity Calc.

03/29/2017 8:21 AM

This is a 18 Ton tool that has 152-.472" circuits running through the tool. That was only for one manifold i wanted to know for. I just wanted to know if it was alright to see what i was sacrificing if i used a smaller line to feed more outlets from a manifold. I have determined that id does not really matter the size of the pipe but how the overall GPM are distributed between the different branches.

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#26

### Re: Water Velocity Calc.

03/29/2017 10:41 AM

Didn't see Bernoulli effect considered for this application. For a straight manifold with taps down the side at 90 degrees, the flow rate will be lower close to the manifold inlet and greater at the dead end of the manifold. This is because the velocity of flow near the manifold inlet causes a reduced pressure. At the blind end of the manifold, velocity is lower and static pressure is higher.

In foundry applications I have seen cases where metal entering the mold cavity from a runner flows into the cavity at the dead end of the runner but actually flows out of the mold cavity and back into the runner at the gate closest to the sprue.

Similarly, I have seen cases where on a long manifold at low pressure and high flow rates where the connections closest to the manifold inlet suck outside air back into the manifold.

Make sure to include manifold velocity pressure effects when balancing the flow rates.

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