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pencilgeek (Industrial) (OP)
11 Jan 13 8:09
Let me start by saying I don't know anything about fluid mechanics. I need to know how many GPM I can push thru a 2" pipe at 100 PSI. Assuming a length of 10 ft. And what would be the velocity. Seems like it would be a simple calculation. Can someone give me a formula?

Thanks
Chuck
Latexman (Chemical)
11 Jan 13 8:28
There are a lot of references that have tables of almost exactly what you want. It's usually in psi/100 ft of pipe for water though. I know Crane's Technical Paper 410 and Cameron Hydraulic Data has it. There are others; you may even have it in one of your old books.

At 100 psi for 10 ft, you are off the chart on the high velocity end. Erosion issues, noise issues, i.e. major problems would be in store for you.

To get in the ball park of normalcy, use 0.1 to 10 psi/100 ft. For 2" the corresponding flow range would be about 10 to 115 gpm.

Good luck,
Latexman

25362 (Chemical)
11 Jan 13 9:19

GPM of what?
pencilgeek (Industrial) (OP)
11 Jan 13 9:30
GPM of water. And yes, the GPM should be high, in like the 200 GPM area. I have seen those books mentioned before in some google searches. Are those the "go to" books for calculations like this?

Chuck
Latexman (Chemical)
11 Jan 13 9:47
From Crane's TP410 website,

"Crane Technical Paper No. 410 (TP-410) is the quintessential guide to understanding the flow of fluid through valves, pipes and fittings, enabling you to select the correct equipment for your piping system.

Originally developed in 1942, the latest edition of Crane TP-410 has been fully updated to reflect the latest knowledge and research in the fluid handling industry. The TP-410 has served as an indispensable technical resource for over 60 years to specifying engineers, designers and engineering students."

I've used it almost 34 years. I recommend it highly. I wish I had had it when in school. And, it's not expensive, $60.

Good luck,
Latexman

Latexman (Chemical)
11 Jan 13 9:56
BTW, for 200 gpm, 2" is probably too small. Depending on what is being done and the quality/type of water, 3" (8.7 fps), 4" (5 fps), or 6" (2.2 fps) may be better choices.

Good luck,
Latexman

bimr (Civil/Environmental)
11 Jan 13 10:29
This question has been asked several times on this forum. Have you done a search?

http://www.eng-tips.com/viewthread.cfm?qid=276054

If you calculate the maximum velocity with Cranes, you will get a velocity around 20-25 ft/sec which equates to a flow of around 200-250 gpm.

Here is a link to a chart that supports this.

http://www.dakotasupplygroup.com/wp-content/upload...
ElSid1 (Mechanical)
11 Jan 13 11:34
Ditto the Crane and Cameron. I tend to use the Cameron more. What is the application for?

Quote:

At 100 psi for 10 ft, you are off the chart on the high velocity end. Erosion issues, noise issues, i.e. major problems would be in store for you.
.
You also have to deal with codes! For example, Civils frequently go into the 20-30 fps on water mains. As a mechanical, the plumbing code limits the velovity to 8 fps. I've had to explain to my civil counterpart why my branch line was bigger than his main. they were not impressed curse
Helpful Member!  cvg (Civil/Environmental)
11 Jan 13 12:38
civils DO NOT "frequently go into the 20-30 fps on water mains". This would be a very infrequent event, and probably only at a fire hydrant flowing at wide open. It is certainly not typical and municipal standards for water mains that I have seen are limited to 5 - 7 fps. where did you hear that and what kind of loons do you work with?
pencilgeek (Industrial) (OP)
11 Jan 13 12:52
You asked what the application is, and I probably should have mentioned it up front. But it is not civil or mechanical engineering. I am trying to figure out how much water I can push thru a plastic injection mold. Hence, the 10 ft. length. I am given water from a 2" hose at 100 psi and I am trying to figure out how many GPM I am being supplied. From this 2" hose it branches off into several smaller drilled lines and eventually back into a 2" return hose. I think there must be 100 calculations I have to do to figure this out, but I am just trying to get past the first step (input GPM). I appreciate all the help you guys are giving me, even though some of it does not apply. I did do a search on here for what I am looking for, but I don't think my terminology was close enough to find what I needed. I did see that another mold designer was looking for similar help in a similar situation, but that thread didn't help me much.

With my application in mind, which book would do me more good? Cameron or Crane? Just looking for an opinion if you have one.

Or, with my application in mind, is there a simple formula?

Thanks
Helpful Member!  bimr (Civil/Environmental)
11 Jan 13 12:52
The velocities in city water mains are always lower than the velocities in building water services.
trashcanman (Mechanical)
11 Jan 13 13:17
Quote:

" I am given water from a 2" hose at 100 psi and I am trying to figure out how many GPM I am being supplied. From this 2" hose it branches off into several smaller drilled lines and eventually back into a 2" return hose."


This is a completely different problem than what first appeared in your 1st post. If I understand what you are describing, the water is to be forced through numerous small drilled lines and back to the return hose.
1. The water pipe IS NOT the bottleneck. The main pressure drop will be through the numerous orifices.
2. You do not have 100 psi to burn. There must be enough pressure available to return to the 2" return hose and back to the source.
3. Determine the pressure drop through the mold (psi vs. gpm. This must be done through field experiments). Leave enough return pressure so you will not cavitate your pump.
cvg (Civil/Environmental)
11 Jan 13 13:21
you cant get input gpm without output gpm. friction loss in the mold through all the drilled holes will reduce your pressure and flow rate. assuming this is a pump, then your pump curve also comes into play. a flow meter would be useful
ElSid1 (Mechanical)
11 Jan 13 14:20
cvg and bimr,
I was tasked with estimating water supply for a mid-rise, no FP, that was a differned tap. My calculations showed a 14" line, was able to "justify" a 12" in my estimates. The main was 12". I was told that the city water mains are allowed to go to higher velocities since they are buried (noise not an issue). I ended requesteing two 6" tap offs closer capacity to the 14", put the civil was not happy. What AWWA standard do the mains fall under?
Latexman (Chemical)
11 Jan 13 14:25
Trashcanman is right, completely different problem. Much more complex to solve theoretically. Is everything existing so you can experiment, or does this need to be figured out for a project?

Good luck,
Latexman

cvg (Civil/Environmental)
11 Jan 13 14:43
allowable flow velocity in city water mains is set by city standard, not AWWA. Every agency I have checked is 5 - 7 fps. Headloss at higher velocities have a large impact on capacity and pumping costs. Keep in mind that cities and water districts must deliver water at minimum pressures to the entire service area through literally miles and miles of water mains. Keeping velocity low and head loss low is essential to be able to do this. However, different standards apply to private service laterals which is what you are describing, not a water main.
pencilgeek (Industrial) (OP)
11 Jan 13 14:49
Thanks guys for being patient with me as I learn how to ask this question. I know how many GPM I need to push thru each mold (it varies depending on a few things), but I want to verify that I can actually push the required GPM thru the mold, at a given 100 PSI with a 2" supply hose. Am I jumping the gun trying to figure out how many GPM I am getting from the supply hose? Or do I need to look at the complete system first? Or do I just buy the book and improve my knowledge before I try to figure it out?

Chuck
Helpful Member!  Latexman (Chemical)
11 Jan 13 15:24

Quote (pencilgeek)

Am I jumping the gun trying to figure out how many GPM I am getting from the supply hose?
Yes.

Quote (pencilgeek)

do I need to look at the complete system first?
Yes.

Quote (pencilgeek)

do I just buy the book and improve my knowledge before I try to figure it out?
Yes.

You also may need to understand "manifolds", "distribution header", "maldistribution", "series flow resistance", and "parallel flow resistance", because it is more complex than either reference mentioned above gets into. You've also got a ways to go before you are ready to do this. It is not beyond you, IMO, but if time is critical and you have the budget, you may want to sub this out to an Engineering Firm.

Good luck,
Latexman

Helpful Member!  77JQX (Civil/Environmental)
11 Jan 13 15:39
If you want to low budget this and have the time to get some learning, EPANet is a free hydraulic simulator that can (probably) handle the hydraulic calculations for the complete system. As with anything else, if you don't understand the inputs, the results you get will probably be garbage. To get meaningful results, you'll need to educate yourself about friction factors, fittings, minor losses, what creates the souce water pressure, etc. But as far as calculating power, EPANet will match programs costing $1,000s.
pencilgeek (Industrial) (OP)
11 Jan 13 15:40
Latexman, thanks for the honesty. Time is not critical and I have never let not knowing how to do something keep me from trying. Question: Is this something I could learn to do if I took a class in fluid mechanics or does a person need degree? I am guessing that, since you recommend farming it out. that I would need a degree in fluid mechanics?

Chuck
bimr (Civil/Environmental)
11 Jan 13 18:17
pencilgeek

Your question seems to be how much capacity does a hose carry, not a piping question. Ask your rep at Grainger to tell you the answer.

Note that you may need to use multiple hoses as well to obtain the flow rate that you desire.

Note, you will get less flow out of a hose because the hoses may be corragated with poorer hydraulic parameters than pipe.
MikeHalloran (Mechanical)
11 Jan 13 18:35
A fluid mechanics class won't help.
A fluid mechanics degree wouldn't help either; it's not what it sounds like.
This is advanced engineered plumbing.

Buy the Crane book; it's not expensive. Don't lend it out; it has a habit of not coming back.
There's an Imperial version and a Metric version; buy whichever one suits you.

The book will eventually yield to study, and will give you techniques for estimating the pressure drop from end to end of one pipe.

Multiple pipes in parallel or series can be estimated using Kirchoff's Laws, which come from electrical engineering but work just fine for fluids, except that fluid resistors are square-law devices.

If you're using a centrifugal pump, you can't just pick a flow number out of a catalog.
You have to model the entire system for a guesstimated but completely arbitrary flow. Once you have a pressure drop for that arbitrary flow, you can use it to compute what is effectively a Cv, then use the Cv to plot the system resistance curve, and graphically find where the pump curve intersects it, which is the operating point for the pump. You can do a lot of it in Excel. The fancy tools just make a solution quicker.

Luckily for you, the typical mold cooling system is normally bled of air, so you won't have to worry about two-phase flow. Not so lucky for you, the typical mold cooling system has a fairly complex internal geometry. You can model it as a combination of orifices and pipes, but when you test your model against a real mold, it won't be super-accurate. I.e., you'll be doing well to get within a factor of 2 either way. Luckily for you, centrifugal pumps are not super fussy about that.

First buy the book, find the examples that apply to you, and work them.





Mike Halloran
Pembroke Pines, FL, USA

Latexman (Chemical)
11 Jan 13 21:30
Mike nailed it. You don't need a class. Buy Crane TP410 and study it. It won't be all you need, but it's an excellent start. After you think you know it, go through the examples. It has a chapter of examples worked out for you. It's great! It's concise. It won't take long. A degree in fluid mechanics? I don't think there is such a thing. Some of the best fluid mechanics I know are Chemical, Mechanical, Aerospace, Civil/Environmental and Petroleum Engineers that focused on the area and taught themselves. That's all. You will meet some of them on this forum. When you are ready with specific questions, we'll be here.

On the complex components, like the plastic injection mold, you may need to drill down into the company that furnished it to find the one Engineer that designed/knows it. He/she will probably know how to characterize the resistance to flow, whether it's a K value or Cv value or two K or three K or equivalent length or whatever. If not, you may have to do some flow tests in the field. If so, yeeeehaaaa! You'll really learn something then.

Also, learn to use Search (between Forum and FAQs) to find old threads on the subjects and keywords you need. It takes time, but it's a gold mine!

Good luck,
Latexman

katmar (Chemical)
12 Jan 13 4:29
The Crane and Cameron books are excellent, but there is no "recipe" in either of them that you can apply mechanistically to solve your problem. Studying either of these sources thoroughly over a few weeks would probably give you most of the required knowledge that you would get in a college course. But you might still be lacking the on-the-job learning and experience that would be required to tackle a real-world problem like this.

One of the biggest conceptual hurdles to overcome it to understand the difference between pressure and pressure drop. Almost all engineers who have worked in fluid mechanics for a few years have this understanding so deep in their psyche that they cannot believe that it is not obvious to everyone. But it is not obvious.

The 100 psi you have is NOT all available to drive the flow through the 10 ft of hose. Part of that 100 psi will be used up overcoming friction in the hose, but some also has to be used to drive the water through your molds and through the return piping. There are techniques in Crane and Cameron for modeling these sequential resistances to flow. A resistance coefficient (usually just called the K value) has to be determined for each section, and for series flow the K values are additive. This is analogous to resistances in an electrical circuit (but more complicated).

If you can measure the pressures at the start and end of your 10 ft of 2" hose then you can calculate the pressure drop across the hose. And once you have the pressure drop you can calculate the flow rate. At 200 gpm the pressure drop would only be 5 or 6 psi across this short piece of hose. Fortunately conservation of mass means that if it is flowing through the hose then the same quantity is flowing through your mold. This means that you can calculate the flow through any section of the circuit for which you have the required information, or as a worst case you have to work with the K value for each element and the overall pressure drop.

Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

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