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Re: Head Loss Calculation HELP!!!
Re: Head Loss Calculation HELP!!! -- airbag club	Post Reply	Top of thread	Forum

Posted by: habashy ® 03/03/2006, 13:37:44 Author Profile eMail author Edit

as u know the flow rate entering the pipe so by applying the continuity equation , Q=const. so.....Qin=Qout+Qholes Qin is knowm... for holes : the hole is considered as an orifice so the flow rate from a hole is a flow rate from an orifice : Qhole=Cd A (p)^1/2 ---------(I) where : Cd : coefficient of discharge from the hole A : hole area p : presure loss across the hole……. Qholes=Qhole1 + Qhole2+….. to calculate the pressure loss across the pipe: Use Hazzen-Williams formula... divide the pipe into number of parts according to the holes numbers by which first part is from the pipe begining till the first hole. calculate the pressure loss in the first part. the second part is from the first hole to the second hole knowing the pressure loss from the first part subistitute by the value of the pressure loss (p)in the formula (I) to get the flow rate from the orifice.. by the same way , knowing the flow rate is (Qin-Qhole1) calculate the pressure loss in the second part and add it to the pressure loss in the first part and get Qhole2 as before.... by the same way get the pressure loss across the whole pipe..

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Re: Head Loss Calculation HELP!!!
Re: Head Loss Calculation HELP!!! -- airbag club	Post Reply	Top of thread	Forum

Posted by: rookie eng. ® 02/28/2006, 10:14:52 Author Profile eMail author Edit

Use the General Energy equation, you will calculate the loss in the pipe with the reynolds number, and refering to the Moody Chart to find your friction factor. Then use this as a value for head loss in the the general energy equation. The Fluids Reference section on this site will be helpful.

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Re: Head Loss Calculation HELP!!!
Re: Head Loss Calculation HELP!!! -- airbag club	Post Reply	Top of thread	Forum

Posted by: Administrator ® 02/27/2006, 17:58:52 Author Profile eMail author Edit

"airbag club" - I think my wife is a long time member.

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Re: Head Loss Calculation HELP!!!
Re: Re: Head Loss Calculation HELP!!! -- Administrator	Post Reply	Top of thread	Forum

Posted by: randykimball ® 02/28/2006, 23:50:11 Author Profile eMail author Edit

Then our wives likely know each other... My wife doesn't watch or read anything that may remotely cause a brain cell to record anything new. ... this, I think, in deliberate protest to my total opposite nature. I'd walk a mile to get the smallest new learned concept. But I love her to pieces..she provides the yang to the ying. later, /←randy→/ The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them.

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Re: Head Loss Calculation HELP!!!
Re: Head Loss Calculation HELP!!! -- airbag club	Post Reply	Top of thread	Forum

Posted by: zekeman ® 02/26/2006, 13:26:47 Author Profile eMail author Edit

Does the flow terminate at the holes. or does it continue on?

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Re: Head Loss Calculation HELP!!!
Re: Re: Head Loss Calculation HELP!!! -- zekeman	Post Reply	Top of thread	Forum

Posted by: airbag club ® 02/26/2006, 15:48:42 Author Profile eMail author Edit

thank you for your reply. the feed pipe is 25' long, 20" diameter, and it has 25 2.5" diameter holes on the top. The feed pipe is capped at the very end, so the water is forced to go through these holes. Water is to travel down the feed pipe, and bleed/leak through the 2.5" diameter holes. i need to calculate the head loss and pressure drop across the 20" feed pipe. any suggestions or help on where I can find these equations? Please HELP!! Thanks!! Modified by airbag club at Sun, Feb 26, 2006, 15:50:03

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Re: Head Loss Calculation HELP!!!
Re: Re: Head Loss Calculation HELP!!! -- airbag club	Post Reply	Top of thread	Forum

Posted by: zekeman ® 02/27/2006, 16:56:42 Author Profile eMail author Edit

How many holes and what is the spacing? This is not a conventional flow problem and you will not easily find it in the engineering literature. But let's see what it looks like for openers.

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Re: Head Loss Calculation HELP!!!
Re: Re: Head Loss Calculation HELP!!! -- zekeman	Post Reply	Top of thread	Forum

Posted by: pwehmer ® 02/27/2006, 23:45:56 Author Profile eMail author Edit

Are you trying to get the same volume flow from each outlet?

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Re: Head Loss Calculation HELP!!!
Re: Re: Head Loss Calculation HELP!!! -- pwehmer	Post Reply	Top of thread	Forum

Posted by: airbag club ® 02/28/2006, 01:11:05 Author Profile eMail author Edit

Thanks for the replies. The feed pipe is 26' long with holes spaced 1' apart. Currently the flow does not evenly distribute through the holes. The holes towards the inlet of the feed pipe are receiving higher flow than the holes at the end of the feed pipe. I want to calculate the pressure drop in the feed pipe after each hole. The reason for these calculations are we want to try to place restrictors on the holes in the front, in order to try to obtain a uniform distribution throughout each hole. Please HELP if you can. Thanks!

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Re: Head Loss Calculation HELP!!!
Re: Re: Head Loss Calculation HELP!!! -- airbag club	Post Reply	Top of thread	Forum

Posted by: zekeman ® 02/28/2006, 20:15:38 Author Profile eMail author Edit

I forgot to ask, what is your water pressure? Your results are not surprising since the flow thru the holes, as you know, is proportional to the square root of pressure and the pressure is falling downstream; maybe resizing the holes downstream would be another way of achieving uniformity. Give us the water pressure and we should be able to get you something.

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Re: Head Loss Calculation HELP!!!
Re: Re: Head Loss Calculation HELP!!! -- zekeman	Post Reply	Top of thread	Forum

Posted by: zekeman ® 02/28/2006, 21:00:30 Author Profile eMail author Edit

Also, how much flow do you want out of each hole? The solution to your problem is vastly easier if we know the water pressure and the outflow required and is the cylinder diameter a constant or can you change it if necessary?

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Re: Head Loss Calculation HELP!!!
Re: Re: Head Loss Calculation HELP!!! -- zekeman	Post Reply	Top of thread	Forum

Posted by: airbag club ® 03/02/2006, 16:57:06 Author Profile eMail author Edit

Okay, so we got word that the head loss through this feed pipe is insignificant. Now I want to calculate the head loss through a smaller pipe. This is all the information I know. 5' long, one inlet, capped end 2.5" diameter 15 1/4" holes evenly spaced across the pipe Flow rate going in is 76GPM. Please help if you can, or point me in the right direction. Thanks!!!

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Re: Head Loss Calculation HELP!!!
Re: Re: Head Loss Calculation HELP!!! -- airbag club	Post Reply	Top of thread	Forum

Posted by: zekeman ® 03/02/2006, 22:55:36 Author Profile eMail author Edit

You say the pressure drop is minimal? If so,do you have empirical data to support that?; I did the problem and got a significant loss, but I made a few assumptions that may not be valid. As for your second problem, are you requiring uniform flow?f so, you can't have the same hole diameters.

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Re: Head Loss Calculation HELP!!!
Re: Re: Head Loss Calculation HELP!!! -- zekeman	Post Reply	Top of thread	Forum

Posted by: zekeman ® 03/02/2006, 23:21:28 Author Profile eMail author Edit

And what about the inlet pressures and how do you know the flow rate . I'm puzzled!! Unless you are saying that you require a certain inlet flow with the hole geometry you specified and then get the pressure you need to support the flow.Very strange. Instead of asking for pressure drop, why don't you state the problem required.

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Re: Head Loss Calculation HELP!!!
Re: Re: Head Loss Calculation HELP!!! -- zekeman	Post Reply	Top of thread	Forum

Posted by: airbag club ® 03/03/2006, 17:17:48 Author Profile eMail author Edit

Hi, sorry for the complicated question and puzzling you. I appreciate the help that you are doing. Okay, so the apparatus is a water filtration media bed. In order to clean the filtration bed, the underdrain system is backwashed. Now, there is no uniformity in this backwash flow, and I am required to help design a system that will help provide uniform flow through these pipes, during backwash. So, the first step is to determine the pressure difference and head loss through the feed pipe and the smaller lateral pipes that it branches off to. Those values I listed above are all that I know, and we were not given an inlet pressure..

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Re: Head Loss Calculation HELP!!!
Re: Re: Head Loss Calculation HELP!!! -- airbag club	Post Reply	Top of thread	Forum

Posted by: WhiteTiger ® 03/04/2006, 04:24:32 Author Profile eMail author Edit

Since it's an obscure one-off problem, how about approaching it empirically? Cut one foot lengths of 20" pvc, split them longitudinally and fit them around your problem pipe, use them as adjustable hole baffles to equalise your flow at each of the holes, then measure how much is blocked off at each one and figure your remaining aperture in square inches. Drill holes with that many sq in. Tiger

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Re: Head Loss Calculation HELP!!!
Re: Re: Head Loss Calculation HELP!!! -- WhiteTiger	Post Reply	Top of thread	Forum

Posted by: zekeman ® 03/04/2006, 08:41:46 Author Profile eMail author Edit

I solved this using the energy equation and also found that the classical frictional pressure drop is negligible but the actual drop is not; the hole diameter distribution results are: 0.249549329 0.255490 0.262309295 0.269847328 0.278246324 0.287691714 0.298431426 0.310806254 0.325300691 0.342632902 0.363925779 0.391063623 0.427536129 0.480842366 0.571821045 0.808676935 The initial head is 17 feet at the first hole assumed at 1/4 inch; corresponding pressure drops (not shown here) comes out of the energy equation. This is a good first start to a difficult problem. You must test this with a 5 foot section of pvc or abs and drill holes at each location and since the pressures are so low pvc may be a good material for the final pipe. The foregoing assumes that you get the 75 gpm by proper flow restrictors after you install the small pipe sectons since each of the 2.5" pipe stations offers restriction to the main flow. I will check my arithmetic and post the solution if you wish after confirming.

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