Parallel Pipes exiting to STP (Hazen-Williams applicability) 1

[adamlcarp]

I'm working on a parallel pipe scenario where there is one inlet and multiple outlets all exiting to free air (STP). Do the constraints of the Hazen-Williams equation still apply (ie all hL values equal)? I've found solutions assuming so but in practice we have found that in a case where all pipes have identical ID, roughness, and length we will consistently see greater dispense volumes from the pipe farthest from the inlet.

A little background: inlet is horizontal, all dispense pipes are vertical exiting the bottom with all dispense inlets at same height. Prior to a timed dispense all pipes and inlet tube (manifold) have been primed and are assumed full of fluid (ie. no air volumes displaced at beginning of dispense).

Thanks in advance for any guidance,
Adam

 

[Latexman]

Use Search (between Forum and FAQs) on terms "flow distribution", "fluid distribution", and maldistribution. There are many threads on this which will help you.

Most likely, the main reason your calculations do not match reality is momentum changes were not included in the analysis.



Good luck,
Latexman

 

[zdas04]

You are never going to find any model that predicts the observations that you've described. The problem is that all of them are dealing with averages and averages of averages. If you assume that Hazen Williams will be within about 10% of observed data (sometimes it is better, sometimes it is worse), I bet that every one of your outlets is within 10% of the average volume. Any number closer than that is just a guess (that +/- 10% uncertainty means that any number in that range has the same value as any other number in that range).

The only way you are going to balance all of the legs is to put control valves on all of the legs and drive each of the control valves based on a flow meter. Just relying on the pipe being "the same" will result in maldistribution.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[BigInch]

"no air volumes displaced at beginning of dispense"
As soon as air gets in, forget HW.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[adamlcarp]

The problem I'm having (and didnt describe as thoroughly as I could) is I'm dealing with pipe diameters of .5588mm +/-.0063mm, dispense volumes in uL and pipe lengths of 24.13mm. We have a self imposed 2.25% variance and have determined that there is a (mostly) linear decrease in dispense volume per tube as you move from the furthest tube in towards the inlet. I'm trying to determine what percentage of our 2.25% allowance is being taken up by a phenomenom the design is creating and what is caused by the tolerance of our dispense tubes.

Using Hazen Williams I've come up with solutions to the variance allowed due to the varying tubing ID's but as I see more threads on use of H-W they seem to all refer to parallel pipes with single inlets and outlets. Is my use of it with single inlet and multiple outlets appropriate or am I off base?

Thank you all for your input

 

[BigInch]

Sorry, but you're making me laugh pretty hard right now. Please excuse the typing.
HW does not apply to such small diameters where surface tension forces are immensely greater than friction.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[BigInch]

With those diameters you can get flow going as high as 300 ft or so, straight up!
Maybe you are not even talking about water for that matter.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[Latexman]

As zdas04 said you need a control mechanism - control valve or needle valves or valves open for different length of times - something.

Good luck,
Latexman

 

[vpl]

You may want to consider use of the Darcy (sometimes called the Darcy-Weisbach formula or the Fanning formula). Also, given your rather small diameters, you're likely to see considerable impact from the roughness of your piping (no matter how smooth you think it is). You might look at Crane's Technical Paper 410 "Flow of Fluids Through Valves, Fittings and Pipe" (the 25th printing), especially Chapter 1. You're basically in the area where friction factors will be much larger than expected simply due to the small diameter of the pipe/ tube. Also, depending on your fluid and flow rates, you could be running into some problems with capilliary action.

Patricia Lougheed

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[BigInch]

Repeat: HW, Darcy, typical flow formulas ARE NOT APPLICABLE.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[vpl]

Right Big, which is why I told him to read the front part of Crane which gives a good explaination of how the pipe diameter affects pipe roughness. He can read up on it and figure out for himself what's happening. It's a pretty widely available reference book (and fairly cheap if not available) and if he's not working with your size pipelines, then it's probably a good one for him to invest in.

Patricia Lougheed

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[BigInch]

Good. Thought nobody was listening.

Abstract
Experiments were conducted to investigate the flow characteristics of water flowing through rectangular microchannels having hydraulic diameters of 0.133-0.367 mm and H/W ratios of 0.333-1. Experimental results indicated that the laminar flow transition occurred at Reynolds numbers of 200-700. This critical Re for the laminar transition was strongly affected by the hydraulic diameter, decreasing with corresponding decreases in the microchannel. In addition, the size of the transition range was diminished and fully developed turbulent flow also occurred at much lower Re. The friction behavior of both the laminar and turbulent flow was found to depart from the classical thermofluid correlations. lite friction factor, f, was found to be proportional to Re−1.98 rather than Re for the laminar condition, and proportional to Re−1.72i for turbulent flow. The geometric parameters, hydraulic diameter, and H/W were found to be the most important parameters and had a critical effect on the flow. Generally, increasing the ratio H/W increases the friction factor. The reduction of the microchannel hydraulic radius decreases the friction factor significantly for a given H/W. There exists a special range of ratio H/W (approximately 0.5 mm) at which the experimental data are lower than the predictions obtained from classical correlations. Continued reduction of channel size increases the difference between fI,expf1,theo at REcri, and the quantity of fI,exp becomes smaller within the region adjacent to H/W = 0.5, and larger when H /Wis out of this region.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[Latexman]

I don't think I'd waste time calculating. It seems they are close to what they want. They just need something to fine tune the outcome. They need an adjustable control.

Good luck,
Latexman

 

[MikeHalloran]

You won't find control valves in that microfluidic world, at least not yet.
Balancing resistances will not work; bubbles, rat hairs, and machining chips will screw it all up.

Speaking of which, bubbles can be used to advantage, e.g. with TFE tubes and optical meniscus detectors. ... but they can be troublesome, too.

Start retooling now for multiple positive displacement plunger/piston pumps, one per dispense stream.



Mike Halloran
Pembroke Pines, FL, USA

 

[Latexman]

Adamlcarp,

Why not work with dispensing equipment vendors?

Good luck,
Latexman

 

[adamlcarp]

Thanks all,

We are manufacturing these assemblies ourselves. My investigation stems from intermittent failures that can plague the line for weeks at a time. It seems as though a redesign of the inlet may be the best solution at this time.