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topic 29731

Required Velocities in DI Distribution


I am designing a 1 mega-ohm DI system for a client and would like to find some literature that talks about appropriate velocities in pure water distribution systems. The client want to keep velocities above 5 (fps) and I am having trouble doing so with the restraints of this system. I would like to be able to show that slower velocities would be acceptable. I have worked on other DI systems where the velocities go as low as 2.5 (fps). Can you help me or offer any insight?

Thanks in advance.

Matthew J. Tanis
engineering consulting - South Burlington, Vermont, USA

First of two simultaneous responses -- 2004

The quality of DI water invariably degrades in storage and in the distribution system. The > five feet per second velocity is usually specified to keep biological deposits from forming on the inside of the piping and in dead legs. This is usually accomplished by taking the water from a continuously recirculating loop. If this and the 5 fps are not possible, or if the water quality degrades below your specifications at the point of use, then you may want to consider using point-of-use mixed bed resin polishing tanks. These will last a fairly long time if fed with water with a nominal resistivity of 1 Meg-Ohm-cm.

Lyle Kirman
consultant - Cleveland Heights, Ohio

Second of two simultaneous responses -- 2004

Hi Matthew,

Firstly my background is plastics ... not D.I. systems at all, so take what I say with a pinch of salt.

5 FPS is slow ... it's walking speed.

DISTRIBUTION ... .If this is what you're after ... have you tried a sparger approach? This will give you totally superlative mixing. Mind you, I'm thinking only of PVC.

On the header pipe ... or on the sparger laterals ... drill some coarse l/8" dia. holes at 2" intervals. Every 5 feet of so make/instal a 3/8" or l/2" thick ring to be around l" larger than the pipe. (the hole sizing and spacing depends on the amount of liquid (or air) being pushed through).

Then get some cloth (yes, cloth) and wind it around and over the rings ... one layer may be sufficient ..,. and tie down with 'cord' (i.e., vinyl or the sailmakers' Terylene).

What happens is that you'd get, courtesy of the large holes, even distribution along the length and myriads of ultra super fine bubbles or perfect liquid dispersion.

If this makes sense to you, please let us know.

freeman newton portrait
Freeman Newton [dec.]
White Rock, British Columbia, Canada

(It is our sad duty to advise that Freeman passed away
April 21, 2012. R.I.P. old friend).

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