What is the formula for determining if you are achieving the 3ft/sec scrub velocity requirement at the loop return?

You need 2 of 3 variables. Flow Rate, Diameter, velocity.

A 1" pipe @ 7.4 gpm will be approximately 3 ft per second.

With flow meters installed I can see ours runs at around 8.7 gpm which is over 3 ft per second. Check out this web site or just google flow calulations.

http://www.1728.com/flowrate.htm

Please keep in mind that a 1" pipe does not necessarily have a 1" ID. A schedule 80 pipe is thicker than a schedule 40 pipe.

For schedulue 80 pipe PVC, 3 ft/s is 6.73 gpm for 1" pipe, 4.04 gpm for 3/4" pipe, and 2.19 gpm for 1/2" pipe. Your flow needs to be greater than these values at all times. Cut these values in half for 1.5 ft/s for direct feed systems.

Florian we have 1" pipe, 10 gpm going out to loop and 6gpm on the return. Tank feed system. what would that give us for flow velocity ?...thanks in advance. I always appreciate the assistance

Joey,

That comes out to 2.447 ft/sec so, you do not meet the 3 ft/sec requirement.

Chuck

Thanks Chuck I appreciate it. It looks like I have to achieve a 7.5 gpm to get the 3ft/sec.the problem is that after we bought the BBraun machines it screwed everything up because the water flow is dynamic not static. We had to adjust back pressure for reuse and sometimes the flow is 8 and sometimes it is 5 ...always moving!!

Florian - I have a question for you. If your loop is 1" all the way through but about the last 10' or so is 3/4 (and this is where the flow meter is installed) then how would you messure that? I believe most loops have this set-up. I have around 8.5 gpm at this point and never below 5.5 gpm at worst case when filling granuflo or bicarb mixers.

What is in the last 10' of piping? Any connections that use water? If no connections, and you have 5.5 gpm going into the 3/4" section, then you have 5.5 gpm in the 1" section just before the 3/4" starts. You need greater than 6.73 gpm to get at least 3 ft/s velocity. Therefore, this case would not meet the requirement. If you have something using at least 1.2 gpm before the 3/4" section starts, then you may meet the requirements.

This requirement is one of those "open for interpretation" requirements. Many clinics turn their flow off at night, especially if they have a direct feed system. Personnally, I perfer flow 24/7. The right velocity is supposed to reduce your chance for bacteria growth. The real question is, how are your bacteria/endotoxin levels? If they are good, then what your are doing is working....but document what you are doing and have your medical director sign it. Your auditor will question you about this.

Florian - I have 7 machines using this water before the 3/4" starts. Like I said, normally with all machines running I run around 8.5 gpm. Only when I use water for Granuflo or bicarb does it drop. I was told, especially with the Granuflo, to throttle down the valve so it doesn't draw so much there for maintaining the flow needed.

I guest I was just wondering if most of my loop is 1" but the flow meter is in the 3/4" section is it aright to use that as my flow in this case?

Thanks for your help

What is the flow when no machines are running? If your have 7 machines running 800 ml/min and 8.5 gpm at the end, then the flow with no machines running would be around 10 gpm. This is also the flow at the beginning of the loop. Basically, if we start with 10 gpm, then you have 9.79 gpm after the first station (800 ml/min is 0.21 gpm being subtracted from the loop flow). 9.58 after the second station, etc. If you have 1" piping after the 7th station, then the flow needs to be greater than 6.73 gpm after the 7th station. If it goes to 3/4" at the Tee that feeds the 7th station, then the flow needs to be greater than 6.73 gpm after the 6th station, and needs to be greater than 4.04 gpm in the 3/4" piping.

Is there a way to find the flow rate without velocity or flow meter. I have a 1" schedule 80 leaving the Ro system and 3/4" schedule 80 returning to it. How do I find the flow so I can calc the velocity?

If your pump is relatively new and has not degraded, you can read the discharge pressure gauge to determine the flow. Use its pump curve to determine the flow.

Thanks Florian
If I figured it right off the label and checked web page, I am hitting 7.5 gpm.

dear all,
Can I calculate pressure base on flow rate and pipe size ?

Thanks a lot.