Flowrate issue through a 3/4" Cu pipe (solar)system

[BPLBPL]

Hi all, I'm currently trying to fix a badly designed closed-loop Solar thermal system running at the below conditions:

3/4 inch, 'L'-type Copper piping(0.785" inside dia)
Flow rate = 2.2 gpm
Pressure head @ 2.2gpm = 28 ft of H2O
Tiny Grundfos pump = UPS 15-58FC

The system is running into air-block and overheating issues because of low flow rate. I'd like to increase the system flow rate to manufacturer-rated flow of 6 gpm thru the collectors. At 6 gpm, the total pressure head would increase to 53 ft of H2O which the current UPS15-58FC can't handle. I am planning to install two Bell&Gossett PL-36 pumps in series or one B&G PL-55 to achieve the desired flow rate.
The pumps info. below:

http://www.grainger.com/Grainger/items/4JA84?BaseItem=4JA10

http://www.bellgossett.com/literature/files/4234.pdf

Now, my concern is with running a high flow rate of 6gpm thru a 3/4" Cu pipe! My calculations indicate that at 6gpm, the velocity will be 4.35 ft/s(which is fine) and Re = 47,000, very turbulent!! Will running 6gpm through a 3/4" pipe lead to noise/vibration problems and erosion/bursting of pipes?? This is a residential project, so noise & pipe-burst are big concerns. Increasing the pipe diameter is the obvious answer, but will prove too, too expensive for the client.

Your inputs on the above issue would be deeply appreciated. Is my pump selection ok? Thank you
bplwithu - A freshly graduated engineer

 

[BPLBPL]

The B&G pump curves for PL-36 and PL-55 pumps below:

http://www.bellgossett.com/productPages/Series-PL-2.gif

 

[BigInch]

I'd say it will probably be OK, 5 is borderline for reasonably quiet ops, but NO faster than 5 ft/sec. You might want to make sure the tubing is well fastened to supports near turns. No guarantees though.

http://virtualpipeline.spaces.msn.com

"We can't solve problems by using the same kind of thinking we used when we created them." -Albert Einstein

 

[gepman]

How hot is your water? See the following for velocity limits in copper as a function of temperature.

http://www.engineeringtoolbox.com/copper-pipes-water-velocities-d_1081.html

I think that you should be concerned about corrosion also.

 

[BPLBPL]

The operating temperature ranges 130-160F. Also, is it alright to install 2 centrifugal pumps one after the other i.e., adjacent to each other such that the discharge of pump1 is in contact with the suction side of pump2 ?

thanks gepman and BigInch.

 

[TBP]

You might consider going with a higher delta-T on the system, to keep the GPM down.

In a closed-loop system, corrosion shouldn't be much of an issue. Pretty much the only time corrosion will come into play is if there's an on-going issue with the relief valve lifting, or leaks, resulting in routinely having make-up water being fed to the system - and they should be repaired promptly anyway.

The Bell & Gossett website has loads of excellent info. Manufacturers have a real interest in having their equipment work properly in any given project. They'd typically far rather help to make the initial installation correct than have to get involved after the fact, to deal with problems that could have easily been avoided.

 

[BPLBPL]

TBP, unfortunately I can't pay around with the deltaT. It's kind of fixed 35-40F.

 

[gepman]

If you look at your temperature it is above the recommended limits of the link that I gave you, especially at any bends. These limits are confirmed by the CDA which is an excellent resource on copper (I use it mainly in electrical switchgear design). See

http://www.copper.org/applications/plumbing/techcorner/designing_piping_systems.html.

Download the Copper Tube Handbook while you are there.

What is the advantage of two pumps in series over a correctly sized single pump? I always prefer a single pump unless there is some overriding advantage to having pumps in series (i.e. change in operating conditions).

I agree the B&G website is very good. Sign up for the pump calculator. I use it all the time.

 

[BPLBPL]

Thanks gepman.
It is very difficult to find a small sized high head pump which can handle 6 gpm at 53 ft of head. Thats why I chose to use 2 pumps in series. Big cost difference as well.

 

[BPLBPL]

Thanks everyone for your replies. I have decided to go with 2 B&G PL-36 pumps in series.

I should complete the installation next week and will let know how things went. Thanks again.

 

[hydtools]

Turbulent flow is good for heat transfer. It keeps mixing fluid for good heat pickup by all the fluid, not just the surface layer fluid as in low velocity laminar flow.

Ted