How to read a pump curve correctly? 3

[ctbailey]

Hello to all.

I am needing to "size" an effluent pump, one which will be used in a proposed onsite septic absorption field. I intend to specifiy "black poly" or polypropelene pipe/tube for the pressure line.

1. I have calculated the elevation head.

2. I have calculated the friction loss. This was not easy, since I could only find one very obscure document which included "plastic" pipe friction values. All my hardcover texts only include steel pipe for friction losses.
I eventually settled on the following:
1 1/2" pipe, "Proline-polypropylene 150" at 30 GPM, loses 5.81 feet/100 ft. of pipe.

Please advise whether this friction loss is acceptable.

3. I calculated the minor losses due to the fittings. I used the "steel" fittings found in the Lindburg book.

4. here's the question:

I have a pump curve, from Meyers Pump - I selected a pump, the Meyers ME00. It's pump curve can be found here:

http://www.femyers.com/pdf/pdf.sse/sse.brochure/k3320-50.pdf

My total head, lets say is 60 ft. (Z+f+m) f=function of GPM

by selecting the ME100, and my CHOSEN rate of 30 GPM, I fall well "inside" the pump curve.

BUT!!! if I follow the chart from the Head of 60' right until I meet the curve, and then down to the Flow rate, I come up with just over 50 GPM.

When I recalculate my friction losses based on the higher flow rate, my total head goes up.

So I enter the pump chart again at the Total Head of now 70+/-ft and slide over to the curve, and down to the flow rate. I come up with 30 GPM.

What am I doing wrong with reading my pump curve? This seems like it could go on for ever. I was hoping one or two iterations and the values would settle down, but this oscillation between two different sets seems wrong to me.

THE REASON FOR MY CONCERN:
I would like to know the flow rate that the pump will flow at, so that I may calculate the time the pump is on, or "Doseing time."

If I can't determine my flow rate, I can't determine my dose time.

Thank you for you help, for this question that I should have learned back in fluids class.

Craig Bailey

___
Craig T. Bailey, PE

http://www.bailey-associates.com

 

[bimr]

You have picked the pump correctly.

Once you have selected a pump, the operating performance of the pump will follow the curve for that particular pump. As pumping head increases with higher flow, the flow through the pump will decrease. The pump performance will always follow the curve.

The problem that you are having is that this particular pump has a steep pumping curve. You may be able to find a pump with a "flatter" curve so that the flow will not vary as dramatically with pumping head.

At such a low pump capacity, you are somewhat limited by availability. You might check into the use of a variable speed motor controller such as the Aquavar by Goulds. Such a system will provide constant flow by varying the motor speed.

 

[Maury]

You haven't completed the analysis. The next step is to develop a System Curve. This is plot of the total head loss through the piping system for various flow rates. I would suggest that you calculate the head loss for 20, 40, 60, and 80 gpm. Be sure to include the static head. Plot these points on the diagram and connect them with a curved line. The actual capacity and head will be where this System Curve intersects the pump curve. The important point is that each of the four pumps must operate exactly on its curve and the piping must operate exactly on the system curve. Where these two intersect is the operating point.

 

[francesca]

On larger pumps, the impellers are usually trimmed to offset the pump curve down to your design point. (The parallel lines on the pump curves are the same pump with different sized impellers. You'll specify the next one up, which has to be trimmed to perform at the design point.)

I wouldn't imagine that trimming the impellers is an option on a small individual pump, though.

As Maury said, plot your system curve, which will tell you how your system will perform. Then you need to make sure that your receiving system can handle the higher than intended flow.

What method did you use for your friction head? Hazen-Williams? Manning's? If you still need your friction loss checked, it's easier to tell us what method and what coefficient you used. Or did you just use a table? In which case, you can probably check your own answer by using the Hazen-Williams formula.

If you calculate your friction loss using a formula, you can use a spreadsheet to quickly come up with your system curve.

 

[ctbailey]

Thank you to all who have responded. I apologize for my slow reply... I've been on a long weekend.

SYSTEM CURVE - That makes sense. I will plot a curve and determine where the two curves intersect. Thank you.

Friction losses were determined using a table of published values. These values were published by a plastic pipe manufacturer. The value I started with was 5.81 feet/100 ft. of pipe, at 30GPM.

I did not use Hazen Williams or Mannings. I will run the numbers using a Hazen/williams Constant of 130, and see where I fall.

___
Craig T. Bailey, PE

http://www.bailey-associates.com

 

[daddymem]

Check with your local Myers distributor for their MAPS computer program. Run your selection and design by the local rep, they are usually very helpful not only in selecting a pump that will work, but a pump this is economical and efficient and "off the shelf" for ease of future repair or replacement.

I ain't never seen a hearse with a luggage rack.-GS

 

[DLauEng]

For the Hazen Williams calc, check out this website...

http://www.lmnoeng.com/hazenwilliams.htm

D Lau