well "head preassure"

hello friends:
i live in the country and around here every one has a well for water. my pump went out and i needed a new one. so i started looking for a new one. in one of my querys i reached vern the well driller/installer. in our conversation i mentioned the well pipe was 2" in diameter and 250' to the pump his comment was that 2" pipe creates a lot of "head preassure". i don't think so, i think that the head preassure is a factor of how far you lift the water and a larger pipe will only make it easyer (less friction).

any comments would help.

thank you
dave

 

[DeltaCascade]

I agree. "Static head" is from the 250' column of water over the pump.

Your supplier appears knowledgeable enough to indicate that it is something to consider! So, yes, "friction head" will also develop, for a given flowrate. "Velocity head" is not an issue for steady state flow, but may be at pump start up.

Crane Technical Paper No.410 has a convenient table which states, for a 2" schedule-40 horizontal pipe at 6 usgpm flowrate and 60F, a 0.044 psi pressure drop per 100' of pipe; summarizing some additional data points:

usgpm ft/sec psi/100ft
6 .574 .044
10 .956 .108
20 1.91 .375
30 2.87 .786
40 3.83 1.67
50 4.78 2.03
60 5.74 2.87
70 6.70 3.84
100 9.56 7.59
200 19.14 28.8
(3" pipe at 60 usgpm gives 2.60ft/s and 0.406 psi/100ft)
(you may have smaller pipe inside diameter??)

You may want to add equivalent length due to bends and fittings and valves. Of course, you will have some backpressure, too (30 psig or so?), which is added to get total static head.

Just add the "static head" for the vertical part of the pipe (and backpressure) to the "friction head". With one atmosphere being 33.957 feet of water at 68F, you have about 7.36 atm of static head, or, 108 psi. Add the "head" due to friction and you have your steady state system curve to match the pump curve to.

It looks like friction head will be small compared to static head, and total head, that the pump must overcome, depending upon the flowrate.

The savings from installing larger diameter pipe can be calculated from the above. Savings would be in reduced energy cost, and possibly also less expensive pump.

(My guess is that you also may want around 3-6ft/sec water velocity as a minimum to help keep the line flushed clean...)

Good luck.

 

[owlscreech]

Neither the head nor the pressure at the well head will be different. The weight of the water would be greater for a 2" line of course. That may be where the well driller is coming from, but it is not relevant here.

Pumping requirements could be different for a larger size line and this was a new installation. I believe the issue here may be horsepower requirements. If you had changed the piping from a one inch to a two inch, you are obviously changing the dynamic system curve to allow more flow at the same head. If the same pump is supplying the water, then the HP requirements will go up some, but not much since the reduction in the dynamic head is very small compared to the sum of the previous elevation and dynamic head.

But it sounds like you are just changing out the pump, so all of that above is academic. Use the same pump design head and HP requirements and there's no problem.

 

[DiaphragmPumps]

I think the point is being missed here. The Head Pressure eg PSI is relative to an area hence pounds per sq inch, so if you were 250' down in the sea or had a 2" diameter pipe on your head the pressure per sq inch is the same. Your pump engineer has made a comment which is both true and untrue depending on what size pipe he is comparing against.

Any centrifugal pump has a flowrate proportional to pressure and therefore the pipe size needs to be considered. If your flow was 20 litre per min thru a 2" pipe your 'head' would equate to roughly 250' static plus 0.4' dynamic (ie friction loss) totalling 250.4' Head, by going larger or smaller this figure will vary, and you need to decide, is the expense of going to a larger bore size, greater than the saving on the pump hp required to generate a flow to compensate.
Eg If it was a 1" pipe the dynamic loss would be approx 7.5' not 0.4' and a 3"pipe would be approx 0.05'
Formula used : hf = f * ( L / D) * (V2 / 2 * g).

Is it possible your supplier is trying to screw you into paying for a new pipe as well as the pump?