Pumping Water -- Please check my pump system design work

  • Thread starter Fish4Fun
  • Start date
  • #1
247
2
Hey Folks! Just need a quick sanity check ... Putting in a new 4in well and submersible pump ... trying to wrap my brain around the "physics" of it .... Here is what I ***Think I Know*** :

1) The Energy required to move any given volume of water to ground level can be calculated using M*G*H .... where H = the distance from ground level to the water level, in this case ~50ft....

2) The Energy required to elevate the same volume of water to a pressure of 65psi (@ ground level) is equivalent to the energy required to lift that volume of water to 65psi/.433psi per foot ==> ~150ft

3) Using 1 & 2 ==> To move 20 gallons of water from 50ft below ground to ground level at a pressure of 65psi is equivalent to moving 20 gallons of water to a height of 200ft (~61M).... 20 gallons of water weighs ~76kg; so the energy required = 76kg * 9.8m/s^2 * 61M = ~45,432J ... Assuming a flow rate of 20 Gallons Per Minute, this would imply 45,432J/60s = ~757W or Slightly more than 1HP

Obviously there are many mitigating factors that could influence the actual size of the pump required, BUT 757W is the actual work required to do the job.... Pump Efficiency and other variables would likely suggest sizing the pump larger perhaps 1.5HP to 2HP...., BUT the general method used to derive the amount of work requisite is sound?

Why am I using physics to figure this out? Major US MFGs offer 4in submersible well pumps from 1/2HP to 5HP .... AND each Power Rating is offered in various "Gallon Per Minute" models with little or no explanation as to the selection of one over the other ... Obviously a 1/2HP pump rated for a constant flow of 25 GPM can only do so with a very nominal amount of "head" ( < 80ft total ) .... while a 1/2HP pump rated at 10 GPM could operate nominally at up to ~200ft of total head. The fact that several distinctly different "GPM" rated pumps are available at any given power rating suggests that various pump models are optimized for use at various depths // output pressures AND Flow Rates .... For any given depth//pressure there is likely only one or perhaps two choices for nominal flow rate at peak efficiency ... At least that is my take on it .... But I just wanted to make sure I was figuring everything correctly ...

Thanks in Advance!

Fish
 

Answers and Replies

  • #3
jrmichler
Mentor
1,362
1,470
Your physics is correct, and you are correct about how pumps are optimized for different conditions. Kuruman's link is an excellent summary of how to specify a pump. Note that the link calculates the required pump head the same way you did, with the addition of line loss.

By varying calculated flow rate from zero to, say, 30 GPM, you can draw a system curve. Calculate two system system curves, one for the pressure at which the pump turns on, and the other for the pressure at which it turns off. Then sketch the system curves on the pump curve. The intersections show the actual flow rate from the time the pump turns on until it turns off.
 
  • #4
247
2
@kuruman ... THANKS! I thought I had searched the entire Internet attempting to find a pdf Exactly like that, LoL ... Obviously I gave up the search prematurely.

@jrmichler ... Thanks for the Reply! Honestly it is somewhat satisfying to note the PDF linked by Kuruman solves the problem of calculating the pump HP in almost exactly the same manner (ie converting "pressure" to "head") .... my experience with wells, pumps and water systems over the years has left me a bit reticent to "trust" specifications like 18GPM without fully investigating the caveats ....

Unfortunately the single biggest caveat cannot be readily calculated; that is the "flow rate of the well" ... I currently have a simple 1 1/4" pipe driven 45 feet into the ground, directly coupled to a 1/2HP jet pump. This aquifer has amazingly good water, but the flow rate is an abysmal 1.75GPM .... the area has four relatively distinct aquifers at various depths .... the first is typically between 20ft and 60ft and varies drastically in quality and flow rate ... the other three range from ~100ft to over 2000ft with at least one being very high in salt and other mineral content making it completely unsuitable for domestic use. Local well drilling companies don't have a great track record with "finding good water" ... they always find an adequate supply of water, but frequently the water requires a multi-stage treatment system to remove iron, sulfur-dioxide and other mineral issues and still the water is has to be heavily filtered to make it palatable .... So, for the last 10 years I have used the 1/2HP jet pump to fill a 550 gallon cistern and used a 1.5HP "pressure booster pump" to provide pressure and volume to our home. Recently the booster pump failed, and I need to either replace it or install a well capable of directly supplying water to our home.

All of the options are expensive and involve some potential risk of failure ... My current thought is to purchase a good-quality US made 1.5HP 4" submersible pump rated for ~20GPM @ ~200ft of head ... I will wash a 4" PVC casing down ~45 feet relatively close to my existing well and test the submersible pump to see what kind of flow I get ... if I can get 10gpm or better then I will call it a win and be done ... If I don't get enough flow then I will move the high-volume submersible pump to the 550 gallon cistern and use it as a "pressure booster pump" ... Assuming the "new" 4in well provides more than 1.75gpm, I will then purchase a 1/2HP 4" submersible pump designed for ~5gpm and use it and the existing 1/2HP jet pump in a redundant system to keep the cistern full.

All this for water, LoL! Yes, County water is available, but the quality is not great ... the "connection cost" is ~$10k and then there would be the associate monthly usage fees ... Hiring a professional well-drilling contractor is roughly the same $10k with no assurance of water quality ... Replacing the existing pressure booster pump is roughly the same $700 as a good quality US made 4" submersible pump ... washing 4" PVC 45 feet into the ground will be an interesting challenge, but plenty of people in the area have done it successfully, so I should be able to manage it ...

Any way ... Thanks for the sanity check!

Fish
 
Last edited by a moderator:

Related Threads on Pumping Water -- Please check my pump system design work

  • Last Post
Replies
1
Views
4K
Replies
11
Views
891
Replies
10
Views
1K
  • Last Post
Replies
4
Views
2K
  • Last Post
Replies
5
Views
1K
  • Last Post
Replies
2
Views
13K
  • Last Post
Replies
2
Views
5K
  • Last Post
Replies
1
Views
408
  • Last Post
Replies
3
Views
3K
  • Last Post
Replies
1
Views
610
Top