PSI drop -- is this normal?

[DeanS]

Hi Cary. CSV1A valve installed before 20 gallon pressure tank.  CSV set at 64 psi @ 2-3 gpm.  CSV installed this summer.

 Irrigation system uses 9 gpm during 20 minute cycle.  PSI starts at about 60 and not 64 psi which I know is normal due to friction loss.  My question is, during the 20 min irrigation cycle, the psi can drop from 60 to 57, sometimes 55.  If household water is used (flush toilet for example) during the 20 min irrigation cycle, the psi could drop a couple more psi until that water is done being used and it rises again to what it was prior to flushing. Is this normal (the psi drop during the cycle as well as dropping a bit more if toilet flushed) and to be expected? I know practically nothing about pumps and read about the charts and head pressure and it makes my head hurt.  The irrigation coverage is excellent and my lawn looks great, I am just trying to see if normal or is it indicative that the new pump (11 months old) might be failing or some other issue. I think it is normal but I am just a dummy accountant.

Here are some key things you may need to know:
- GPM output of irrigation system is about 9 gpm; well recovery is 3.4 gpm. So each minutes lose 5.6 gpm
- this equates to feet in the well loss of : 5.6 gpm X 20 mins divide by 1.5 factor = 75 feet of well lost during 20 minute sprinkler cycle ; 112 gallons lost
- well is 200 feet deep; static water level is at 15 feet; pump is at 180 feet; well guy says when full the well holds approx 240 gallons
- pump is Goulds 3/4 HP model 10CS07412CL

I am thinking the 75 feet (112 gallons) lost in the well during the cycle means less head pressure for the pump so the 4-6 psi drop is normal in this situation. But as i said, I am a dummy accountant not a well water guy and pump charts make my eyes hurts, so figured you might be able to help me. 
 The CSV has been great overall as it does what it is supposed to do. So I am very happy in that regard.

Thanks.

[Cary Austin]

The CSV1A itself has 5 PSI of reduced pressure falloff at 10 GPM.  So, when using 10 GPM you will have 5 PSI less than when using 2 GPM. 

Also, the lower the water level in the well, the less pressure your pump can produce by 1 PSI for every 2.31' it drops.  So, as the water level in the well drops by 23', the pump produces 10 PSI less pressure.  Conversely, as the water level rises in the well the pressure increases by the same amount.

You can turn up the CSV and pressure switch by 5 PSI to make up foe reduced pressure fall off.  But you need a larger pump to make up for a decreasing water level.  However, the water level can only decrease by so much as it is a low producing well, and it is probably a good thing that the flow and pressure drop off as the well is pumped down.

[DeanS]

So interpreting what you are saying, the psi drop I am experiencing is normal and to be expected in a low producing well?

[Cary Austin]

Yes. Sorry. Lol