That is centrifugal territory for sure.
Can you power the pump only when a valve is open?
This makes things a lot easier.

Yes, they just sit there until something is opened and pressure drops. I want a pump that powers up instantly, tries to maintain a set pressure, and does NOT operate on a high low range of pressures, like 20-40, 30-50 etc.

I would start with the pump you have.
Just get rid of the pressure switch and air tank.
Power the pump ONLY when a valve is open.
This will require a contactor that closes and powers the pump when any of the valves are open. Irrigation controllers usually have a signal output for this.
Not sure what your controller is.


Do not dead head the pump or power it against a closed output. Bad things happen.
You can use a pressure relief valve plumbed back to the tank to prevent this problem.
Set it at 40 psi and use it as your pressure regulator.

If the cast iron pumps arent holding up to the chemicals there are reasonable stainless models available.
Bur cam 506518SS looks ok
True value and amazon has these. About $200

Thanks Nematode, I'll look into it. The pumps only get fresh water.

You seem to want an exact output pressure, a relief valve might get you there, but if it is not precise enough, a pressure reducing valve can be used in addition to the relief valve.
Wilkins nr3xl is adjustable 15-75 lbs. 1" should do the job.

OK here is another way.
Use a cycle stop valve and a very small pressure tank.
My irrigation is set up this way.
Pump is powered always and has a 40/60 pressure switch.
Valve opens, pressure quickly drops from 60 to 40, pump turns on and stays on with pressure at 50psi until the irrigation valve closes. Pressure builds to 60 and pump turns off. No need for pressure relief valve. Cycle stop valve is adjustable.

Im not sure if the pressure variation is a problem.

One more and I promise I'll quit. Insomnia sux.
506532SS bur cam.
Drop it in and remove your pressure tank. Its a "constant pressure" unit.
Looks like a cycle stop type valve on the output with an electronic control.

Cant find the exact pressure spec but there is a customer service number in the install instructions.

[url]https://www.google.com/url?sa=t&source=web&rct=j&url=http://pdf.lowes.com/installationguides/772910065324_install.pdf&ved=0ahUKEwj8hvKYh87SAhVq54MKHRhWAMkQFggwMAM&usg=AFQjCNFqgoDxPOACXPhpTaYuAZdEQjL_Cg&sig2=5OMEU3Hp1qzO6VMtOCRalA[/url]

$270

One thing you need to do first is to get rid of the irrigation valves you have maybe.
You need ones with flow control.
This will give you the exact output pressure you want.
These are the ones I use.
They can be used from .025 to 30 GPM.
[URL]https://www.sprinklerwarehouse.com/Irritrol-Sprinkler-Irrigation-Valve-p/2500tf.htm[/URL]
All irrigation controllers have what is called a Master valve/Pump start station.
What you will see on them is the connections for the power (maybe) two connections for the rain sensor (jumped out), the master valve and then the stations for how ever many stations you have.
And a common for all valves.

There are two ways you can connect the pump start relay.
One is to wire it parallel with the master valve.
The other is to wire it parallel with the valve that controls the drip system.
Both will work.
Connecting it to the master valve the pump will always come on anytime a station comes on.
Connecting it to a station it will only come on when that station comes on.

Here is a description of one.

[LIST][*][B]Electrical relays for both low voltage (24V ac) control switching and high voltage (120V ac or 240V ac) main power contacts[/B]
- Allows remote pump switching using 24V ac output from an irrigation controller’s master valve/pump start circuit
- Opens and closes main power contacts for pumps (1HP at 120V ac 1 Phase or 2HP at 250V ac 1 Phase)
- Note: 2HP at 120V ac will exceed maximum amp rating.[*][B]Highly efficient 0.1 Amp operating requirement[/B]
Draws less holding power than most solenoid valves[*][B]Can also be used with the Irritrol PC Control system for switching control of low voltage landscape lighting[/B]
Saves the expense of an extra timer and puts irrigation and landscape lighting control in one location on the owner’s computer[/LIST]

Thanks guys, I really appreciate the time you spent helping with this, I will let you know what happens.