low pressure shutoff - whats the right way

[SilverLeaf]

I'm in the process of building a 4040 RO homemade system, based on several of the plans I've seen here. I think I have a good handle on most parts of the system. But one thing I just can't wrap my head around is the shutoff valve(s), what their actual purpose is, and how to configure them. A couple questions:

1) For the low pressure shutoff valve between the feed pump and the actual Procon pump, do I:

A: wire it to shut off both pumps if/when it experiences low pressure
or
B: wire it to just shut off the Procon pump. (this would leave the feed pump running, which would cause a problematic buildup of pressure, right?)

I'm assuming the purpose of this shutoff is just to make sure that the Procon pump never runs dry. Is that correct?


2) Is it necessary to have a high pressure shutoff valve between the Procon pump and the RO membrane (and if so, does this again just wire to one pump or both)? If I understand it correctly, my procon pump has a 250 PSI relief valve in it; wouldnt this preclude the need for a shutoff if the pressure is building too high?

[Clinkis]

You want both presure switches to shut of BOTH pumps. And yes you want your HP switch between your HP pump and the membrane. Typically you don’t want pressure relief on your procon pump. Usually you want a procon pump without relief.

[ronintank]

I am just finishing my ro unit and hope to start it this weekend.
I bought a pump switch with a adjustable low pressure cut off switch built into it.
So i will run the power into the switch and have the switch power an outlet that i plug my pumps into.
The low pressure cut off switch has a little lever on the side that you have to pull up on to connect the contacts.
At that point i will only have my feed pump plugged in, once the pressure is above my min. shut off setting i can let go of the lever and the contacts in the switch will stay connected and feed power to my outlet.
After all the air has been pushed thru the ro unit plug in the high pressure pump into the same outlet and both pumps will be protected by the low pressure cut off switch.
My pump has the adjustable high pressure by-pass for over pressure protection.