Ro pressure

[cat tech]

I’m in the process of building my first ro unit , small little 3- 400gpd set . I notice that most of the
Set ups for these small portable units use a needle valve to control system pressure.
Is there a reason why a inline pressure relief valve could not be installed to keep a constant system pressure
( ie a flow through 100 psi relief valve) ????

[maple flats]

You really want to be able to adjust the pressure. For example, when cleaning the membranes you will not use the same pressure and as the membranes get cleaner the pressures will drop.
Just consider this, why is it that the RO manufacturers don't use a pressure relief valve?

[RedMapleCreek]

There are several reasons that a pressure relief valve is not appropriate for that service.

Number one being as Maple Flats stated that you need to be able to vary the setting. With small hobby ROs you will generally adjust the needle valve to achieve a matching flow of concentrate and permeate.

Number two is that a pressure relief valve does not control pressure to a steady value. It is intended to protect and relieve against over pressure and once it pops it does not re-seat until the pressure has dropped a bit below the set pressure. In other words your system pressure will oscillate significantly as it opens and closes.

Number three is the needle valve is a fraction of the cost of a relief valve.