Duo 60 Desallator pressure problem


I'm having an issue with the watermaker where the pressure becomes erratic after running normally for 15 to 90 minutes.  You could watch a video of the pressure dial here:  https://www.dropbox.com/sh/63wu632zckpu9u1/AAB2tVJwzWvCiczr2W_oWvoza?dl=0

First I'm wondering if anyone else has had this trouble?

The things I've done to try to solve the problem include: replacing the prefilter, cleaning the seachest, checking seawater supply by disconnecting the hose from the watermaker into a bucket, checking connections in case it was sucking air on the supply side.  None of these indicated a problem.

Since the pump motors are constant speed motors, the pressure must be regulated by a "dump valve" on the back of the control panel, which is what the pressure dial must be adjusting.  So the brine line is attached to the dump valve, and it regulates how much pressure is relieved to the waste line which controls the pressure in the membranes.  Has anyone had to replace this valve?  I would expect all the other Desallators operate in a similar fashion.

The other possibility that has occurred to me is there must be a logic board involved to monitor the pressure and will perform an auto shut down (opens the dump valve) if the pressure goes too high or too low.  It seems logical that the logic board could fail in a way that it sends erratic signals to the dump valve.  Anyone have trouble with that?

I don't have an idea on how to test either of these, or if my understanding of how the system operates is even true.  Any information would be appreciated.  

Does anybody have any other possibilities I could look into?  



Wanderer, SM#477



There is a mechanical, adjustable back pressure valve on the brine output.  That is controlled by the knob on the panel.  

While the logic board does monitor the pressure, but it has no control over it.  All it does is shut down power to the pump if the pressure is too high.

The most likely causes are on the pump side with water supply, check valves, or motor speed.  A bad back pressure valve COULD cause pressure problems, but I doubt very much they would vary with time.  Those valves are very simple.

Bill Kinney
SM160, Harmonie
On the hard at LMC,
Fort Lauderdale, FL


Thanks Bill.  It's a very curious problem.