I tested two different cheap Chinese diaphragm pumps during my first run of RO. I would have to rate both as failing to meet my expectations. That doesn't mean that I won't use them for RO, but I won't know for sure about that until I do more testing and I'm about ready to shut down for the year. The reason for my displeasure with these pumps was the same for each. The pumps are not sealed well enough for operating at the pressures they are advertised for.
The first was a 12VDC 130 psi diaphragm water pump from China that cost $15. It started to leak around the seams between the diaphragm covers and the body of the pump at about 95 psi. I reduced the pressure to 85 psi and the leaks became mere weeps. After the pump was running for about an hour, I noticed very faint smoke coming from the motor. I then noticed that sap was getting inside the motor. The path was from one of the small weeps from one of the poorly sealed diaphragm covers. So I have to consider that the smoke is possibly just from the burning of sugar inside the motor. Maybe reorienting the motor to avoid moisture will suffice if I can't satisfactorily solve the leak issue. I intend to try to glue the covers on with JB weld or seal them with food grade sealant and see if I can get the pump to operate at 120 psi, which is just below the rating of the 1/4" tubing and the filter and RO vessels. I could have tried to remount the motor just to see if the smoke would go away, but because I was actively concentrating sap, I decided to just replace the pump with a spare cheapo sap vacuum pump, solving the pump orientation issue in the process.
The second pump is a 12VDC 100 psi diaphragm water pump. It also started to leak around the seams but this time at about 80 psi. This pump reorientation satisfactorily prevented sap from getting into the motor. I ran the pump for over 9 hours at 70 psi and successfully concentrated the sap to the point where its volume was reduced to 55% of its initial volume. I intend to try the alternate sealing process on this pump. This length of time for the small amount of sap (30 gallons) that I started with just isn't satisfactory. If I can't get this pump to operate at 95psi with excessive leaking, I would consider to be only a backup pump.
If neither of these pumps rises to the occasion, there is a similar cheapo pump rated at 160 psi that would be the next step up before going to a more expensive SeaFlo or Aquatec pump. The goal is to get to 120 psi with a continuous duty pump that doesn't leak too much.
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