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Old 18-10-2022, 14:58   #1
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Watermaker. Low pump RPM = high PPM?

TLDR;
1) Is it possible that 20% lower HP pump RPM can cause higher PPM? From 200 to 1000?


2) Do some new membranes have lower quality?


3) Can you shot a membrane by running it at high pressure (1000psi)?



I have a Desallator watermaker. After only 1 year of usage its brushes were gone. Manufacturer asked $200 for brushes only. I said **** off to them and bought an 220V engine for $200. The main difference is that it has 20% lower RPM.

Also at some point for 1-2hours membranes were at abnormal pressure. Probably >1000psi.
I checked PPM and it was 1000ppm. I decided that due to my mistake I shot the membranes and need to buy new ones.

I bought new ones from Amazon for half the price, no brand name but "made in USA".
After assembling I checked PPM and it became 2500!
The membranes on Amazon have few reviews and apparently people were happy with them.


In one the threads here I read that a lower RPM can cause higher PPM. I made a different pulley so the pump RPM is now 20% more then originally was designed. PPM lowered to 1200 but still too high.
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Old 18-10-2022, 17:35   #2
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Re: Watermaker. Low pump RPM = high PPM?

I believe lower RPM in your motor can can increase the PPM of your system. I had a 12 V motor that was found Is both the quantity and quality of wooder decreased. At all came back when I changed the mode
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Old 18-10-2022, 18:13   #3
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Re: Watermaker. Low pump RPM = high PPM?

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Originally Posted by motion30 View Post
I believe lower RPM in your motor can can increase the PPM of your system. I had a 12 V motor that was found Is both the quantity and quality of wooder decreased. At all came back when I changed the mode


As far as I know the total dissolved solids (tds) has nothing to do directly with pump rpm or motor size. ( just flowrate across the membrane) Really it’s about the pressure/ flowrate across the membranes. I would believe from prior experience that 1000 psi is excessive, especially if it is less salty than full seawater ( thinking brackish like in the ICW). It sounds to me like you have a membrane issue. The membranes should have an associated flow rate for good water production. Too much pressure/flow rate will exceed the membrane capability and produce higher tds water or even rupture the membrane. We produce about 30 gph at 800 psi. Good luck. Safe travels
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Old 19-10-2022, 15:58   #4
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Re: Watermaker. Low pump RPM = high PPM?

Motor speed rpms are set to produce the proper pressures and flows from the pump it is attached to. Like with most pumps, if you slow down the motor the pumps pressures and flows will drop off. This will certainly affect PPM readings that will show higher than expected PPMs. Running membranes at 1,000psi is the absolute top pressure they can handle without doing damage to them. Most watermaker systems are designed around using the 800psi range as a good psi point. This gives your watermaker room for psi fluctuations that normally occur with changing temperatures and salinity of the water you are operating in. The colder the water the higher the pressures. Most systems are not self-adjusting for this pressure differential thus the 800psi range is somewhat of an industry standard. The fact that your new membranes are giving even worse PPM readings may be more to the installation or quality of the membrane. Also make sure the membrane is for saltwater use. You can easily buy a brackish water membrane from the same people. Look for the SW2540 or SW2521 on your membrane SW= For Salt Water.
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Old 19-10-2022, 19:09   #5
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Re: Watermaker. Low pump RPM = high PPM?

When you start your watermaker, the pressure should be very low and gradually increase the pressure (over 30 sec to 1 minute) until you make the recommended flow rate. Too much pressure will tear the membrane. Each membrane brand should have a data sheet that gives the flow rate. Going to 1000 psi will make more water, but hurt the membrane. Also water temp needs to figure in. Membranes are rated at 77°F. Warmer water more product, colder water less.
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