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Old 18-03-2020, 15:59   #1
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what would cause very low pressure on a watermaker?

gday fleet
once again mining the vast knowledge bank. thanks in advance. i have an aquamaxx 12 vdc watermaker. the pressure suddenly dropped to around 300 psi and even with the pressure regulating tbar completely closed i cannot get the required 850 psi. the pump appears to be running normally and there are no leaks that i can find. i thought it might be plugged prefilters so i changed them and still no joy. my question is: what are the possible causes of pressure failure? if you need anymore info just ask.
thanks a million once again
shala and her frustrated skipper
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Old 18-03-2020, 16:25   #2
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Re: what would cause very low pressure on a watermaker?

Haven’t had a similar problem on my watermaker, but have decades of experience with moderate pressure (3000 psi) hydraulic systems.

Several possibilities occur to me:

1) you state that there are no obvious leaks. Maybe your membrane failed, and the “leak” is to the freshwater output. If this were the case, you would have greatly increased output, maybe even blowing the usually low pressure hoses and fittings, and of course the output would be salty.
2) even with new pre-filters, your feed water sub-system might be clogged. Do you have a low pressure feed pump? Is thruhull open, and not clogged? Easy check - remove low pressure feed line from high pressure pump, turn on system, lo pressure feed pump, and see if you have copious salt water flow out the end of the low pressure line. Don’t run it too long, the high pressure pump won’t like being run dry.
3) If neither of these checks shows any problem, the high pressure relief valve may be stuck partially open. I’m guessing this is your ”pressure regulating tbar”. You might dismantle this, check for debris, failed o-rings, broken springs, lack of movement in internal mechanism. Make sure you mark lines before dismantling so you put the assembly back together correctly

4) Go see Rich at Cruiser’s Supply La Paz.
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Old 18-03-2020, 16:29   #3
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Re: what would cause very low pressure on a watermaker?

Hmm.....

Pump kaput ?

Pressure valve kaput ?

Pressure relief valve kaput ?
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Old 18-03-2020, 16:52   #4
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Re: what would cause very low pressure on a watermaker?

On a pressure pump like this, the first place I go is the internal check valves. A spot of dirt, corrosion, or failed seat and they don't work right.
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Old 19-03-2020, 09:33   #5
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Re: what would cause very low pressure on a watermaker?

Had a similar problem, mine was a ruptured membrane caused by generator automatic shut down. Pressure needs be slowly decreased or sudden decrease will cause ruptured membrane. Mine is an RO Watermaker, Rich Bourn is knowledgable.
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Old 19-03-2020, 10:08   #6
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Re: what would cause very low pressure on a watermaker?

Potential problems already pointed out but some different terminology may help in explanation. .

First potential listed above is your Membrane could have possibly failed ( or developed a tear or hole ) or a cartridge seal ( o-ring ) to the housing could have failed or rolled upon assembly.

The Salt Water is going to take the path of least resistance - as all fluids do - and bypass any possibility of generating a resistance to flow. Pressure is the result of resistance to flow.

Any and all relief valves built into the system could have failed, again bypassing the intended path of flow. Broken belief valve springs are possible. Debris caught between the valve seat and poppet also causes an open circuit. Relief valve seat / poppet erosion and / or corrosion would produce the same result.

Your High Pressure pump could be worn to the extent that " Internal Leakage " is taking place thus preventing any appreciable amount of flow to develope. When this happens the pump body it's self should get fairly warm if not hot because a significant amount of the power to spin that pump is being lost within the pump body.

Wasted Energy = Heat ! You can locate defective relief valves by localized heat at that point in the circuit. Use of a infrared thermometer would be the safest and easiest way to check for this.

Word of note, if the fault is at the Membrane you probably won't see any heat because the flow never had any resistance thus their is no significant pressure drop across the defect and no resulting generation of heat.

Pre Filters and Charge Pump Circuit all needs to be checked. I would have a Compound Gauge installed between the Discharge of the Charge Pump and the High Pressure Pump Inlet ( actually just before the inlet ) so you can see at a glance the condition of that circuit.

Please excuse me if this is too rudimentary, my experience is over three decades within the Filtration Industry. My capacity within that industry includes R&D, Systems Designer and Fluid Systems Decontamination Engineer.

Good Luck
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Old 19-03-2020, 17:43   #7
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Re: what would cause very low pressure on a watermaker?

Had the same exact issue on our watermaker (CruiseRO). Turned out the hi press relief valve had a broken internal spring, and was bypassing all the pressure. Removed the Hi-
Press Relief Valve and put a plug in its place....and instantly back to normal ops. I just need to watch the pump press, to ensure it never climbs above the 800-850psi range. Has worked perfectly for the last year.
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Old 19-03-2020, 22:45   #8
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Re: what would cause very low pressure on a watermaker?

thanks all
i will rip the system down tomorrow and advise what the findings are. will do some checks mentioned before dismantling. thanks again
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Old 19-03-2020, 22:59   #9
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Re: what would cause very low pressure on a watermaker?

Quote:
Originally Posted by sailcrazy View Post
Had the same exact issue on our watermaker (CruiseRO). Turned out the hi press relief valve had a broken internal spring, and was bypassing all the pressure. Removed the Hi-
Press Relief Valve and put a plug in its place....and instantly back to normal ops. I just need to watch the pump press, to ensure it never climbs above the 800-850psi range. Has worked perfectly for the last year.
47 years in industrial water treatment.

RO has of course 3 streams. 1 in, 2 out. You're right to watch that pressure because by plugging that relief valve, you just turned your RO into a filter. 1 in and 1 out.

To the OP, I looked through the other post and didn't see this.
Do the cheapest thing first. Check to see that that you're not in rinse mode.
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Old 20-03-2020, 03:22   #10
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Re: what would cause very low pressure on a watermaker?

We have an elderly low-output watermaker, and have had problems with it not getting up to pressure when we are moving "fast". (6 knots - for us, that's fast! Be kind!) Seems to get air in the intake. We havent cured the problem, we just dont use it when moving faster than about 4 knots.
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Old 20-03-2020, 18:19   #11
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Re: what would cause very low pressure on a watermaker?

Quote:
Originally Posted by Noisykate View Post
We have an elderly low-output watermaker, and have had problems with it not getting up to pressure when we are moving "fast". (6 knots - for us, that's fast! Be kind!) Seems to get air in the intake. We havent cured the problem, we just dont use it when moving faster than about 4 knots.
Sounds like your RO raw water intake thru hull is too shallow. It maybe so bad that when you are moving fast and heeled over, it isn’t even below the waterline.

Do you have a thru hull that is deeper below the static water line that you can use as a raw water intake? If not, you may want to move your RO raw water intake thruhull deeper during your next haulout.
High pressure pumps don’t like air bubbles in their feed lines.
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Old 21-03-2020, 03:51   #12
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Re: what would cause very low pressure on a watermaker?

Dear AndyEss
Thanks for you suggestion and interest -
Unfortunately, the intake is well below the waterline already, and moving it is not an option. On the upside, now we've worked out what the problem was, we have resolved it by simply being selective about when we run the watermaker. For us, that was the simplest and most sensible option. I just mentioned it for the OP - easy, zero-cost thing to check before diving down the rabbit hole of expensive repairs and refurbishment.
Cate.
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