Can a Watermaker Be Used While Underway ?

[senormechanico]

I have a hard time believing that using a watermaker in fresh water would be a problem.
Wouldn't increased flow due to reduced pressure amount to virtually the same power consumption and therefore, the same amount of generated heat in the membrane?

Spectra's website even mentions using a swamp as a water source. Ewww..

[s/v Jedi]

The correct way for using your watermaker in brackish or fresh water is to adjust the pressure for the rated product output of your membranes. So, if your membranes are spec'd for 40 gph you should adjust pressure so that output is 40 gph. In the tropics (warm water) I reach that output at 200 psi or sometimes even less.

cheers,
Nick.

[beau]

I have put together a system using a 6.5 hp gasoline pressure washer(brass pump) (ceramic plungers) (Rated at max 3,500psi)
I have a 3 gallons a min feed pump with filters etc and 2 x 2.5 inch x 40 inch pressure membrane in series, a pressure regulator at the end of the system and a pressure gauge . (800psi?)
I believe it should put out approx 25 gallons and hour of fresh water and is intended to be used once a week.
Cost less than $2,000.00
Its not running yet but hopefully next week.
Am I on the right track??

Any comments woud be appreciated.

[Tellie]

Originally Posted by beau

I have put together a system using a 6.5 hp gasoline pressure washer(brass pump) (ceramic plungers) (Rated at max 3,500psi)
I have a 3 gallons a min feed pump with filters etc and 2 x 2.5 inch x 40 inch pressure membrane in series, a pressure regulator at the end of the system and a pressure gauge . (800psi?)
I believe it should put out approx 25 gallons and hour of fresh water and is intended to be used once a week.
Cost less than $2,000.00
Its not running yet but hopefully next week.
Am I on the right track??

Any comments woud be appreciated.

Please keep us updated as you go along.
My two cents and it's two cents too expensive.
The presures and flow seem to be in the right range. The issue with DIY units is they are very hands on systems. But the cost differences make it attractive to many. All water makers need to be fresh water flushed. Especially if you are using a brass pressure washer pump. Salt water in a dark and oxygen deprived environment causes all kinds of problems with close tolerance parts. So a fresh water flush is advised after each use. A system as powerful as yours will use a lot of the water you just made. If you have sufficent tankage this is no problem. Both membranes need to be fluhed as well. Depending on how you plumbed the membranes together will depend on the amount of fresh water needed. I'm not a big fan of running the pressures up to 800psi. I like it more in the lower 700 range. You will notice over time as you use your water maker the pressure differences on your gauge depending on where you are, salinity, temperature and water condition. Many times these differences cause pressure spikes. Membranes are rated for 800psi but these spikes can go well above this if you are set at 800psi. You will lose a bit of production with a lower pressure setting but I believe if all is working well you should easily see your 25gph goal and your membranes life will be better preserved. Good luck and let us know how it works.

[foggysail]

Originally Posted by beau

I have put together a system using a 6.5 hp gasoline pressure washer(brass pump) (ceramic plungers) (Rated at max 3,500psi)
I have a 3 gallons a min feed pump with filters etc and 2 x 2.5 inch x 40 inch pressure membrane in series, a pressure regulator at the end of the system and a pressure gauge . (800psi?)
I believe it should put out approx 25 gallons and hour of fresh water and is intended to be used once a week.
Cost less than $2,000.00
Its not running yet but hopefully next week.
Am I on the right track??

Any comments woud be appreciated.

INTERESTING! Because I to am building my own system, a couple of points caught my attention. For example you did not mention the pump's specification nor your propulsion for your pump and the pump's flow rate at a particular RPM.

The reason for me flagging these points is that a pump will output whatever it is designed for at a particular RPM; it will have a linear output as a function of RPM regardless of the pressure needed for that flow. Now of course if the pressure rating for the pump get exceeded, it could destroy itself and of course, it will require HP to get high pressure. If your pump is designed to operate with a gasoline 6.5 HP engine, it may not satisfy your plan if operated it with a different propulsion system based on the HP required at the RPM your propulsion system operates.

Just trying to help here, not trying to discourage you. If your not already aware, the HP needed is = (gal/min)*(psig)/(1714*eff) where efficiency is about 85%.

Foggy

[foggysail]

Tellie---

You raised an important point regarding operating pressure!!!! I took a run through ROSA and was awed at the low pressures the program gave in its readout. Did not go further because other things are far more pressing at this point for me. YEAH, I agree operating at 700 might be best for a system. But how about product flow rate? Is that not important also or is just simply setting the pressure to about 700 and then what you get is what you get??? Is there any real reason to use ROSA???

Thanks--

Foggy

[beau]

Tellie, Thank you for your critique, I appreciate it.

I assume, from what you say, that you can run the system at the lowest pressure that produces a reasonable fresh water flow?

I am aware this is a hands on unit which is why it is larger than most, and will only be operated once a week, when I can produce a weeks supply of fresh water, and the flushing etc in fresh water is only once a week.

Foggysail, the gasoline motor and pressure pump came as one unit.(pressure washer)
It has an output of 3 gallons a minute, which is needed for sufficient flow through my membranes. The maximum pressure is rated at 3,500psi but I only intend to use what i need to produce fresh water.
It has a brass pump and came with a spare set of seals plungers etc.

I do not know yet what the flow rate will be at 700psi but I am advised it should be at least 3 gallons a minute or more, but that can all be contolled by the motor throttle and the pressure regulator?
A don't think a flow of more than 3 gallons a minute should be a problem anyway, as long as I control the pressure?
Comments please?

[foggysail]

Sorry Beau-- I misread your post. I thought you were salvaging the pump from the gas engine/pump combo. Whomever engineered the engine/pump combo did a good job. At its rated capacity of 3 gal/min @ 3500psi, the needed HP is a little over 7; must pumps are never operated at their capacity pressure and you will be much lower than 3500.

The engine's HP is at a particular RPM and it probably is close to the RPM needed for 3 gal/min. But again, because your operating pressure is much lower I doubt you will have a problem. Good luck with your project!


Foggy

EDIT: Beau-- the flow rate is determined by RPM only. The pressure will be whatever it needs to be to make the fluid flow bounded only by exceeding the pump's rated pressure (failure) and the available HP driving the pump.