watermaker refit problem

[rapanui 2]

hello I am rebuilding a water maker with new HP pump and new membrane housing, the problem: the HP pump give me 0.5 gpm the membrane is 2,5 inches X 40 inches long,what fresh water production can I get?? thank you for reply

[Sailing August]

Just a guess. Maybe 30-35 gallons an hours.

[a64pilot]

I don’t think you have enough run that big of membrane, but I suspect your close. I think you need as a min .8 GPM
However if you fed it 1.6 GPM, it would make 20 gallons per hour. That is the max that membrane is capable and 1.6 GPM is excessive, but apparently an excess isn’t harmful.
However you may get close to 20 GPH, but I don’t know if not having enough flow will shorten the membranes life.

Look at Cruise RO watermakers to get an idea
http://www.cruiserowaterandpower.com/WaterMakers.html
His 20 GPH is a single membrane with a 1.6 GPM pump, the 30 adds a second membrane, the 40 adds I believe a 2 GPM pump.

So of course it’s a function of both how much membrane surface area you have and Water flow, I believe as a rough example you need three times the raw water flow than what your product water flow will be. Too low a flow rate will I think reduce membrane life, too much isn’t harmful but wastes power.

[rapanui 2]

Thank you for your reply ,so I will need to upgrade the HP pump next power is 1gpm wil it be enought?

[Captain Bill]

Your flow rate is probably too low for a 40 inch membrane. Membranes are usually rated for about 15% product water but can usually be pushed to 20% safely. Your 30 gallon per hour pump will get you about 6 gph of product water. You will get more membrane life by using a 21 inch membrane. You might get up to 9 gph with a 40 inch membrane but your membrane won't last. If you have a power cat, why are you limiting your self by using such a small pump? Power usage is not usually a major consideration on a power vessel. A 2.5 x 40 inch membrane is rated for 720 GPD or 30 gallons per hour. A 1.5 hp motor power a 2.5 gpm pump and you won't be stressing your membrane. You should also note that typically plunger type HP pumps don't cost any more for a low output than a higher output of the same basic design. The only real difference between them is the camshaft used to move the plungers. The power required to drive them goes up, but not the price.