Watermaker - Pressure vs PPM

[nfbr]

I have a new seawater pro watermaker - >40gal/hr

at 800PSI I get about 240PPM
at 900PSI I get about 195PPM

Flow does increase 10%, so maybe it's just dilution.

Does anyone have information on feed pressure vs PPM?

Thanks

[Tellie]

First running at 900psi is pushing the membranes capability. It will work for a while but the way membranes are designed they are looking for a certain amount of flow and pressures. Over flowing or pressurizing a membrane will cause damage. 800 psi should be the limit. Just because the numbers tend to look better when the pressure is raised does not mean it should be done. You can increase the pressure over 1000psi and get even better numbers but the membrane will be toast sooner than later. You also have to consider the temperature of the water you are drawing from. As temps change so does the density and this effects watermakers and their internal pressures as well. We see a lot of damages of over pressurization and flow, especially on these, industry coined, DIY watermakers where the end user can keep cranking up the pressures because the numbers look better. Keep it at no more than 800psi. 240 PPMs is a great number.

[BillKny]

The answer is complicated, and takes a bit of thought.

The RIGHT pressure depends on the salinity of the feed water, and the water's temperature. You NEED to know the maximum pressure rating for your membranes, and the maximum flow rate. NEITHER should be exceeded.

From the Dow Filmtec Membrane Manual:

In principle, the operation parameters of seawater plants are adjusted the same way as in brackish water applications. However, the maximum allowed feed pressure as specified on the product information sheet, and the permeate TDS are often the limiting factors.
Decreasing feedwater temperature can be compensated by increasing the feed pressure up to the maximum. Once the maximum pressure is reached, a further decreasing temperature causes the permeate flow to decrease.
Increasing temperature is compensated by lowering the feed pressure. This is only possible, however, as far as the tolerated permeate TDS is not exceeded. Alternatively, increasing temperature can be compensated by taking a number of pressure vessels out of service. By reducing the active membrane area, the feed pressure and the permeate TDS are kept about constant. A system analysis has to be run to make sure that maximum element permeate flows are not exceeded. When some vessels are taken out of service, they have to be properly isolated and preserved.
An increase in the feedwater salinity can be compensated by increasing the feed pressure up to the maximum. If further pressure increase is not possible, than a lowered permeate flow and system recovery has to be accepted. A lower feedwater salinity allows to decrease the feed pressure and/or to increase the system recovery and/or to increase the permeate flow.

[phorvati]

New watermaker....i had numbers on 100-200 in cold new england waters the first year. As migrated my way south to FL, bahamas...and a year or two later, I was happy with 450.