The heart of the system is the high-pressure plunger pump, similar to what is used in the car wash and other pressure-washing industries. The rpm and flow rate are important, with the ideal flow being 2.5 to 5 gallons per minute. An electromagnetic clutch is not necessary, or even desirable in my opinion. A robust commercial-quality pump is necessary. I don’t like plastic, but the body does not have to be stainless steel or titanium. The latter two may last a lifetime, but that is overkill for most of us. Bronze and even brass will last for 15 to 20 years, as long as it is flushed with fresh water after each use. We use a commercial brass Hypro pump, costing about $470, whereas a titanium or stainless-steel pump can cost $1,500, and bronze is about $800. This is worth considering if you are on a tight budget.
The next most expensive parts are the membranes and their tube housings. Again, quality is important; the industry standard 40-inch Filmtec 2540 membranes cost about $200 each. I bought our original Codeline fiberglass tubes for about $50 each at a used marine parts store. Purchased new, the price is about $250. There are other options, but stainless-steel end fittings are best and used tubes must be free of defects and leaks.
The remaining parts include a rugged 12V boost pump to make sure the high-pressure pump never runs out of salt water. Jabsco’s Water Puppy is suitable, which is what I use.
Two stainless-steel, oil-filled gauges are necessary. One is a high-pressure gauge for keeping track of the feed water pressure through the membranes (about 800 psi). The other gauge is a low-pressure gauge for monitoring filter feed water pressures (about 10 psi). Both gauges should be supplemented with flow meters to help determine when it is time to clean or replace membranes and filters.
We use two flow meters, one for the saltwater brine flow out of the membranes and one for the freshwater product flow. For longest life, it is important not to produce more than about 20 gph through each membrane. So, on a two-membrane system, the product flow should be kept to no more than 40 gph while operating.
Finally, there is the high-pressure regulating valve, several three-way “Y” valves, numerous stainless-steel and plastic fittings, and both high-pressure and low-pressure hoses to suit your installation. No rocket science is required; just time and common mechanical/electrical skills to do basic plumbing and maintenance work.