Owner, Green Bay Packers
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The problem with having a suction line on a pump mounted above it is that "lift" is now entered into the equation. A centrifugal pump has a theoretical lift capability of 25 feet. Any amount of suction lift reduces the capability of the pump. BTW, an elbo located in that suction line adds the equivalent of 5' of lift. Now head is a different matter. The centrifugal pump can "push" water much more effectively than it can pull or lift it. Most are, in fact, designed to have a certain amount of head pressure against discharge. A sometimes easy way to pick up prime on a centrifugal that may be air-bound is to throttle the discharge down to almost closed until pumping is acheived.
The chief advantage to centrifugal pumps is volume pumped. A secondary is simple, trouble free, rotational motion design. A centrifugal designed for pumping water with debri should have open, versus closed, impellers. The slight loss of pump capacity will be offset by the reduction in impeller fouling. Sewage ejection pumps are frequently of this design.
The problem with diaphram, and all "positive displacement" pumps, is capacity. For a given size, and given size prime mover, we cannot get the volume out of a positive displacement pump that we can out of a centrifugal pump. With this trade-off in mind, it may make good sense to mount the positive displacement pump at the lowest point in the bilge where it can perform a "stripping" function, while mounting the high capacity centrifugal higher where it will be able to do it's job without as much risk of fouling impellers. Coincidently, that is roughly how you pipe up an oil tanker for discharge. The other problem with positive displacement pumps, be they diaphram, piston, or gear, is maintenance. These pumps by the nature of their design, wear more. Most of them involve reciprocating components and that always means friction and wear.
Ironically, I just saw the first "Wilden" pump I've seen in a number of years-my auto mechanic has one for pumping oil. Wilden is the world class leader in air-powered diaphram pumps. NO merchant ship is without one, for a variety of uses. They come in all sizes-my mechanics was about 10" square. The size seen on merchant ships are usually the size of a portable generator. The pumps are made of aluminum and thus light and portable. It is child's play to rebuild one and parts are available world-wide. You do need air to power one though, which brings me to my point. In the post mentioned earlier, the surveyor mentioned having a bilge pump running off the engine. I did not have time to read his idea thoroughly, but it occured to me that it might be easier to mount a compressor on the engine-similar to a Peterbilt. With a supply of air you could run a Wilden pump anywhere-in your bilge, in the bilge of the boat along side, in your dingy, hell you could lower it over the side and use it for wash-down if you wanted to get crazy. Wilden does make attachments, via cam-lock fittings, to fit a suction line on at the strainer box, in essence making a water shop-vac out of it. Larger boats could use this to good effect. Compressor draw on the engine would be the limiting factor I suspect. Giulietta could also then use a pneumatic wrench instead of his electric winch buddy!
The best line check valves I know of are made by Maas-Midwest and available in the sizes required. They generally come threaded female, each end-but male x female are probably available. They are an all bronze spring check (no plastic) and are of superior construction and reliability. I recommend an oversized check valve for bilge application due to the possibility of small stuff/line debri. If discharge is 1", I'd go 1-1/4" on the line check. I advocate mounting the check valve as close to the pump as possible. If the hose above the check is already full of water it will aid in "loading" the impeller via head pressure when the pump kicks in. It will also be much more effective at stopping water running back through the pump into the bilge. Mounted at the through-hull the water in the line will slowly seep back through the pump and into the bilge-although it will not allow water from the through hull to pass. The valves are more effective against head pressure than thay are against vacuum on the other side.
That's probably enough to provoke comment-and I haven't even gotten to the employment of ejector pumps(probably not what you may think they are), but I think they'd be of limited use on a boat. Although the USCG still requires their carriage on merchant ships.