When using an emerged versus a submersed pump on a water feature there is a field of technology everyone should be familiar with: the use of check valves in plumbing.
Check valves or one-way valves are devices that hold water static in plumbing when the pump(s) are off. Their lack of appropriate use is the cause of much time loss and frustration in re-priming lines, replacing plumbing and flooded equipment.
By judicious use of check valves a would-be waterscaper can keep their sanity by keeping their plumbing lines primed; i.e. full of water. Without them all manner of havoc is manifest: loss of prime, motors, hair (from pulling out).
So what are these magical devices, how do they work and when and where do you use them?
Check valves are specialized fittings that are attached in line with your plumbing; when water stops being pumped, they are actuated, preventing the water from back-flowing. The most often used and available types of check valves are: Swing, spring and ball-types. See illustration.
All check/one-way valves basically function the same. The force of moving water unseats the check allowing flow through the valve. When flow stops, the weight of water (or in the case of the spring check, with the additional "push" of the spring) closes the valve.
Where & When:
So what's the big deal you ask? Hah! How many of us have sucked on hoses, put hoses against them, swore at, begged, cried and prayed that their pump, drain lines would work; all for naught because you couldn't get enough liquid in them?
Well, check valves can change all that! As elucidated in the last article, on Plumbing, there are two possible spatial arrangements for a pump that's not underwater. These are above and below the water level of the lowest basin. In either case your desire is to have the ability to clean the pump trap or filter, do repairs or removal of the pump/filter system without the loss of prime or flooding.
If your water effect's pump/filter system is below water level, you will want to have a shut-off valve before the pump to stop water flow while you're working on the system. Similarly, you will want to provide a shut off or check valve immediately after the pump/filter to prevent the line from back-siphoning and thereby draining the upper basin and/or discharge line. See illustration.
A shut-off valve may be used on the discharge line rather than a check valve, but these are more expensive and flow-restricting. In the second scenario where the pump/filter system is above water level, two check valves may be used instead of one check, one shut-off. These will be cut in as per the previous illustration.
Now my views re what types of check valves are best per a given application: Most commonly available are brass and PVC bodied check valves. By and large the PVC are more appropriate for systems with less than 3" plumbing. This is especially true with lower flow rate, low head pumps with PVC lines.
For systems with fish and/or plants in them, swing-type checks are the hands down favorites. Various flotsam and jetsam will usually not clog a swing check as readily, they're cheaper and have the longer service life.
For non-biological, "poisoned" systems with no life in them, a more preferred type may be low tension spring checks. These are especially useful in a low head situation where there is little difference between the highest and lowest water levels. With the added force of the spring, a positive seat is assured. What may be otherwise lost is some pressure and volume compared to a swing-type check valve.
As per the Plumbing section, the best available, most-appropriate technology in valves are plastic (usually PVC), schedule 40 or rarely 80, ball valves. True union valves that will allow easy removal of parts without cutting are more money, but strongly suggested.
With a submersed pump, you may still want to install a check valve on the discharge line to prevent back-siphoning if the discharge point is below water level.
Many people believe their pumps are self-priming or "it will start after a while" running dry. This is a dangerous notion. Do not run your pump dry. It hurts the life of the motor, ruins the pump and can be extremely dangerous. If you discover your pump has been running without water in it, turn it off to allow it to cool down. Discover the cause(s) of the problem. Cure them, re-prime the lines & then turn the pump back on.
So there you have it; everything you always wanted to know and more regarding check valves. Next time you're bemoaning having to prime your lines for lack of them, think of this article and smile.