Solenoid valves: the electrical hands
A solenoid valve opens or closes a line on an electrical signal. It is the actuator for level control, dosing interlocks, flush cycles and safety shutoffs. Four specification points catch people:
- Coil voltage: 24 V DC, 24 V AC and 220 V AC are all common stock; a right valve with the wrong coil is a dead valve (though coils are usually swappable, so check before replacing the whole unit).
- Normally closed vs normally open: NC (power to open) is the default and fails safe for supply duty; NO suits duties where loss of power must leave water flowing. Choose deliberately: "what happens during load-shedding" is the design question.
- Minimum operating differential: most general-purpose solenoids are pilot-operated diaphragm valves that need a pressure difference (typically ~0.3 bar) across themselves to open and seal fully. On gravity feeds and tank outlets that differential does not exist, and the valve hunts or refuses to close. The fix is a direct-acting (zero-differential) model, decided at ordering, not on site.
- Water quality: dirt blocks diaphragm pilots, so a strainer upstream is cheap insurance.
Non-return valves: the one-way streets
A non-return (check) valve permits flow one way and stops the reverse. The locations that are non-negotiable:
- At every chemical injection point: the injection (non-return) valve on the dosing line where it enters the process pipe, protecting the pump and line from process pressure, plus a non-return in the process line upstream of the quill so chemical cannot run back to the source. Neither stops syphoning. Chemical syphons forward, through the check, and that needs the back-pressure/anti-syphon valve at the pump (the dosing articles).
- On every pump discharge: stopping back-spin, drain-back and reverse flow through stopped pumps, especially with parallel pumps where one runs while another idles.
- Backflow protection where a treated or chemical-bearing system connects to a potable supply. Municipal connections feeding treatment plant deserve proper backflow prevention, not just a swing check.
- On borehole risers (as foot valves or inline checks) to hold prime and column.
Character notes: spring checks seat reliably in any orientation; swing checks are gravity-dependent and slam-prone on fast reversals; and a fouled check that almost seals is invisible until the symptom appears somewhere else: the softener that drains overnight, the doser whose back-pressure valve no longer holds and syphons, the pump that starts flooded. When flow behaves impossibly, suspect a check valve first.
Relief valves: the pressure fuse
A pressure relief valve opens at a set pressure and discharges to a safe path, protecting whatever cannot protect itself. The non-negotiable location is any line a pump can dead-head. Every positive-displacement pump (which includes all metering pumps) will generate whatever pressure its casing allows against a closed valve, until the weakest component fails. Centrifugal booster systems, geyser and vessel installations, and thermal-expansion-prone closed sections (sun-warmed pipework between two closed valves included) all carry the same logic.
Set-point discipline: relief set comfortably above working pressure and below the weakest component's rating, remembering the derated ratings of warm PP housings and the dead-head pressure of the pump installed, not its duty point. Pipe the discharge somewhere visible and safe: a relief that dribbles unseen into a drain hides the overpressure problem it exists to announce.
Rule of thumb: solenoids chosen for voltage, fail-state and minimum differential; a non-return wherever flow must be one-directional (injections, pump discharges, backflow points); a relief wherever a pump or the sun can raise pressure against a closed valve. None of the three costs much, and all of them are cheaper than what they protect.