Solenoid valves: the electrical hands
A solenoid valve opens or closes a line on an electrical signal — 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 — 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: dirty water and diaphragm pilots disagree; 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 genuinely 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; 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: any line a pump can dead-head — and every positive-displacement pump (which includes all metering pumps) will generate whatever pressure its casing allows against a closed valve, until the weakest component volunteers. 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 actual pump, 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 — all of them are cheaper than what they protect.