Three duties, three kinds of physics
Water infrastructure valves manage large, slow, silty flows where surge and airlock are the enemies. Gas valves manage a compressible, invisible, flammable fluid where the enemy is leakage measured in bubbles. Fire valves must do nothing for years and then work instantly at full pressure. The engineering inside each family follows its enemy.
Waterworks mechanics
A sluice valve lifts a wedge clear of the bore, so an open valve costs almost no head, which matters when pumping water over kilometres. Non-rising stems keep the mechanism inside the body for buried chambers; gunmetal seat rings resist the dezincification and corrosion that raw water inflicts on iron. The subtle actors are the air valves: as a rising main fills, air collects at high points, throttling flow like a partial closure; a large-orifice air valve expels it during filling, while a small-orifice valve vents the air that continuously comes out of solution under pressure. On draining or column separation, the same valves admit air to stop vacuum collapsing the pipe. Reflux valves at pump stations close on flow reversal to keep the column from spinning pumps backwards; their closing speed against the decelerating column decides whether shutdown is quiet or hammering.
Why surge dominates water design
A kilometre of DN600 main carries tonnes of moving water; stop it abruptly and pressure spikes travel the line at the speed of sound in water. Slow multi-turn closure of sluice valves, controlled check valve dynamics and correctly placed air valves are the three defences, which is why waterworks specifications obsess over operating gear ratios and air valve sizing more than shutoff class.
Gas tightness mechanics
Gas escapes through paths water never finds, so gas valves rely on resilient sealing throughout: polymer-seated ball valves whose seats conform under preload, solenoid valves whose sealing disc presses a moulded elastomer onto a machined orifice. External sealing, stem O-rings, body joints, is engineered and tested to the same seriousness as the seat, because external leakage is the hazard. Solenoid shut-off valves in burner trains are held open electrically against a spring; any power loss, flame failure or interlock trip de-energizes the coil and the spring closes the valve in milliseconds, the fail-safe logic on which burner safety rests. Pressure in a gas train falls across regulators in stages, so each valve is specified for its band; a valve tight at millibars may not be the right component at bar-level inlet pressure.
Fire hardware mechanics
A landing valve is a globe-pattern valve feeding an instantaneous coupling: globe geometry because it throttles well, letting a firefighter control flow from a pressurized riser without slam. The isolation valves on hydrant mains are locked or electronically supervised open, because their only failure that matters is being found closed during a fire. Riser check valves hold the system charged. Everything in the family is built for the long-idle-then-instant-duty profile: gunmetal and stainless resist seizing corrosion during the idle years, and periodic testing regimes exist because a fire valve's first real operation must not be its first operation ever.
What this means for buyers
Specify water valves for head loss, surge behaviour and buried-service corrosion; gas valves for certified tightness, seal compatibility and fail-safe action; fire valves for approvals and guaranteed readiness. State medium, pressure, standard and approval per line in the enquiry, and each family arrives built for its own enemy.