An altitude valve is a self-operating valve that keeps a water tank from overflowing without needing a level sensor, a controller, or any electricity. It senses, through pressure, when the tank has reached its full level and shuts by itself, then reopens on its own when the tank draws down. On tanks fed from a pressurised main, it is the simple hydraulic guardian that stops treated water spilling over the top. This guide explains how an altitude valve closes at a set level and reopens on drawdown, the difference between single- and double-acting types, its pilot mechanism, and how position feedback into SCADA confirms fill status.
Altitude Valve in one line: An altitude valve is a hydraulically operated control valve that automatically closes to stop a tank from overflowing when its water level reaches a set point, and reopens to let the tank refill when the level drops. It works on water pressure alone, using a pilot that senses the tank's level as a pressure and drives the main valve open or shut, so no external power or level instrument is required. Altitude valves are common on tanks fed from a pressurised distribution main where overflow must be prevented automatically.
The problem an altitude valve solves is straightforward: a tank fed from a pressurised main would keep filling past full and spill over the overflow unless something stops the flow at the right level. An altitude valve does exactly that, and it does it hydraulically, reading the tank's level as a pressure. The water level in a tank corresponds directly to a static pressure at the base, so as the tank fills, that pressure rises, and the altitude valve is set to close when the pressure indicates the tank has reached its target full level. Once shut, it holds the tank at that level and prevents any further inflow, so treated water is not wasted over the top.
Reopening is just as automatic. As customers draw water and the tank level falls, the sensed pressure falls with it, and when it drops below the valve's setting the altitude valve opens again to admit water and refill the tank. The whole cycle of closing at full and reopening on drawdown happens with no operator action, no electrical controller, and no separate level transmitter; the valve is driven entirely by the water pressure it senses. This self-contained operation is the altitude valve's great appeal, because it protects against overflow even where power or telemetry is unavailable or has failed.
Altitude valves come in single-acting and double-acting forms, distinguished by whether they let the tank drain back out through the valve. A single-acting altitude valve controls flow in one direction only: it lets water into the tank and shuts at the full level, but it does not permit water to flow back out through itself, so the tank drains through a separate connection or check arrangement. A double-acting altitude valve allows flow in both directions, admitting water to fill the tank and then, once the main pressure drops below the tank level, letting the tank feed back into the system, which lets the tank support the system's pressure during high demand.
What makes the valve act is its pilot. The main valve is a large, pressure-operated valve whose opening and closing are commanded by a small pilot control that senses the governing pressure and directs water above or below the main valve's diaphragm to stroke it. In an altitude valve, the pilot is set to the pressure corresponding to the desired tank level, and it plays the sensed tank pressure against that setting to decide whether to hold the main valve open or drive it closed. Because the pilot does the sensing and the main valve does the work, the whole assembly regulates the tank level using nothing but the water's own pressure, which is why these are classed as automatic hydraulic control valves.
An altitude valve needs no telemetry to do its job, but operators often want to know what it is doing, and a simple position or limit switch on the valve provides that. A limit switch reports whether the main valve is open or closed, and that single contact, brought into a SCADA system, tells the control room whether the tank is currently filling or has reached its full level and shut. This is a lightweight way to gain visibility of a tank's fill state without installing a full level instrument, which is useful on tanks where a continuous level transmitter is not justified or not present.
That position feedback is genuinely informative because the valve's state is a proxy for the tank's condition. If the altitude valve is reported closed, the tank has reached its set full level; if it is open, the tank is drawing down and refilling. Watching that status change over time shows the tank cycling and confirms the valve is actually operating rather than stuck. A valve that stays open when it should have closed, or one that never opens, becomes visible as an anomaly, which matters because a failed altitude valve can either let a tank overflow or leave it unable to refill, both of which are problems the operator wants to catch early.
Because these valves sit on tanks that are often remote and lightly instrumented, feeding their position into a cloud SCADA platform such as Merobix lets operators fold even these simple hydraulic devices into a central view alongside pumps, pressures, and levels. The same hosted monitoring used across water and other industries turns a bare open-or-closed contact into a trended, alarmable signal, so a valve that fails to close and threatens an overflow, or one stuck shut that starves the tank, raises an alert. In this way a purely mechanical, self-operating valve gains a layer of remote confirmation, giving operators assurance about fill status without the cost of a full level-instrument installation.
An altitude valve senses the tank's water level as a pressure and is set to close automatically when that pressure indicates the tank has reached its full level, stopping any further inflow. It holds the tank at that level until customers draw water and the level falls, then reopens to refill. Because it runs on water pressure alone with no electrical controller, it prevents overflow even where power or telemetry is unavailable.
A single-acting altitude valve controls flow in one direction only, letting water into the tank and closing at the full level, so the tank must drain through a separate path. A double-acting altitude valve allows flow both ways, filling the tank and then letting it feed back into the system when the main pressure drops below the tank level, so the tank can support system pressure during high demand.
Yes. A simple position or limit switch on the valve reports whether it is open or closed, and that contact brought into SCADA tells operators whether the tank is filling or has reached full and shut. This gives a lightweight confirmation of fill status without a full level transmitter, and it also reveals a valve that fails to close, which risks overflow, or one stuck shut, which starves the tank.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.