Pressure Valve

A Pressure Valve (also known as a breather valve or equalization valve) is a small but essential component installed in the housing of AGL fixtures, junction boxes, and other sealed enclosures. Its primary function is to equalize the pressure difference between the inside of the fixture and the external atmosphere that occurs due to temperature fluctuations. As a fixture heats up (from sunlight, lamp operation, or ambient temperature changes), the internal air expands, creating positive pressure. Conversely, during cooling or rapid temperature drops, the internal air contracts, creating a negative pressure (vacuum) that can pull moisture and contaminants into the enclosure through seals. The pressure valve allows controlled air exchange to equalize these pressure differentials while incorporating a hydrophobic membrane or barrier that prevents moisture ingress, maintaining the fixture's IP67/IP68 environmental rating.

Sealing a fixture tightly enough to earn an IP67/IP68 rating creates a problem the seal itself cannot solve: a sealed enclosure that heats and cools with the day-night cycle will still try to exchange air with the outside, and if there is no deliberate path for that exchange, the air will find one anyway — forcing its way past gaskets and threaded joints, gradually wearing down the very seals meant to keep moisture out. The pressure valve's purpose is to give that unavoidable air exchange a single, controlled route, fitted with a hydrophobic membrane that lets air pass while blocking liquid water, rather than letting it happen uncontrolled through the enclosure's weakest points. Left unaddressed, the result of pressure cycling without a valve is exactly what the fixture's environmental seal was meant to prevent: condensation inside the lens, fogging that degrades the beam pilots rely on, and corrosion at the electrical terminations.

Pressure valves are used in virtually every sealed AGL enclosure where temperature fluctuations can cause pressure differentials. Common applications include:

• Runway & Taxiway Light Fixtures: Inset and elevated fixtures where thermal cycling from lamp operation and sunlight causes significant pressure changes.
• Threshold & End Light Housings: Sealed optical chambers requiring pressure equalization to prevent lens fogging.
• Approach Lighting Fixtures: Barrettes and single-unit approach lights with sealed optical systems.
• PAPI & VASI Optical Units: Precision optical assemblies where condensation on lenses would compromise glide slope indications.
• Obstruction Light Housings: Sealed beacon and LED obstruction light enclosures.
• Junction Boxes: Underground and above-ground junction boxes where moisture ingress is a concern.
• Isolating Transformer Housings: Sealed transformer enclosures where thermal cycling can cause pressure differentials.
• CCR & Control Cabinets: Outdoor cabinets where temperature and humidity fluctuations require controlled breathing.

The valve fits into a threaded or gasketed port in the fixture's sealed optical housing, positioned so it communicates with the internal air volume without sitting directly in the optical path or against the lamp/LED assembly. Orientation follows the same principle as a moisture vent — fitted so condensation or splashed water on the housing exterior can't pool directly against the valve — and the surrounding gasket is checked for a full seal during installation so the valve doesn't become an alternate leak path into the housing. Being a passive, unwired component, no cabling is involved; installation is limited to correct torque on the threaded fitting and a post-installation pressure/seal check.

The pressure valve operates on a simple but effective principle: it allows air to move in and out of the enclosure to equalize pressure while preventing moisture and contaminants from entering. Key functional aspects include:

• Hydrophobic Membrane: The valve incorporates a microporous or hydrophobic membrane (typically made of PTFE or expanded PTFE) that allows air molecules to pass but blocks liquid water and water vapor, preventing moisture ingress while allowing pressure equalization.
• Pressure Equalization: When internal pressure exceeds external pressure (heating), the valve opens slightly to release air. Conversely, when internal pressure drops below external pressure (cooling), the valve allows air to enter, preventing vacuum formation that could compromise seals.
• Self-Cleaning Design: Many pressure valves are designed with a self-cleaning feature where the air movement through the valve helps remove dust and debris from the membrane surface, maintaining long-term performance.
• IP Rating Maintenance: The valve is designed to maintain the enclosure's IP67/IP68 rating by allowing pressure equalization without compromising the watertight seal.
• Condensation Prevention: By equalizing pressure and allowing humidity to escape, the valve helps prevent condensation from forming inside the fixture, which could cause lens fogging, corrosion, or electrical shorts.

The valve is typically installed in a threaded port on the fixture housing, often positioned at the highest point of the enclosure to facilitate air movement and moisture escape. It is a passive device that requires no external power or control.

Like the moisture vent it is often used alongside, the pressure valve has no control logic and no connection to the ALCMS — it is a passive mechanical boundary on a sealed enclosure, positioned to let internal air pressure equalise with the atmosphere as the enclosure heats and cools through its duty cycle, while blocking liquid water and dust from entering the other way. Because it is a purely passive part of the enclosure's mechanical architecture, its condition (valve seat clean and moving freely versus stuck or clogged) is verified only through physical or visual inspection during scheduled maintenance, the same as the moisture vent and the other sealing components on the same housing.

The valve body is machined from stainless steel, nickel-plated brass, or UV-stabilised engineering plastic, with a threaded connection sized to mount directly into the base can or fixture housing wall. At its core sits a hydrophobic expanded-PTFE membrane, a microporous barrier that allows air to pass freely for pressure equalisation while its pore structure repels liquid water even under submersion pressure. The membrane is bonded or clamped across the valve's internal air path, protected on the outer face by a perforated cap or louvred cover that shields it from direct water jets, debris, and physical damage while still permitting airflow. This construction lets the valve vent the small pressure differentials created by temperature-driven expansion and contraction of air inside the sealed base can, without which repeated thermal cycling could otherwise stress or breach the fixture's main gasket seals. Chemical-resistant seals at the threaded connection maintain the valve's own IP67/IP68 rating against the fuel, de-icing fluid, and moisture exposure typical of its below-grade or fixture-mounted location.