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What is a Relief Valve?

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  • Written By: Lori Kilchermann
  • Edited By: Lauren Fritsky
  • Last Modified Date: 02 November 2016
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    Conjecture Corporation
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A relief valve is a device used to bleed off excess pressure in a pneumatic or steam line. Typically constructed with adjustable tension springs, the relief valve has a panel or valve that will open as the pressure reaches a predetermined point. It is often referred to as a blow-off or pop-off valve due to the sound the escaping pressure makes as the relief valve is pushed open. The typical relief valve is adjustable as well as rebuildable to allow the valve to work in a vast assortment of manufacturing operations. While the common relief valve will reset itself after venting excess pressure, which allows the system to remain functional, some emergency pop-off valves must be manually reset after opening.

With machines such as air compressors, excess air pressure inside of the holding tank can lead to dangerous explosions and injury or death to nearby operators. While the typical air compressor is constructed with a pressure-sensitive safety switch intended to turn the compressor on and off as the air pressure builds, it can occasionally fail. When this happens, the compressor could potentially build up enough air pressure to cause the tank to rupture. By installing a relief valve in the compressor's tank, the valve can open and exhaust any excess pressure prior to the tank failing and potentially injuring persons and property.

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Many pneumatic presses and other types of manufacturing machinery depend on a relief valve to bleed off excess pressure as the machine performs its usual task. When a pneumatic press is operated, a loud swoosh of air can be heard escaping the press after ever pressing cycle. This is the sound of the blow-off valve releasing the high air pressure required for the press to accomplish its task. The press regains the high pressure needed to operate as the press is being loaded by the press operators. Once loaded and ready, the press cycles through another pressing sequence and once again exhausts the air pressure and returns to the starting position.

With steam-operated machinery, the process is much the same. A relief valve vents the built-up steam from the lines or the machinery after each duty cycle and avoids over-pressurizing the components within the machine. The sound of the steam being released is often very noisy and sharp as the valve opens and closes at a high rate of speed, allowing the steam pressure to escape instantly with no gradual seepage of pressure. The closing of the valve is just as abrupt and often makes a louder sound than an opening valve as the pressure escaping through the relief valve is capped off almost instantly.

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