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WINTERIZE A BUILDING
This article describes Water Tank Pressure Relief Valves - how to find buy, or where to install safety valves on water tanks and on well water systems. We explain where a pressure relief valve is usually installed on well water pressure tanks, how the valve works, and why pressure relief valves are an important safety feature on well tanks.
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WATER TANK PRESSURE RELIEF VALVES: installation, inspection, troubleshooting, repair, code requirements for
Safety warning: all pressurized tanks should have an appropriate pressure relief valve installed to avoid possible injury should the tank burst. In this photo the brass fitting to the right of the black tank drain is a pressure relief valve installed in the correct location.
Notice that there is no shutoff valve of any kind installed between the relief valve and the pressure tank?
If the pressure relief valve is leaking this is an UNSAFE condition (corrosion or mineral deposits could block the valve and prevent its operation in an emergency). The valve should be replaced.
Early in our building inspection career plumbers in our area were skeptical when we identified the absence of a pressure relief valve as a safety concern (and a building or plumbing code violation). Then in about 1984 a local plumber was killed by a bursting water pressure tank.
The water pump was a submersible unit located (remotely) inside of the water well. The plumber was leaning over the water tank repairing a fitting, unaware that the water pump had turned on, and unaware that the pressure control switch had been damaged or modified so that it failed to turn off the water pump at a safe pressure.
The water tank burst, metal fragments severely injured the plumber who, working alone, bled to death. After that terrible event all of our local plumbers took this relief valve requirement seriously.
How to Buy & Install the Right Water Tank Pressure Relief Valve for Water Storage Tanks or Water Pressure Tanks
Question: What size or capacity of pressure relief valve should be installed on a water pressure tank?
My water tank pressure switch is set to 20 - 40 psi. How do I gage the size of a relief valve I need to put for my tank? - D.R.
Reply: typical water tank pressure relief valves are 1/2" or 3/4" in diameter orifice and are set to open at 75 psi.
The water tank pressure relief valve shown in our photo (above) is marked indicating that it will open at water pressure equal to or greater than 75 psi. That pressure is pretty standard and you'll see the 75 psi figure on valves used on water tanks and on some tankless water heaters too.
According to the Water Systems Council, [quoting, paraphrasing, and expanding - Ed.]
High pressure in a building water supply system can result if the water pressure switch that controls the well pump is damaged, or if the pump stays on too long.
Most submersible well water pumps can develop pressure exceeding the rated working limits of the water pressure tank or building water distribution system piping, fittings, and fixtures. Excess water pressure can damage water system component as well as the pressure tank if not corrected immediately, and can also cause leaks within the piping system or at faucets and fixtures. Therefore on all building water supply systems using a private well and pump, a pressure-relief valve is recommended.
Water pressure relief valves are spring-controlled and open only in response to system pressure. That is, water temperature is not a controlling factor for these valves. For water service, these important safety devices are generally adjusted to relieve pressure higher than 75 psi. 
Watch out: water pressure tank fatality: we [Ed] are aware of a fatality that occurred in the Hudson Valley of New York State when a plumber, working alone on a water pressure tank and pump system was killed as the water pressure tank burst. There was no pressure relief valve on the water system; a submersible well pump was installed; the well pump pressure control switch stuck in the "on" position.
We suspect that because the submersible pump was located inside the well and thus there was little or no operating noise when the pump was on, the plumber had no idea that the pump was running. When the pressure inside the water tank exceeded the tank's design specifications the tank burst open. The plumber, leaning over the water tank to perform some task, was cut and killed by the "shrapnel" formed when the water tank burst. - ED.
Examples of water tank and similar pressure relief valves include
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