Archive for October 10th, 2010

Pressurized Containers – Overpressure Devices

Sunday, October 10th, 2010

     Have you ever come home to a basement full of water?  The sinking feeling in your stomach at the moment of discovery is soon followed by a cascade of other emotions:  fear, anger, and you probably had a few choice sailor’s words to round off the experience.

     What’s just happened?  Well, it may very well have been a water heater explosion, and the water on the floor may be just the beginning of the damage.  Perhaps you even have a hole blown into the side of your house!  Watch this video for excellent graphic footage of just such an explosion:


     You probably didn’t realize that the water heater in your home has the potential to become a pressure vessel, and with that present all of the potential dangers that a pressure vessel presents.  Remember our discussion on the Boyle-Charles Law a few weeks ago?  We learned that in the fixed volume environment of a pressurized container if the temperature keeps climbing, the pressure keeps building, and the outcome of this coupling is precisely what we’re observing in the video.  The water in the water heater has turned to steam, causing pressure to build in the vessel until rupture occurs.  

     None of us wants to come home to a basement filled with water, much less a hole blown through our house by a rocketing water heater, so how can we prevent it from happening?  One answer is to have your water heater regularly serviced by a qualified plumber.  The plumber would make sure that the overpressure device on the water heater tank, namely the relief valve (a.k.a. T&P valve), is in proper working order.

     Now you may have noticed a circle drawn around the water heater’s relief valve in the video.  As their name implies, relief valves are used to relieve pressure buildup, generally of liquids.  If the pressure within the water heater reaches a certain limit, set by the heater’s manufacturer, the relief valve will automatically open to vent off the pressure.  A pipe on the outlet of the valve safely directs the water and steam that is let off to the floor where it can flow down to a drain.  That’s why floor drains are usually located in close proximity to water heaters. 

     Besides relief valves, overpressure devices come in many configurations, including:  safety valves, rupture discs, fusible plugs, and temperature limiting controls.  They may be used singly or jointly in order to perform the same basic function, that is, to keep the pressure within a vessel from building to the point where it may fail.  Next week we’ll explore the other overpressure devices mentioned and where they are generally employed.