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releif valves

bo ramsour
bo ramsour Member Posts: 6
hi; a customer brought this to my attention and i could not answer it ...

  a boiler normally has a relief  valve  only.. whereas a water heater or a storage tank must have a temperature and  pressure valve. , now the state inspector in Colorado's is requiring that all boiler relief  valves must be pipped in a vertical postion, and i can understand this , but why  can a t/p valve bell allowed to be  piped  in horizontally .

 my main question is that  why do you need a t/p valve on a tank or water heater but not on a  boiler...

 thank you

 bo ramsour - denver


  • icesailor
    icesailor Member Posts: 7,265
    edited May 2014
    Relief Valves:

    Potable water storage tanks require a Temperature/Pressure Relief valve with the temperature probe in the top third of the tank to measure the temperature of the water in the tank. Under pressure, the water temperature can be well over the boiling point but kept from boiling from the pressure. If the potable water in the  tank is over the boiling point, and the pressure drops to a point where the water can boil, the entire tank can vaporize into steam. The temperature sensing tube on the T&PR valve is usually 210 degrees, and the water in the top of the tank is the hottest water. So, before the water gets hot enough to boil at 212 degrees, the valve opens letting cold/cooling water in. Theoretically, the cold water entering, will require more BTU input energy that the heat source can provide. That's part of the reason that water heater manufacturers started supplying the proper relief valve with water heaters. The AGA rating of the valve must be higher than the input of the heater. The valve relief pressure is supposed to be a percentage higher than the working pressure. So it drips water and not steam to allow for thermal expansion over the working pressure.

    As far as the Pressure relief valves on boilers, I think that ASME codes require that relief valves be installed in a vertical position. Because you will have steam on the entire seat rather than only partially when in a horizontal position. You are protecting boilers for their rated working pressure, not their temperature. You can run a hot water boiler at 240 degrees with 12#+ pressure and not have the relief valve blowing and leaking all the time. But it doesn't matter so much what the boiler temperature is, just the pressure. It shows it that way in all the installation manuals I have ever seen. Many boiler manufacturers of packaged boilers all come with the relief valve mounted in a vertical position.

    All plumbing codes that I am aware of require that the temperature sensing part of the valve ne DIRECTLY inserted into the top 1/3 of the water storage tank.


  • ChrisJ
    ChrisJ Member Posts: 14,946

    I don't think this answers the vertical vs horizontal question but I feel it's an excellent video on the general subject.

    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • icesailor
    icesailor Member Posts: 7,265
    edited May 2014
    Relief Valves:

    MythBusters did a show on exploding water heater tanks. They used a 30 gallon electric. They had to go through a lot to get it to blow, but when it did, it blew a building apart.

    I had a friend who owned a house with a side arm heater like the one shown in a string on The Wall at this time. One Saturday morning, he went to work for the morning, came home and took a shower. He had a oil stove side arm heater that had no shut-off. He went off to a wedding in the Afternoon. He had forgotten to shut off the heater. He came home at 2:00 AM and found his house gone. Or, at least the vertical walls were horizontal and no roof. They never found the tank. Somewhere in the woods, the tank was hiding. Unless the tank achieved an altitude and trajectory high enough and far enough to get to the ocean. About 1/4 mile away. Good thing he wasn't there for the Event.

  • Jack
    Jack Member Posts: 1,046
    The first week...

    of my apprenticeship (Local 109) they showed the film of the water heater blowing up, reaching sub-orbital status and blowing the house down. It is an image that does stick with you.