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Thread: Sewer Lift Station

  1. #1
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    Sewer Lift Station

    I have a project that includes a new sewer lift station that pumps both residential and commercial waste water. The wet well has a vent that terminates about a foot above grade and there is a hatch for access to pumping/control equipment in the wet well. The lift station is in a residential area and the owner wants to disguise it. There will be a structure around the wet well that looks like a house; however the roof only extends about three feet inside the footprint of the structure (if you stand on the wet well hatch and look up you see sky). There are no louvers or fans.

    I believe this falls under NFPA 820 table 4.2.2 row 19 which is a pump station that is not physically separated from the wet well, which would require the entire area inside the structure to be Class I Div 1. The trouble I am having is that the structure does not really meet the definition of "enclosed space" or "not enclosed." I am concerned that there is no way for any gasses to dissipate within the structure.

    Any thoughts?

    Enclosed Space. The interior space of any tank or unit process that is closed to the atmosphere, excluding vents or pressure relief, or the area around any open tank of unit process surrounded by a builidng or other structure constructed with a roof and solid walls.

    Not Enclosed. Any tank or unit process open to the atmosphere or the area around any open tank or unit process housed in a building or other structure constructed with a roof and having at least 50% of the wall area open to the atmosphere. Fixed open louvered panels with effective openings greater than 50 percent of the wall area and evenly distributed over the wall area are considered open to the atmosphere.

  2. #2
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    This is an either/or situation and it sounds from your description that it is “enclosed” especially if you are concerned about disappating the gases.
    "Bob"
    Robert B. Alexander, P.E.
    Answers based on 2014 NEC unless otherwise noted.

  3. #3
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    Quote Originally Posted by rbalex View Post
    This is an either/or situation and it sounds from your description that it is “enclosed” especially if you are concerned about disappating the gases.
    I would think that the practical effect of the partial enclosure (essentially a open roof as I understand it) would depend on whether the gases produced are lighter than air (hydrogen, methane) or heavier than air (butane, propane, or more complex hydrocarbons).
    What are the likely gases from a sewage pump station?

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  4. #4
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    Quote Originally Posted by rbalex View Post
    This is an either/or situation and it sounds from your description that it is “enclosed” especially if you are concerned about disappating the gases.
    I'm not sure about that. The "structure" is really a fence with a projection at the top that shades an area only up to 3 feet on the interior. So, if you had a 15' x 15' footprint, the total area is 225 square feet. If the "roof" covers a strip 3 feet wide on the perimeter, the center that's open to the sky is 81 square feet (9' x 9').

  5. #5
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    Quote Originally Posted by GoldDigger View Post
    I would think that the practical effect of the partial enclosure (essentially a open roof as I understand it) would depend on whether the gases produced are lighter than air (hydrogen, methane) or heavier than air (butane, propane, or more complex hydrocarbons).
    What are the likely gases from a sewage pump station?

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    Sewer gases are generally methane (lighter than air) and hydrogen sulfide. Without substantial openings in the side of the structure, it is enclosed. An HVAC engineer may determine the level of dispersion, but it is unlikely that it isn't Division 1
    "Bob"
    Robert B. Alexander, P.E.
    Answers based on 2014 NEC unless otherwise noted.

  6. #6
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    Thank you.

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  7. #7
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    Quote Originally Posted by gadfly56 View Post
    I'm not sure about that. The "structure" is really a fence with a projection at the top that shades an area only up to 3 feet on the interior. So, if you had a 15' x 15' footprint, the total area is 225 square feet. If the "roof" covers a strip 3 feet wide on the perimeter, the center that's open to the sky is 81 square feet (9' x 9').
    The description was "It looks like a house" with no description of the ventilation available. It sounds like solid walls.
    "Bob"
    Robert B. Alexander, P.E.
    Answers based on 2014 NEC unless otherwise noted.

  8. #8
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    Quote Originally Posted by rbalex View Post
    The description was "It looks like a house" with no description of the ventilation available. It sounds like solid walls.
    From the OP: "...however the roof only extends about three feet inside the footprint of the structure (if you stand on the wet well hatch and look up you see sky)..."

  9. #9
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    Quote Originally Posted by gadfly56 View Post
    From the OP: "...however the roof only extends about three feet inside the footprint of the structure (if you stand on the wet well hatch and look up you see sky)..."
    Yes - so? Reread the definition of Not Enclosed in the OP. While the open roof is a consideration, the walls must also be at least 50% open to the atmosphere; otherwise, you have an Enclosed Space for the purpose of area classification.

    I do note I made an error in my response to you, there is, in fact, a description of "ventilation" in the OP; i.e., " There are no louvers or fans." That is there is no ventilation.
    "Bob"
    Robert B. Alexander, P.E.
    Answers based on 2014 NEC unless otherwise noted.

  10. #10
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    Quote Originally Posted by rbalex View Post
    Yes - so? Reread the definition of Not Enclosed in the OP. While the open roof is a consideration, the walls must also be at least 50% open to the atmosphere; otherwise, you have an Enclosed Space for the purpose of area classification.

    I do note I made an error in my response to you, there is, in fact, a description of "ventilation" in the OP; i.e., " There are no louvers or fans." That is there is no ventilation.
    OK, just so I clearly understand.

    If I have an area surrounded by a stockade fence and open to the sky, that would be "enclosed",

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    but if I surround the same area with a chain link fence, still open to the sky, that is "not enclosed"?

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    I note that the qualifier "with a roof" is part of the definition of "enclosed" and "not enclosed". My claim is you don't have a roof.

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