Sewer Lift Station

[Bwas]

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.

 

[rbalex]

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.

 

[GoldDigger]

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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[gadfly56]

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').

 

[rbalex]

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

 

[GoldDigger]

Thank you.

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[rbalex]

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.

 

[gadfly56]

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)..."

 

[rbalex]

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.

 

[gadfly56]

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",



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



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.

 

[drktmplr12]

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.

how would hydrogen sulfide be dispersed? it is heavier than air and will collect within the walls without any form of ventilation. to me the "house" is now a confined space, an extension of the wet well. anyone entering it should have a sniffer before they walk in.

 

[gadfly56]

how would hydrogen sulfide be dispersed? it is heavier than air and will collect within the walls without any form of ventilation. to me the "house" is now a confined space, an extension of the wet well. anyone entering it should have a sniffer before they walk in.

Diffusion is your friend. Heavier than air gases only "settle" when they have no way of escaping. It would depend on how many lb_m/hr you were generating as to whether you could accumulate any concentration of interest. That in turn would depend on the open roof area as well as local wind speed or lack thereof. This would be an interesting field exercise for some ChemE students.

 

[rbalex]

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The relevant definitions:

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 building 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.

NEC 500.5(B)(1) Class I, Division 1. A Class I, Division 1 location is a location
(I) In which ignitible concentrations of flammable gases, flammable liquid-produced vapors, or combustible liquid-produced vapors can exist under normal operating conditions,

As I mentioned in my initial response this is an either/or situation. It is either an Enclosed Space or Not Enclosed. While a "roof" is mentioned in both OP definitions, without 50% open walls, it is not Not Enclosed and Enclosed Space is the only other option. This is especially true without sufficient identifiable ventilation. The Enclosed Space definition is not exhaustive and, in fact, mentions a few cases where a roof is not essential.

A Class I, Division 1 location only requires that ignitible concentrations can exist under normal operating conditions. It doesn't require it to be a constant presence, for long periods of time, or even frequent,

Most sewer gases are under constant generation. In the OP case, "dispersion" is not relevant. Note it is not mentioned in any of the definitions above while a minimal ventilation is hinted at for the OP definitions.

I would have no problem defending my classification position for the OP installation in court for both safety and cost-effectiveness. Considering it Not Enclosed is not defensible.
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[Bwas]

Thanks for all the input. The structure is a house without a complete roof; block and stucco construction. I feel the space is not "not enclosed" so the prudent approach is to classify the space. I can just also see where I may get some push-back as the space does not meet the letter of the law for enclosed either.

My co-worker mechanical engineer said that he would need to provide supply and exhaust fans, along with supply and exhaust ductwork, to prevent short cycling from the open roof and properly ventilate the space. The lift station is literally feet away from other houses on each side, so I doubt the neighbors would appreciated the fans running 24/7 right outside their bedroom windows. On top of that, ventilation can only reduce the classification to Class I Div 2, not eliminate it.

My concerns are for both methane and sulfides, so a combination of heavier and lighter than air.

 

[kwired]

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.

Why not extend this vent to outside the structure? It looks like a house, typical house has sewer vents so that shouldn't look out of place. Then you don't have open vent within the space to be concerned about.

 

[rbalex]

Why not extend this vent to outside the structure? It looks like a house, typical house has sewer vents so that shouldn't look out of place. Then you don't have open vent within the space to be concerned about.

This is an excellent recommendation.

 

[GoldDigger]

Why not extend this vent to outside the structure? It looks like a house, typical house has sewer vents so that shouldn't look out of place. Then you don't have open vent within the space to be concerned about.

And presumably whenever the hatch is open for maintenance a door or other substantial part of the wall is open?

 

[gadfly56]

​

The relevant definitions:


As I mentioned in my initial response this is an either/or situation. It is either an Enclosed Space or Not Enclosed. While a "roof" is mentioned in both OP definitions, without 50% open walls, it is not Not Enclosed and Enclosed Space is the only other option. This is especially true without sufficient identifiable ventilation. The Enclosed Space definition is not exhaustive and, in fact, mentions a few cases where a roof is not essential.

A Class I, Division 1 location only requires that ignitible concentrations can exist under normal operating conditions. It doesn't require it to be a constant presence, for long periods of time, or even frequent,

Most sewer gases are under constant generation. In the OP case, "dispersion" is not relevant. Note it is not mentioned in any of the definitions above while a minimal ventilation is hinted at for the OP definitions.

I would have no problem defending my classification position for the OP installation in court for both safety and cost-effectiveness. Considering it Not Enclosed is not defensible.
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OK, I can accept that argument.

 

[rbalex]

And presumably whenever the hatch is open for maintenance a door or other substantial part of the wall is open?

It would need to be worked under a confined space and possibility a hot work permit but that would be true in any case.

 

[smoothops10]

Why not extend this vent to outside the structure? It looks like a house, typical house has sewer vents so that shouldn't look out of place. Then you don't have open vent within the space to be concerned about.

NFPA 820 Annex has a figure showing exactly this. The house with a hatch in it is still the same rating as the wet well. Wet Well and house would have to be 'physically separated' to be unclassified. Physically Separated - A gastight partition between two adjacent spaces, or two nonadjacent spaces, with no means of gas communication between the spaces.