septic tank pumps

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augie47 said:
You might want to review this thread;

Sewer wet well conduit for submerged pumps.

I am an electronic technician for the City of Lakeland Water Utilities. We are interested in finding out how to obtain an exemption in the NEC 501.15 (4) Class 1 Division 1 Boundary. Our situation is this. We have 185 sewer wet wells in the city. They have a rigid aluminum conduit that is...
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You know the specific gravity of methane is like 0.55, so it really doesn't tend to collect in wet wells
 
Per 500.4 classifications are determined by design engineers and not electricians or inspectors. That said, every wet well I've encountered other than small residential installations have been Class 1 Div 1 by engineering design
 
Well there is always the possibility especially with a combined (sanitary and storm) system of someone dumping something that doesn't belong into the system like gasoline or solvents
 
NFPA 820, Table 4.2.2 Collection Systems, (LINE 10a] states various residential collection systems have, "possible ignition of flammable gases and floating Class I liquids [flash point < 37.8°C (100°F)]." Oddly enough, NEC Section 500.5.(B)(1)(1) would describe that as Division 1; that is, it could have, not necessarily does have an explosive atmosphere under normal conditions. NFPA 820 however, would only classify it as Division 2.

There would need to be a bit more analysis, such as tank volume and general ventilation.

[Edit Add]: H2S is also a possible component. It is toxic and has a greater LFL/UFL range than methane.
 
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rbalex said:
[Edit Add]: H2S is also a possible component. It is toxic and has a greater LFL/UFL range than methane.
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I've never seen dangerous levels of H2S in a municipal wet well. Now wastewater systems for paper mills is a whole different story. I've seen levels in tanks so high that we had to remove sludge for 3 days working 24 hours before we got the levels down far enough that we could enter the tank.

H2S is some really bad stuff. Can kill you really quick

Hydrogen sulfide is a colorless, flammable gas that smells like rotten eggs at low concentration levels in the air. It is commonly known as sewer gas, stink damp, and manure gas. At high concentration levels, it has a sickening sweet odor. At extremely high levels, a person can lose their ability to smell the gas and become unaware of its presence. This condition, known as olfactory fatigue, can also occur when people have been exposed to hydrogen sulfide for a longer period of time. Hydrogen sulfide is heavier than air, so it can build up in low-lying areas and enclosed spaces.
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Joethemechanic said:
Methane has a stoichiometric air-gas ratio of 9.53 to 1 on a volume basis (the weight ratio is 17.2 to 1)
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The lower flammability limit is much more important than the stoichiometric ratio.

Joethemechanic said:
You know the specific gravity of methane is like 0.55, so it really doesn't tend to collect in wet wells
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In a septic tank, where can it go? The inlet pipe discharges underwater; the outlet pipe discharges downhill and underground, and the access hatch doesn't have much free-flowing area for methane to escape and outdoor air to replace it.
 
drcampbell said:
The lower flammability limit is much more important than the stoichiometric ratio.


In a septic tank, where can it go? The inlet pipe discharges underwater; the outlet pipe discharges downhill and underground, and the access hatch doesn't have much free-flowing area for methane to escape and outdoor air to replace it.
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None of the wet wells in sewage lift stations I've ever worked on have the inlet pipe underwater, and I've worked on lots of them. And they are supposed to have vents

wet well.jpg
 
drcampbell said:
The lower flammability limit is much more important than the stoichiometric ratio.


In a septic tank, where can it go? The inlet pipe discharges underwater; the outlet pipe discharges downhill and underground, and the access hatch doesn't have much free-flowing area for methane to escape and outdoor air to replace it.
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Indeed it is, technically it only requires 25% of LFL
 
Joethemechanic said:
None of the wet wells in sewage lift stations I've ever worked on have the inlet pipe underwater, and I've worked on lots of them. And they are supposed to have vents

View attachment 2566473
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Again NFPA 820 is "The Standard." I won't do your homework on this one, but "vents" are generally required. Of course, NFPA 820 isn't necessarily locally adopted, but if there is an employer/employee relationship in the equation, Fed or State OSHA will often have something to say under their "General Duty Clause." I only linked to the FedOSHA text, but most State OSHA's have similar verbiage.
 
Joethemechanic said:
None of the wet wells in sewage lift stations I've ever worked on have the inlet pipe underwater, and I've worked on lots of them. And they are supposed to have vents
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Who said anything about wet wells or lift stations? The question was about septic tanks.

Though I am a little puzzled by the presence of a pump in a septic tank. I thought they were usually gravity-fed.
 
drcampbell said:
Who said anything about wet wells or lift stations? The question was about septic tanks.

Though I am a little puzzled by the presence of a pump in a septic tank. I thought they were usually gravity-fed.
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I just assumed he was working on a lift station of some sort. Unless it's some kind of crazy system where the pump intake is near the top, and it's pumping water up to a leach field. That would be a new one for me though. It should still need a vent to work properly
 
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