Grinder Pump and Sump - Is area above it classified? REf NFPA 820

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rcwilson

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We have a project on an industrial site with an existing sump and grinder pump that pumps the flow from a single toilet and wash basin up to the main sewer system. The sump and all components in it (level switches) are suitable for Class 1 Div 2. The pump motor sits on top of the sump with the pump down below. The existing installation was considered non-classified. The sump is flush with the floor in a utility substation service building that houses transformer oil processing equipment, but the entire building is considered non-classified. (Transformer oil is not considered a flammable or explosive liquid, similar to diesel fuel. No oil drain goes to this sump, unless someone dumps oil in the toilet).

As part of a new installation we changed the pump to a different voltage, using a motor listed for Class 1 Div 2.

Our PE stamped the drawing showing the area as non-classified, quoting NFPA 820, Table 4.2 "Collection Systems", Line 9, which shows a Not Normally Ventilated (NNV) pumping unit in an enclosed area serving one but not more than five dwellings is unclassified. The table lists grinder pumps specifically and the description fit the installation. We could not locate another NFPA reference that discussed classifying grinder pumps.

The AHJ has objected to the area classification because Table 4.2 Line 9 is for "Individual Residential Pumping Units,...(e.g., grinder pumps...)" and this is an industrial facility.

All the other categories in NFPA 820, Table 4.2 are for larger municipal sewer systems, pumping stations and mixed residential and industrial wastewater where possibility of flammable liquids is greater than a single toilet. All categories recommend the enclosed space (building) above the larger sumps as either Class 1 Div 1 or 2. The recommendation from the AHJ is to classify the entire existing building. That is not practical or necessary.

Question -Is there any other reference we could site to back up our engineering judgment that this single toilet grinder pump installation does not require classification of the entire existing building?
 
Not my area of expertise, so cannot provide any reference without researching. But short of getting the building de-classified as is, might purging the sump with a vent to outside be an option?
 
Good thought. The sump is vented to the outside with a 2" line.

I just got a picture of the sump. It is only 3' x 3' with a round (12"?) access hole on top that is covered by a plate that mounts the pump motor and level switch conduits. The plate seals the access hole. There is no manhole or manway.

The only access to the sump is through that round hole. The probablility of a sewer gas build up inside the sump is quite small and if there was, it would discharge through the vent line and not get into the building. The adjacent floors slope away from the sump preventing any spilled transformer oil or cleaning solvents or nasty liquids from entering the sump. Risk of a hazardous atmosphere is small.

I think what concerned the AHJ was the PE referencing an NFPA table entry that only applied to dwellings. (We have not been albe to communicate directly with the inspector. AHJ and project are in a different state, a couple time zones away). I believe a letter from the PE stating that the sump is unclassified without referring to Table 4.2 or NFPA 820 might be sufficient to resolve the issue.
 
rcwilson said:
.......................................). I believe a letter from the PE stating that the sump is unclassified without referring to Table 4.2 or NFPA 820 might be sufficient to resolve the issue.
Click to expand...

Thats would be my recommendation.
Give the AHJ something to CHA.
 
Well, let's look at it from a tangent .....

There's a firm called Weil Pump, that has been dedicated to making sewage pumps for nearly a century. They are known for making a better-than-average, quality pump. Your sewage is their bread and butter, so to speak.

They do make pumps listed for classified locations, but the vast majority - and most of theirs are fairly large- are not listed for hazardous locations. This includes their submersible pumps; very few are 'explosion proof.'

Looking at the bulk of sewage pumps with a discharge smaller than 2", it's pretty hard to find one that is listed as 'explosion proof.'

From this I derive that very few sewage pits pose a hazard of ignition. While this equation might change in specific places where large volumes of sewage are collected and aerated -such as at a treatment plant- a simple sump .... well, I can't ever recall one going 'boom.'

Indeed, I'd be more concerned about the area around the connections of any gas-fired appliance.
 
Thanks for the comments. Your reasoning matches our thinking.

We ended up classifying the sump internals and the end of the vent pipe as Class1 Div 1. The area 12" above the sump was given a Class 1 Div 2 rating. This made sense since the sump top was not flush with the floor but recessed in a 3' x 3'x 12" deep pit covered with a grating. With this solution the inspector is happy and there is no impact on existing wiring, plus the installation meets Code requirements for seals, intrinisically safe equipment, etc.

We also learned to be careful about which documents we formally cite in backing up our engineering judgement.
 
A Minor Caution

A Minor Caution

When it appeared you were heading for an unclassified designation, I left this thread alone since that is what I believed it should be.

I don?t believe it will make much difference except for documentation but you should consider Section 500.5(B)(2)(3). If the area around the vent is Class I, Division 1 as you indicated, there will also be a Division 2 envelope around it. This is the same reason it was appropriate to have the Division 2 envelope above the sump. However, the envelope should probably extend 12? around the edges of the opening too.
 
Thanks for the comment, Bob.

We did extend the Division 2 classification around the pit and the vent. The existing layout could meet code requirements with the extended classification.

We always put a Class I Division 2 bubble around a Class I Division 1 area like two concentric spheres at vents. Usually, the vents are for high pressure fuel gas or safety relief vents that require much larger diameter bubbles than this atmospheric pressure sump. Extending the classification at the vent doesn't affect anyhting unless someone installs a light fixture on the roof line near the vent.

What I did not like about this solution was having a Divsion 2 classified area inside a building. Usually that makes the entire building space classified unless we do an analyisis of air changes, natural ventilation, and add combustible gas monitoring. But this installation is safe and the AHJ is satisfied, so we will leave it alone.
 
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