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OT: Mechanics Pit Ventilation

kb0thn

Stainless
Joined
May 15, 2008
Location
Winona, MN, USA
Hi Guys,

I am putting a 14ft long, 3.5ft wide, 6ft deep concrete mechanics pit into my home shop. I have some heavy equipment and a lift for 40k+ lb machines isn't going to happen. And the pit is already in the process of being built, so merits of pits versus lifts aren't helpful at this point. This is something that will get used for 10's of hours per year.

Once of the frequently cited concerns about pits is ventilation. I don't want to go overboard, but I do want to take it into consideration. I am forming the walls today, so I have the opportunity to put in some penetrations without too much difficultly.

I am thinking that a 3" or 4" PVC pipe out from near the bottom of the pit at one end, horizontally under the slab, and then 10ft up a wall to a duct blower that is sucking air. Shop air would be drawn through the open top of the pit and anything hanging out at the bottom would be sucked out. As the building will eventually be heated, my plan is to exhaust the air back into the building so I can potentially do a fairly high flow fan and not waste a bunch of heated or cooled air.

Thoughts? Thanks!

-Jim

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Yup, but I would get more 4" pvc pipe, and make a couple of runs down there,
with multiple risers. It's cheap now, very expensive later.

Try some simple 4" computer fans on the end of the risers (up top, not down
in the pit) and see how it draws.
 
find a large fan and bolt it to the wall and point it up at 45 degrees. or down to push air into it.
you can also have a tube fan down low to push up and out under the slab near the door. 4" is plenty to move 100CFM+
 
find a large fan and bolt it to the wall and point it up at 45 degrees. or down to push air into it.
you can also have a tube fan down low to push up and out under the slab near the door. 4" is plenty to move 100CFM+

If you put the fan down in the pit, it's always in the way.
And, if you turn it on with vapors present, it could spark an explosion.

small fans with no starting switches to spark (muffin fans) and up & out
of the fumes.
 
Run some larger conduit down there to allow electricity and air to be installed later. Put a heavy angle iron edge on the top so you can weld something for uprights for a barrier. I used 1 1/4 pipe couplers welded into the angle and I can thread in stands for chain fencing. The angle also allows you to put boards across to cover part of the pit. Very handy when working on both the top and bottom side of an vehicle.
 
Water drainage as well?

It is extremely hard packed clay at the top of a hill. 15ft from the nearest side of the building. There is zero possibility of water coming in from the ground. But I am planning on running a drain pipe into the pit so I can have a sump pump sitting on the floor. I thought about doing an actual sump for it, but with it on the floor, it won't collect and pump out inadvertent fluid spills.


or down to push air into it.

Yeah, a ceiling mounted fan would sure be an easy way to push some air into there and presumably dilute or push out any bad air in the pit. At least when it is open.

Run some larger conduit down there to allow electricity and air to be installed later. Put a heavy angle iron edge on the top so you can weld something for uprights for a barrier. I used 1 1/4 pipe couplers welded into the angle and I can thread in stands for chain fencing. The angle also allows you to put boards across to cover part of the pit. Very handy when working on both the top and bottom side of an vehicle.

All great ideas. I have most of those in the plan. But adding some substantial sockets for a railing makes sense. Thank you! I do have a massive formed 1/4" steel step rail. 1000 lbs worth of it. It has an 8" high step to accommodatepit_detailed.jpg a timber cover. My design criteria is to be able to put 15k on a single portion of the cover. I thought timber could do that, but it isn't looking that way. So now contemplating formed steel cover segments. The step also serves for making rolling oil drain tray, jacking platform, etc. Will have upwards facing LED lights the full length of each side. And electricity and air.
 
Why not just use a confined space portable blower...cheap and easy.

I actually have one that came on a utility service truck I had. 12V fan and flexible duct that pops out of it. Seems cumbersome to pop that in each time. Wouldn't end up using it and would defeat the purpose.
 
Some bad air sinks and some rises so consider how both will escape. The heavy stuff that will choke you tends to sink. The explosive stuff tends to rise. Sounds like you have the heavy asphyxiating stuff figured out. I would also provide some passive vent for explosive gasses that might leak from a welding tank, dripping fuel etc into a sealed pit with roof closed. Maybe as simple as a tee near the top in the lower vent pipe.
One thing many people forget in venting is the vent pumps out air fine and dandy but the replacement air flow should be planned as well. I assume intake at one end discharge at the other. Of course with a open trench no really an issue. But think about it for when it is closed off. I wonder if rust will be a problem. Seems similar to a basement shop so it may be damp and musty/rusty.
Bill D.
 
Some bad air sinks and some rises so consider how both will escape. The heavy stuff that will choke you tends to sink. The explosive stuff tends to rise. Sounds like you have the heavy asphyxiating stuff figured out. I would also provide some passive vent for explosive gasses that might leak from a welding tank, dripping fuel etc into a sealed pit with roof closed. Maybe as simple as a tee near the top in the lower vent pipe.
One thing many people forget in venting is the vent pumps out air fine and dandy but the replacement air flow should be planned as well. I assume intake at one end discharge at the other. Of course with a open trench no really an issue. But think about it for when it is closed off. I wonder if rust will be a problem. Seems similar to a basement shop so it may be damp and musty/rusty.
Bill D.
so you've never set foot in a pit before.....
 
I have no experience with pit ventilation, but for a 6 ft deep pit in Minnesota (could be different in, say, Texas), I wonder how serious the ventilation issue you cite is going to be. Since you seem uncertain yourself about this, it might be too committing, or limiting, to go ahead with installation of conduits etc. at this construction stage, before knowing exactly what, or if, you're going to need. Might be some merit in just completing the pit and then assessing what is most appropriate when you get a handle on the scope of the problem. What sort of ventilation/AC do you have in your shop?

-Marty-
 
No expert but...

Holes collect heavy gas so ventilation from lowest point a must.

Explosion proof outlets should be used.

Google OSHA regulations as most are based on past bad events.

Do slope the floor such that it drains.

Place a sump in the corner with plenty of room to collect scrap and room for sump pump as stuff will spill and mopping is best way to clean.

Dig trench to outside of shop for air if possible.

The fan can be mounted outside at end of trench so no need for intrinsic safe unit and allows for whatever meets the flow requirements.

Having the fan blow outside reduces risk of heavy gasses creeping back into the pit.

Make space (pockets in walls) for proper fire extinguisher of proper type.

Smoke and CO detectors at both ends.

Pocket in wall for air and power reels.





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I would also provide some passive vent for explosive gasses that might leak from a welding tank, dripping fuel etc into a sealed pit with roof closed.

I was planning on leaving pit fan on or at least on the shop lighting circuit so it ran at a pretty regular interval. The cover is going to be far from hermaticaly sealed, so I figure there will be plenty of air leakage in. I did contemplate running another 4" piece of PVC. But it's a bit of work and expense. 50ft+ of schedule 40 4" pipe $, digging more trench to get it under the high traffic portion of the slab, sloping it, etc.

so you've never set foot in a pit before.....

I think this is directed at Marty ... but I've actually never set foot in a pit before either. Spent quite a while on the internet researching and have talked to people with experience working in pits. Trying to be ventilated, deep enough, bright enough, and dry enough. But definitely open to other suggestions. I'm 36 and hope to be using this until I die. Have limited time, and somewhat limited budget (this thing better be done for under $10k), but definitely want it to be pleasant and relatively safe to work in.

it might be too committing, or limiting, to go ahead with installation of conduits etc. at this construction stage, before knowing exactly what, or if, you're going to need. Might be some merit in just completing the pit and then assessing what is most appropriate when you get a handle on the scope of the problem.

The penetrations into the pit are pretty easy at this stage. A few hours of work to get a conduit roughed in and to the nearest wall. The next pour puts about 20k lbs of 8-5/8" thick heavily reinforced concrete walls in. I have a core drill and appropriate bits. But adding a conduit through the wall and before backfill will take twice as long. Once I back fill with flowable fill the outside of the pit is now surrounded in low strength concrete and it will become a major project to add anything down low in the pit. Once I pour the slab over the top, the only way anything is going to be added is with directional drilling or rod pushing. Lots of work and lots of expense!

I just went through this with the ICF (poured concrete) house my wife and I are building. Added about 20 penetrations after the fact (details not worked out prior to pouring walls) and that easily took a man-week of labor or two.

Thanks for the suggestions, y'all!
 
No science here, but I have a similar pit that I installed in my home shop. Great for servicing just about anything, even just for changing mower blades. I did put a piece of 8" pipe in for potential fan but have never felt the need to install. Depends what type of work is going on in shop I guess. I did put a sump pit in the floor of pit. I Ran my floor drains into it. WE have sand and I left bottom of pit open, have yet to install sump. Allows enough drainage for snowmelt off of vehicles etc. Pour lip into floor for timber cover. Mine is 2" oak and actually remains covered most of the time. Definitely conduit into four outlets lights etc. Comercial ones I have seen have lights and even tool boxes recessed into sides. Wish I would have put lights into my walls.. The biggest thing to consider in my mind is placement. Make sure it is in the right place. Can you get your equipment in and still close door. No matter what part of a machine or vehicle you are working on....
 
Explosion proof outlets should be used.
Would rapidly be defeated as soon as I plug something into it. Personal shop. Not doing it class 1 div 1.

I'm improving on my current situation of laying on my back on gravel or in the mud or on snow to work on equipment.

Vapor tight LED lights are going to be about 5ft below the floor of the pit / 1ft below shop floor. I was planning on a couple of outlets at that height as well. But I am "new school" and use cordless tools for 90% of my wrenching, so not even sure I will end up using outlets much at all. I could even eliminate the outlets by just pulling overhead cord reel into the hole.



Dig trench to outside of shop for air if possible.
The shop building is a 55' x 74' pole shed. Clear inside to 14' up and pitched roof above it. As a 50 year old pole shed, there are holes you could throw a cat through. There is no limit on makeup air with this building. But I am planning on demolishing the pole shed and doing a modern steel framed building in its place in the future. I am considering adding another 4" pipe for air in to the pit. But there are a whole host of issues with exchanging a lot of air in Minnesota. Summer time can be 100F with 90% RH. Winter time can be -30F with 5% RH. That is a lot of expensive air for the 10+ months a year that we are heating or air conditioning.


The fan can be mounted outside at end of trench so no need for intrinsic safe unit and allows for whatever meets the flow requirements.

I am not picturing this. PlastikDreams mentioned this as well. At 14ft long trench I will have the vehicle covering most of the trench and hanging over one end. I'm picturing a box fan at one end of the trench somehow pointing down?

Having the fan blow outside reduces risk of heavy gasses creeping back into the pit.

Pipe from bottom of pit will go up end wall of building. Planning on terminating it inside the building, but could poke it through wall or roof later if needed.

Make space (pockets in walls) for proper fire extinguisher of proper type.

Wall construction doesn't lend itself to pockets deeper than 2-5/8". But this is a good idea. Thanks!

Smoke and CO detectors at both ends.

Where would make sense to place these? I do like the idea of the smoke detector so if something is smoldering in the hole while I am out of the hole, I will hear about it.

Pocket in wall for air and power reels.

I have a ships ladder on one end of access. Planning on having a bulkhead on one side of it for the utilities. Planning on building some drawers into the ships ladder for a set of wrenches and spill cleanup materials. And putting the sump pump underneath.

Thanks!
 
............... into the pit so I can have a sump pump sitting on the floor. I thought about doing an actual sump for it, but with it on the floor, it won't collect and pump out inadvertent fluid spills.

Just a minor note to mentally file for possible future reference. If you have a major water leak and the pit fills with water you can use that sump pump to get out MOST of the water. Submersible sump pumps like the one I think you may be planning need to be underwater for cooling. Once the water level starts to go down below the top of the pump it will overheat and self-toast. I completely fried one learning that lesson. Hopefully you will never have that situation.

Steve
 
Once the water level starts to go down below the top of the pump it will overheat and self-toast. I completely fried one learning that lesson. Hopefully you will never have that situation.

Interesting! So in an actual 1ft^2 sump it sucks down the last 10 inches of water so quickly it doesn't matter? But in a bigger hole it spends too long getting down the body of the pump and overheats?

Thanks
 
I put a floor drain in mine, also air coming in, electricity and used oil going out. Never got around to hooking any of them up but the conduit is there.
 
Looks like very nice work on the pit and carefully thought out!

Have you thought about protection from inadvertently walking off into the pit when not in use? Shops that I inspected when I worked for a workers compensation company had chains and pipes that dropped into tubes set around the pit.

These employers of course had employees and were under OSHA rules. You may not have employees, but I'm just sayin'.
 








 
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