Trench Drains

[madvb]

Not sure if this question is posted in the right place. Does anyone know if trench drains that are installed inside a building (auto repair shops) require p-traps and vents? This is drained to a sand/oil interceptor located outside the building. I figured the sand/oil interceptor acts like a trap and it has its own vent.

Thanks. Sorry if this is not in the correct forum.

 

[gepman]

I am not sure if this is the right forum but a lot of work is MEP so you will probably have the right audience.

Your answer would depend on the plumbing code under which this work falls but where I have done work the answer would be yes, you need P traps and vents.

 

[PEDARRIN2]

A trench drain is considered a floor drain which normally will require a trap - unless it is in a freezer which it dowsn't sound like it is.

Whether it will require a vent is up to the plumbing code and the other fixtures on the pipe network. If you are in an area governed by the ICC, you can get away with not venting it if there are no stacks (vertical piping more than ~8 ft)

The fact that you have a sand interceptor may require the vent. Solids in the stream tend to invalidate the no vent methods in the code.

 

[Golestan]

you probabely will need p-trap and vent. And might even need a trap primer. check with your plumbing inspector authority.

 

[ChrisConley]

Hmmmm, I'm going to be the only person who disagrees. Up in my neck of the woods the oil interceptor is considered a trap because... it is a trap. And we're not allowed to double trap fixtures.

That goes for grease interceptors as well.

 

[gepman]

ChrisConley

Does this mean that a restaurant with a grease trap does not have traps inside the restaurant?

An auto repair facility can be considered an electrical hazardous location (Class I Division II) with 24" of the floor (due to fuel spills). Heavier than air gases could go down the drain pipe without a trap and surface in another room of the same building which had a trench drain without a trap.

All the industrial facilities that I have had to put trench drains had traps with or without a sand or grease interceptor. Most had large interceptors just outside the building.

 

[GMcD]

Check to see if the local Codes allow a "trade waste" trapping setup. That's where you can take all the drains from one space through one central interceptor/sump which is the primary trap, so you don't have to put traps in at every floor drain in the room. This is a typical arrangement for the product cooler drains in most grocery stores and many repair shops where an oil interceptor is required. The key is that this is only allowed for the drains in a single room or space and only for certain applications.

So in the example of an auto repair shop, all the drains from the open bay shop floor can be installed without traps, and routed to a central oil interceptor which acts as the trap before the main drain routes to the sanitary main drain. The oil interceptor has a vent on the downstream side for sewer gases that may come up from the sanitary sewer. You cannot take a floor drain from another room that is separate from the shop and connect it to the trade-waste drain system. It only applies to drains from a single space.

 

[PEDARRIN2]

Sometimes the fixture and the interceptor have to be trapped because the interceptor can become air bound. So "double trapping" does happen.

The main requirement for trapping is to prevent sewer gases or other gases from traveling down the pipe to occupied areas. Typically the only time a code will allow you to not trap a fixture is if it is in an area that is subject to freezing. A lot of times you indirect this waste to a drain that is trapped.

Again, take a look at your plumbing codes or talk to the plumbing inspector. They will tell you what you can do in your area.

 

[madvb]

Thank you all for your responses. I figured the sand/oil interceptor is a trap itself as ChrisConley mentioned. I didn't put a trap, vent, and trap primer in my trench drains, but wasn't really sure. The contractor mentioned it, so I figured I should ask. I'll check with the plumbing inspector to be sure.

I didn't know that we are not allowed to double trap fixtures. Is that in the UPC anywhere?

 

[ChrisConley]

gepman (Electrical)

Does this mean that a restaurant with a grease trap does not have traps inside the restaurant?

No, what it means is that the sink that feeds into the kitchen sink mounted grease inteceptor doesn't need a trap. We're not allowed to put a big grease trap on the outside of a kitchen, we're required to use 'point of use' traps (to prevent double trapping, which our inspectors are very picky about).

I've heard in other places they put their grease intercetors outside the building... which just isn't an option in -40 design conditions.

gepman (Electrical)

Heavier than air gases could go down the drain pipe without a trap and surface in another room of the same building which had a trench drain without a trap.

One of the requirements for this method is that all of the floor drains served are in the same space, as GMcD states.

PEDARRIN2 (Mechanical)

Sometimes the fixture and the interceptor have to be trapped because the interceptor can become air bound. So "double trapping" does happen.

Any trap can become air bound Peddarin, which is why we vent (and often prime) the interceptor, unless you mean something different. Can you elaborate more on 'air bound' inteceptors? I've never heard the expression.

 

[PEDARRIN2]

It is similar, as you say, to a trap that is not vented.

A grease or kitchen waste interceptor can become "air bound" if the air in the influent pipe that is brought into the interceptor through the influent pipe cannot get out of the interceptor. The trapped air builds up and gradually the water level in the interceptor lowers, thus making the interceptor ineffective. Thus you have to trap the interceptor, either on the interceptor itself or on the outlet piping to let the air out. You also have to trap the drain to prevent sewer gases from entering the occupied space. Thus the "double trapping".

We design our interceptors with an open sanitary tee, inside the interceptor on both the inlet and outlet pipe. This opens the inlet, interceptor, and outlet to allow the air to flow up and down the piping network. Then we do not have to vent the interceptor.

In some jurisdictions that I have worked in, they require an exterior grease interceptor. This has its plusses and minuses. If it is located too far from the kitchen, then most of your grease never makes it to the interceptor. And it is also difficult to put these in for remodels of buildings in the middle of the city, which do not have room outside. The plus is that it can be more easily monitored by the AHJ to verify that it is being cleaned/emptied. I prefer them outside since the larger the tank, the more grease it can separate and hold. But I have never worked where the design conditions were that cold so I can understand the aversion to an exterior unit.