NFPA 25 - Main Drain Test

[MedullaOblongata]

Hello,

Per NFPA 25 Table 13.1 Main Drain Tests are required Annually/ Quarterly.
We are trying to size the floor drain/sewer line to drain water from the Main Drain during the test.
The confusion is how much water flows from the 2" drain when its fully open. The city won't let the main drain to flow to the sidewalk.
Any insight on how the test is carried out if there is flooding issue during the test would be helpful.
Is it any good to size the floor drains/ sewer line for this particular test (taking in consideration the volume of water)? Also P-trap would slow down the flow.

Thanks !

 

[Oremus]

Can you get the assistance of the civil guy or the plumber? At this point you've kind of left the world of fire sprinklers and entered into the gray area between where the sprinkler trade meets the plumbing trade and nobody wants to take responsibility for it.

I know RPz backflow preventor drains can get quite big when properly sized. It's not uncommon to see a 6" drain piped to outside underneath the port of a 4" rpz depending on the water pressure.

 

[cdafd]

Yes they do but out alot of water

Most floor drains cannot handle just opening the valve with the pipe over the floor drain

Forgot if 13 allows direct connection. I am thinking that not directly to the sewer connection, but like in a multi story bldg the floor taps are tied into a drain line, so seem like you could do that with the main drain

 

[cdafd]

Looks like on the low end a two inch flows maybe around 50 gallons a minute, but not for sure I know it is alot

 

[DRWeig]

Old set of department stores (big ones, 300,000+ square feet) I worked at long ago -- the main drains were 2", teed directly from 8" pipes, 100 psig normal static pressure. The drains were in the basements, piped directly to 8" sewer pipes. I'm not a plumbing engineer, but was told by one back then that the short length of 2" drain pipe would flow several hundred GPM when opened fully, maybe even 400 GPM. Perhaps an experienced ME plumbing guy can verify.

First test I witnessed was followed in about 10 seconds with screams from the basement ladies' room. Turned all the toilets into geysers. We payed the ladies a healthy sum later on to compensate. We had the drains cleaned and tried again when the store was closed. This time no geysers. Policy from then on was: 1) clean the drains before sprinkler testing; and, 2) test only after hours, and make sure the restrooms are empty.

Get some confirmation from a good plumbing engineer -- this post is just memories from an EE.

Good on ya,

Goober Dave

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[Jondoeiowa]

Perhaps try an Air Gap? When we had a similar situation we had an engineer rule that a 2'-0" long piece of 8" was to be placed before the 6" building drain. We then placed our 2" main drain 1'-0" down into this 8" perhaps check with your plumber or engineer about this. Might work for this situation.

Jon

 

[SprinklerDesigner2]

Over the years the question of flows from a main drain test has come up several times.

Throwing something together quick I think most standard 2" main drains (angle valve with 7' of pipe with single 90 deg elbow and 45 deg elbow) will have a K-factor of between 50 and 60.

To be conservative using a k-factor of 60 a 2" main drain will discharge 300 gpm @ a 25 psi residual pressure and 600 gpm @ 100 psi residual pressure.

I think this is a little on the high side but we would want to error that direction if we would dare to tell someone what we think a full flow main drain would produce. In actual practice I think the flows would be closer to 250 gpm @ 25 psi and 500 gpm for 100 psi residual pressures.

I used a 130 c-value on pipe and .90 discharge coefficient through a circular orifice .... both introduce a conservative margin which is why I think the actual k-factor is closer to 50 than 60.

Am I close or off base?

For the record I would not tell anyone what I thought the flows would be... I would avoid any liability by letting the civil or project engineers figure this one out.

 

[SIfrank]

It might help to discharge into some kind of reservoir to remove some of the turbulence before entering the floor drain.
Perhaps you could jury-rig a series of 50 gal.plastic drums piped together with 4” PVC as low down as possible with a 3” discharge directly over a 4” floor drain. No guarantees though.