IPC vs Manning Equation for Sewer Piping 3

[nuuvox000]

Hello, I have used the drainage fixture unit method of the international plumbing code to size a building drain. The civil engineer thinks it is too big and has asked if we can do some calculations to show that a smaller pipe size may be used. (He thinks the IPC is too conservative). I used the manning equation and assumed half-full flow in the pipe and it does seem to show that a smaller pipe size can be used but no one in my office has used anything but the IPC so we're a little unsure of standing behind the manning equation in this case. I'm just curious if anyone has used the manning equation to size building drainage sanitary sewer lines and how it worked out for you. Is a half-full flow appropriate? Would you assume more or less than this for any reason?
Thanks!

 

[fel3]

The Fixture Unit Method (FUM) and Mannings Equation (MEQ) are two totally different things and you can't substitute one for the other.

The FUM estimates peak flow for a building or group of buildings based on the number and type of plumbing fixtures, type of facility (public or private), and whether you are looking at water demand or sewage flow. The FUM is not a pipe sizing tool. However, the result from the FUM (peak flow) is used when sizing pipes. Since I'm working at home under lock-down and my notes are at work, I can't comment on the amount of conservatism built into the FUM. My recollection is reasonable conservatism, not extreme conservatism. I hope someone else can comment on this point.

MEQ is an empirical equation used to estimate the hydraulic conditions in a pipeline experiencing uniform flow. MEQ is not capable of estimating the peak flow because it knows nothing about the source of the flow. For this reason, assuming a half-full pipe does not provide an estimate of the peak flow. The main benefit of assuming a half-full pipe is that the cross-sectional area and wetted perimeter are easy to calculate. This was a big deal in the days before programmable calculators and computers. Depth ratios other than 0%, 50%, and 100% require trig, and solving for the depth ratio require iterating the trig equations. I have a cardboard slide rule for solving MEQ, but I prefer to use a program I wrote in the early 1980s for the HP-41CV, which I later ported to the HP-42S.

The FUM is the normal "standard of practice" for estimating peak flows from buildings and is part of the code you are working under. So, unless you have another method you can properly defend to the client and building department (and there are cases where this is appropriate), I would say it's the method you should use. Also, use the code method for sizing the plumbing inside the building. MEQ won't help you on the vertical runs.

FYI, every mechanical engineer I have dealt with over the years uses the FUM almost exclusively and I have used it myself for site projects such as industrial parks and prisons. However, when I am modeling water distribution systems and sewer collection systems, I typically use zoning-based flow factors that are derived from historical data (e.g. water meter records, water well production records, wastewater treatment plant flow records, temporary manhole meters, etc.).

Once you get outside the building, it's time to employ MEQ. However, for all of my multi-discipline projects, I was the one sizing the sewers outside the building, not the mechanical engineer. Regardless, knowing the peak flow (estimated using the FUM), pipe roughness coefficient, maximum allowable depth ratio (often dictated by the local jurisdiction), minimum allowable slope (often dictated by the local jurisdiction), it is possible to solve for the minimum required diameter (and easy if the maximum allowable depth ratio is 50%), then round up to the next standard size (or down if you are very close to a standard size and the local jurisdiction will accept a slight increase in the depth ratio).

If this size is smaller than the building plumbing, then upsize and recalculate the new and smaller depth ratio. Then check the flow velocity to make sure it's above the minimum required by the local jurisdiction (I typically see 2.0 ft/s or 2.5 ft/s). If this size is larger than the building plumbing and you have the vertical room to do this, you might increase the slope to the point that a smaller pipe matching the building plumbing can work.

I hope this helps.

Fred

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[PEDARRIN2]

As fel3 stated, two different worlds and very rarely should they intersect.

And the use of MEQ is included in the FUM in the IPC and assumes between 1/2 and 2/3 full based on assumed correlation between fixture use and actual flow rates. IPC also only allows certain pipe slopes based on pipe size. MEQ is used a little more explicitly in storm pipe sizing because you don't have to worry about venting.

When you get outside, the civil uses MEQ (after he asks the mechanical/plumbing designer to give him peak flow) and sizes it based on different slopes and different percent of full flow.

Inside the building you are "bound" by the plumbing code which the plans examiner "might" let you modify, but then he typically wants a PE stamped/signed letter indicating what deviation from the code was taken and that you are taking full responsibility for it. That happens when they are nice to you. Otherwise you just follow the code.

 

[nuuvox000]

Thank you both very much for the great responses! I think that makes things more clear; we decided to stick with the IPC FUM after all.