Christoph Lohr: Welcome to The Authority Podcast: Plumbing & Mechanical. When talking about the built environment, we would do well to remember: We shape our buildings, and afterwards, our buildings shape us. Therefore, on each episode, we’ll discuss the latest trends from IAPMO in plumbing and mechanical safety, sustainability and resiliency. Join me, your host, Christoph Lohr, and together we’ll explore the ways we can make our buildings shape us for the better.
And welcome back to The Authority Podcast: Plumbing & Mechanical. I’m your host, Christoph Lohr. And joining me is my friend and colleague, Mr. Dan Cole. Dan, welcome back to the show. It’s been awhile, but we’re excited to have you come back and join us for another recording.
Dan Cole: Good to be here.
Christoph Lohr: Dan, for our listeners, can you tell our listeners a little bit about your role at IAPMO and also a little bit about your background?
Dan Cole: I’m senior director of Technical Services and Research at IAPMO. I overlook four publications that we publish as supporting material for the codes, which is the Illustrated Training manuals for the plumbing and mechanical codes, and also study guides for the plumbing and mechanical codes. I also work with special projects that come up and needed research on the technical level involved in a number of committees inside and outside of IAPMO. My background is, I was a journeyman plumber in the state of Illinois licensed for over 20 years, was a plumbing contractor for about 10 years, became a plumbing inspector for a jurisdiction in Illinois before I came on board with IAPMO.
Christoph Lohr: Excellent, and Dan, I think you’re setting yourself a bit short. I was going to say, after having worked with you now for a number of years, I would say you’re also the world's foremost expert on Dr. Roy Hunter, I feel.
Dan Cole: Yes, he became an object of study for me back when I was a plumbing inspector in trying to understand his design curves for peak demand.
Christoph Lohr: Yep. Well, and I think it was that research that led to the creation of the Water Demand Calculator®.
But we're not going to be talking about water sizing today. We're actually going to be talking about storm sizing, and specifically IAPMO is publishing a new white paper on storm drainage systems, which as of this recording should be close to being released in the next month or so. Can you tell our listeners what this storm drainage pipe sizing white paper is about?
Because obviously with your work in pipe sizing, I think you’re definitely a household name. So can you tell us a little bit about the storm pipe sizing white paper?
Dan Cole: Yeah, these kind of challenges really drive me. I like them, I like to look into the origin of how things were sized in the past. And once you discover the origin, you kind of take another look at the parameters of things and have they changed?
And if they have changed, does it affect things? For example, the sizing of the storm drainage systems are dependent upon flow capacity equations, and when we talk about flow capacity, we’re talking about the flow rate through a pipe that flows freely by gravity. In other words, the flow does not back up where it causes pressure. It’s not a pressurized pipe.
It flows freely as an open channel flow. So we’re looking at the capacities, and there are equations that will compute the capacities and calculate velocities and flow rates and pipe conduits. One of the variables in the equations is the coefficient for roughness. This paper explores how the roughness of different types of pipe material will change the computational results that are dependent upon the roughness coefficient of the pipe.
Christoph Lohr: Excellent. I was going to say, in a summary for our listeners — and would love to hear it in your words — is it fair to say that this white paper explores how different pipe materials will have different capacity of storm water to flow through them?
Dan Cole: Oh definitely, yes. Different types of pipe material will have different inside diameters, and they will have different types of flow resistance based on the material. And those are the two main contributors when you compute flow capacity in pipes. That’s what we were exploring, just how significant will flow capacity change when you change these two parameters?
Christoph Lohr: Excellent. So the paper discusses then these equations that you mentioned?
Dan Cole: Yes, the paper is structured around three types of equations — one for the vertical pipe equation and two for the horizontal equations. These equations do have names, especially the horizontal equations. They have names commonly known as the Manning equation and the other one is the Darcy-Weisbach equation. The equation for the vertical piping doesn’t have a common name, but it was derived by the National Bureau of Standards. If we're keeping consistent with the authors of the equations, we could call the vertical equation the Wylie-Eaton equation, because they were the ones that created the equation for vertical pipe capacities.
Christoph Lohr: Excellent. For our listeners, let me go ahead and I would say we’re going to share the screen here as we’re recording. For our listeners, Dan, if you can just make sure those are listening at audio only, what page should I go to, roughly, for those equations? I’m assuming starting down on flow capacity equations, page six?
Dan Cole: If you want to start with Table 1.0, we started constructing a table that lists all the different types of pipe material allowed by the codes for storm drainage. For example, ABS, cast iron, even concrete pipe, copper, ductile iron, polyethylene, all different types of plastics, PVC and so forth. We added two columns to this table for the roughness coefficient for both the Manning equation and the Darcy-Weisback equation.
Christoph Lohr: And those are on the right-hand side, right, Dan?
Dan Cole: Yes, and they’re quite different from each other, these coefficients. The Manning's coefficient for roughness, in terms of n, it’s a dimensionless coefficient, whereas the Manning equation uses an f variable, which is dimensional because it is in feet; it can be measured in feet. So these are really two different types of developing coefficients of roughness, and again, which is the reason why we wanted to compare the two equations. How different are they from each other?
Christoph Lohr: And is one of the equations more conservative than the other? You might have mentioned that; I might just not have heard.
Dan Cole: The Manning equation, what we discovered, is more conservative than the Darcy-Weisbach. That means that it computes less flow capacity than the Darcy-Weisbach, so it’s more conservative. But the difference between the two equations is really not that much. It’s a difference between 3% to 9%, depending on the type of material.
So if you go from, the smoothest PVC pipe is a 3% difference between the two equations. If you compare with cast iron, it’s 9%. They’re comparable with each other, Manning’s being more conservative.
Christoph Lohr: Gotcha; that makes sense. And so the Wylie-Eaton formula that we’re kind of describing here, that’s Equation [1] here that I’m showing on the screen, right? We don’t have to read it out, but that’s for the vertical distribution, correct? Vertical piping?
Dan Cole: Yes, and this is a difficult equation because in Equation [1] you don’t see a coefficient of roughness in this equation; you see a ratio, an annular ratio of cross-sectional section of the pipe and diameter.
So where is the coefficient of roughness? Well, it’s in the constant, beginning with the equation. In this one the constant is 27.8, so the coefficient of roughness is hidden in that constant. You have to dive deeper and kind of disassemble this equation and discover — and the paper does this, it discovers how this equation was created, where the coefficient of roughness is, and if you change that coefficient of roughness, it’s going to change that constant number.
Christoph Lohr: You said the roughness variable changes for different types of pipe material. Does it impact the outcome of the equations?
Dan Cole: Significantly so, and that’s one of the prominent features of this paper. When you change the coefficient of roughness for the equations, especially in the paper comparison, like two extremes, the smoothest and the roughness — so it’s between PVC pipe and cast-iron pipe — there is a significant difference of computational results when you change that friction of coefficient.
Christoph Lohr: Excellent. If I’m understanding this correctly — and kind of a summary then — pipe material indicates two things that impact flow predominantly, which is the internal diameter — because that changes for pipe material for a given size; for instance, PVC and cast iron, I think it’s the two main materials. For the same pipe size, they have different internal diameters, so that’ll impact how much water can flow into it.
I’m assuming a bigger internal diameter will allow more flow and a smaller internal diameter will have less flow.
Dan Cole: Yes.
Christoph Lohr: The other item you’re talking about is the roughness, so again, looking at PVC and cast iron, PVC — plastics being smoother, cast iron being rougher — the flow rates will change because rougher will have more friction, will reduce the flow rate, where smoother piping will have more flow rate. So that’s kind of the two main criteria where pipe material really has a big impact on the flow for a given pipe size. That’s correct?
Dan Cole: Just for an example, in the paper there’s Table 2.2 for vertical stacks and it compares PVC with cast iron.
Christoph Lohr: Let me go ahead and share that. I'll put it up on screen. Right there, is that the one you're talking about?
Dan Cole: Yes, so there's different annular ratios. The Wiley-Eaton paper suggests that you do not exceed one-third of the annular ratio for vertical stacks.
We looked at an annular ratio of a quarter and 7/24 and one-third just to see the variation of flows, but when we when we look at specifically between cast iron, PVC and especially the one-third annular ratio, we’re seeing, for example, let’s just look at 4-inch pipe. Four-inch pipe, PVC has a flow capacity of about 429 gallons per minute.
If you use cast iron, that’s reduced to 172.5. This is an example of unintended consequences. The plumbing codes typically choose one pipe material for their sizing charts — either cast iron or PVC — but they don’t tell you in the codes what pipe material those charts are based from. If you are sizing a system that uses PVC material for the sizing chart and you want to use cast-iron pipe as your material, then you’re allowing or estimating the capacity of 428 gallons per minute for 4-inch, when the true capacity is 172.5.
Christoph Lohr: Wow. And just to be clear, Dan, flow rate, flow capacity of a piping is just one of many considerations when a design professional or an installer is utilizing a material. Obviously there’s other concerns outside of that. But definitely understanding what that limit is is one very important part of that concept in terms of the overall design of a storm system and its impact on resiliency and public health and safety.
Dan Cole: Absolutely. And if we have charts and tables of different types of material that will show what the flow capacities are, then the designer can choose his material and know with confidence that, OK, this is a flow capacity of this material compared to this other material. And it may determine which pipe material he wants to choose for his project without running the risk of oversizing or undersizing his pipe.
Christoph Lohr: That makes sense; obviously that is very significant then. When you’re just looking at those values, the difference is, we’re talking, it seems like for some of these values between 20 to 50% going from one material to another in terms of potential undersizing or oversizing; undersizing in terms of flow capacities for a given pipe size.
Dan Cole: Yes, and vice versa. If the sizing table of the code is based on cast-iron pipe and you choose PVC pipe, then you may be oversizing your system and the cost expenditure would be more because the table is limited to cast iron, you want to use PVC, which allows a greater flow capacity, but the code is saying you’re stuck with this flow capacity for that pipe material.
Christoph Lohr: That makes sense then. In terms of storm piping, ultimately, if I understand the code correctly, storm piping, the entire purpose for it, is to take water off of a roof and bring it down below grade right to the city storm. My head is telling me that it's much, much better to be oversized than it is undersized in terms of the pipe size itself; you don’t want to be exceeding that capacity. Is that correct?
Dan Cole: Yeah. When you look at it from a safety perspective, absolutely. The more conservative, the better.
Christoph Lohr: And then I’m also thinking part of storm sizing — also from my days as a designer — I remember that we looked at tables for rainfall rates, the 100-year rainfall event. I know it’s a little bit off topic, but it does relate. We’re also seeing some changing water patterns too, or rain patterns. I was just looking up some information, some studies on how some rainfall rates are more intense in certain areas; not ubiquitously so, but certain areas are seeing higher-intensity rainfall rates. That could have a potential impact here too, I would think.
Dan Cole: It would. The paper doesn’t, like you said, the paper doesn’t discuss rainfall rates and whether or not they need updating. The Uniform Plumbing Code rainfall rates are based on a 100-year, one-hour rainfall mapping from it’s called Technical Paper Paper Number 40, which is Rainfall Frequency Atlas of the United States for Durations from 30 minutes to 24 hours and Return Periods of 1 to 100 years.
And that’s published by the U.S. Department of Commerce back in 1961. You’re right, we are seeing more intense, shorter-duration rainfalls; for example, five- to 10-minute downpour bursts with the potential to overwhelm rough drainage systems. It’s definitely worth looking into and maybe we’ll need for another white paper to consider updating rainfall rates in the codes, but that is definitely something to look at.
Christoph Lohr: I love that point, Dan. What level of confidence can our listeners and readers have in this white paper?
Dan Cole: Well, we had that concern too. We wanted to make sure that our computations were correct and what we're seeing, because it's quite significant, is some of these differences between materials can be significant. We sent this paper out for peer review and it was peer reviewed by 11 engineers, academicians, regulators.
Their names are included in the paper with acknowledgment and thanking them for their review. And furthermore, one peer reviewer submitted a code change based on this white paper.
Christoph Lohr: Excellent. What does that proposal look like?
Dan Cole: Well, what we’ve seen from the proposal is that more tables are being proposed for stormwater drainage, so there's four sizing tables for vertical rainwater conductors. There's four sizing tables for horizontal rainwater piping.
Each table is labeled for specific types of material and like I said earlier, this gives the designer greater latitude in choosing what type of material he would want based on the flow capacities that he's looking at based upon the roof design. And it gives him better confidence when choosing a pipe material that he knows what the flow capacity is of that particular material.
Christoph Lohr: Excellent. At the time of this recording, we’re still before the code hearings in San Diego, May 6 through 10. But this white paper will be coming out. How can someone obtain a copy of this white paper?
Dan Cole: It’ll soon be published. We’re just finalizing, finalizing some proofing, but then it will be available as a free download on IAPMO’s website.
Christoph Lohr: Excellent. As we’re kind of working, wrapping up the conversation here, Dan, one thing that I kind of want to circle back around to is different pipe materials that are selected by model codes can have an impact on the sizing tables. In the white paper itself, there’s really very limited conversation on one code versus another.
But what did you find in terms of model codes out there and what the base material is and what the impact was?
Dan Cole: We don’t compare codes in the white paper; we’re just comparing equations and so forth. But what I have observed is that the UPC tables are based on cast-iron pipe. Here you have a very conservative pipe-sizing table. Like you mentioned, Christoph, earlier, it is better to be more conservative than it is to be undersizing your pipes. Being based on cast-iron pipe material, if a designer wants to use PVC, it’s going to be bigger pipe — more than what’s needed — but it’ll be sufficient to clear the roof of the rainwater. On the other hand, I did see that the IPC, the International Plumbing Code, their tables are based on PVC pipe material.
There is the unintended consequence that I mentioned earlier, that if the designer is using cast-iron pipe and he’s using that table for PVC capacities, then that pipe is going to be way undersized and it may be detrimental to the system.
Christoph Lohr: If it’s udersized, ultimately roof ponding, more weight on the roof if the structural engineer didn’t account for that, that could be potentially some concerns in terms of resiliency, and even pipes bursting I’m assuming could be in play. Most storm piping, it’s not soldered together. It’s not fused together. They’re using the hubless couplings, I think is one of the most common, especially in cast-iron systems. PVC would be glued, I would think, but cast iron, what we’re talking about here, it’s typically a mechanical kind of coupling that’s holding it together with, those have a rating, I think, too. Safe systems, but you don’t exceed on too many occasions the capacities of those systems, I would think.
Dan Cole: Yeah, ponding is a huge issue for structural roof conditions. When I was a plumbing plans examiner, I saw plans come in with only primary drainage. The argument was that there was only a 2-inch lip on the roof and the water would spill over if it exceeded the capacity of the primary drains. So I sent it back to the structural engineer and I said, “Verify and provide proof that the roof structure can handle ponding and it’s sloped in the middle” so it would be more than two inches in the middle, that this structure would be able to handle the ponding up to sometimes six inches in the middle before it would overflow the ends of the roof. The plans came back with secondary drainage every time. Every time it came back with secondary drainage because the roof wasn’t able to handle that; water’s heavy and it’s not able to handle that kind of ponding.
So it is significant for the roof drainage system to keep up with the flow capacity to prevent ponding on that roof.
Christoph Lohr: Definitely. Well, Dan, this has been really informative. I have learned a lot from you and I think our listeners, I know, have learned a lot from you. The next time we have you on Dan; we had you first on for the Water Demand Calculator and now we’re having you on for storm sizing. What do you think we’re going to be talking about the next time we have you on the podcast?
Dan Cole: Oh, who knows? Drainage systems, rainwater tables, who knows?
Christoph Lohr: I love it, I love it. That’s a very coy answer. I love it, Dan. Well, on behalf of The Authority Podcast: Plumbing & Mechanical, and on behalf of IAPMO, Dan, I would say it was my absolute pleasure and privilege to have you on here with me again.
Thank you so much for your time and thanks for sharing your insights and expertise with our listeners.
Dan Cole: And likewise, enjoyed being here.
Christoph Lohr: Thanks for joining us on this week’s episode of The Authority Podcast: Plumbing & Mechanical. Love this episode of the podcast? Head over to iTunes to subscribe, rate and leave a review. Please follow us on Twitter @AuthorityPM; on Instagram @theauthoritypodcast; or email us at iapmo@iapmo.org. Join us next time for another episode of The Authority Podcast: Plumbing & Mechanical.
In the meantime, let’s work together to make our buildings more resilient and shape us for the better.
