Last week, I posted a Main Drain Estimator tool that runs a supply-side hydraulic calculation in loops until it balances. This tool is showing promise as being much more in line with expectations for estimating flow through a main drain.

Based on your feedback from this past week, I've updated the tool to incorporate three different main drain models that insurers have used to estimate main drain flow so that you can compare our results to others.

One important note: all other three models depend on using a 2-inch main drain. If the actual size is different, then the comparison is not applicable (results in N/A in our tool).

I ran an error analysis based on our method; the c-factor is the biggest driver of variability. Using a C-factor that's off by 10 of the real value would result in an error of about 7% of our predicted flow. It's not terrible, but accurate C-factor use without looking inside the pipe would naturally be a source of uncertainty here.

One direction I'd like to take next is to chart the main drain results against a prior flow test and/or against a hydraulic placard. We could add accommodation to account for underground losses when comparing a flow test at the street, and we would be able to represent the main drain test, the flow test, and the hydraulic placard based on riser data on the same chart. 

In theory, should the main drain be clear of obstruction, this could offer a very quick gut-check on whether the main drain results are within expectations from an original water supply (which is the purpose of a main drain test). It could also provide an initial flag if the water supply has degraded below the system design, which would warrant some further evaluation.

There's certainly plenty of nuance and debate here about NFPA 25 inspection and testing scope, but for practical purposes from a building owner or insurer's perspective, a quick-use utility tool that could help find those problematic properties could offer some benefit to the industry.

Check out our updated tool below, and feel free to comment on the updates or your thoughts. Thanks!

Last week, I wrote about why estimating the flow through a main drain is more complex than just calculating the resistance of one open orifice to how much flow comes out.

The problem with simply using an open orifice is that we calculated the maximum possible flow from that opening. That was what I wanted in order to hand off a maximum possible flow for a plumbing designer to accommodate, but the maximum calculation is problematic if we want to estimate how much actual flow comes from a main drain.

In last week's comments, we shared different ideas and models too (thank you!).

Essentially, at least in theory, the flow from the open end of a main drain is restricted at the opening but also throttled by the pipe path along the main drain (including the length of pipe, friction, and any obstructions), the main drain valve, and the parameters of the riser.

Additionally, our riser gauge measures the normal pressure even when water flows. It's not a pitot gauge.

Considering that, I took the conceptual outline from last week and built an iterative tool that takes all the input information we need and estimates flow from a main drain.

What this does is take the main drain configuration, take the main drain residual pressure we get, assumes and loops a pressure balance, and turns out a theoretical flow from the main drain.

The caution here is that this is an estimation, and we haven't proven what input values are most-accurate from real-world tests. For instance - how much of the pipe is obstructed, on average? What c-factor best represents real-world conditions? What would an error analysis suggest about our range of possible flow?

All these can be tested and figured out in time, but in the meantime I wanted to offer up the first draft of the tool for your exploration and feedback:

Give it a spin, and let me know what you think.

If you find a bug, let me know and we can discuss improvements in the comments.

Thanks as always for being part of our community here! Hope you like this one.

​- Joe

One of the curiosities I have every time I run a main drain or see one run is how much flow the system is actually discharging.

From the amount of discussion and inquiries one of our tools has generated, I know many of you are curious about it, too.
 
HOW MUCH DOES THE MAIN DRAIN ACTUALLY FLOW?
For one – if we knew with some certainty how much flow came through the main drain, then we could actually complete a backflow forward-flow test entirely just by opening up the main drain all the way.

That’s the theory, at least, that I’ve heard some people point to as to why they don’t provide another fixed means of forward flow.

For a lower hazard system; say a system whose greatest challenge is still Light Hazard – it’s not unfathomable that a fully-open 2-inch main drain could flow at least the system demand (which might be as low as 120 gpm for a minimum quick response (QR) reduction area and 30% overage, no hose allowance included).

Even for an Ordinary Hazard Group 2 system using a QR area reduction and 30% overage, the system flow may still be in the 220-250 gpm range. Would a fully open 2-inch main drain be enough to handle it?
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Or what if that main drain was upsized to 2-1/2 inches?
 
This piques the curiosity, right?
 
IS FORWARD-FLOW ACHIEVABLE THROUGH A MAIN DRAIN? IF SO, WHEN?
It would be very nice to have an idea if forward flow was achievable for some of these lighter-weight systems just through the main drain.
 
The Drain Flow Estimator

We introduced a tool we called the Drain Flow Estimator tool a while back (https://www.meyerfire.com/blog/a-new-fire-sprinkler-test-drain-flow-calculator), which was built to estimate the maximum possible flow from an inspector’s test or a main drain.

That tool only uses one calculation: discharging water through an open orifice:

Let’s say we have a very large water storage tank and poke a hole in the side of it near the bottom. How fast does water drain from the tank?

We have a formula for that. It’s Q = 29.84 C d^2 √p.

That is, we have a flow (gpm) that is constrained by the type of opening (C, the discharge coefficient), the diameter of the opening (d, in inches), and the total system pressure at the opening (p).

​We use this regularly when we conduct fire hydrant flow tests. The equation translates pitot pressure to how much flow comes out of the opening. We took measurement inaccuracy into account and built this out into its complete tool for converting pitot pressures to flows (https://www.meyerfire.com/blog/new-pitot-to-flow-rate-converter-with-precision).

THE PROBLEM
​Here’s the problem with using only that equation to estimate flow from a main drain – it’s the maximum possible flow.
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Now, it meets the need that we had for building the tool—to estimate the maximum possible flow so that we could size drains appropriately (hint: don’t run an inspector’s test or main drain to a janitor’s sink).

It serves its purpose of estimating the maximum possible flow.

However, the Drain Flow Estimator tool doesn’t provide a realistic amount of flow through the main drain or inspector’s test and drain, only the maximum.

That’s problematic if we want to know the actual flow through a Test and Drain or a Main Drain, as discussed earlier.

Why is it the maximum and not actual?

#1 PIPE CONSTRICTION
That is, we’re not accounting for the pipe's constriction between the opening and the riser, the friction loss within the riser itself, the loss through the elbows along that path, or the constriction at the valve opening.

#2 TYPE OF PRESSURE
Another thing we’re not really considering is the type of pressure that’s measured. When we take a pitot pressure measurement, we insert a tube into the centerline of the water flow.

The pressure measurement taken from a pitot gauge accounts for the static pressure of the water (the normal pressure that is exerted in all directions) and the velocity pressure caused by the forward motion of the water.

​That’s what a pitot gauge is measuring—the total pressure.

​A gauge on a riser does not measure total pressure; it measures normal pressure. That is, it doesn’t matter if the water is standing still or moving at 20 feet per second. The gauge is only measuring the pressure that runs perpendicular to the pipe in the normal direction.

#3 LOCATION OF PRESSURE
The last source of error is where the pressure is measured.

For a hydrant flow test, we measure the pitot pressure immediately after the hydrant opening. We use the formula and convert it to a flow, knowing the pressure right at that opening.

If we instead use this same formula for an open orifice but add a pressure upstream at the riser, then we’re using a higher pressure than what will be available downstream at the opening of the main drain.

If we want to know the maximum possible flow, that’s probably fine. That’s the extreme case.

But if we want to know the actual flow through the drain, then that’s problematic; it’s another source of error.
 
BUILD A TOOL THAT CAPTURES ACTUAL?
So, how would we construct a tool that estimates the actual flow through a main drain?

Well, in theory, we could work an iterative loop like this:

1. Assume a pitot pressure right at the opening of the main drain (the 45-degree elbow discharging to the outside, for instance).
2. Translate the pitot pressure to a flow based on the characteristics of the opening.
3. Knowing the flow through the opening, we could calculate the friction loss as we move upstream through the main drain.
   1. We will gain pressure as we move upstream, just as in a hydraulic calculation.
   2. This would require an estimated C-factor for the pipe, knowledge of how the main drain is routed, and information about the pressure loss at a main drain valve.
4. At the point where the drain connects to the riser, we could determine the pressure at that intersection, and we still know the flow.
5. We calculate the friction loss going upstream through the riser until we reach the riser pressure gauge.
   1. We would need to have an estimated C-Factor for the pipe and know the routing of the riser.
6. Now, we would have a total pressure where the gauge is located.
7. We also know the velocity of the water movement in the riser, which we can use to translate our total pressure into the normal pressure, which is what the gauge is reading.
8. We would compare the estimated normal pressure calculated to the actual normal pressure in our real-world test.
   1. If our estimated pressure at the riser is too high, we restart our loop and assume a lower initial pitot pressure.
   2. If our estimated pressure at the riser is too low, we restart our loop and assume a higher initial pitot pressure.
9. We iterate this loop until we’re close enough for our needed accuracy.

 
As a result of this process, we would have an iterated, balanced supply-side hydraulic calculation that estimates the flow coming through the main drain.
 
If you love the math or the theoretical exercise – weigh in on your take. Open to ideas on this.

DOWNSIDE & POTENTIAL MISUSE
Now that’s great Joe, so go ahead and build it (typed in sarcastic voice font).

We can build it (and probably will because I’m curious). If we do, I’d want to go out to a parking lot and validate this in the real world 30 different ways (looking at you Fire Sprinkler Podcast).

But beyond that, there’s a fundamental issue with a calculator like this – it’s still an estimated amount of flow based on a pressure measurement at the riser but with the flow coming out downstream some distance later.

AN ESTIMATE BUT NOT REALITY
It’s not a measure of the actual flow through the opening; it’s only a calculated estimate.

The downside of not being a measurement is that if there’s some wrong assumption—say a C-factor or number of elbows or whatnot—then we introduce inaccuracy.

But it's probably hard to detect. What if we have some type of pipe constriction that we can’t see from the outside? Say there’s a large rock or dirt buildup, or the coupon that was cut for the main drain tap is actually smaller than it should be.

That constriction would throttle the actual flow down but maintain the same or higher pressure upstream at the gauge.

That is – it would look like it’s flowing more water than it actually is.

The advantage of measuring the actual flow out of a main drain is that we know with some certainty what the flow is achieving and not an estimate.

Fundamentally, I know of two quick(er) ways to measure the flow, even for a main drain.

There’s the bucket test, in which you flow into a large 55-gallon drum and time how long it takes to fill it up. Divide your bucket size by the time it takes to fill up, and you then have your average flow.
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​Then there’s the pitot measurement. Connect a test hose with an adapter to the main drain, measure the pitot pressure, and convert it to a flow.

​Either way, you could measure the flow coming from the opening. That’s far more accurate, of course, than a tool that estimates and incorporates a handful of assumptions.
 
THE UTILITY?
Is this kind of tool, that provides a theoretical balanced supply-side flow with the supporting math and documentation, something that would be of interest?

Do you see the harm in having an estimate doing more harm than good here?

Do you ever use a main drain for forward flow on less hazardous systems, and if so, do you verify what that flow is?

Curious on your thoughts about this as a challenge in the lens of trying to create helpful resources and not circumvent or obstruct good practices.

As always, appreciate you being here and being part of the community.

​- Joe

Last week, we posted results on what sprinkler contractors need as part of a set of biddable construction documents.

One of the top needs that sprinkler contractors expressed was whether the owner had any insurance-driven criteria that applied to the project.

THE SURPRISE
This likely isn't a surprise if you've encountered it on a job: a building is designed, bid specs are applied, bids are collected, the contract is awarded, sprinkler shop drawings are created and submitted, and then out of the blue [BOOM!], a review comes in from FM Global.

FM Global? Did someone know that this was an FM Global job?

No discredit to the FM Global team whatsoever - they do an excellent job in establishing a higher level of excellence and have propelled our industry for years - but shouldn't we all have known that FM would be a part of the project from the beginning?

That answer, of course, is yes.

CHALLENGES WITH INSURER-DRIVEN CRITERIA
It can be tough to grapple with if you've been on a project where that's been a surprise. It can also put a building owner in a difficult position of mediating what their insurer wants them to provide against the increased cost of doing so via change order.

As we've discussed, delegated design is one key area where a consultant provides tremendous value in coordinating and pre-planning these asks well before bid day, which would create a smoother project experience. Instead, missing or ignoring insurance criteria altogether can set the project back in schedule and cost.

From the consultant's side - it's not always easy to get a straight answer from a building owner. 

VARIABILITY AMONG BUILDING OWNERS
Big developers or large corporate clients are often very informed on their design standards; they may even have a complete set of standards themselves ready to distribute.

Smaller or first-time building owners are often less likely to carry insurance criteria that stipulate much in terms of fire protection above code minimum. 

But what about the projects in between? 

What about the corporate client building in the area for the first time? The regional grocery chain? The distribution center? Mid to large retailers? Restaurants? Healthcare? Manufacturing? Hotels?

Anything in this range could carry insurance that mandates a standard above NFPA 13 in certain areas, including critical ones like sprinkler design criteria.

Just because an insurance company isn't FM Global doesn't mean that FM Global Standards don't apply; many other carriers could still follow FM Global criteria or even have a more comprehensive program like XL GAPs (or something similar).

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BEST PRACTICES FOR GETTING THE INSURANCE QUESTION ANSWERED
What I'm most curious about is what has been your most successful process for getting this information from a building owner.

As a consultant, I had my best luck when we'd have a design meeting, and the owner or owner's representative was in the room, and I could ask directly about any insurance mandates above code minimum.

The line I used often sounded like, "Do you carry any requirements or standards above code minimum, like FM Global?"

If the owner or owner's representative was familiar with FM Global, there'd usually be a quick yes and they could confirm fairly quickly.

Honestly, all other cases would get a blank stare. I'd explain that the insurance criteria above the code were not the norm but that we'd want to incorporate it if there were any that applied.

In most cases, this was enough information to work from, but I never liked the inexact nature of going by a mostly uninformed answer. I found it to at least elicit a response, unlike emails, which tended to never get returned, but still - there's got to be a better way.

THE BIGGER QUESTION: WHAT WORKS BEST FOR YOU?
From last week, we know that insurance criteria are a major factor in determining what should be in the bid documents.

So my question to the consultants here is: What have you found to be the best approach to getting this information from an owner?

What method has worked to (1) actually get a response from the right person and (2) get a response that's usually accurate?

Let us know in the comments here. I have my lame approach but I'd much rather collaborate and share ideas on what works so that, as a whole, we can do a better job of creating a quality set of bid documents.


Thanks for continuing to advocate for the industry. Hope you have a great rest of your week!

- Joe