Wednesday, April 15, 2009

Revit MEP (Plumbing) Piping Flow: another second class citizen

Warning: What follows is a rant fest filled with WAY more information about MY ideas of how both plumbing design and Revit MEP work. Any deviation from reality on either subject can wholly be attributed to a 14 month old daughter, a pregnant wife and a looming Revit configuration deadline. Please excuse my definite lack of plumbing design knowledge. See one, do one, teach one, I always say.

Revit MEP is ALL about propagating information throughout the model.

Spaces can be assigned a required Air Flow value.
Air Terminals know what space they are in.
Spaces know how much Air Flow is coming from the Air Terminals it contains.
Duct connected to an air terminal knows how much air is flowing through it (based on what the terminal is providing to the room.)
Variable Speed Fans (VSF) know the SUM of the flow from all the terminals connected to it downstream (Revit is doing the math on the fly.)
And so on and so forth all the way back to the Roof Top Unit (RTU) that SUMS all the VSF (and therefore all the terminals) that are connected to it.

We can tag and schedule this information fairly easily. And, it's used in sizing calculations.

Sounds keen.

Enter Plumbing pipe systems. (Cue ominous musical score.)

On the surface they work similarly to HVAC duct systems. But any plumbing designer will tell you (you have to listen closely they probably won't repeat it) that flow through a plumbing systems is different than duct systems. It's even different than flow through hydronic, or process systems.

The difference is DIVERSITY. No we're not talking about multiculturalism. Simply put, a system that contains 1 watercloset (WC) (that's toilet to the rest of us) has a 100% chance that all WC will be flushed at exactly the same time. Therefore the system is designed to accommodate the flow of that WC. A system with 100 WC has about a 0% chance that ALL will be flushed at the same time (aside from High School pranks.) Plumbing designers use a diversity factor along with Fixture Units (FU) to determine the most likely (design) flow in their systems. FU are assigned to fixtures (WC, sinks, etc) by Authorities Having Jurisdiction (AHJ(why is it always a TLA(Three Letter Acronym?))) so they can vary by locale. In short the designer totals the FU on a system and applies the diversity factor (which they get from a table) for the number of actual fixtures and that tells them the flow they need and therefore the pipe size. Whew! (Did I really have a 3rd level parenthetical statement back at AHJ?)

Why does all this matter? Well, for one, Revit doesn't get to look at that diversity factor table when it's computing flow. I can't even imagine the math used to derive flow from FU without it. All in all Revit does a darn good job of computing that flow and reporting it out to us. Connect a bunch of sinks to a pipe that connects to a water heater and the pipes will total the FU (and the resultant flow) from however many it is actually connected to.

Now the rub. The pipe that connects to the equipment (water heater) reports it's FU and flow fine, but NEITHER THE FU OR THE FLOW propagates to the equipment. You can create family or shared parameters based on both FU and flow. However, when mapped to the same parameters on the family's connectors (similar to how you cause Air Flow to propagate through VSF) you receive NO report. And, it will not propagate upstream.

At least I can't get it to work (and I tried every combination of connector settings related to those two parameters.) After talking with an Autodesk rep, it came to my attention that Revit MEP wasn't designed to pass flow through equipment in plumbing systems (where flow is calculated from FU.) So that means, I can't schedule equipment (like water heaters or softeners) and have the schedule show the actual flow demand on the unit (based on the system it is connected to.)

A bug? Unforeseen functionality need? I think it's just that the Revit code jockeys took it the wrong way when they were told to work on the FU systems...

BTW, this has been logged as a functionality request to the development team. Here's to making plumbing flow as useful as it's first class cousins Air Flow and (hydronic) piping flow. :)


Dave Baldacchino said...

Great post Erik. I've heard that plumbing didn't work as engineers want it and now I understand why better.

Erik said...

I think that the Terminus (air terminal,plumbing fixture, power receptacle) to Source (RTU,Water heater, Panel) flow that Revit MEP "likes" (and is recommended by Autodesk) doesn't work that great for engineers either.

At a schematic level they want to do some rule of thumb design and get place holders in the model to allocate space and convey intent.

Revit MEP starts to bog down and balk when you get too many objects in the model with connectors on default systems.

That, and modeling (or heaven forbid, changing) sloped piping sucks! :)

paul said...

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Duckman said...

What you said is true. However REVIT does not take into consideration many things when it "sizes" a pipe. The fixture unit method is fine when it comes to small, average building types but does not take into consideration such things as building type (an office building demand curve is much different than a sports stadium demand curve), pressure drop (elevation and friction loss), etc.

Pressure pipe sizing is much more complicated than I can explain here, just let it suffice to say that I wouldn't trust REVIT to size a pipe. Lawsuits are not on my wish list.