Plumbing in Revit - Fixture Unit to Flow Conversion

[ceyhunyuzuak]

Hi all, hope you're doing well;

as an HVACR BIM Engineer I have pretty much never used plumbing discipline until several of days ago. 

recently as I didn't have much to do in terms of projects I wanted to go ahead and learn some plumbing. well, I went through the IPC documents, that Revit turned out to be strictly dedicated to, and figured Revit converts FU to Flow using the table below in the picture (the right one, that begins with 5FU).

I approached this rather arrogantly in the beginning and I thought I would come up with such a workaround that can be used to do not only automatic pipe sizing (without Dynamo involved) but also pressure loss calculations, therefore pump sizing. I failed horribly. the workaround I came up with needed another workarounds which also needed another workarounds some of which affects the first workaround in this sentence 😁

I've never felt more desperate in my professional life than Revit Plumbing discipline made me feel.

Is there any hope? or should I just give up?

TL;DR : Dear Autodesk, please let us do FU to Flow conversion.

[iainsavage]

You don’t need workarounds or Dynamo.

Revit will do it fine if the systems are connected properly.

If possible upload your project and I’ll take a look at it.

[ceyhunyuzuak]

it indeed does the work but it only takes the IPC as reference and as you can see in the picture it begins with 5 FU which is equivalent of 56.78 L/min. that means even though you have 0.5 FU you will get 56.78 L/min flow rate in the pipe whereas for other standards minimum flow rates can be as follows;

for other IPC table, that you can again see in the picture, >11.36 L/min

for UNI 9182 (systems with flush tanks) >18 L/min

for UNI 9182 (systems with flushometer valves) >108 L/min

for prEN 806 (systems with flush tanks) >6 L/min

for prEN 806 (systems with flushometer valves) >33 L/min

for Règles DTU 60.11 >6 L/min

for DIN 1988 teil 300 >30 L/min

for lower FUs, let's say for a urinal, if I want to use 3/4" pipe, 56.78 L/min flow will yield 36.6 kPa/m friction value. You can imagine how this would look in the pressure loss report 🙂 

You might say "then you use flow instead of FU". but in this case Revit will directly accumulate the flow rates rather than keeping them rates under a fashion of certain algorithm or tables.

[iainsavage]

I'll clarify my earlier comment - Revit will convert the fixture units to flows properly per the IPC method and it will automatically size pipes based on those flows.

Whether the IPC method is correct is a different debate.

In the case of your urinal, its not Revit/Autodesk's fault if that ridiculous value is the flowrate dictated by the IPC method. 

If you are suggesting that Revit should include different options for calculation methods, I agree, but it doesn't.

Similarly most other calculation methods are based on ASHRAE data, with no alternative.

Being in UK, my "workaround" is that I use the predominantly flush tank option because we don't normally use direct flush valves on WCs and the pipe sizing results generally seem to come out okay with that method.

[ceyhunyuzuak]

in my honest opinion it is quite understandable for Revit to stick to certain standards (mostly ASHRAE, as you also pointed out) when it comes to complex calculations such as heat gain/loss where you have literally tens of parameters that can greatly affect the outcome. But in this case what we have here is only 2 parameters, FU and Flow.

it's true that one can handle pipe sizing with ease, but in terms of generating an acceptable pressure loss report it turned out to be a disappointment for such a great software to fail to do such easy task.

[iainsavage]

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@ceyhunyuzuak wrote:

it turned out to be a disappointment for such a great software to fail to do such easy task.

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The software is not failing in the task. It is following the IPC calculation method. If there is any failure it is in the IPC method for not making allowances for branches with small flows on a system which has flush valves.

That said American urinals have monster flush valves on them (1” or 1.25”?) and the flow is probably the same as a WC in that case, so your example urinal doesn’t mirror American plumbing practice (which is what the IPC is for - ignore the “International” label)

In the past I used to use other calculation software such as Cymap and Hevacomp and they only used one calculation method as well, based on CIBSE. So I’d say it would be pretty unusual to get a software package which allowed selection of all possible design codes from around the world.

As I said, use the flush tank option and play about with the appliance FUs to get reasonable values in your system.

PS: the bigger question is why there are so many different probability methods and why they yield such different results - is water usage pattern really so different from country to country, or is there a flaw in the theory used to calculate probability?

[ceyhunyuzuak]

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@iainsavage wrote:

PS: the bigger question is why there are so many different probability methods and why they yield such different results - is water usage pattern really so different from country to country, or is there a flaw in the theory used to calculate probability?

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I know, right? 🙂

As I mentioned above plumbing discipline has rather recently got into my circle of interest but the first thing I noticed was that lack of correlation between different standards. I thought to myself like "maybe that depends on the behaviour of people who live in different countries, maybe a cultural thing, even."

You are right. Revit is doing what it's coded with a great success. what has been coded, on the other hand, is another story.