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By Mark Graf, Derek Main and Robert Linzmeier
Depending on how advanced your utilization of Building Information Modeling (BIM) systems is, you may or may not have used Revit Systems to help calculate and size plumbing systems. This article looks at the requirements to correctly setup, implement, and, most importantly, generate actual calculations that can be used to confirm and size plumbing piping systems.
For the purpose of simplicity, this article focuses solely on domestic water piping with calculation ability in Revit 2014. The domestic water piping system is more advanced than sanitary waste and vent, storm, natural gas, and specialty systems that have some calculation ability but are not currently 100 percent developed for our specific use.
So what is a Revit System?
Simply defined, Revit Systems are:
“Pre-defined families specific to a Revit project with existing parametric, graphical, and documentation requirements. These systems cannot be created in, deleted from, loaded into, or saved out of a current project.”
Revit Systems have been around for quite a while (my first experience was Revit MEP 2008), but never easy to use correctly. Earlier versions of the systems were limiting as to what types of systems could be created and lacked piping systems functionality. In Revit MEP 2012, functionality increased with the ability to create additional systems, much improved operability when creating and connecting systems, and greatly enhanced calculation options and procedures.
Systems in Revit MEP 2012 gave the user the ability to differentiate and filter various pipe types and hold information such as fixture units, pressure loss, flow, etc. while allowing the use of Revit’s sizing calculator. More advanced parameters, such as fluid type, temperature, flow conversion method, etc., could now be incorporated. Subsequent versions of Revit 2013 and 2014 have had steady improvements to system calculations focused on expanding formulas and ease of use.
Creating a system for calculation
Rather than give you a step-by-step account of system creation, I will point out a few important steps we have found to make the process easier.
Make sure to keep track of the systems when creating them. When modeling a new segment of pipe, Revit assumes you are creating a new system and if you don’t assign these segments to a previous or “master” system, you will end up with many systems, which will prevent you from using the calculation procedures to their full capabilities. We found it beneficial to have only one system for each pipe type (i.e. cold water, hot water, hot water return).
Fixtures are not assigned or connected to a system until you pipe from them.
Abbreviations and other graphic overrides can be assigned to systems to help with identifying and labeling.
It is also important to understand the calculation process for the piping systems previously created. The first step is to ensure the system is complete and not disconnected anywhere. You can do this by hovering over a pipe in the water system and pressing the “tab” key until the entire system is selected or by selecting the system through the system browser. If there is a disconnect in the system, or pipe segments are on the incorrect system, the software will not calculate the flow correctly. Often, this results in a calculated flow of zero and users must go back and fix the disconnect. There is also a “Show Disconnects” button that can be used.
In order to use the calculator, you must select the desired piping and then find the “Duct/Pipe Sizing” button under the “Modify/Pipes and Analysi”s tab. This will generate a new window asking for various parameters that future calculations will be based on. Revit allows you to calculate based on velocity, friction or both (keep in mind friction loss is built into the individual pipe material properties). There is also a portion of the window that allows the user the ability to determine how the branches are calculated. Choosing the “match connector size” option seems to work the best as the “calculated and larger of calculated and connector” option provides mixed results.
After the sizing method and parameters have been input, click “OK” and Revit will size the piping. Note that Revit will use the pipe size it thinks is needed, but pipe sizes for any segment can always be manually changed if the user desires. This allows for flexibility that may need to be built into any given project, such as future expansion, unknown loads, etc. Revised sizing due to plan or material changes are also very easily implemented.
Those steps will almost get you accurately sized piping systems. However, there are a few things left to consider.
The first caveat we have discovered with the process is how the system calculates small branches on certain systems. Domestic water piping systems in Revit are based on Water Supply Fixture Units (WSFU) and either predominantly flush valves or predominately flush tanks are used to size a system, just as you would manually size a domestic system. When using the calculations to size piping, users must assign one or the other to a system and use it for the entire system.
It is imperative to select one way or the other and not a combination of both on a given system to ensure you obtain accurate branch pipe sizes. For example, the cold water system contains two sets of two back-to-back public lavatories fed together which would account for 1.5 WSFU per fixture or 6 WSFU total. Knowing your project specifications call for 0.5 gpm lavatory faucets you would end up with a total of 2.0 gpm of flow required. However if you are using the predominately flush valve sizing method for the rest of the cold water system, which contains water closets and urinals, the software will calculate 6 WSFU as 23 gpm, resulting in a size of the branch piping in the range of 1 inch to 1-1/2 inches when real world experience states that only a ¾ inch pipe at most is needed. It’s not a function of the calculator giving the wrong output; it is merely a lack of the sizing calculator having the flexibility to account for two separate sizing methods for a single system. This would be an example of where the user would be required to review the sizing and manually update where needed.
The second caveat is that in no way does the software take into account project specific available water supply pressure when calculating piping systems. As we all know, pipe sizing is a direct result of many factors including friction head loss, density of fluid, coefficient of friction, length of pipe, diameter of pipe, velocity, gravitational acceleration, and pressure. While the software does allow the user to account for some of these variables, it fails to account for pressure. While the sizing outputs are able to be generated to a close proximity, the lack of ability to input project specific pressure(s) will always result in a fundamentally incorrectly sized water piping system. Additionally, pressure loss in fittings, valves, and equipment is taken into consideration in the majority of Revit families and systems, but many times specialty families of equipment do not include pre-determined pressure losses. The user must be aware the families contain the correct pressure loss figures so the system calculations can account for this. Hopefully, future versions of the software will add in pressure requirements/availability consideration.
Even with all these software improvements, we should never forget they are strongest when combined with an organized approach to laying out, managing, and checking work by hand, or with Microsoft Excel. As with any computer based calculation software, users will always find themselves in a jam if they rely entirely on its output and don’t do a bit of old fashioned engineering to rest assured projects are being designed correctly. Specific code requirements should always be implemented as well when using systems. As we all know, different versions of codes don’t always rely on sizing requirements that have the same loads and this is where it is important that the connectors are updated with the correct values as required.
Our initial use of systems and calculation tools in the latest versions of Revit has absolutely resulted in a much more efficient and streamlined process for designing piping systems. It’s a platform that enables us to ensure the front-end work is done to determine final outcomes before the first pipe is modeled. Our team has also been able to promote a more consistent project design with the use of BIM systems and calculations across offices. BIM systems also make it easier for QA/QC reviews to take place as reviewers are able to follow and verify the validity of systems and sizing calculations. Acknowledging the few shortcomings outlined above, our team is confident the software will continue to evolve to meet the needs of plumbing engineers and designers.
There probably never should come a day when we exclude the designer from the project entirely, but these systems and automated calculations continue to evolve and strengthen our use of BIM.
Mark Graf, CPD, CDT is senior associate with CannonDesign.