I’ve been poking my nose around Revit Forum a little more lately and a post in the MEP forum prompted me to write a little bit about hydraulic pipe sizing. I speak with a lot of guys in MEP locally and they give me the impression that they think pipe sizing is some kind of dark art, in actual fact it’s not that difficult at all.
In my response to the original post, I just created some generic plumbing fixtures to show the example, but I thought I’d take some time to go into a little more depth on sizing domestic hot and cold water services, my examples are going to be somewhat based around AS/NZS3500 Parts 1 and 4.
The Basics of Pipe Sizing
Before you begin, to perform pipe sizing for a plumbing system, you need to have a good set of families that have either flow rates or fixture unit ratings applied to the piping connectors. A good set of families are the building blocks of good pipe sizing results.
In the past I’ve mentioned that I have these basic families for plumbing fixtures that are simply place holders for the pipe connections. There is no need for elaborately modelled fixtures in an engineering model. If you like, you can even copy/monitor the architect’s fixtures and then map them inside your model to your engineering fixtures. The idea of these plumbing fixture families is explained really well in an AUGI article that Dave Benscoter wrote back in early 2013.
The placeholders for the piping connections are controlled by parameters for height, spacing, diameter and offset from the wall (for drainage). A lot of these adjustments may not be required on most jobs, but the ability to take the modelling further is there ready to go when needed. The family allows for automated tagging of fixture abbreviations, pipe sizing calculations and if you go to the effort of making a family type per specified fixture, you can generate accurate penetration drawings as well.
Another alternative that I have used is rather than model all the pipework up to and including the plumbing fixtures is that I will have an isolation valve box family where I fill out the fixture units of the fixtures it is serving. Just remember, if you’re doing this and you’re entering a flow rate it should be probable simultaneous flow – also called diversified flow – for which I usually rely on the Barrie Smith ‘Blue Book’.
If you’re using out of the box (OOTB) Revit families, check the loading units (which are called fixture units in Revit) and check that they match the requirements of AS/NZS3500.
In my example, the plumbing fixture connectors have their direction set to in and the hot water unit has the cold water set to in and the hot water set to out. The cold water inlet of the hot water unit also has an instance parameter where I can enter the downstream probable simultaneous flow.
You need to make sure that there are no open ends in your system, in 2014 and newer, you can use the Cap Open Ends tool if required.
The small box in my example is to cap off the end, it is set to out as it is essentially the water connection to the site.
For the purpose of the demonstration, I’ve created all the distribution pipework at 50mm. The connections on the basins are 15mm and the hot water unit I’ve used is a Rheem 613050 so both the inlet and outlet are 32mm.
For the cold inlet to the hot water unit, you can set it up to work on either fixture units or a litre per second rating. If using a litre per second flow rate, make sure that it is based on a probable simultaneous flow value.I’ve used probable simultaneous flow in my example, which based on the Barrie Smith Blue Book is 0.21l/s
Once you have all your pipework and fixtures connected, select the pipework that you want to size and you will see the Duct/Pipe Sizing icon on the ribbon.
The next step is to input the data of how you want to size your pipework.
The options available to you are to size by maximum velocity only, or you have the option to select either and/or maximum pipe friction in pascals per metre.
You can also set additional constraints using the drop down box, the options are calculated size only, match the connector size, or to use the larger of either the calculated or connector size.
Finally, you can also restrict the maximum pipe size that is allowed in your system.
I am simply going to size the piping based on a maximum velocity of 3.0m/s (AS/NZS 3500.1 Clause 3.4).
In the constraints section, restricting the sizing to that of the connector allows you to achieve the 15mm branch to a single fixture rule (AS/NZS 3500.1 Clause 3.5.2) however keep in mind that Revit will size the pipework as 15mm until the next branch (tee) fitting so if your fixture is further than 3m from a branch you will need to adjust the pipe size manually yourself. Revit will also oversize branches of pipework connected to a larger fitting as shown below with the HWU. You should always check your sizing results and never rely on Revit to do all the work for you.
As you can see, other than our oversized pipework from the HWU highlighted in red, the results are quite good.
You could argue that my 25mm incoming connection is too large to serve only 5 hand basins. The reason it sized to 25mm is because of my flow value on the cold water inlet to the HWU. If I change the cold water connection to use fixture units, and manually enter 5 fixture units, the incoming pipework is reduced to 20mm.
Analysing Your Pipework Velocities With Revit
You can create a view in Revit that analyses the velocities of your pipework (amongst other things). This can assist you when reviewing your pipework sizing with easy to follow graphical displays of your pipework velocities. You can find the Pipe Legend tool on the Analyze tab of the ribbon.
The results give an easy to follow display of your pipe velocities by colour, you can clearly see that the 32mm pipe from the hot water unit has a velocity of 0.9m/s
Manually adjusting the pipe size to 20mm, you can now see that the velocity of the pipe exiting the hot water unit is now 2.7m/s or changing to 25mm, the velocity is 1.5m/s. Based on velocity, either pipe size is acceptable under the Australian Standards.
Creating a Pressure Loss Report
Finally, you can also run a Pipe Pressure Loss Report which is found on the analyze tab.
The pressure loss report is generated in HTML format, I am simply going to select the cold water system only and I’m going to display the system information and the critical path.
The resulting report shows that we have 51.1kPa of pressure loss along the critical path. Referring back to AS/NZS3500.1 we require a minimum pressure at the outlet of 50kPa (Clause 3.3.2) which tells us that we need a minimum of 101.1kPa at our connection point.
Conversely because the maximum allowable static pressure at an outlet is 500kPa (Clause 3.3.4) if we had 600kpa available at the connection point we would need to install a pressure limiting valve to comply with Australian Standards.