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Plumbing Feature of the Month: Pipe Sizing

Monday, January 10th, 2011

For any type of plumbing or piping system, there are two steps to sizing the pipes. First, you have to calculate the flow in each pipe section. Second, you have to take that flow and convert it into a size. In Design Master Plumbing, we made the decision for our initial release to automate the first step, but not the second. Calculating flow is a perfect task for a computer: a simple calculation you have to perform a large number of times. Sizing the pipe based upon the flow is more difficult for a computer, because determining which sizing table to use is so complicated.

To calculate the flow, the flow value at each fixture must be defined, similar to setting the airflow in CFM at each diffuser in Design Master HVAC. The flow values are then added up through the pipe system back to the source. You can view the flow values either by displaying them on the pipe labels or by querying the pipes. With the flow values calculated for you, sizing the pipes by hand is a relatively quick process.

One feature of our flow calculations that is often overlooked is the possibility of having multiple different flow categories for a pipe. This is important for handling diversity in some plumbing sizing codes.

Consider how water supply pipes are sized in the Uniform Plumbing Code. Most fixtures are assigned a specific fixture unit (FU) value. The flow in a pipe is the sum of the FU’s of all the connected fixtures. However, the flow to some fixtures depends upon the number of fixtures connected. For example, the flow for a single hose bib is 2.5 FU. Each additional hose bib is 1 FU.

Using flow categories, you can create a “Hose Bibb” category. Each hose bib you insert is assigned a value of 1. When you size your supply pipes, you know the number of hose bibs connected to each pipe. If there is 1 hose bib, you add 2.5 FU to any other flow that is in the pipe. If there are 2 hose bibs, you add 3.5 FU (2.5 for the first and 1 FU for the second).

In a future release, Design Master Plumbing will be able to size pipes automatically. Until then, you will have to size the pipes manually based upon these flow values. But calculating the flow automatically, and updating the flow automatically when the design changes, makes the sizing significantly faster and reduces the likelihood of errors.