Four Steps to Size Sanitary Sewers & Vents 🏢🔧
SYED SAAD ALI
👷♂️ Mechanical Design Engineer | Crafting Tomorrow's Buildings with Ingenuity and Insight 👨💻
Hello, Mechanical Engineers. As you are aware that sanitary sewers and vents play crucial roles in maintaining the quality of the indoor environment of a building. Correct sizing is essential for their effective functioning, ensuring that waste and venting systems operate smoothly. Fortunately, the International Plumbing Code (IPC) provides a set of charts that make it easier to size these systems accurately. Here’s a step-by-step guide to getting it right:
Step 1: Add Up the Total Number of Drainage Fixture Units (dfu) for Each Sanitary Branch
Start by determining the dfu for each sanitary branch. IPC Table 709.1 lists the dfu value as a load factor and the minimum size of the trap in common plumbing fixtures. For example, let’s assume you need to add a new restroom with 10 water closets and four lavatories to an existing building. Here’s how you calculate the total dfu:
Step 2: Determine the Sanitary Branch Sizes Using the dfu Values
Next, size the branch using IPC Table 710.1(2). You need to know the number of branch intervals tying into the same stack and use the pipe size with an allowable dfu value greater than the calculated dfu. In this example, the table shows that the minimum pipe diameter for a horizontal branch with 44 dfu is 4 inches. A useful tip is that water closets typically have 4-inch waste connections, so any branch with a water closet will be at least 4 inches.
Step 3: Calculate the Size of the Main Building Drain Using the Total dfu Values
To size the building sanitary main, use IPC Table 710.1(1). Determine the desired slope using the pipe size with an allowable dfu value greater than the total dfu, which is 44 in our example. According to the table, the minimum slope for a building drain with 44 dfu is 1/8 inch per foot of pipe, and the minimum pipe diameter at this slope is 4 inches.
Step 4: Size the Vents Using an IPC Table and the dfu Values
Finally, size the vent piping using IPC Table 916.1. You must know the total vent piping length to select the appropriate vent pipe size. In our example, let’s assume the restroom has its own vent to the roof with a total piping length of 25 feet. According to IPC Table 916.1, for a 4-inch waste stack with a dfu up to 140 and with 27 feet or less of vent piping length, the correct vent pipe size is 2 inches.
Conclusion :
Understanding how to use the IPC tables enables engineers to quickly determine the appropriate, code-approved sizes for sanitary mains and vents. Properly sizing these systems ensures efficient, effective, and compliant drainage and venting in any building project. Mastering this skill is essential for MEP engineers to deliver high-quality and reliable plumbing systems.
By following these four steps, you can confidently size sanitary sewers and vents, ensuring optimal performance and compliance with industry standards.
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