A New, Balanced Main Drain Estimator [Tool]

Last week, I wrote about why estimating the flow through a main drain is more complex than just calculating the resistance of one open orifice to how much flow comes out.

The problem with simply using an open orifice is that we calculated the maximum possible flow from that opening. That was what I wanted in order to hand off a maximum possible flow for a plumbing designer to accommodate, but the maximum calculation is problematic if we want to estimate how much actual flow comes from a main drain.

In last week's comments, we shared different ideas and models too (thank you!).

Essentially, at least in theory, the flow from the open end of a main drain is restricted at the opening but also throttled by the pipe path along the main drain (including the length of pipe, friction, and any obstructions), the main drain valve, and the parameters of the riser.

Additionally, our riser gauge measures the normal pressure even when water flows. It's not a pitot gauge.

Considering that, I took the conceptual outline from last week and built an iterative tool that takes all the input information we need and estimates flow from a main drain.

What this does is take the main drain configuration, take the main drain residual pressure we get, assumes and loops a pressure balance, and turns out a theoretical flow from the main drain.

The caution here is that this is an estimation, and we haven't proven what input values are most-accurate from real-world tests. For instance - how much of the pipe is obstructed, on average? What c-factor best represents real-world conditions? What would an error analysis suggest about our range of possible flow?

All these can be tested and figured out in time, but in the meantime I wanted to offer up the first draft of the tool for your exploration and feedback:

Give it a spin, and let me know what you think.

If you find a bug, let me know and we can discuss improvements in the comments.

Thanks as always for being part of our community here! Hope you like this one.

​- Joe