Water Wednesday - Hydraulics

Category: Engineering

Written By: Keith Rutherford

Date: May 5, 2021

Water Wednesday - Hydraulics cover image

Temperatures are starting to climb, and it is time to get those sprinkler systems back in order. This Water Wednesday will tackle some hydraulic truths.

I was standing in the checkout line at one of the big-box DIY stores with some 1” diameter PVC in my cart when a curious customer asked what I was doing with such large pipe. I explained to him that the water pressure at my house is not great, so I wanted to be sure I could get the most pressure and flow possible to my sprinklers. He proceeded to explain to me that I needed smaller pipe to accomplish this, not the larger pipe. He used the example of putting a nozzle on the end of a hose and how the water sprayed much farther through the smaller hole than it did through the open end of the hose. His logic made sense and I understood how he could make the connection of smaller pipe equaling higher pressure. Of course, I knew the pressure loss in a smaller pipe is higher than in a larger pipe and could prove it to him mathematically but there was not time standing in the check-out line.

I explained that even though it sprayed farther, the flow rate was lower and asked if he had ever noticed that filling a bucket was faster with an open hose than through a nozzle. He insisted it was faster with the nozzle. The conversation ended when it was his turn to check out.

Here is the hydraulic proof in layman’s terms that larger pipes can flow more water and maintain higher pressures:

1. To get the same amount of water through a smaller pipe, the water must flow faster (higher velocity). Think about rush hour traffic and a two-lane road vs. rush hour traffic on a one-lane road. To get the same number of cars through in a specific period of time, the cars on the one-lane road need to travel faster. The one-lane traffic will likely move slower than it needs to make the quota of cars in the same time as the two-lane road. This is the pressure loss.

2. The pressure loss is calculated using the velocity of the water through the pipe, squared. You can see in the graph how the multiplier for pressure loss increased rapidly at higher velocities.

Armed with this knowledge you can identify the options for achieving a greener lawn (without AstroTurf).

1. Use larger pipe to get the same amount of traffic through at a lower speed. Higher flow rate with less pressure loss means more sprinkler heads on the zone.

2. Use smaller pipe and get fewer cars through the pipe. This means fewer sprinklers per zone in order to maintain adequate pressure.

At some point the larger pipe stops making a difference because the square of the velocity is small. 

Designing pipe networks with a lower velocity will create a more efficient system. In pumped systems, minimizing pressure loss mean lower horsepower pumps and a lower electric bill. You only pay for the pipe size once, but you pay for the electricity for the higher horsepower pump forever.