#PRV There are a few different schools of thought when it comes to sizing PRVs and specifically servicing High-Low demands in water delivery systems. My preferred method is to start with the end in mind. Many PRV systems that I see are over sized. Some feel that you are always safe going line-size. However, if your system requires 375 GPM, but your line size would dictate a PRV that would deliver 1000 GPM, then what’s the problem? We have the requirements more than covered right? ... Wrong.
In my experience, this philosophy causes a host of problems, most notably: exaggerated costs for equipment that is oversized, the potential for harmful magnified cavitation in the oversized valves, and extreme wear and tear on a system that will exhibit a condition known as wire-draw, present when you have too low of a flow of water squeezing through a valve intended for a bigger purpose, adding appreciable noise to the system.
When given the opportunity to size a PRV or a High-Low system, I will start by understanding the actual flow requirements, often leading to a pressure reduction system smaller than the line size. All PRVs will exhibit a degree of fall-off. With Automatic Control Valves (ACVs) that fall-off can be minimal (mostly around 5 PSI). The pressure drop is required to initiate flow in the valve. Direct acting PRVs are a little different and the valve will exhibit greater pressure drop proportionally to system flow.
ACVs will flow extremely high volumes of water. For example, our Wilkins ZW209 in 4” will comfortably flow 800 GPM with little pressure fluctuation. However, with the exceptional flow rates that valves like this typically have, they also have a minimum flow characteristic - meaning that we recommend that a minimum amount of water pass through the valve to prevent the inherent problems we discussed earlier referring to oversized valves (wire draw, noise etc…) To meet the minimum flow characteristics of the larger ACV, I feel the best practice is to bypass that larger valve with a Direct Acting PRV. This is where we want the pressure drop (fall-off)
Using the example before with the 4” ZW209 (ACV) with a peak flow of 800 GPM, it would have a minimum flow of 50 GPM. So to properly size the bypass valve, we want to select a direct acting PRV with a 5 psi fall off when its flow is equal to the minimum flow of the ACV. The Wilkins 500XL in 2” can easily flow 100+ GPM. but if we use it to bypass the ACV and set the static pressure 5 PSI higher than the ACV. That means it will open first when there is downstream pressure drop (demand). It will flow the first 50 GPM and at that time, it will lose 5 PSI. When system flow requirements exceed 50 GPM, the larger ACV will seamlessly kick in, cushioned by the smaller valve for consistent pressure and flow.
For more information or assistance in sizing a high-low PRV system in Alberta or BC, Canada, please feel free to contact me.
Dan Cramer: email@example.com
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