Fall-off pressure is the reduction of pressure at the outlet of a pressure reducing valve (PRV), when downstream fixtures are open to allow flow through the PRV. If, for example, a PRV is set at 70 psi, that is the static pressure setting and represents the PRV downstream pressure in a no-flow condition. As soon as fixtures start to open, water begins to flow through the PRV and the downstream pressure will fall. More open fixtures means more flow through the PRV, resulting in more fall-off pressure. The amount of fall-off pressure depends on the inlet pressure of the PRV, the size and design of the PRV and the flow rate through the valve (sum of all fixtures).
Fall-off pressure is an important consideration when sizing a PRV for a given application. A PRV that is undersized might have too much fall-off which will result in insufficient pressure at the fixtures and unhappy water users! Be sure to consider the fall-off pressure through the valve, and piping losses all the way to the farthest fixtures.
Most PRV manufacturers' literature have some type of chart or graphs showing fall-off pressure for their valves; some are hard to understand. Caleffi's 535H Series PRV literature makes sizing and selection very simple. Take a look at Technical Brochure 01265/16 NA; Graph 1 tells you at a glance which size valve to pick based on pipe velocity between 3 and 6 feet/second (recommended velocity). Graph 2 tells you what your fall-off pressure is based on the GPM flowing through the PRV.
Our Coffee with Caleffi webinar from July 2016, Applying Pressure Reducing Valves in Plumbing Systems featuring guest speaker Julius Ballanco, P.E., is all about PRVs; feel free to take a look at the webinar archive at your convenience.
We look forward to your comments, post one below, and contact us if you have any questions!