1" Npt Relief Valve 10-20 Psi Spring The Type P289 Back Pressure Regulator Functions As A High Flow Relief Valve With An Adjustable Set Point. The P289 Can Be Used In Place Of A Standard Relief Valve To Provide Protection Against Over Pressurization In The Downstream System. The Design Of A Large Diaphragm Area And A Pitot Tube Booster Allow The Valve To Respond Quickly And Relieve The Excessive Pressure Smoothly, Especially In Low-Pressure Settings. Exact Replacement For Fisher 289/ Fisher 289H/