Bronze & Iron Valves - rymca.com

AHEAD OF THE FLOW ®www.nibco.comRevision 6/1/03Sizing the AutomaticStop-Check F-869It has been said that the automatic stop-check is the mostsophisticated valve in the NIBCO line. One reason for thisstatement is the procedure required to select the proper sizestop-check for a given installation.The obvious way to determine valve size is to match the pipediameter. For the stop-check this is not correct. In many boilerinstallations, a valve smaller than pipe size could and shouldbe selected. There are cases where a 3" stop-check should beused on an 8" header.Size specification of the stop-check used on the boiler systemis usually done by the architect or engineer, but it should bepossible for the NIBCO salesman to assist the wholesaler orcontractor in selecting the right valve if there is some doubt.In order to size a stop-check the technical information neededis saturated steam gage pressure in pounds per square inchand actual required steam volume flow in pounds per hour.This information is then applied to the flow rates on the followingchart to determine proper valve size.Another technical question that comes up regarding our automaticstop-check is what pressure is required to openthe disc on various sizes?Due to the design of the valves, all sizes open with a .76 psidifferential across the seat, providing the valve is properlyinstalled (handwheel is in the open position), the valve hasnot been damaged (for instance, the pilot on the disc has notbeen bent) and no foreign particles from the line have causedthe working parts to become inoperative.In the event that a boiler does shut down the stop-check mustprevent backflow into the boiler, the weight of the disc anddash pot piston, through the force of gravity, promote rapidsealing so steam and condensate cannot backflow into theboiler even at low pressures. The weight of the disc/piston isvery helpful, especially in low pressure systems. In the NIBCOdesign stop-checks, you will note that the disc is mounted topermit the disc to settle into the seat area for positive sealingin case there is any misalignment due to assembly or unevenheat expansion. This feature also permits field repair of thevalve seat without fear of misassembly by field mechanics.If you have questions on this or any other NIBCO valve, callNIBCO Technical Service at:1-888-446-4226The line pressure required to open the valve is a function ofthe disc/piston weight and the size of the port diameter. Forinstance, on a 6" valve the weight of the piston/disc is 21.5pounds. The diameter of the opening is 6" or an area of 28.2square inches. Therefore, the minimum pressure required toopen the valve, disregarding friction which is minimal, wouldbe:21.5 pounds for weight of disc/piston------------------------------------------------------------------ = .76 psig28.2 square inchesNIBCO INC. WORLD HEADQUARTERS • 1516 MIDDLEBURY ST. • ELKHART, IN 46516-4740 • USA • PH: 1.800.234.0227TECH SERVICES PH: 1.888.446.4226 • FAX: 1.888.234.0557 • INTERNATIONAL OFFICE PH: +1.574.295.3327 • FAX: +1.574.295.3455www.nibco.com59