NIBCO Check Valve Automatic Stop F869B - Manual (Page 13)

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13
Sizing the NIBCO F-869-B Automatic Stop-Check Valve
It has been said that the automatic stop-check valve is the most sophisticated valve in the
NIBCO line. One reason for this statement is the procedure required to select the proper size
stop-check valve for a given installation.
The obvious way to determine valve size is to match the pipe diameter. For the stop-check
valve this is not correct. In many boiler installations a valve smaller than pipe size could and
should be selected. There are cases where a 3” stop-check valve should be used on a 8”
header.
Size specification of the stop-check valve used on the boiler system is usually done by the
architect or engineer, but it should be possible for the NIBCO salesman or technical services to
assist the wholesaler or contractor in selecting the right valve if there is some doubt.
In order to size a stop-check valve, the technical information needed is saturated steam
pressure, maximum and minimum required steam volume flow in pounds per hour. This
information is then applied to the flow rates on the chart attached to determine proper valve
size.
Boiler “turn down ratio” must also be considered. If the turn down ratio is 4:1, this means the
boiler maximum output can be reduced to 25%.
Another technical question that comes up regarding our automatic stop-check valve is what
pressure is required to open the disc on various sizes?
The line pressure required to open the valve is a function of the disc and dashpot piston weight
and the size of the port diameter. For instance, on a 6” valve the weight of the piston is 7
pounds and the disc weighs 12-1/2 pounds for a combined weight of 19-1/2 pounds. The
diameter of the opening is 6” or an area of 28.2 inches squared. Therefore, the minimum
pressure required to open the valve, disregarding friction which is minimal, would be:
19.5 pounds for weight of piston = .7 PSI
28.2 inches squared
Due to the design of the valves, all sizes open with a .7 PSI differential across the seat,
providing the valve is properly installed (handwheel is in the open position), the valve has not
been damaged (for instance, the pilot on the disc has not been bent) and no foreign particles
from the line have cause the working parts to become inoperative.
In the event that a boiler does shut down, the stop-check valve must prevent backflow into the
boiler. The weight of the disc-piston, through the force of gravity, promotes rapid sealing so
steam and condensation cannot backflow into the boiler even at low pressures. The weight of
the disc and piston is very helpful, especially in low pressure systems. In the NIBCO stop-check
valves, you will note that the disc is mounted to permit the disc to settle into the seat area for
positive sealing in case there is any misalignment due to assembly or uneven heat expansion.
This feature also permits field repair of the valve seat without fear of improper assembly by field
mechanics.