Design Features & Benefits
Crane Nuclear Two-Piece Tilting Disc Check Valve – Principle of OperationThe core of a CNI Two-Piece Tilting Disc Check Valve consists of a pivoting circular disc secured within a cylindrical housing. The pivots are located just above the center of the disc, offset from the plane of the body seat. This design imparts a bell-crank action to the movement of the disc, allowing the disc to clear the seat as it rotates. The seat is a circular bevel type and the disc moves in and out of contact with no sliding or mutual wearing of the seat and disc edge. The disc seat lifts away from the body seat while opening, and drops back into contact as it closes, with no sliding or mutual wearing of the seat and disc edge. A clearance within the pivot mechanism allows the disc (when closed) to make contact with the body seat, producing excellent sealing.
Design and constructionThe design and construction of this valve results in four principal advantages over standard type swing check valves, namely:
- Closing without “slam”
- Reduced head loss
- Minimized wear of moving parts
- Improved Tightness
Closing without “slam”
The CNI Two-Piece Tilting Disc Check Valve will close without slamming when installed with normal piping arrangements. The special design features which limit slamming are as follows:
The angle of the seat is so inclined that the distance the disc must travel from its wide open position to its seat is reduced to the minimum consistent with overall design objectives, thereby enabling fast closing before reversal of flow can occur. The disc is pivoted just above its center, providing a near balance between the upper and the lower “halves” of the disc. This dampens the movement of the disc toward the seat as the action of the flow against the top of the disc partially offsets the force against the bottom. The seat and disc rings are beveled and the disc drops into the seat in the body as it closes, further cushioning any impact. Unlike the action of an ordinary swing check valve, the disc reaches its seat at the time of zero velocity in the line. This prevents slamming which can cause severe systemic problems due to the ensuing vibration of pipelines and connected equipment, including a potential rupture of pipe joints.
Reduced head loss
A very important advantage of the CNI Tilting Disc Check Valve is the mitigation of pressure loss that in other valves is attributable to friction. The reasons for this are threefold: Due to being hinged just above its center of gravity, friction at pivoting surfaces is minimized. The lightweight disc’s “Aerofoil” shape provides additional lift and negligible drag on flow (of medium) when in the open position. The body is streamlined with no pockets or obstructions to perturb smoothness of flow, and at the seat it is enlarged by just the right amount needed to compensate for any reduction in volume of flow that might otherwise be caused by the disc. With the disc in full open position, the pressure drop across the CNI Two-Piece Tilting Disc Check Valve will be approximately 0.3 times the velocity head. This constant will generally hold for either liquids or gases. This value applies to steel valves having a stop angle of 5°. The CNI Two-Piece Tilting Disc Check Valve will be fully open at velocity heads of approximately 0.25 psi. To maintain disc stability, velocity heads for gaseous flow should not be less than 0.03 psi. Reducing the size of the tilting disc should be considered at these values. A test on a 22 inch CNI Two-Piece Tilting Disc Check Valve under 450 pounds gas pressure, carrying 3,200,000 feet of gas per hour, showed a drop in pressure of slightly less than 7 inches of water. This kind of performance results in substantial savings in the cost of power for pumping.
Minimum wear of moving parts
CNI Two-Piece Tilting Disc Check Valves are extremely simple in design and construction. They have only one moving part - the disc, and as was detailed above (under Principle of Operation), the mechanism of its motion practically eliminates physical wear: The pivot pins and bushings are made of bronze or specially hardened and corrosion resisting alloys, practically impervious to wear.
The CNI Two-Piece Tilting Disc Check Valve pivot pins allow the disc ring to make a complete seating contact with the body ring without sliding or rubbing, as the seating surfaces of these rings are beveled and very precisely machined. This minimizes the possibility of wear and potential leakage after a long period of service, while maintaining excellent sealing under all pressures.
Valve line placement
The CNI Two-Piece Tilting Disc Check Valve operates equally well in either a horizontal or a vertical position, provided the flow is upward if vertically oriented (downward flow would neutralize its disc balancing characteristics). The valve must be positioned not less than ten (10) pipe diameters (of straight pipe) apart from the discharge of a pump, and not less than five (5) pipe diameters (of straight pipe) away from a fitting, pump inlet, or other line disruption.