Analysis of Sealing Force in a Bi-Directional Pressure Butterfly Valve
A bi-directional pressure butterfly valve is primarily composed of a valve body, a disc (or butterfly plate), a stem, and a sealing ring. The eccentric distance between the center of rotation of the stem and the sealing surface of the valve body plays a crucial role in achieving proper sealing. When the medium flows in the forward direction, the stem applies a necessary sealing force, FMF (in Newtons), to the disc’s sealing surface. However, due to the influence of the medium force, FMJ (also in Newtons), the actual sealing force at the interface increases to FMZZ. This means that the total sealing force FMZZ is the sum of the required sealing force FMF and the medium-induced force FMJ.
During reverse flow, the force exerted by the stem on the sealing surface is again the combination of FMF and FMJ. In this case, the force applied by the stem is greater than during the forward sealing process, which can impact the overall performance of the valve.
Analysis of the Impact of Stem Bending Deformation on Sealing
Since the stem experiences higher forces during reverse flow, it's essential to evaluate how bending deformation affects the sealing performance. When the medium flows in the opposite direction, the stem must overcome both the sealing force and the positive pressure from the fluid. This results in a simplified beam-like load distribution on the stem.
Under the combined effect of the sealing force and the medium pressure, the stem may bend. During forward flow, the stem bends toward the sealing surface, which enhances the sealing effect. However, during reverse flow, the stem bends away from the sealing surface, causing the disc to move away as well. This reduces the effective sealing force and compromises the seal. As a result, the valve may fail to maintain a reliable seal when the medium flows in the reverse direction. To address this issue, a new type of bi-directional metal-seated hard seal butterfly valve has been developed.
In this improved design, the upper and lower wedge shafts are positioned on the top and bottom of the valve chamber. The cross-sectional area of these wedges is increased, which raises the moment of inertia (I) and improves the rigidity of the stem. This reduces the deformation amount δ2 without significantly increasing fluid resistance. Additionally, if the clearance between the wedge shaft holes and the stem is equal to or larger than δ2, the deformation of the wedges will not cause the stem to bend.
The upper and lower wedges further push the stem, increasing the wedge force. This allows the disc to generate a stronger sealing force, compensating for any minor displacement caused by δ2. As a result, the reliability of the reverse-sealing performance is significantly improved.
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