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This feature involves an anomaly found in the Valve Signature curve of a Fisher 10" design 8510 butterfly valve with metal seal and a size 70 Type 1052 spring and diaphragm actuator. The 8510 is an "eccentric" design butterfly valve. This means that the centerline of the disk is offset from the centerline of the valve body, thus allowing a camming action as the disc swings open and closed. Because of this offset, the disc only makes seal contact during approximately the first 5º rotation (from the closed position) and thus minimizes seal wear. To better understand this, let’s take a look at what a typical Valve Signature should be for this design.
Notice the bulge over the first 5º rotation. This is normal and represents an increase in friction (torque) as the disk rotates into the seal. The minimal friction seen throughout the remainder of the travel is associated with the packing and to a lesser degree, the bearings. The friction value should remain somewhat constant throughout this portion of the stroke. Now let’s take a look at the signature that is associated with this case study.
Notice that a bulge had occurred at approximately 20º rotation. In appearance it looks similar to the profile of a "snake that had swallowed a pig". This is an indication that seal contact had been made at 20º as apposed to the normal 5º position. The decay in friction after the bulge indicates that the disc had passed through the seal. Now what could have caused this? Keep in mind that the 8510 utilizes a lever mechanism to transfer the linear motion of the actuator to the rotary motion of the valve. The lever is connected to the valve shaft via a spline connection. The shaft for a 10" 8510 contains 24 splines. Doing the math, we find that each spline position represents 15º rotation (360º / 24 = 15º). In this case, it appears as though the lever had been errantly installed on the shaft one spline off during valve maintenance. Being off by one spline in the counter clockwise direction would cause seal contact to occur at 20º instead of the normal 5º rotation. This would also allow for 15º additional travel prior to the actuator stop being contacted. The additional travel allowed the valve disc to travel through the seal. By traveling past zero, through the seal, the seal would have been ruined and thus the valve would no longer provide tight shutoff. Proper lever-to-shaft installation alignment must be made, plus the seal must be replaced in order to bring this valve back into performance specifications. |
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