Pendulum Effect No. 3

EDITORIAL NOTE: In addition to good design practices the following article points out areas for Inspectioneers to pay attention to, like those in figure 1, and the potential for fretting in the absence of suitable softer, but durable, fretting insulators like those in the last paragraph.

In a past issue, we discussed one solution to the instrument line block valve pendulum problem. This was where the valve assembly can be mounted remotely from the vibrating product line, such as at-grade. This issue covers two possibilities where the valves need to remain close to the vibrating line.

For 1-inch instrument lines, instead of using the common installation as shown in Figure 1, that shown in Figure 2 is suggested. This design has had a more favorable history of failure-free operation. The key is to support the block valves downstream from the branch connection and to keep the nipple for the relief valve short. It has been found that the supports must be substantial because vibration is generally in several directions.