Valve trim retrofits: solving chronic control problems

摘要

Only through a thorough investigation will the root cause be identified; then you can take intelligent problem solving action. Such investigation would look at what causes the noise, vibration, and damage and what causes the poor flow-control and instability. In many cases, the best solution lies in trim retrofitting, (See Figure 1, on page 22). This involves replacing the trim and frequently, the bonnet and actuator as well. The original valve bodyremains in place. This is especially desirable when the body is welded into a heavy wall piping system or is in an inaccessible location. Every effort should be made to retrofit a valve without machining the valve body. This simplifies the installation process and saves time and money. The body should, however, be repaired to correct any damage and be brought back to its original condition. This is critical if the body-sealing surfaces have been damaged or if the minimum wall thickness has become less than required. Sufficient body flow area must be available. Usually, this is the case in which the original trim design was of a cage guided typeo or, as is frequently the case, the valve was oversized in first place. If a valve has been oversized, a reduction of the flow capacity factor generally permits a smaller trim size to be used. This may provide the necessary flow area in top-guided valve designs. Another solution to inadequate flow area is increased stroke that can be provided by new bonnet and actuator. This increased stroke frequently has the benefit of improved flow control since the same actuator signal range operates in a longer flow area.