278
Table 28-1 Common Failure Modes of Inverters ME PROBLEM
THE CAUSES AecevDeoel Thm Too short
BE
CONTROL VALVES Although there are limited common control valve failure modes, the dominant problems are usually related to leakage, speed of operation, or complete valve failure. Table 29-1 lists the more common causes of these failures. Special attention should be given to the valve actuator when conducting a root cause failure analysis. Many of the problems associated with both process and fluid-power control valves really are actuator problems. In particular, remotely controlled valves that use pneumatic, hydraulic, or electrical actuators are subject to actuator failure. In many cases, these failures are the reason a valve fails to properly open, close, or seal. Even with manually controlled valves, the true root cause can be traced to an actuator problem. For example, when a manually operated process-control valve is jammed open or closed, it may cause failure of the valve mechanism. This overtorquing of the valve’s sealing device may cause damage or failure of the seal, or it may freeze the valve stem. Either failure mode results in total valve failure.
280
Control Valves
Table 2%1 Common Failure Modes of Control Valves
THE PROBLEM
THE CAUSES DWDebris Trappsd In Valve seat
I Excessivewear
!I
@ Packed Box Too Loose pacldng Too -Damaged Valve Stem Bound Vahe Undedzed
IO 10 10
I
Corroskn
DirvDebrisT m d In Valve seet
Mechanical Damaple sdsndd Failure W m T m of Valve (N-0. N-CI Source: Integrated Systems, Inc.
Io
281
30 SEALS AND PACKING Failure modes that affect shaft seals normally are limited to excessive leakage and premature failure of the mechanical seal or packing. Table 30-1 lists the common failure modes for both mechanical seals and packed boxes. As the table indicates, most of these failure modes can be attributed directly to misapplication, improper installation, or poor maintenance practices.
MECHANICAL SEALS By design, mechanical seals are the weakest link in a machine train. If there is any misalignment or eccentric shaft rotation, the probability of a mechanical seal failure is extremely high. Most seal tolerances are limited to no more than 0.002 in. of total shaft deflection or misalignment. Any deviation outside this limited range will cause catastrophic seal failure. Misalignment
Physical misalignment of a shaft will either cause seal damage and permit some leakage through the seal or it will result in total seal failure. Therefore, it is imperative that good alignment practices be followed for all shafts that have an installed mechanical seal. Process- and machine-induced shaft instability also creates seal problems. Primary causes for this failure mode include aerodynamic or hydraulic instability, critical speeds, mechanical imbalance, process load changes, or radical speed changes. These can cause the shaft to deviate from its true centerline enough to result in seal damage.
282
Seals and Packing
283
Table 30-1 Common Failure Modes of Packing and Mechanical Seals THE PI
Source: Integrated Systems, Inc.
Chemical Attack
Chemical attack (i.e., corrosion or chemical reaction with the liquid being sealed) is another primary source of mechanical seal problems. Generally, two primary factors cause chemical attack: misapplication or improper flushing of the seal.
284
Root Cause Failure Analysis
Misapplication
Little attention generally is given to the selection of mechanical seals. Most plants rely on the vendor to provide a seal that is compatible with the application. Too often there is a serious breakdown in communications between the end user and the vendor on this subject. Either the procurement specification does not provide the vendor with appropriate information or the vendor does not offer the option of custom ordering the seals. Regardless of the reason, mechanical seals often are improperly selected and used in inappropriate applications. Seal Flushing
When installed in corrosive chemical applications, mechanical seals must have a clear-water flush system to prevent chemical attack. The flushing system must provide a positive flow of clean liquid to the seal and also an enclosed drain line that removes the flushing liquid. The flow rate and pressure of the flushing liquid will vary, depending on the specific type of seal, but must be enough to assure complete, continuous flushing.
PACKED BOXES Packing is used to seal shafts in a variety of applications. In equipment where the shaft is not continuously rotating (e.g., valves), packed boxes can be used successfully with no leakage around the shaft. In rotating applications, such as pump shafts, the application must be able to tolerate some leakage around the shaft. Nonrotating Applications
In nonrotating applications, packing can be installed tight enough to prevent leakage around the shaft. As long as the packing is properly installed and the stuffing-box gland is properly tightened, there is very little probability of seal failure. This type of application requires periodic maintenance to ensure that the stuffing-box gland is properly tightened or that the packing is replaced when required. Rotating Applications
In applications where a shaft continuously rotates, packing cannot be tight enough to prevent leakage. In fact, some leakage is required to provide both flushing and cooling of the packing. Properly installed and maintained packed boxes should not fail or contribute to equipment reliability problems. Proper installation is relatively easy and routine maintenance is limited to periodic tightening of the stuffing-box gland.
