Seal Design

Advancing the state-of-the-art sealing systems are new suites of computer programs such as C'StedySM(1) used to analyze the performance of both contacting and non-contacting seal designs during steady-state and transient conditions. This type of finite analysis considers all of the operating conditions, the fluid sealed, the materials of construction, and seal geometry. The outputs from the program are seal distortion, temperature distribution, friction power, actual PV (pressure X velocity), leakage, the percentage of face in liquid or vapor, and fluid film stability (see Figure 1). This type of analysis requires accurate fluid and material properties. The results from the program can predict the success or failure of a given installation.

For example, a mixture of liquid hydrocarbon made up of ethane, propane, butane, and hexane has to be sealed. This is a new application. The operating condition is 1,300 psig at 70°F. The shaft speed is 3,600 rpm. To determine the performance of this seal prior to installation, an analysis must be made. The results of this study indicate stable operations for a contacting seal, as shown in Figure 2. This study was used to predict seal performance. Actual field results from this difficult service were excellent at startup and during equipment operation.

FIGURE 1 C'StedySM fluid film model for a mechanical seal (John Crane Inc.)

1Service Mark of John Crane Inc.

FIGURE 2 C'Stedy output for a successful seal on high pressure light hydrocarbon service (John Crane Inc.)
FIGURE 3 The basic components of a mechanical seal

These state-of-the-art computer tools not only predict performance, but they also can be used to determine any short seal life. By using a series of calculations per second, this type of analysis can be used to create an animation that will visibly show changes to a seal at startup and during fluctuations in operating conditions. A seal can be examined for stable and unstable operations. This is a useful analysis tool for critical applications.

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Survival Treasure

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