New Center

When testing a newly built piping system, the pipes and valves are first tested: two leak tests, one 150% hydrostatic test and one N2He (nitrogen, helium) leak test. These tests cover not only the flanges connecting the valve to the pipe, but also the cover and body interfaces, as well as all cock/spire components in the body.

To ensure that the cavities in parallel gate valves or ball valves are fully pressurized during testing, the valves should be in the 50% open position, as shown in Figure 1. So far so good, but for the most commonly used globe valves and wedge gate valves, is this really possible? If the valves are partially open, as shown in Figure 2, the pressure in the cavity will act on the shaft root. Spindles are usually made of graphite. When testing with small molecular gases such as helium at 150% design pressure, it is usually necessary to tighten the pressure bonnet bolt to obtain normal test results.

The problem with this operation, however, is that it can oversqueeze the disc, resulting in increased stress needed to operate the valve. As friction increases, so does the degree of operating wear.

Serious injury

If the valve position is not in the upper seal seat position, tightening the pressure valve cover will force the valve shaft to tilt. The tilt of the valve shaft may cause it to scratch the valve cover during operation and cause scratch marks.

Further tightening of the pressure bonnet is a common practice if misoperation during initial testing results in a leak of the valve shaft root. Doing so may result in serious damage to the pressure valve cover and/or gland bolts. FIG. 4 is an example of the pressure valve cover bending due to excessive torque applied to the gland nut/bolt. If excessive stress is applied to the pressure valve cover, it may also cause the valve cover bolt to break, as shown in Figure 5.

  Figure 4: The pressure valve cover is bent due to excessive tightening torque on the bonnet nut/bolt.

Figure 5: The bonnet bolt is pulled off.

Figure 6: A tilted stem can lead to scratching and serious leakage.

Figure 3 illustrates the best way to avoid this. For initial testing, run the valve in full open position, where the valve is in the upper seal seat.

Then loosen the nut of the pressure valve cover to relieve the pressure acting on the valve shaft root. A preliminary test in this condition will tell you if there is a problem with the stem and/or cover seal. If the performance of the upper seal seat is poor, consider replacing the valve. In general, the upper seal seat shall be a proven metal-metal seal.

Two points worth noting

After completing the initial test, you need to apply the appropriate compression stress to the stem stem while ensuring that the stem does not overstress the stem. In this way, excessive wear of valve stem can be avoided and normal service life of disc root can be maintained. Two points are worth noting: First, the compressed graphite disc will not return to the state before compression even if the external pressure is removed, so there will be leakage after the compression stress is removed. Second, when tightening the stem disc, ensure that the valve position is in the upper sealing seat position. Otherwise, the compression of the graphite disc may be uneven, resulting in a tendency for the stem to tilt, which in turn causes the stem surface to chafe and the stem disc to leak seriously, as shown in Figure 6. Such valves must be replaced.