When testing the newly constructed pipeline system, the pipes and valves need to undergo preliminary tests: two leak tests, one 150% static hydraulic test, and one N2He (nitrogen, helium) leak test. The objects of these tests include not only the flanges connecting the valves and pipes, but also the valve covers and valve body interfaces, as well as all the stop valves/valve cores in the valve body. 

To ensure that the cavities within parallel gate valves or ball valves are fully pressurized during the test, the valves should be at 50% open position, as shown in Figure 1. So far, everything seems normal, but can this really be done for the most commonly used stop valves and wedge gate valves? If these two types of valves are in the half-open position as shown in Figure 2, the pressure in the cavities will act on the valve gland. The main gland is usually made of graphite material. When tested with small molecule gases such as helium at 150% of the design pressure, it is usually necessary to tighten the pressure valve cover bolts to obtain normal test results. 

However, the problem with this operation is that it may overly compress the packing, resulting in an increase in the stress required to operate the valve. As the friction increases, the degree of wear on the packing during operation also rises. 

Serious injury 

If the valve position is not at the upper sealing seat position, when tightening the pressure valve cover, there will be a tendency to cause the valve shaft to tilt. And the tilt of the valve shaft may result in scratching the valve cover during operation, and cause scratch marks. 

If incorrect operations during the initial test result in leakage from the valve shaft packing, the usual approach is to further tighten the pressure valve cover. However, doing so may cause severe damage to the pressure valve cover and/or the valve cover bolts. Figure 4 shows an example where excessive torque was applied to the valve cover nut/bolt, causing the pressure valve cover to bend and deform. If excessive stress is applied to the pressure valve cover, it may also cause the valve cover bolts to be pulled apart, as shown in Figure 5. 

Then, loosen the nut of the pressure valve cover to relieve the pressure acting on the valve shaft packing. Conduct a preliminary test under this condition to determine if there are any problems with the valve stem and/or valve cover seals. If the performance of the upper sealing seat is poor, consider replacing the valve. In summary, the upper sealing seat should be a verified metal-metal seal. 

Two notable points 

After the initial test is completed, it is necessary to apply appropriate compressive stress to the valve stem packing, while ensuring that the packing does not exert excessive stress on the valve stem. This can prevent excessive wear of the valve stem and maintain the normal service life of the packing. Two points are worth noting: Firstly, even if the external pressure on the graphite packing is removed, it will not return to its pre-compressed state. Therefore, leakage will occur after the compressive stress is unloaded. Secondly, when tightening the valve stem packing, ensure that the valve position is at the upper sealing seat. Otherwise, the compression of the graphite packing will be uneven, causing the valve stem to have a tendency to tilt, which will result in surface abrasion of the valve stem and severe leakage of the valve stem packing. Such valves must be replaced.