Mechanical Seal Technology (continued)
EagleBurgmann and GIW worked together early in the project to carefully design the mechanical seal to meet the needs of the pump and also to account for the special challenges of the deep tunnel project.
“The seal for this project was specifically designed for purpose,” Haselbacher explained. “On the one hand, it is a simple semi-split case design—only it is much bigger. However, there are special issues with a vertical system. It must be straightforward and as safe as possible but engineered to purpose.”
In the HGH semi-split seal design, repairs can be made with the split parts. This is critical for this particular application, Haselbacher said. “Semi-split mechanical seals are used if there is sufficient axial clearance to pull off the seal housing and seat housing until the sliding parts are accessible. That is, only the wearing parts—such as the seal faces and O-rings—are split,” Haselbacher explained.
“In the case of the GIW seal housing, Part 4 is made split, for the case of repair, as space is limited. The seal faces, O-rings and springs can be replaced without complete disassembly. In the case of the GIW pump, the inflatable seal has to be activated.”
Haselbacher explained that the mechanical seal for this project was designed with an inflatable seal that blows up “like a tire” 2 to 3 bars above product pressure to prevent leakage. This is a safety precaution. The pump cannot run with the seal inflated. “The flowback device is state-of-the-art,” Haselbacher said. “This helps keep the seal chamber free of particles.”
The seal usually has a flush in with clean water (plugged Bore 1 in Figure 1) with about 15 to 20 liters per minute. This is to keep the area around the seal as clean as possible since a bell-shaped seal chamber cannot be used in this application, Haselbacher said.
“In case of necessary repair by opening ‘the flush out’ for cleaning (Bore 2) so the service personnel can clean up the seal chamber before opening. We flush the seal chamber with clean water.”
The plugged bore (Bore 3 in Figure 1) is for venting.
“Venting will always occur before startup of the pump to assure that the room is free of air and gas,” Haselbacher said. “The pump is vertical, so dry running of SiC/SiC faces is often the reason for seal failure. With a seal like this, liquid at the faces must always be assured. The expectancy of the life of the seal is until total pump revision. The pressure-less quench (see the green areas in Figure 1) on the inside of the faces provides an additional backup.”
The two seal faces have a stationary design that can move from the top using springs to close the gap. This provides leakage protection if there is movement of the shaft, Haselbacher said.
Both seal faces are designed floating in between two O-rings. This is to allow for easy replacement in case of service and to uncouple the faces, Haselbacher said. The seal is serviced through the bearing bracket and disassembly is not needed to make repairs.
The maximum operating limits of the seal (depending on size, speed and pressure) are:
- Shaft diameter: 50…750 mm
- Pressure: 25 bar
- Temperature: 150 C
- Sliding speed: 20 meters per second