Face-seal shaft seals

Mechanical Face Seals This type of seal forms a running seal between flat precision-finished surfaces. It is an excellent seal against leakages. The seahng surfaces are planes perpendicular to the rotating shaft, and the forces that hold the contact faces are parallel to the shaft axis. For a seal to function properly, there are four sealing points ... 

It doesn t matter who manufactures the seal, almost all seals have the same component parts, because all parts have to perform the same fimetions. The common parts in all seals are the gland, the stationary face, the rotary face, the secondary seals, the spring, and the fastener to the shaft. Let s look at how mechanical seals are designed (Figure 13-3, next page). 

Install the pump back plate and seal chamber assembly. Mount the dial indicator on the shaft and place the needle onto the outer diameter of the lip or face of the seal chamber (Figure 14-24). An alternate method would be to place the indicator needle inside the seal chamber bore. Rotate the shaft. This will verify that the shaft is concentric with the seal chamber bore. If it is not concentric, the seal may rub against the bore when the pump is started. 

With the indicator still in this same position, place the needle onto the lip or face of the seal chamber (Figure 14-25). Rotate the shaft. This... 

Face lubricated-type seals must be connected from the source of lubrication to the tap openings in the seal gland before startup. This is another predetermined environmental control feature that is mandatory for proper seal function. Where double seals are to be operated, it is necessary that the lubrication feed lines be connected to the proper ports for both circulatory or dead-end systems before equipment startup. This is very important because all types of double seals depend on the controlled pressure and flow of the sealing fluid to function properly. Even before the shaft is rotated, the sealing liquid pressure must exceed the product pressure opposing the seal. Be sure a vapor trap does not prevent the lubricant from reaching the seal face promptly. 

In the process industries the conditions at the pump seal are often harsh and more complex seals are needed. Mechanical face seals are used, Figure 5.14. They are generally referred to simply as mechanical seals, and are used only on rotating shafts. 

To obtain satisfactory seal performance, the shaft stiffness shall limit the total deflection imder the most severe dynamic conditions over the allowable operating range of the pump—with maximum diameter impeller(s) and the specified speed and fluid—to 50 pm (0.002 in.) at the primary seal faces. This shaft deflection limit may be achieved... 

Mechanical seal performance depends on the runout conditions at the mechanical seal chamber. Seal chamber face runout is a measure of the perpendicularity of this face to the pump shaft axis. This runout (TTR) shall not exceed 10 pm per 20 mm (0.0005 in./in.) of seal chamber bore (Appendix K). 

In order to ensure a proper seal against the process medium In the pumping chamber the shaft seal Is of the axial face seal type - a sealing... 

The shaft seal assembly consists of two face-type, mechanical seals in series, with controlled leakage bypass to provide the same pressure differential across each seal. The seal assembly is designed for 2500 psi differential and to reduce the leakage pressure from Reactor Coolant System pressure to the volume... 

Load actual pressure at a sealing face in the case of a shaft seal, the sum of the elastomeric lip s inherent beam force, the hoop force (as a result of lip stretch upon installation) and the garter spring tension, all of which contribute to shaft loading at the contact point. 

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