A seal is a device that prevents one fluid from leaking into another. Compressor can be divided into:
Internal seals, which are designed to prevent the movement of gas within the compressor casing
External seals, which are designed to prevent the escape of gas from the compressor casing to the atmosphere
The labyrinth seal is the simplest and most common. A series of sharp teeth are machined on the inside of a circular metal liner fitted to close tolerances around the shaft.
Some leakage from the high pressure side to the low pressure side is allowed. The amount can be reduced by machining teeth on the shaft that interlock with those on the liner so that they almost touch.
Another form has teeth machined on a shaft sleeve that presses into a soft metal sleeve fitted to the casing. The rotating teeth cut grooves in the softer metal. The leakage across a labyrinth seal depends upon the pressure on each side, the number of teeth, and the clearance between the teeth and the shaft.
Maximum clearances are generally about 0.002 inch of shaft diameter. Seal operation at pressure differentials below 50 psig use 3 to 6 teeth, while higher pressure may have as many as 20.
FIGURES 24a & b LABYRINTH SEALS
SIMPLE LABYRINTH SEAL INTERLOCKING AND STEPPED
LABYRINTHS WITH INJECTOR AND
EJECTOR AND RESTRICTIVE RING
Typically uses of labyrinth seals in a centrifugal compressor are:
- Inter stage seals to stop the flow of gas from the tip of an impeller to its eye
- To maintain the differential pressure across the balance drum
- To control the escape of compressor gas into the seal system of an oil film
- To control the loss of lubricating or seal oil along a shaft
Centrifugal compressors require seals around the rotating shaft to prevent gases from escaping where the shaft exits the compressor casing.
As shown in Figure 32, these seals use oil, which is circulated under high pressure between three rings around the compressor shaft, forming a barrier against the compressed gas leakage.
The centre rotating ring is attached to the rotating shaft, while the two rings on each side are stationary in the seal housing, pressed against a thin film of oil flowing between the rings to both lubricate and act as a leak barrier. “O-ring” rubber seals prevent leakage around the stationary rings.
Very little gas escapes through the oil barrier; considerably more gas is absorbed by the oil under the high pressures at the “inboard” (compressor side) seal oil/gas interface, thus contaminating the seal oil. Seal oil is purged of the absorbed gas (using heaters, flash tanks, and degassing techniques) and re circulated.
The recovered methane is commonly vented to the atmosphere.
FIGURE 32 – LIQUID FILM SHAFT SEAL – WET SEAL.
An alternative to the traditional wet (oil) seal system is the mechanical dry seal system. This seal system does not use any circulating seal oil. Dry seals operate mechanically under the opposing force created by hydrodynamic grooves and static pressure.
As shown in Exhibits 26a and 26b, hydrodynamic grooves are etched into the surface of the rotating ring affixed to the compressor shaft.
When the compressor is not rotating, the stationary ring in the seal housing is pressed against the rotating ring by springs. When the compressor shaft rotates at high speed, compressed gas has only one pathway to leak down the shaft, and that is between the rotating and stationary rings.
This gas is pumped between the rings by grooves in the rotating ring.
The opposing force of high-pressure gas pumped between the rings and springs trying to push the rings together creates a very thin gap between the rings through which little gas can leak.
While the compressor is operating, the rings are not in contact with each other, and therefore, do not wear or need lubrication. O-rings seal the stationary rings in the seal case.
FIGURE 26a – DRY SEAL
Putting two or more of these dry seals together in series, as shown in Exhibit 2b, is called “tandem dry seals,” and is very effective in reducing gas leak-age. This type of seal has less than one percent of the leakage of a wet seal system vented into the atmosphere and costs considerably less to operate.
FIGURE 26b – DRY TANDEM SEALS