This article was originally published in the June issue of COMPRESSORtech2. We only publish a fraction of our magazine content online, so for more great content, get every issue in your inbox/mailbox and access to our digital archives with a free subscription.
By Neetin Ghaisas and Sourav Majumdar
Dry gas seals are an integral part of modern centrifugal compressors. Specifying engineers sometimes do not give enough attention to the scope, definition and selection of the most appropriate gas seal type, its control system and auxiliaries for a given service application. Several parameters such as the composition, the thermodynamic state conditions of seal or buffer gas, the type of barrier gas, the situations requiring a slow roll, suitability for bidirectional rotation and the system rating must be evaluated before finalizing the design of a dry gas seal system. Ideally, this review should involve the vendor with the unit responsibility, the gas seal manufacturer, and the machinery engineers who work for the machine owner and engineering contractor. Selection of the right type of dry gas seal and its support system contributes to long-term reliability and availability of process plant centrifugal compressor trains.
Some of the key topics discussed in this paper include the principle of operation of dry gas seal faces, the methods of seal gas control, slow roll and challenging sealing environments.
Dry gas seal faces
A dry gas seal consists of a rotating, hard-face mating ring with either a machined or etched circumferential spiral groove pattern and a primary (stationary) ring in softer ma- terial. The primary ring is radially restrained but can move in the axial direction. Under static and depressurized condition, the springs behind the primary ring keep the seal faces closed and in contact. In the static and pressurized condition, the sealed gas penetrates across the faces at the tip of the groove. A sealing dam maintains uniform pressure distribution between the seal faces and helps to provide the hydrostatic lift that results in a low startup torque, lower heat generation, and less parasitic power.
In the dynamic condition, the progressively shallow spiral grooves draw the sealed gas toward the center dam and create gas film pressure to separate the faces so they become noncontacting during running. Special geometry of the spiral grooves provides a uniform hydrostatic and hydrodynamic pressure distribution and maintains the proper seal face gap for gas film stiffness. These features assist in quick liftoff, even during low-speed operation (10 fps [3.05 m/s]) for pressures less than 50 psig (345 kPa guage) and allow the seal faces to adjust rapidly to the changes in the process conditions.
Depending on the shape of the circumferential grooves, the seals can be unidirectional (Figure 1) or bidirectional (Figure 2). A bidirectional seal provides protection from reverse rotation in situations such as the failure of dis- charge check valve and eliminates the need for spare seal cartridges on each end of a between-bearings centrifugal compressor. The spare seal cartridge can be installed at either end of the compressor. A sealing dam and land are shown in Figures 1 and 2. Figure 3 illustrates the pressure profile across dry gas seal faces during dynamic condition.
Slow speed operation of seal faces (slow roll)
The rotors of between-bearings centrifugal compressors, gas turbines and steam turbines experience elastic deflection or bending during standstill condition. The external load, acting perpendicular to the axis of the rotor that results in flexure or bending of the rotor, is either due to gravity force or due to the combined effect of gravity and thermal differential across the rotor based on its operating environment. A slow roll of the driven compressor and driver steam turbine train at speeds from 2 to 500 rpm, including coast down, or ratchet slow roll at 0.125 turn per minute (typically not more than 50 rpm) to gradually relax the bow in the rotors before starting up the train requires the dry gas seals to be designed in a way that provides a face separation during slow roll and in unpressurized or pressurized conditions inside the seal chamber.