The faces of a dry gas seal can contact during slow roll if hydrostatic, or hydrodynamic forces are not adequate to achieve seal face liftoff. The integrity of seal faces that are subjected to contact during slow roll is a function of the rotor speed, dew point of the gas, seal face material properties, contact loading and the duration and frequency of such conditions.
In a noncontacting slow roll, the gas film stiffness is less than during a steady-state running condition. Dry gas seal faces are more susceptible to rubbing if the sealed gas has entrained liquids. The other detrimental conditions include axial displacement of the shaft and reverse pressurization of the inner seal. Partial face contact can occur during coast-down at speeds below the seal face liftoff speed.
By proper balance of the seal face load factor, applied spring force, and friction coefficient of the seal face materials, dry gas seals have been designed to be suitable for slow roll operation at the sealed pressure ranging from 3 to 450 psig (21 to 3100 kPa guage) and axial displacement of ± 0.06 in. (1.5 mm).
Types of dry gas seals
A dry gas seal is essentially a mechanical seal with a rotating mating ring and a stationary seal ring. A representative layout of a dry gas seal assembly and basic seal parts are shown in Figures 4 and 5. The three principal types of dry gas seals for turbomachinery applications are discussed in the following paragraphs. On the inboard of a primary seal, a process gas labyrinth is provided. Both types of primary seal gas control methods, viz., seal gas flow control method and differential pressure control method, ensure positive flow of seal gas across the process labyrinth to avoid reverse flow of the process side gas into the dry gas seal cavity. The source of the seal gas is either a clean process gas, an inert gas or a sweet gas that is compatible with the process gas. A continuous supply of the seal gas at steady-state conditions is essential to proper functioning of the dry gas seal. The seal gas supply pressure should be at least 50 psi (3.4 bar) greater than the highest sealed pressure.
It is a very common practice to use seal gas tapped from the compressor’s discharge. However, positive flow of the seal gas is not available when pressure rise across the compressor is inadequate during process and operational upsets and in other transient conditions such as startup and shutdown. Alternative systems such as Ampliflow can be used to boost the seal gas pressure to help avoid contamination across the primary seal faces.
A single-seal design is used in those compressors that handle gases that are neither flammable nor toxic and do not create an environmental harm. Some examples include carbon dioxide, air and nitrogen. A labyrinth-type seal can be added to a single seal to reduce the leakage of the sealed gas in the event of a seal failure.
A double-seal design has two opposed seals with a suitable barrier gas introduced at a pressure at least 50 psi (3.4 bar) higher than the sealed process gas pressure. This arrangement is suitable for sealing dirty gases or where external leakage is not permissible (for example, toxic gases and hazardous gases like hydrogen sulfide) or where seal gas consumption must be minimized. A double-seal arrangement is also used in compressors with a limited axial space for the seal cartridge or when compressor suction pressure is close to the vent system pressure. In this case, the seal system is required to be equalized to some intermediate pressure above the vent system pressure. The standard operating pressure limit of double seals is much less than that of single seals and tandem seals.