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Dry gas sealing is an indispensable part of modern centrifugal compressors. Designated engineers sometimes do not pay enough attention to the scope, definition and selection of the most suitable gas seal type, its control system and auxiliary equipment for a given service application. Before finalizing the dry gas seal design, several parameters must be evaluated, such as the composition of the seal or buffer gas, the thermodynamic conditions, the type of barrier gas, the conditions that require slow rolling, the applicability of bidirectional rotation, and the system rating system. Ideally, this review should include suppliers with unit responsibilities, gas seal manufacturers, and mechanical engineers working for machine owners and engineering contractors. Choosing the right type of dry gas seal and its supporting system contributes to the long-term reliability and availability of the processing plant's centrifugal compressor unit.

Some of the key topics discussed in this article include the working principles of dry gas sealing surfaces, sealing gas control methods, slow rolling, and challenging sealing environments.

The dry gas seal consists of a rotating hard-faced mating ring with a machined or etched circumferential spiral groove pattern and a main (fixed) ring made of a softer material. The primary ring is restricted in the radial direction, but can move in the axial direction. Under static and reduced pressure conditions, the spring behind the primary ring keeps the sealing surface closed and in contact. Under static and pressurized conditions, the sealing gas passes through the face at the tip of the groove. The sealing dam maintains a uniform pressure distribution between the sealing surfaces and helps to provide hydrostatic lift, thereby reducing starting torque, generated heat and parasitic power.

Under dynamic conditions, the gradually shallow spiral groove sucks the sealed gas into the center dam, and generates a gas film pressure to separate the faces, making them non-contact during operation. The special geometric shape of the spiral groove provides uniform hydrostatic pressure and hydrodynamic pressure distribution, and maintains an appropriate sealing surface gap for the stiffness of the gas film. Even in low-speed operation (10 fps [3.05 m/s]) at pressures below 50 psig (345 kPa gauge), these features help with rapid lifting and allow the sealing surface to quickly adjust to changes in process conditions.

Depending on the shape of the circumferential groove, the seal can be unidirectional (Figure 1) or bidirectional (Figure 2). The two-way seal can prevent reverse rotation in the event of a failure of the discharge check valve, and there is no need to install a spare seal box at each end of the centrifugal compressor between the bearings. The spare seal box can be installed at either end of the compressor. Figures 1 and 2 show the sealing dam and land. Figure 3 illustrates the pressure distribution of the dry gas sealing surface under dynamic conditions.

The rotors of centrifugal compressors, gas turbines, and steam turbines between bearings will deflect or bend elastically when stationary. The external load acting perpendicular to the axis of the rotor causes the rotor to bend or bend, which is either due to gravity or the combined effect of gravity and thermal difference on the rotor, depending on the operating environment of the rotor. The driven compressor and driving steam turbine train slowly rolls at a speed of 2 to 500 rpm, including coasting, or rolls slowly with a ratchet at 0.125 revolutions per minute (usually no more than 50 rpm) to gradually loosen the bow in the rotor to start the train The dry gas seal is required to be designed to provide surface separation during slow rolling and under unpressurized or pressurized conditions in the sealed cavity.

If the hydrostatic force or hydrodynamic force is insufficient to achieve peeling of the sealing surface, the dry gas sealing surface may contact during slow rolling. The integrity of the sealing surface contacted during slow rolling is a function of rotor speed, gas dew point, sealing surface material properties, contact load, and the duration and frequency of such conditions.

In the non-contact slow roll, the air film stiffness is less than the stiffness under steady-state operating conditions. If the sealing gas entrains liquid, the dry gas sealing surface is more susceptible to friction. Other disadvantages include axial displacement of the shaft and reverse pressurization of internal seals. Partial surface contact may occur at a speed lower than the peeling speed of the sealing surface during sliding.

By properly balancing the load factor of the sealing surface, the applied spring force, and the friction coefficient of the sealing surface material, the dry gas seal design is suitable for slow rolling operation at a sealing pressure of 3 to 450 psig (21 to 3100 psig). kPa table) and ± 0.06 inches (1.5 mm) of axial displacement.

The dry gas seal is essentially a mechanical seal with a rotating mating ring and a stationary sealing ring. Representative layouts of dry gas seal components and basic seal components are shown in Figure 4 and Figure 5. The three main types of dry gas seals used in turbomachinery applications are discussed in the following paragraphs. Inside the main seal, a process gas labyrinth is provided. The two main sealing gas control methods, namely, the sealing gas flow control method and the pressure difference control method, ensure that the sealing gas flows through the process labyrinth in a forward direction, and prevents the process side gas from entering the dry gas sealing chamber in the counterflow. The source of the sealing gas is clean process gas, inert gas or desulfurization gas compatible with process gas. Continuous supply of sealing gas under steady-state conditions is essential for the normal operation of dry gas seals. The sealing gas supply pressure should be at least 50 psi (3.4 bar) higher than the maximum sealing pressure.

It is a very common practice to use seal gas extracted from the compressor exhaust. However, if the pressure at both ends of the compressor rises insufficiently during process and operational disturbances and during other transient conditions (such as startup and shutdown), the forward flow of the sealing gas is not available. Alternative systems such as Ampliflow can be used to increase the sealing gas pressure to help avoid contamination of the main sealing surface.

Single-sealed compressors designed to handle gases that are neither flammable nor toxic and do not harm the environment. Some examples include carbon dioxide, air, and nitrogen. A labyrinth seal can be added to a single seal to reduce the leakage of sealed gas when the seal fails.

The dual seal design has two opposite seals and introduces a suitable barrier gas when the pressure is at least 50 psi (3.4 bar) higher than the sealing process gas pressure. This arrangement is suitable for sealing dirty gas or not allowing external leakage (for example, harmful gas such as toxic gas and hydrogen sulfide) or the situation where the consumption of sealing gas must be minimized. The dual-seal arrangement is also used for compressors with limited axial space in the sealed cylinder, or when the compressor suction pressure is close to the discharge system pressure. In this case, the sealing system needs to be balanced to some intermediate pressure higher than the exhaust system pressure. The standard working pressure limit of double seal is much lower than single seal and tandem seal.

In the tandem sealing device, the main sealing surface undergoes full pressure breakdown. The secondary or outer seal usually operates at a pressure lower than the primary (inner) seal; however, it is designed to withstand the full pressure when the primary seal fails. The primary vent leads to the torch of the factory. If nitrogen is used as the separation gas, the secondary vent is usually open to the atmosphere.

A series seal with an intermediate labyrinth is used to prevent the process gas from leaking into the atmosphere. The clean buffer gas introduced in the middle labyrinth is slightly higher than the pressure of the main exhaust port, creating a pressure difference, which helps prevent the process gas from migrating to the secondary sealing surface, and helps to remove the leakage and sealing gas in the main exhaust port. Leakage of separation gas in the secondary exhaust port (Figure 6). In most hydrocarbon and critical process applications, tandem seal arrangements are more widely used than single and double seal arrangements.

The main function of the barrier seal is to prevent the lubricating oil from the bearing seat from migrating into the dry gas seal cavity and to prevent the process gas from leaking into the bearing oil. Two common types of barrier seals include labyrinth and segmented double carbon rings. Carbon ring seals can be contact or non-contact. The non-contact design is preferred to avoid local heating and protect the carbon ring seal from premature wear. The barrier seal should be suitable for bidirectional rotation. Figure 7 shows the arrangement of the carbon ring separation seal in the dry gas seal box.

The clean and dry separation gas is continuously injected into the isolation seal to protect the lubricating oil from migrating from the outer bearing seat. The separated gas should be filtered to 5 microns of solid particles, and should be 99.98% free of entrained liquid particles of 3 microns and larger.

Use nitrogen or air as the separation gas. For safety reasons, it is recommended to use nitrogen as an inert gas as a separation gas. If nitrogen is not readily available at the job site, nitrogen-making equipment should be considered. Most of the air flow through the auxiliary exhaust port of the dry gas seal is separated gas and a very small amount of sealing gas. When air is mixed with combustible process gas, the air may create an explosive atmosphere in the dry gas sealed secondary vent. If the mixture of process gas and air is within the explosive limit and there is an ignition source in the secondary vent, combustion may occur in the secondary vent.

If air must be used as the separation gas, the dry gas seal control system should be designed to create a lean or rich environment in the secondary vent. This precaution is necessary to minimize the risk of explosive mixture formation in the secondary vent. An oil-lean environment is created by injecting air into the secondary vents, which results in the hydrocarbon-to-air mixture being below the lower explosive limit (LEL) of the mixture.

Injecting process gas into the secondary exhaust port creates a rich environment. It causes the hydrocarbon plus air mixture to exceed the upper explosive limit (UEL) of the mixture. Generally, mixtures containing 5% to 15% methane (CH4) in the air or 17% to 54% CH4 in O2 are considered explosive mixtures. The fuel/air mixture with less than 5% to 17% of fuel (hence the excess oxidizer) is a lean mixture, while the fuel/air mixture with greater than 15% to 54% of the fuel (excess fuel) becomes a rich mixture.

If there is a rich mixture in the exhaust port, the dry gas secondary exhaust port must lead to the factory's flare. The amount of air or process gas that needs to be injected into the secondary vent is determined by engineering research, and it must evaluate all possible operating conditions, including failure of the primary seal. Figure 8 shows the relationship between hydrocarbon percentage and molecular weight. The area above the UEL line in the figure represents a concentrated mixture, the area below the LEL line represents a dilute mixture environment, and the area between the UEL and LEL lines in the figure is an explosive mixture area.

The flow rate of the sealing gas to the main seal is controlled by the flow control method or the pressure difference control method. The main purpose of these two control methods is to make the sealing gas actively sweep the process gas maze to prevent the process gas from entering the dry gas seal in countercurrent.

The flow control system controls the supply of sealing gas to the seal by adjusting the flow of the sealing gas through the upstream orifice of each seal. It includes a valve with a remote flow controller that can compare the sealing gas flow with each seal and adjust the flow control valve to measure downstream of the flow orifice according to the "high selection reference to the pressure of each seal" .

The differential pressure control system controls the supply of sealing gas by adjusting the sealing gas pressure to a fixed value higher than the sealing (reference) pressure (usually 10 psig [70 kPa]). It includes a differential pressure control valve with remote control. There is a bypass pipeline with a manual shut-off valve around the control valve.

The dry gas seal control system must have sufficient range and controllability to maintain a minimum gas velocity of 16 fps (4.9 m/s) within the internal process labyrinth and the labyrinth gap, ranging from minimum to maximum, and a maximum of two times Maximum design clearance. This is essential to avoid contamination of the sealing surface and to keep the required sealing surface temperature below the upper limit of safe operation. No matter what type of seal gas control system is used, a proper balance of seal gas consumption and seal gas flow rate is essential. High sealing gas flow requires more energy and makes the control system inefficient. It also requires relatively large system components.

The flow control system minimizes the consumption of sealing gas and can maintain an acceptable minimum sealing gas velocity. In addition to dual seal arrangements and very low seal pressure (less than 100 psig [690 kPa]) seals, flow control systems are generally ideal for all seal types and applications. In addition, it does not need to measure such an important reference pressure in a differential pressure control system.

The sealing gas entering the main sealing area must be clean and dry (99.98% free of entrained liquid particles of 3 microns and larger), and should be filtered to at least 10 microns of solid particles. In addition, in the entire dry gas seal system, a dew point margin (overheating) of at least 36°R (20°K) is essential. In order to determine this margin, a phase diagram computer simulation of the dry gas seal system from the main seal gas supply point to the main exhaust port must be performed to evaluate any possibility of seal gas condensation. The temperature of the seal gas must be measured at the location where the seal gas enters the seal, not at the source of the seal gas supply. Figure 9 shows some phase diagram curves.

In order to achieve the above-mentioned sealed gas quality, it is usually necessary to integrate the sealed gas processing system with the entire dry gas control system. The sealing gas adjustment hardware consists of a unit that provides clean and dry sealing gas. Coolers, moisture pre-filters and sealed gas heaters (if necessary) are used to provide dry sealed gas. Moisture demister and dual filters remove sealed gas. In many compressor applications, in order to avoid sealing pollution under transient conditions such as startup, shutdown, process sequence, slow rolling, recovery, and precipitation, a device that can increase the air supply pressure and generate a sufficient positive pressure difference is required. The difference between the seal gas supply pressure and the seal pressure should be at least 50 psi (3.4 bar) to avoid contamination of the primary seal.

Figures 10 and 11 show cross-sectional views of the sealed gas heater and coalescing pre-filter. The schematic diagram of the sealed gas booster is shown in Figure 12.

Many remote compressor locations cannot use nitrogen as a source of separation gas. In this case, a nitrogen generator can be installed on the job site to provide pure nitrogen (purity between 95% and 99.5%) from compressed air. Most dry gas seal manufacturers provide nitrogen generators as a stand-alone unit or a unit integrated with the dry gas seal control panel.

When the sealing gas is a hydrocarbon or a hydrocarbon gas mixture, the main exhaust port from the dry gas sealing system is directed to the torch of the factory. The normal torch pressure is generally close to atmospheric pressure, and the maximum pressure can be as high as 50 psig. If the compressor suction pressure is lower than the maximum main seal discharge pressure, the seal cavity must be balanced to an intermediate pressure higher than the compressor suction pressure.

The bypass safety valve provided on the main seal exhaust line should be installed as close as possible to the dry gas seal. The size of the pipes and vents of the dry gas seal system should be designed to prevent excessive pressure in the bearing housing when the seal fails.

Unless the hydrocarbon and air mixture is above the upper explosion limit, the dry gas sealed auxiliary exhaust port will open to the atmosphere. As mentioned in the previous discussion, for obvious safety reasons, there is a dense mixture in the secondary ventilation chamber that needs to be transported to the flare of the factory.

The materials of dry gas sealing surfaces, secondary sealing elements and metal parts must be suitable for all operating conditions and gas compositions of the application. For example, when selecting dry gas seals for centrifugal compressors in liquefied natural gas (LNG) services, the following factors need to be considered:

• Use bidirectional silicon nitride to rotate facing the silicon carbide/carbon fixed surface (depending on pressure, size and speed)

• Amorphous diamond-like carbon coating on the fixed surface of silicon carbide to improve its hardness, lubricity and chemical and thermal stability

• Special Teflon and spring energized Teflon washers and O-rings for low temperature applications

• Low-speed peeling surface technology for tertiary sealing.

• Sealed dry nitrogen rating (-94°F [-70°C]).

• The flow control method of the sealing gas.

• Separately seal the flow meter on the gas supply line.

• Sealed gas regulation system including supercharger.

Sulfur is present in chemical components, such as hydrogen sulfide (H2S) gas. The melting point of elemental sulfur ranges from 235° to 246°F. Sulfur solidifies at lower or higher temperatures and pressures, depending on its content in the saturated gas. Also in a gas stream containing H2S and/or sulfur vapor, the deposition of solidified sulfur occurs due to the chemical reaction supported by the catalyst and changes in gas pressure and temperature. An example is the separation of sulfur when H2S reacts with traces (3% to 5%) of oxygen present in nitrogen, and nitrogen is used as a source of separation gas. Figure 13 shows some possible locations of sulfur deposits in dry gas seals. Figure 14 includes an image of a seal assembly contaminated with sulfur.

In centrifugal compressors that process H2S gas, consideration should be given to integrating the gas conditioning device with the overall dry gas seal control system. Using a coalescing pre-filter to clean the sealing gas and using a sealing heater to increase the temperature of the sealing gas supply to 250°F (121°C) are two effective measures to prevent sulfur solidification and deposition. Similarly, adding a booster device helps to avoid the migration of unclean gas to the sealed area under any transient conditions.

The following discussion applies to seals, which are stored in suitable packaging, and seals that have been installed in the machine for a long time but have not yet been put into use. During storage, the function of the main part of the seal may undergo changes such as aging and deformation. These conditions can destroy the performance of the seal. The recommended protection and preservation measures are listed below.

• The storage place should be dry (relative humidity <65%) and free of dust. The temperature of the storage location should be between 58° and 78°F (14° and 26°C).

• The influence of radiation, heat (direct sunlight), steam and ozone must be avoided. Special storage containers with nitrogen protection should be used to protect the seal.

• Corrosion protection agents should not be used because they will directly affect the function of the seal.

• For storage periods of 36 months or longer, consideration should be given to re-qualifying the sealed cylinder before installation. The elastic auxiliary sealing element should be replaced and the sealing surface should be checked for changes, which may adversely affect the sealing performance.

• Spare gaskets (uninstalled and individually packaged gaskets/O-rings) should be protected from heat sources such as direct sunlight, artificial light with ultraviolet spectrum, and heating fixtures/appliances.

• Washers/O-rings should not be stored below 50°F (10°C) and should be heated to 50°F (10°C) before installation.

• Precautions should be taken to protect stored dry gas seals from all sources of ionizing radiation that may damage the seals and their components.

All images are provided by Flowserve Corp.

About the author: Neetin Ghaisas, PEng, is a senior researcher on Fluor's rotating equipment. Contact him: neetin.ghaisas@fluor.com. Sourav Majumdar is the General Manager of North American Compressor Technical Sales at Flowserve Corp. in Calgary, Alberta, Canada. Contact him: smajumdar@flowserve.com