September 2010 Vol. 237 No. 9
Features
Bigger Dry Seals for Super-Size LNG Compressors
The increased global demand for liquefied natural gas (LNG) is having a significant impact on companies that both produce and supply equipment to this market, resulting in rapid advances in machine design, capacity and efficiency. In the LNG process, natural gas is converted to liquid form, making the gas easier to store and transport. When the LNG is needed for use, it is regasified and distributed as pipeline natural gas.
As fossil fuels become more expensive and less readily available, natural gas is becoming a viable alternative especially in countries such as China and Japan, where propane has been the most used fuel. LNG terminals are under construction in parts of the world that normally would not have been able to have access to natural gas.
According to “LNG Tanker Market Highlights,” a May 2008 report from Marsoft, a market analysis firm to the maritime industry, “New LNG liquefaction plants are expected to come online at an average rate of three or four per year from now until 2012.” The report goes on to say, “Global LNG demand projections generally remain strong, with base case demand projected to grow by more than 70% from 2007 to 2012, and supply projected to grow by more than 80% over the same period.”
An increase in LNG demand has a direct impact on LNG technology, including the main turbo compressors in use at LNG plants and the mechanical shaft seals responsible for optimum compressor reliability. As the LNG market grows, so does the equipment used to move and pump the gas. Bigger compressors allow more LNG to be moved more efficiently and economically.
Bigger Compressors, Bigger Seals
As compressor manufacturers developed the technology to make larger compressors, compressor components also had to be made larger. This posed new challenges for the motor, engine and gas turbine industries to produce bigger pieces of equipment to drive the larger compressors. With the increase in sizes of both the compressor driver and shaft, the mechanical sealing industry has also had to meet the challenge of making bigger dry gas seals for these “super-sized” compressors.
Although it might seem to be an easy solution, seal manufacturers cannot just scale-up the size of an existing gas seal design to make a bigger seal. Larger diameter dry gas seals require specific design characteristics to meet reliability standards. Some of the significant design aspects of larger seal design include the significance of machine coast-down; the importance of adequate clearance between seal component parts; effective seal separation; and seal validation testing.
Specific Design Considerations
The scale of investment in plant equipment does not often allow for the building of redundancy into systems. For the compressor industry, the requirement for the near-permanent availability of compressors greatly affects critical components such as dry gas seals. The reliability of these components is dependent on how they are operated, the seal technology and the materials used. In special large-diameter, dry-gas seal applications, it is typically slow roll conditions, compressor train coast-down and low-compressor suction pressures that present the greatest challenge.
Dry Gas Seals
If the seal faces are separated by the pressure differential across the seal, only hydrostatic lift takes place. This will occur when the opening force of the pressure between the seal faces is slightly greater than the closing force of the seal. The pressure that builds from the shallow grooves in the rotating seat separates the seal faces. The spring force and the friction of the secondary sealing element mostly determine the low-pressure closing force of the seal.
Contacting Conditions And Wear
During contacting conditions, it is important to keep the energy transfer between the stationary face and the rotating seat low to minimize wear. The energy transfer is a result of speed, the contact pressure and the friction coefficient. The applied sealing pressure, axial spring force and the friction of the balance diameter-sealing element determines the contact pressure. (Figure 1).
Along with the material combination of the seal face and rotating seat, the sealed gas influences the friction coefficient. As the dewpoint of the gas gets lower, the friction coefficient and the energy transfer into the tribological system increases. Therefore, it is recommended to use hard face technology with diamond like coatings to reduce the friction.
Coast-Down
As the size of the rotor increases, the inertia of the compressor train also increases. Therefore, the rotor will experience a certain coast-down speed profile. Figure 2 shows a typical measured coast-down speed profile of an LNG compression train after shut down. In this instance, the shaft size is 350 mm in the seal area.
Figure 2: Measured Coast-Down Speed Profile of an LNG Train.
Figure 3 shows a cross section of a typical dry gas seal for an LNG compressor. Usually the inboard seal takes the full pressure differential, whereas the outboard seal is pressurized on a low-pressure differential to the second stage vent by the intermediate gas.
Figure 3: 350-mm Shaft Seal.
Taking into account that during the coast-down period the pressure differential across the outboard seal is low, the seal faces need to be separated at low speeds over the duration of the coast-down. In any case, for a certain period, while the seal faces are not completely separated, asperity contacts occur first and as the speed approaches 0 RPM, the surfaces contact fully.
By lowering the axial load, the touchdown speed is reduced. Therefore, contacts occur on a much lower energy transfer in the tribological system, where neither surface polishing nor wear occur. This contributes to an increase in seal reliability and robustness.
Contacting Slow Roll Conditions
Some of the large-diameter applications use compressor drivers, mainly steam turbines, which require specific attention in the cooling process after shut down, and in the heating process before start up, to avoid uneven and rapid temperature gradients in the shaft. With a slow-turning process shaft, permanent shaft bending can be avoided.
For the dry gas seals, two different slow roll conditions should be considered: 1) slow roll without hydrodynamic seal separation; 2) slow roll with hydrodynamic seal separation.
The second case will be not discussed in detail; the upper speed limit is given by the compressor design and should be lower than the speed the compressor requires to produce head. The lower speed limit for this case is given by the hydrodynamic lift-off capabilities of the dry gas seal. This is in the area of approximately 1 m/s tip speed at the outside diameter of the seal rotor.
Contacting slow roll conditions are best described by the first case, where the seal does not separate hydro-dynamically. In a tandem seal configuration, the inboard seal can be pressurized with sufficient pressure to generate hydrostatic lift. However, the outboard seal is pressurized by the inert gas with a low-flare pressure. This low-pressure differential is usually insufficient to generate hydrostatic lift and the seal surfaces will contact. As described for the coast-down conditions, it is essential to minimize the energy transfer into the tribological system to minimize wear.
The main contributing factors are speed, material combination, gas lubricity, and contact pressure. Lowering the speed can reduce the amount of wear. A limiting factor for minimum continuous turning speed is the oil-lubricated, tilt-pad bearing. Continuous barring, or slow roll with linear speeds below the hydrodynamic lift-off speed of the seal, will transfer energy into the tribological system. The higher the speed, the higher the energy transfer. From a seal design perspective, the only method to reduce the energy transfer is to reduce the contact pressure between the seal faces. In addition, the internal friction of the balance diameter-sealing element and the axial spring force need to be optimized to cope not only with the normal operating conditions but also with contacting slow roll conditions.
Discontinuous shaft barring, or ratchet slow roll, is sometimes used. This will usually have no effect on the dry gas seal, and wear will not occur.
Low-Suction Pressure Applications
These applications occur mainly in ethylene and propylene compressors, which are comparable to LNG sealing applications but are special with regard to low-sealing pressures in normal operating conditions. To comply with the safety notice for tandem seals applied in sealing pressures below 20 bar, a pressure differential of approximately 2 bar across the outboard seal needs to be maintained.
In this case, special precautions should be taken to maintain positive pressure differentials across both the inboard and the outboard seal. The positive-pressure differential will avoid backpressure conditions for both seal stages and provide sufficient aerostatic lift-off. This lift-off is required for film stabilization during slow-roll and transient-operating conditions. The clean gas flow to the process side is relatively high.
As an alternative, a single-seal configuration, as shown in Figure 4, provides a simpler and more robust seal and system solution. The inboard seal continually provides the necessary minimum positive pressure differential by the clean gas. The clean gas flow and, subsequently, the internal compressor mass flow losses are minimized. From a safety aspect, the barrier seal is capable of handling the full process pressure differential even in dynamic conditions. In the unlikely event of a seal failure, the compressor can be safely shut down.
Figure 4: Alternative Solution for Low-Suction Pressures.
Future Sealing Solutions
Over the last decade, the shaft size of turbo compressor units for LNG applications has increased significantly. Current projections indicate that further increases in scale will continue to increase compressor efficiency and reduce production costs. Key to success is working with a seal manufacturer who has made investments in reliable, large-diameter sealing solutions.
Author
Emery Johnson builds on more than 14 years of experience in the dry gas seal industry as he leads the day-to-day sales and manufacturing of EagleBurgmann’s growing dry gas seal business. He is a graduate of Texas A&M University, a member of ASME and API and he is applying for his P.E. license in the state of Texas.
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