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.