AN250 LNG compressor passes tests for Tangguh

By Roberto Chellini

Since 1985, GE's Oil & Gas business has specialized in the design and manufacture of centrifugal and axial compressors used in LNG refrigerating plants. Our AN200 axial compressor is currently used as the first mixed refrigerant (MR) compression stage in several LNG lines in the range of 3.8 million tons per year (MTPY); the machine is designed to handle 220,000 m3/h of gas. But, given the promising forecasts for the LNG industry, we decided to design a new machine that could provide the higher volumes necessary for growth.

Building on the AN200’s established design procedures and experience gained from its many years of successful service, our engineers achieved a 20% capacity increase with the new design. By adding a 0 stage at the front of the flow path and removing the last stage, the resulting axial compressor has a design capacity of 249,000 m³/h.

When we received an order for the 4.1 MTPY Tangguh LNG project in Indonesia, it created an opportunity to move this new design into reality.

 

Design Specifications

Similar to the AN200, the AN250 has a welded casing made with 3.5 Ni steel for low temperature operation. After heat treatment, this casing can operate in conditions as low as -100°C (design point -95°C), with an MR gas operating temperature near -30°C (-45°C during transients) at the compressor inlet.

The stator and rotor blades are made of 17-4-PH, which is also resistant to low temperatures. Variable Stator Vanes (VSV) in the first seven stator rows enable angle adjustment between -35° and +10°. The system allows flow variation from 180,000 to 280,000 m³/h at a constant turbine speed of 3,600 rpm. The single-shaft FR7EA gas turbine can be operated from 96-101% of its rated speed of 3,600 rpm, although the operator prefers using the Variable Guide Vanes (VGV) for compressor control.

Testing

Since this was the AN250’s first installation, we felt it was important to initiate a comprehensive internal program before the unit’s official test for customer approval.

A major advantage of our Florence facilities is the capacity for large-scale, full-load string testing. The whole compressor train was thus assembled on an outdoor test bed, reproducing the Tangguh plant’s configuration including auxiliary equipment (anti-surge line, etc.). The compressor was heavily instrumented with sensors for measurement of the mechanical and thermodynamic characteristics.

 

 

 

The intensive three-week program explored all possible operating conditions following a well established Compressor Test Validation procedure. An impressive amount of data has been collected by the test bed data acquisition system and will be analyzed in detail in order to fully assess the AN250’s operating envelope and identify areas for potential improvement.

Preliminary results

The new VSV system and 0 stage (stationary and rotating blades) were both fully acceptable, with stresses below the set limits under all conditions. Stages exposed to higher stresses exhibited the same behavior as in the AN200.

The complete machine performed very well both in terms of mechanical operation and performance – with a measured efficiency that exceeded the target.

Final testing

Upon completition of the internal test program, our BP, KBR, JGC and KJP customers were invited to witness the official test covering all contractual specifications. This test was concluded in only 2 days with full customer satisfaction.

The AN250 was pressurized with a gas mixture reproducing the MR gas used in the Tangguh plant; the MCL1002 centrifugal compressor was kept unloaded but rotating to simulate its load on the train; and the gas turbine was started with the starter-helper steam turbine fed with steam from the test facility boilers.

The whole train was run at full speed (3636 rpm) for four hours with a power of about 75 MW – among the highest required in this type of full-load test. The axial compressor was run with an inlet pressure of 3.5–3.8 bar-a, and a discharge pressure of 19-21 bar-a, for a compression ratio of 5.5-5.6 at the design point.

The efficiency of the AN250 recorded during the test is better than the guaranteed efficiency of the Tangguh contract. The test results also show a 1% gain over the AN200, resulting from the introduction of the 0 stage. This high efficiency which was also maintained in at off-design conditions, will enable plant output to increase by 0.3-0.4%.

Conclusion

Testing explored the entire performance map with full customer satisfaction. This complex testing was completed smoothly in just two days. This was made possible through a combination of our technical experience and capabilities and seamless integration between our design and testing teams. The powerful AN250 axial compressor is now ready for service in Tangguh.

For more information, please contact:

Renato Mascii
renato.mascii@ge.com

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