September 2009 Vol. 236 No. 9

Features

High Efficiency Pipeline Compressor Packages - Efficient, Dependable And Off-The-Shelf

Most natural gas reserves are not in the vicinity of consumers. One method to transport gas to the places where it is needed is by pumping it through pipelines.

Transmission lines cover the long distances and local distribution networks carry gas locally. To overcome the pressure losses in the pipeline, compression equipment is necessary at defined distances distributed over the length of the pipeline.

Compressor stations in long-distance pipelines are typically in remote locations without electrical infrastructure, so the compressors are usually driven by gas turbines. On the other hand, compressor stations in the distribution networks may be located near to the consumers and, thus, near to cities with easy access to electrical interconnects. For these compressors, electric motor driven units are often preferred because of environmental aspects, such as, for example, lower emissions.

For compression trains a high degree of standardization is possible. And, in order to minimize the amount of gas consumed on its way to the consumer, compression efficiency is of the utmost importance.

The implementation time for new pipeline systems is increasing, leading to shorter construction times. Among many considerations, the delivery time for pipeline compression packages has become more and more crucial.

In order to meet these requirements Siemens Energy has developed standard pre-engineered, pre-documented pipeline compressor packages. These packages have been designed in accordance with international standards with a range of options to meet climatic and environmental conditions, from desert to temperate to arctic ambient conditions. By taking this approach rotating equipment has been moved from being a tailor-made engineered piece of equipment to a predetermined commodity. The result of this is that engineering, procurement and manufacturing times are significantly reduced.

Pipeline Compressor Packages
Siemens Pipeline Compressor Packages (SPCP) are pre-engineered gas turbine and motor driven pipeline compressor packages up to 30 MW with lead times down to six months depending on size. These pre-engineered packages combine the well proven standard ranges of Siemens Gas Turbines (SGT™) and Siemens Turbo Compressors (STC™). The pipeline compressors match the Siemens gas turbines’ power and speed to provide optimum performance and are available in five sizes based upon the gas turbine power ranges: SPCP-100, driver SGT-100 ISO power 4.9 MW; SPCP-200, driver SGT-200 ISO power 7.6 MW; SPCP-400, driver SGT-400 ISO power 13 MW; SPCP-600, driver SGT-600 ISO power 25 MW; and the SPCP-700, driver SGT-700 ISO power 30 MW, soon will be available.

The gas turbines cover the power range 5 to 30MW and offer the best efficiency in their class with either dry low emissions or standard combustion systems. All of the subsystems such as filtration, silencing, fuel, lube oil, wash, ducting, etc. are pre-designed and have been thoroughly tested and proven over many years.

The pipeline compressor has to accommodate the various pipeline conditions that exist and needs a greater degree of flexibility. The compressor consists of a vertically split type casing, with separate end covers, which can be rotated through 180? to provide left or right hand inlet; tilting pad bearings and tandem dry gas seals common to all rotor combinations; a common gas seal panel mounted on the base plate; and internal bundle designs for optimized aerodynamic and thermodynamic performance.

Main compressor design parameters are the molecular weight of the natural gas (i.e. the gas composition), the suction and discharge conditions as well as the driver speed and power. The driver speed and power are set by the gas turbine chosen. The compressor design is based on natural gas molecular weights in the range of 16 to 20 kg/kmol, suction pressures between 25 and 70 bars and a pressure ratio up to 2. Casing design pressures therefore depend on the power class and are, for the small pipeline packages SPCP-100/-200, 110 bars again, for all others, 125 bars again.

The operating speed range goes from 50% to 105% of the gas turbine nominal speed giving the benefit of wide operation flexibility.

The compressors have been pre-engineered by: choosing one compressor casing per gas turbine size, one nozzle flange size per casing, defined inlet casing sections and volute sizes, and two directions for nozzles (left- or right hand side).

This pre-engineering allows one to have the documentation instantly available and to stock long delivery material in order to achieve shorter delivery times.

Picture 1: Two pictures showing SPCP compressor variants.
Thermodynamic And Aerodynamic Design

Gas turbine frame size is derived from the total power requirement of the compressor station. The individual internal bundle version related to the head and actual volume flow requirements is chosen from the pipeline compressor selection charts. Based on this pre-selection, the aerodynamic part is custom-designed to achieve highest efficiency and best operating range for the customer-specific application. This design includes the inlet geometry in front of the first impeller and the return guide vane cascade for the two/three stage design. However, because compressor efficiency is crucial, the aerodynamic parts are designed for the specific project:

• Inlet geometry in front of first impeller.
• Impeller diameter and contour.
• Radial diffuser and return guide vane cascade.

This approach allows these pipeline compressors to reach world leading efficiencies in the design point up to 90%. The resulting application area – compressor head versus actual compressor suction flow rate – is shown in Picture 2.

Picture 2: The STC pipeline compressor chart shows the compressor head versus the suction inlet flow for the five SPCP sizes.
Package Design
The SPCP-100, -200 and -400, which are below 15MW, are available as 1-stage and 2-stage designs whereas the SPCP-600, 25 MW class, is available as 1-, 2- and 3-stage designs in order to cover the process conditions described above. A typical cross section of this compressor is shown in Picture 3. The overall compressor weight is about 34 tons.

The SPCP-700 fits the 30MW class gas turbine. It is based on the same design rules and is shown in Picture 3.

The typical package design for a SPCP-400, 15MW class, is shown in Picture 4. The photograph shows one of the packages built for a pipeline station in Egypt. The SPCP-400 compressor covers actual volume flows in the range of 8,000 to around 37,000 cubic meters/hour and heads from 30 kJ/kg for the 1-impeller design up to around 80 kJ/kg for the 2-impeller design. This is also shown in the application map in Picture 2.

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Picture 4: SPCP-400 package design. This is a unit built for a pipeline station in Egypt.

The smallest package is the SPCP-100 in the 5MW class shown in Picture 5. The photo is taken from one of the trains built for a pipeline station in Hungary.

Picture 5: SPCP-100 package design. This picture is of a unit built for pipeline station in Hungary.

Picture 6 shows exemplary efficiencies versus reference volume flows measured for a SPCP-100 compressor. Very high efficiency equivalent to axial inlet machine have been achieved and an excellent turndown range for maximum flexibility can be seen.

Picture 6: SPCP-100 measured efficiencies versus volume flows.

Finally it has to be noted that the same pipeline compressors can be used together with low speed variable speed drives. The horsepower ratings follow the Siemens gas turbine philosophy

Drives system are asynchronous motors up to 18,000 HP typically supplied by Siemens in Norwood, OH or synchronous motors up to 30 MW manufactured by Siemens in Berlin, Germany. Variable Speed drive systems can be either the multi cell perfect harmony from Siemens in New Kensington, PA or the LCI drives out of Berlin. In addition, the customer has the option to obtain the complete electrical integration of the motor drive compressor which includes E-house, LV, MV and HV transformers and related equipment with a completion guarantee from a single supplier.

Picture 7: Typical layout of a pipeline compressor with variable speed drive.

Main Benefits
Customer benefits of pre-engineered pipeline compressor packages include significantly shorter delivery times, instant availability of engineering documentation for faster and easier facility planning, and the use of well-proven compressor and driver products either gas turbines or electric drives, and all under single source responsibility.

Outlook
Siemens Pipeline Compressor Packages offer the opportunity to reduce lead time significantly. Beyond this major improvement, the SPCP also provides an excellent tool for pre-selecting gas turbine-compressor sets during pre-feasibility study and FEED. Engineering effort and time are greatly reduced by the use of selection charts as shown in Picture 3 for standard application ranges and standard documents and drawings enable engineers to develop projects with great accuracy and increased speed. Siemens Pipeline Compressor Packages provide the best technical and cost efficient solution for transmission and distribution needs and are able to meet short delivery time requirements.