Enzo Giacomelli, Fabrizio Bernardini
Nuovo Pignone S.p.A.
Florence, Italy
Hubert Andre
Pipeline Engineering GmbH
Essen, Germany
Use of variable-speed driver systems, such as gas turbines, enhances the functional flexibility of reciprocating compressors.
This article contains a review of the main design aspects for gas-turbine systems followed by a description of some typical applications of turbine-driven reciprocating compressors, with special emphasis on a recent installation in Germany.
Experience suggests an increasing application of gas turbine-driven compressor arrangements as a result of their energy-saving advantages and the high reliability obtained with special design procedures and the availability of modern full-load test facilities.
FLOW-CONTROL FUNCTION
In many natural-gas applications, such as reinjection and storage, flow rates must be controlled to meet service requirements. The use of gas turbines as drivers with variable speed between 70% and 100% of nominal speed is widely accepted for centrifugal compressors.
When a reciprocating compressor is selected, based on technical evaluation of the service and influenced specifically by the size of the plant, it is quite common to find gas engines or electric motors as drivers.
Recently, the use of large compression units with gas turbine-driven reciprocating compressors for offshore installations also has been common.
This use results from the availability and reliability (higher than 99.5%) of reciprocating compressors which, successfully employed for natural gas and other industrial gases for large flow rates and high pressures, do not require a standby machine.1
Reciprocating compressors also offer the advantage of accepting large variations in flows and pressures during operation and are unaffected by changes in the gas molecular weight. The operating flexibility of these machines allows variable production requirements to be met without major modifications or reduction in their usual efficiencies.2
During the last few years, in order to increase flexibility, variable-speed-drive systems have been used more often, made up both by electric motors with variable excitation frequency and by gas turbines.
This fact is particularly important in, for instance, the energy peak shaving achieved through storage of natural gas in depleted gas wells or in artificial caverns. Usually the natural gas is stored in the summer to compensate for variable winter demand, and variable flow rate requirements are commonly encountered.
The combination of gas turbine and reciprocating compressors offers several advantages for both energy and capital expenditure savings.
As the arrangement is generally more complex than with other drivers, all of the most advanced design techniques must be applied to ensure reliable operation.
Use of control, safety, and automatic systems allows turbine and reciprocating compressor trains to be employed in fully automated plants.
These parameters were taken as a basis for evaluation of decisions to use similar trains for heavy duties, as for example in a storage plant.
PROCESSING APPLICATIONS
Although the use of turbines (steam turbines for the most part) as drivers for reciprocating compressors is not as frequent as that of electric motors or gas engines, in the past few years there has been positive experience with large units for ammonia synthesis, hydrocracking, and polyethylene plants.
The design of the train requires careful consideration from torsional and the alignment viewpoints. Quill shafts together with parallel-axis reduction gearboxes have commonly been used.
For one hydrocracking application with a steam-turbine driver, an epicyclic reduction gearbox was used to provide more compact arrangement.
While each application was characterized by the need for a certain capacity control, the main reason for using a steam turbine was the possibility of increasing plant efficiency by utilizing the steam already available for process reasons.
On the basis of this experience, it has been possible to satisfy the need for compact and reliable units, which has led to the supply of gas-turbine-driven reciprocating machines for offshore platform reinjection services and for onshore applications.
The most recent applications have been built with a double-train epicyclic gearbox that reduces the high speed of the turbine, generally within the range of 8,000 15,000 rpm, to 230-430 rpm on the compressor crankshaft (see cover).
A special low-speed coupling, highly flexible from a torsional point of view, has been selected to optimize operation. A torque-limiting device is installed between the compressor coupling and the low-speed gearbox connection flange.
DESIGN CRITERIA
The complex arrangement of gas turbines and reciprocating compressors requires use of advanced computer design techniques.
The main aspects to be investigated are torsional analysis of the train; lateral analysis; close alignment between compressor, reduction gearbox, and turbine; and safety and reliability of operation of the most critical components.
Additionally, correct design requires system component selection with both thermodynamic and mechanical evaluation, a choice of materials that takes into account process-gas requirements, and a special computerized analysis related to pressure pulsation, piping vibration, and cylinder-valve optimization.
Depending on the application, this analysis may have more or less influence on the reliability of the system, and thus different approaches may be required.
In the supply of offshore turbine compressor trains, special attention must be devoted to minimizing the weight of the whole unit, obtaining optimum balance of inertial loads, realizing a very compact skid assembly, and thoroughly investigating the vibration of the machines, auxiliaries, and pipings.
This is the case of three sets operating since 1984 on a platform in Malaysia for reinjection of 37,000 Newton cu m/hr of associated gas at 280 bar (4,060 psi).
Each 130 metric-ton package is mounted on a 12-m long, three-point skid. The four-crank, two-stage compressor is driven by a gas turbine rated at 3,700 kw ISO. Unbalanced inertial forces and couples are minimized by a "flat" compressor crankshaft arrangement with both cranks of each cylinder pair positioned at 180 and the two pairs on the same plane.3
A major contribution to the compactness and to the weight limitation derives from the selection of an epicyclic gear, an installation in which all the components of the train are on the same axis, and the use of a flexible coupling instead of a quill-shaft arrangement.
In applications with this type of machine, special alignment procedures must be followed, and adequate clearances are necessary on the bearings.
This was taken into consideration for a module for a platform in Angola, consisting of two reinjection units with aircoolers, scrubbers, dampeners, and a triethylene glycol (TEG) system provided for dehydration of process gas.
Six cylinder, four-stage reciprocating compressors handle 23,500 kg/hr of natural gas with pressures of 7 345 bar (102 5,003 psi).
Alignment is always an important matter for any combination of compressor and driver. Particularly in the case of gas turbines, proper consideration is required for each of the following concerns:
- The vertical plane, where commonly the axis position of the compressor, gear, and turbine will change between start-up (cold condition) and design operating conditions.
- The axial direction because of thermal expansion of the compressor and gear shafts between their thrust bearings.
As for the main features of compressor and cylinders connected with the service, it should be recalled that, while storage must face the problem of high pressure, reinjection must often take into account the presence of sour gas or difficult lubricating conditions due to the gas quality.
As the reinjection service is very critical, particular care is needed in designing the cylinder.
For successful operating life, the cylinder configuration and the sealing elements must be designed to ensure correct lubrication and high cooling efficiency. A three piece cylinder has often been selected, provided with long tie bolts to reduce overstressing to a minimum in case of liquid entrained with the gas.
A long-stroke crank with low speed has proven to be the best choice for packing and piston-ring life for such a heavy duty application.2 4
TORSIONAL ANALYSIS
Use of reduction gears and couplings requires particular care to prevent critical operating conditions resulting from torsional vibrations. Generally it is necessary to avoid resonant conditions with the first harmonics, which normally have the highest displacement magnitudes.
The best situation is achieved when the first torsional natural frequency is below the operating range of the compressor (Fig. 1). Thus, during normal operation the stress level will be low, but careful considerations must be made for the transient states (start-up or shutdown).
In such cases, in fact, the first critical speed must be passed through. And especially when the train is being shut down under emergency situations, the presence of gas may maintain a high load on the cylinders and consequently on the crank mechanism and the whole group during the pass through the first critical.
In order to have a natural frequency below the normal operating range, a special low-speed coupling with low torsional stiffness must be used.
The study of transient conditions is carried out with a computer program which analyzes the behavior of a nonlinear model, the use of damping coefficients defined for each component, and the application of several exciting torques as a function of time.
The mathematical model will show whether safe running can be expected during start-up (Fig. 2a), normal shutdown and under all operating conditions (Fig. 1), as well as in the event of emergency shutdown (Fig. 2b) when the compressor stops before being depressurized.
And the model will indicate the cases in which high torque values could arise. This depends on the operating pressures and the compressor arrangement (cylinder and crankshaft).
To solve this problem, the train is provided with a special low-speed coupling equipped with a torque-limiting device, which will automatically release the compressor in the event of high torque. The coupling can be easily locked again manually and the compressor restarted once the emergency is over.
PRODUCTION
A quality-assurance system based on procedures and covering engineering and the entire manufacturing process must be followed for such critical units.
To provide even greater certainty of a product offering the guarantee of functionality, safety, and reliability, however, facilities and equipment for testing the entire groups prior to shipment to the job site are needed.
Each type of machine must be tested in either unloaded or fully loaded conditions with the same process gas or one of equivalent thermodynamic characteristics.
The entire control system with its logic is also normally tested. The customer benefits by significant economic advantages insofar as the groups are assembled in the shop instead of in areas where environmental conditions may be severe, entailing very high labor costs.
Furthermore, it is possible to anticipate and solve problems which might otherwise cause difficulties and loss of time on the jobsite at the start-up stage.
One package for Malaysia was subjected to a test that demonstrated compliance of the actual behavior with the design parameters. The skid was resting on its three-point supports in the same position as onboard the platform. Before start up, a complete checking program was carried out to ensure perfect alignment of the turbine-gear compressor axis.2 Checks were carried out on starting sequence and gas and fire protection panels.
With the train running at full speed and unloaded, further checks were performed on the control panel, local instruments, run out of piston rods, noise, skid vibrations, and train torques.
Finally, mechanical and process shutdown procedures were simulated to verify the behavior of the group during possible emergencies arising in field operation. Special care was devoted to the torsional measurements.
A module for Angola, 25 m long, 13 m wide, and weighing 850 metric tons, was more thoroughly examined. Each compressor was tested separately at full speed, under unloaded and fully loaded conditions, and finally with both trains and the entire module operated at full load.
STORAGE COMPRESSORS
Within the context of the expansion of the natural gas storage facility operated by Ruhrgas at Epe, Germany, it was necessary to raise the power installed for gas compression from 3 mw to 11 mw.
Two integral gas-engine compressors had been installed originally. The extension was based on the following operating data: suction pressure range, 40-70 bar; discharge pressure range, 7 200 bar; and gas to be compressed, high or low BTU gas.
The design of the units and the maximum power requirement were based on a suction pressure of 55 bar and a discharge pressure of 150 bar at a flow rate of 100,000 Newton cu m/hr. A major requirement was high flexibility and high efficiency of operation under partially loaded conditions.
The concept was to cover two identical compressors for individual and parallel operation of the existing integral gas-engine compressors. The following alternatives were thoroughly investigated:
- Integral gas-engine-driven compressors
- Reciprocating compressors driven by variable-speed electric motors
- Reciprocating compressors driven by gas turbines
- Turbocompressors driven by electric motors
- Turbocompressors driven by gas turbines.
Pollution levels had to be kept within the limits specified in the 1986 West German air-pollution control regulation, regardless of whether gas turbines or gas engines were selected as compressor drivers (mainly CO = 100 mg/cu m, NOx = 300 mg/cu M).
UNIT SELECTION, FEATURES
On the basis of a comprehensive machinery selection study, it was found that a reciprocating compressor coupled to a gas turbine by a two-stage epicyclic gearbox would be the most favorable solution from both technical and economical viewpoints.
The economic analysis included a present-value calculation covering operating costs on the basis of 4,000 hr/year. A matrix procedure was used for the technical evaluation.
The criteria considered were design, flexibility, partial load behavior, availability, project handling and installation, and after-sales service and manufacturer's references.
In addition to economic advantages, the decisive factor in favor of gas-turbine-driven reciprocating compressors was flexibility and high efficiency at partially loaded operation.5 6
Nuovo Pignone MS 1002 R regenerative cycle gas turbines, with 34% efficiency, rated 4,620 kw ISO, drive the four cylinder machines used to compress natural gas from the pipeline into artificial caverns for storage.
The engine unit of the two shaft type is equipped with a 15-stage axial compressor, a single combustion chamber, and both single-stage high pressure turbine and power turbine.
This power turbine's variable nozzles keep efficiency levels high at partial speeds and loads, thus further enhancing the efficiency benefits provided by the regenerative cycle.
A shell-and-tube recuperator is installed in the exhaust stack to improve efficiency. This solution provides the advantage of boosting the efficiency of an industrial, heavy duty gas turbine design to that approaching the level of aeroderivative turbine models.
A two-stage epicyclic gearbox with gear ratio of 31:1 steps down turbine shaft speed to the compressor speed of 333 rpm.
An indicating-type, torque metering system is installed on the high-speed side, between gas turbine and gearbox, to measure power output under operating conditions and for torque monitoring.
In addition to the previously described torsional analysis, further study was necessary for the high-speed shafts, considering the presence of the torque meter.
The lateral analysis (Fig. 3) indicates the critical speeds as a function of the bearing stiffness. The results have been compared with similar solutions which performed satisfactorily.
STATION CONTROL; LOAD TEST
The capacity control of the compressors is obtained by valve unloaders and clearance pockets on each cylinder. Furthermore, the speed variation and the installation of two units give the plant additional flexibility .
A control system starts up and shuts down the units and distributes load to ensure operation of the compressors at optimum efficiency within their operating curves.
This is part of the central control system, including documentation and trouble shooting function, station control, and long-term monitoring of the plant.
Some parts of the plant have local control systems which implement open and closed loop control as well as monitoring, optimization, data logging, and data processing functions.
The pumping station is equipped with a distributed control system, controlled and monitored from a central control room.
A test was performed in the workshop before shipment, with all the main job items of one train (reciprocating compressor, gearbox, low-speed coupling, high speed coupling, and gas turbine).
The compressor was run under full load in a closed loop with throttling valve and cooler. The natural gas was from the national network, the molecular weight of which was within the contract values.
The sequence was the following:
- Operation at 100% speed with 55-110 barg of pressure rise
- Normal shutdown and start-up
- 105% speed with 55-110 barg of pressure rise
- 100% speed with 55 198.5 barg of pressure rise.
The suction-discharge pressures and temperatures were measured with an automatic data-acquisition system.
During the unit run up and run down, the design torsional natural frequency was checked with strain gauges placed on the low-speed shaft of the gearbox. Signals were transmitted by a tele metering system to stationary equipment for complete analysis (Fig. 4).
The actual torsional natural frequency was 150 cycles/min (cpm) with good agreement with the design value of 151 cpm. Torsional transients for start-up and for shut down at full load were recorded.
A turbine lateral critical speed check was also carried out.
The test results gave satisfactory performance.
ACKNOWLEDGMENT
The authors wish to thank Nuovo Pignone S.p.A., Pipeline Engineering GmbH, and Ruhrgas for permission to publish this information.
REFERENCES
- Giacomelli, E., and Tesei, A., Compressors: A Worldwide Overview, Hydrocarbon Technology International, 1987.
- Traversari, A., Pecchi, S., and Partisani, A., "Design of Packaged Reciprocating Compressors for High Pressure Reinjection of Associated Gas for Offshore Oil and Gas Processing Platforms," First European Fluid Machinery Congress, The Hague, Mar. 24 26, 1981.
- Chellini, R., "Turbine Driven Recip. in Reinjection Service," Diesel & Gas Turbine Worldwide, May 1984.
- Raubenheimer, D.S.T., "Offshore Gas Compression: An Operator's Experiences," First European Fluid Machinery Congress, The Hague, Mar. 24-26, 1981.
- Urban, M., and Andre, M., "Gas Turbine Driven Reciprocating Compressors for a Natural Gas Storage Plant," ASME International Gas Turbine and Aeroengine Congress and Exposition, Amsterdam, June 6-7, 1988.
- Giacomelli, E., Bernardini, F., Pini, A., and Andre, H., "Application of Gas Turbines for Large Reciprocating Compressor Drive," ASME Gas Turbine and Aeroengine Congress and Exposition, Toronto, June 4-8, 1989.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.
