Since 1995, new technological features introduced into very-high-pressure gas reinjection compressors have simplified auxiliary systems, reduced operating costs and improved rotor dynamic behavior.
Natural gas reinjection is used to maintain wellhead pressure with the gas obtained during extraction of crude oil, allowing the production of crude to be kept practically constant. The pressure required is normally more than 3,000 psi and up to 10,000 psia. The compressors are barrel-type, but train compositions vary greatly. Typically the driver is a gas turbine, although the number of electric motor-driven reinjection trains has increased in recent years.
Nuovo Pignone has been involved since the early 1970s in the natural gas reinjection business with centrifugal compressors. The first high-pressure machine was a back-to-back compressor rated for a 6,100-psi discharge installed in Algeria. In 1975 a train for a North Sea platform was successfully full-load tested up to 10,000 psi.1
More recently, five reinjection trains designed for a discharge of 9,100 psi were installed in Venezuela2, and in May the first of three trains for a Kazakhstan plant was tested up to 9,300 psi. GE Nuovo Pignone has delivered more than 240 trains for reinjection service with a discharge pressure higher than 5,000 psi.
From 1970 to 1995, the technology of compressors did not change significantly. Since then, many technological features have been introduced, including dry gas seals, alternative impellers-to-shafts design assembly, spark-eroded impellers, honeycomb seals and an improved capacity to control rotating stalls.

Dry gas seals
Dry gas seals have been used on Nuovo Pignone centrifugal compressors for low-
and medium-pressure application since
1988. They eliminated some problems associated with oil seals, leading to:
• simplification of the system through elimination of the overhead tank, oil pumps, traps and gas separators;
• reduction of mechanical losses within the compressor and energy consumption due to pumps; and
• elimination of oil transfer in the compressed gas.
Particularly with high-pressure applications, the cost of a seal oil system has a dramatic impact on the total cost of the train. Moreover, oil pumps rated for 3,500 to 5,000 psi can create reliability problems.
The gas from the formation normally contains a percentage of hydrogen sulfide, which can contaminate the oil flowing through the high-pressure rings and the oil contained in the overhead tank. This can cause the failure of oil rings and the bearings' anti-friction materials and lead to the need for a seal oil separate from the lube oil, further increasing the system's complexity. Furthermore, the contaminated oil needs to be reconditioned or eliminated, which can impact cost and safety.
The application of dry gas seals eliminates these issues. But introduction of dry gas seals also presented engineers with another challenge: the machine's lateral stability.
In terms of rotor dynamics, the selection of dry gas seals strongly affects the compressor's behavior. Oil seals, if suitably designed, can provide extra support and stiffening for the shaft. Therefore, the introduction of dry gas seals removes a significant contribution to system stability. Since the dry gas seal system also normally increases the compressor's bearing span, the rotor dynamic design is more critical compared with the design of a compressor equipped with oil seals.
Alternative impellers-to-shafts design
GE Nuovo Pignone reinjection compressors are equipped with standard-tested 2D impellers. For a given diameter, the impeller's geometry is fixed for different applications. Each impeller is separated from the others by a spacer shaped to create the needed aerodynamic path at the impeller entrance.
To increase the shaft's stiffness, and therefore its insensitivity to external disturbances, the spacers have been eliminated and the hub diameter modified. Two rings - the front one in two pieces, the rear in one piece - maintain the axial position. A lining of hard material protects the shaft. These features ensure the aerodynamic path doesn't change.
This configuration is particularly effective in increasing the stiffness of the shaft, but a different technology is required to produce the impellers. Traditionally low-flow-coefficient, 2D impellers are slot-welded from the rear of the hub (blades are machined on the shroud) due to the presence of the shroud nose, which is the same diameter as the blade leading edge. Increasing the hub diameter and therefore the seal diameter of the slot welding from the exterior becomes difficult. Different alternatives have been considered, and spark erosion has been selected.
Spark-eroded impellers
Starting from a monolithic center-drilled steel disk, impellers for high-pressure reinjection are manufactured by spark-erosion using numerical control machines. The process tool and the impeller have opposite polarity electrodes. The mean is normally oil or a specific fluid with high resistivity. Beginning at the disk's external diameter, different electrodes are used to manufacture the impeller's blades and cavities. These have the same shape as the vane. The machining starts with a roughing erosion, followed by a finishing phase using a specific tool to create an accurate geometry.
In addition to the possibility of increasing the hub diameter, the main advantages of spark-erosion are:
• no structural discontinuities;
• high structural strength;
• high dimensional accuracy; and
• fine surface quality.
These points are particularly important for high-pressure reinjection machines. Due to the high pressure and density involved, pressure pulsation originated by the asymmetry of aerodynamic field, particularly in the discharge scroll, can cause significant periodic forces.3 The absence of any metallurgical discontinuity within the impeller is clearly a step forward.
In terms of dimensional accuracy, the compressors under discussion are normally characterized by a low flow coefficient. From a geometrical point of view, this is translated into narrow aerodynamic passages, sometimes on the order of 0.3 to 0.4in. The distortion associated with slot welding can cause modification of the blade width of 5% and higher for these low-flow impellers. With spark erosion, the accuracy is 1% to 2%, allowing a more accurate matching between the expected and tested performances.
Honeycomb seals
Research on the influence of honeycomb seals on rotor dynamics is in progress, and the analytical instruments are continuously updated. Nevertheless, the experience of different compressor manufacturers has revealed the honeycomb seals' capability to improve the stability of turbomachines.
Test results show honeycomb seals develop larger destabilizing forces than labyrinth seals, but they also develop higher direct stiffness and direct damping values.4 Honeycomb seals on the balancing drum and the interstage bush of back-to-back machines is a standard feature for Nuovo Pignone high-pressure reinjection compressors.
Rotating stall control
Diffuser rotating stall is associated with low-flow-coefficient compressors and typical of high-pressure reinjection machines. Consequences of diffuser rotating stall are pressure pulsation and low frequency vibrations (10% to 30% of the synchronous frequency), which may limit the operating envelope.
Theoretical analysis as well as experimental investigation on real machines has allowed GE Nuovo Pignone to optimize the diffuser inlet where the rotating stall takes place. A rapid pinch increases the gas velocity at the diffuser inlet, facilitating gas flow toward the outlet without reverse flow.
The first high-pressure centrifugal compressor incorporating the above features was full-load-tested in March 1996 and continues to run on a North Sea platform. With a back-to-back configuration, it is equipped with tandem dry gas seals designed for 2,400 psi dynamic and 3,000 psi static. During the test the compressor delivered at 7,500 psi.
Since 1996, 19 compressors equipped with tandem and triple dry-gas seals with discharge higher than 5,800 psi and up to 9,300 psi have been full-load-tested and put in operation. They have demonstrated excellent stability and an increasing degree of reliability.

Bibliography
1. "Vibrations in Very High Pressure Centrifugal Compressors," Ferrara, P.L., ASME paper 77-det-15.
2. "High Pressure Natural Gas Compression: Venezuela on the Top," Maretti, Armenzani, Sguanci, XII Jornadas de Gas Puerto la Cruz, 1996.
3. "An assessment of the forces acting upon a centrifugal impeller using a full load, full pressure hydrocarbon testing," Borer, Sorokes, McMahon, Abraham, proceedings of the 26th Turbomachinery Symposium.
4. "Handbook of Rotordynamics," Ehrich, F.F., McGraw-Hill Inc., 1992.