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How high-pressure pumps play in future
40
Feature
WORLD PUMPS
September 2013
Operating
How high-pressure pumps play in future Modern plants use vertically-split centrifugal pumps for highpressure services. New high-pressure pumps are designed with special focus on critical areas such as rotordynamics, performance, potential excitations, safety, reliability, auxiliary systems, fabrication processes, and seals.
A
vertically-split casing with a heavybolted head is a normal design for high-pressure pump applications. The heavy-bolted head design can be used for many services, but this design could not be practical for very high pressures because there is not enough space on the pump head for the heavy bolts required. The commonly used configuration for very highpressure applications is a shear-ring coverhead that eliminates the bolts at the pump head. The end-covers are retained by the shear-ring segments. The casing of very high pressure pumps is often forged from a suitable grade of steel.
High-pressure pumps generally use closedtype three-dimensional impellers. Both the efficiency and the pressure coefficients are higher for three-dimensional impellers. Impellers are usually manufactured from low-alloy steels. In special cases, suitable
Figure 1: An example of a vertically-split high-pressure pump.
www.worldpumps.com
alloys could be used. In-depth investigations of the material characteristics should be carried out to examine the pump integrity. For high-pressure pumps, the thrust force generated in the impeller assembly is much greater than can be handled in a thrust bearing. Therefore, the thrust forces should be balanced by using double entry impeller series (a back-to-back arrangement) and balance piston(s). A recommended design is a back-to-back impeller arrangement with two balance pistons. The remaining axial force is the difference between the forces which are a magnitude larger, and an advanced double-acting tilting-pad thrust bearing should be used. Electric motor drivers are used in high-pressure pump trains. High-pressure pumps are usually high-speed machines. Too often, the gear unit could be required to increase the speed of the electric motor. One successful design is a two-end electric motor arrangement using a dedicated gear unit at each end, to drive two pump casings. Using this configuration, an optimum speed can be achieved for each pump casing. Each vertically-split pump casing can easily be opened from each side. For high-pressure pump trains, the steam turbine driver or the gas turbine driver (in very special cases) could be used. The steam turbine speed can be matched with the high-speed pump and a direct drive arrangement could be possible. For a large high-speed pump, a small/
0262 1762/13 © 2013 Elsevier Ltd. All rights reserved
Figure 2: An example of a high-pressure pump. An enclosure is provided for safety increase and noise reduction.
medium gas turbine could be used (it is employed in special plants/locations). Since the gas turbines are available in standard models, a gear unit may be required for the speed match.
High-pressure requirements Vibrations at the high-pressure pumps are usually controlled by employing very stiff rotor designs. The sufficient stability of the rotor should be assured under any anticipated operating or malfunction conditions. Maximizing the rotor stiffness is achieved by limiting the number of impellers per pump casing. The best recommendations include the shortest possible bearing span and the maximum shaft diameter.
WORLD PUMPS
Feature September 2013
Modern high-pressure pumps (which are usually high-speed machines) only use tilting-pad bearings. These bearings inhibit bearing cross-coupling excitation forces and instabilities. A specific attention should be given to the rated head to include a suitable margin covering liquid property uncertainties, fouling, and performance deterioration in high-pressure services. The material selection, stress analysis, fatigue study, and quality control of pump shafts are important. Sufficiently low stresses (as low as around 15-30% of the minimum yield strength) are recommended for regions of the shaft which might be wetted by the pumped liquid. The impeller cracking caused by the welded fabrication is a widespread problem. The electro-discharge machining process (instead of the welding) could be employed for the fabrication of some critical high-pressure impellers. Modern finite element (FE) studies using fine meshing techniques are required, with a special focus on highly stressed localized areas.
movement of seal mating faces should be ensured under all operating conditions. The seal design can become problematic when extreme operating conditions are present and a large number of design parameters have to be optimized for different operating modes. In addition, there are rarely isolated effects/parameters. Moreover, the effects are often contradictory.
Contact Amin Almasi [email protected]
Figure 3: An example of a vertically-split high-pressure pump. It is a high-pressure feed water pump.
The best vendor shop test package for high-pressure centrifugal pumps is a shop performance test using the job liquid. Some vendors may offer a substitute liquid, which is not acceptable. Such a performance test can provide very valuable information about the likely behaviour of a high-pressure pump such as certain aspects of the dynamic excitations, instability, and realistic effects of the seal or bearing. The high pressure presents a challenge to seal applications. Special attention is required because of the possibility of damage upon a rapid depressurization and an explosive decompression. At high pressures, there are tremendous forces caused by the liquid load acting on the seal in both the radial direction and the axial direction. To ensure the maximum stability of the seal at such high loads, the cross-sections of the metal sleeves in the seal cartridge should be large enough. If only one sleeve is used for a high-pressure seal, a relatively small cross-sections would be too weak (with improperly distributed load, and high localized stresses) to handle a high axial load. In a high-pressure design, the sleeves are split to several sections (two or three) to ensure the maximum stability of the seal. The seal gap should be designed as small as possible. However, a suitable www.worldpumps.com
41
Feature
WORLD PUMPS
September 2013
Operating
How high-pressure pumps play in future Modern plants use vertically-split centrifugal pumps for highpressure services. New high-pressure pumps are designed with special focus on critical areas such as rotordynamics, performance, potential excitations, safety, reliability, auxiliary systems, fabrication processes, and seals.
A
vertically-split casing with a heavybolted head is a normal design for high-pressure pump applications. The heavy-bolted head design can be used for many services, but this design could not be practical for very high pressures because there is not enough space on the pump head for the heavy bolts required. The commonly used configuration for very highpressure applications is a shear-ring coverhead that eliminates the bolts at the pump head. The end-covers are retained by the shear-ring segments. The casing of very high pressure pumps is often forged from a suitable grade of steel.
High-pressure pumps generally use closedtype three-dimensional impellers. Both the efficiency and the pressure coefficients are higher for three-dimensional impellers. Impellers are usually manufactured from low-alloy steels. In special cases, suitable
Figure 1: An example of a vertically-split high-pressure pump.
www.worldpumps.com
alloys could be used. In-depth investigations of the material characteristics should be carried out to examine the pump integrity. For high-pressure pumps, the thrust force generated in the impeller assembly is much greater than can be handled in a thrust bearing. Therefore, the thrust forces should be balanced by using double entry impeller series (a back-to-back arrangement) and balance piston(s). A recommended design is a back-to-back impeller arrangement with two balance pistons. The remaining axial force is the difference between the forces which are a magnitude larger, and an advanced double-acting tilting-pad thrust bearing should be used. Electric motor drivers are used in high-pressure pump trains. High-pressure pumps are usually high-speed machines. Too often, the gear unit could be required to increase the speed of the electric motor. One successful design is a two-end electric motor arrangement using a dedicated gear unit at each end, to drive two pump casings. Using this configuration, an optimum speed can be achieved for each pump casing. Each vertically-split pump casing can easily be opened from each side. For high-pressure pump trains, the steam turbine driver or the gas turbine driver (in very special cases) could be used. The steam turbine speed can be matched with the high-speed pump and a direct drive arrangement could be possible. For a large high-speed pump, a small/
0262 1762/13 © 2013 Elsevier Ltd. All rights reserved
Figure 2: An example of a high-pressure pump. An enclosure is provided for safety increase and noise reduction.
medium gas turbine could be used (it is employed in special plants/locations). Since the gas turbines are available in standard models, a gear unit may be required for the speed match.
High-pressure requirements Vibrations at the high-pressure pumps are usually controlled by employing very stiff rotor designs. The sufficient stability of the rotor should be assured under any anticipated operating or malfunction conditions. Maximizing the rotor stiffness is achieved by limiting the number of impellers per pump casing. The best recommendations include the shortest possible bearing span and the maximum shaft diameter.
WORLD PUMPS
Feature September 2013
Modern high-pressure pumps (which are usually high-speed machines) only use tilting-pad bearings. These bearings inhibit bearing cross-coupling excitation forces and instabilities. A specific attention should be given to the rated head to include a suitable margin covering liquid property uncertainties, fouling, and performance deterioration in high-pressure services. The material selection, stress analysis, fatigue study, and quality control of pump shafts are important. Sufficiently low stresses (as low as around 15-30% of the minimum yield strength) are recommended for regions of the shaft which might be wetted by the pumped liquid. The impeller cracking caused by the welded fabrication is a widespread problem. The electro-discharge machining process (instead of the welding) could be employed for the fabrication of some critical high-pressure impellers. Modern finite element (FE) studies using fine meshing techniques are required, with a special focus on highly stressed localized areas.
movement of seal mating faces should be ensured under all operating conditions. The seal design can become problematic when extreme operating conditions are present and a large number of design parameters have to be optimized for different operating modes. In addition, there are rarely isolated effects/parameters. Moreover, the effects are often contradictory.
Contact Amin Almasi [email protected]
Figure 3: An example of a vertically-split high-pressure pump. It is a high-pressure feed water pump.
The best vendor shop test package for high-pressure centrifugal pumps is a shop performance test using the job liquid. Some vendors may offer a substitute liquid, which is not acceptable. Such a performance test can provide very valuable information about the likely behaviour of a high-pressure pump such as certain aspects of the dynamic excitations, instability, and realistic effects of the seal or bearing. The high pressure presents a challenge to seal applications. Special attention is required because of the possibility of damage upon a rapid depressurization and an explosive decompression. At high pressures, there are tremendous forces caused by the liquid load acting on the seal in both the radial direction and the axial direction. To ensure the maximum stability of the seal at such high loads, the cross-sections of the metal sleeves in the seal cartridge should be large enough. If only one sleeve is used for a high-pressure seal, a relatively small cross-sections would be too weak (with improperly distributed load, and high localized stresses) to handle a high axial load. In a high-pressure design, the sleeves are split to several sections (two or three) to ensure the maximum stability of the seal. The seal gap should be designed as small as possible. However, a suitable www.worldpumps.com
41