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Pump FAQs
42
Pump FAQs
WORLD PUMPS
December 2014
Pump FAQs Q. What are some differences between simplex and duplex air-operated pumps? A. For a duplex air-operated pump, compressed air is directed into the pump air chamber via the air distribution system. The compressed air is kept separate from the liquid by a diaphragm. The diaphragm, in turn, applies pressure on the liquid and forces it out of the pump discharge. While this is
occurring, the opposite-side air chamber is depressurized and exhausted to atmosphere and liquid is drawn into the pump suction. The cycle repeats, thus creating a constant reciprocating action, which maintains flow through the pump. (See Figure 10.4.2.1). A simplex pump operates in much the same manner except that no liquid is discharged during the recharge stroke of the diaphragm or bellows.
A spring return is typically used to return the diaphragm and provide energy for the suction stroke. Intake and discharge valves direct flow into and out of the pumping chamber. It should also be noted that simplex pumps require a controller to cycle the compressed air/gas into the air chamber of the pump and to exhaust the air out. The controller can be mechanical or electrical, such as a repeat cycle timer and solenoid valve. The speed of the pump is set by the controller, and the maximum speed is determined by the manufacturer.
Before the pump starts up, any stretch that occurs is due to rotor weight, the sum of the static forces. The thrust load will increase after the pump starts up due to the addition of the dynamic forces. The dynamic forces creating thrust on a vertical turbine
+F Downthrust
DS
To determine the maximum pumping speed on a duplex pump, increase the air supply while the pump rate of flow increases. When the rate of flow no longer increases, throttle back the air supply until the pump rate of flow starts to decrease. This point is the optimum pump cycle rate achievable under these system conditions.
Drf Figure 2.3.3.2.3a – Enclosed impeller plain top shroud.
+F Downthrust
DS
For more information about air-operated pumps see ANSI/ HI 10.1-10.5 Air-Operated Pumps for Nomenclature, Definitions, Application, and Operation.
Drf Figure 2.3.3.2.3b – Semi-open impeller.
D rb Back ring Balance holes
Q. How does axial thrust compare amongst different impeller types for a rotodynamic vertical pump?
Figure 10.4.2.1 – Duplex pump operation.
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A. The net axial downthrust force is carried by the pump shaft. The shaft will stretch, i.e., elongate, under this load.
+F Downthrust
DS Drf
Figure 2.3.3.2.3c – Enclosed impeller with back ring and balance holes.
WORLD PUMPS
Pump FAQs December 2014
pump enclosed impeller (Figure 2.3.3.2.3a) are due to the difference in pressure distributions on the upper and lower shrouds along with the force from the change in momentum of the flow through the impeller. The semi-open impeller (Figure 2.3.3.2.3b) has only an upper shroud. The difference in pressure distributions along both the backside and the vaned side of the shroud is
typically greater than between upper and lower shrouds of an enclosed impeller. Semiopen impeller axial thrust is higher than that of the enclosed impeller. The axial flow pump impeller (propeller) has no upper or lower shroud; vanes are attached directly to the hub. The axial thrust generated is primarily from dynamic forces created by interaction of the propeller vanes with liquid.
The impeller back ring with balance holes configuration (Figure 2.3.3.2.3c) reduces the axial thrust. Back rings may be cast integrally into impellers with a top shroud. They are used when pump total axial thrust requires reduction. The flow through balance holes in the impeller hub shroud, combined with the leakage past the balance ring, reduces efficiency. The exact efficiency reduction depends on the individual design and pump
size and specific speed. The effect of increased leakage through clearances due to wear of the back ring arrangement may be an increase in downthrust and should be considered in sizing the thrust bearing. For more information about axial thrust for vertical rotodynamic pumps, see ANSI/ HI 2.3 Rotodynamic (Vertical) Pumps for Design and Application.
Newly Published Power Plant Pumps Guideline Guidelines for Application & Operation to Maximize Uptime, Availability, & Reliability A one-of-a-kind guide exclusively for the application and operation of pumps in combined-cycle power plant service. This guide: t Identifies the styles of pumps used in combined-cycle power plants t Describes the basic operation of combined-cycle plants t Provides recommendations for operating pumps at the highest efficiency and greatest reliability t Places an emphasis on boiler feed, boiler feed boosting, condensate, condenser circulating, boiler circulating, and heater drain pump applications.
Now available for $150 at the HI eStore www.eStore.Pumps.org/PowerPlantPumps www.Pumps.org
www.worldpumps.com
43
Pump FAQs
WORLD PUMPS
December 2014
Pump FAQs Q. What are some differences between simplex and duplex air-operated pumps? A. For a duplex air-operated pump, compressed air is directed into the pump air chamber via the air distribution system. The compressed air is kept separate from the liquid by a diaphragm. The diaphragm, in turn, applies pressure on the liquid and forces it out of the pump discharge. While this is
occurring, the opposite-side air chamber is depressurized and exhausted to atmosphere and liquid is drawn into the pump suction. The cycle repeats, thus creating a constant reciprocating action, which maintains flow through the pump. (See Figure 10.4.2.1). A simplex pump operates in much the same manner except that no liquid is discharged during the recharge stroke of the diaphragm or bellows.
A spring return is typically used to return the diaphragm and provide energy for the suction stroke. Intake and discharge valves direct flow into and out of the pumping chamber. It should also be noted that simplex pumps require a controller to cycle the compressed air/gas into the air chamber of the pump and to exhaust the air out. The controller can be mechanical or electrical, such as a repeat cycle timer and solenoid valve. The speed of the pump is set by the controller, and the maximum speed is determined by the manufacturer.
Before the pump starts up, any stretch that occurs is due to rotor weight, the sum of the static forces. The thrust load will increase after the pump starts up due to the addition of the dynamic forces. The dynamic forces creating thrust on a vertical turbine
+F Downthrust
DS
To determine the maximum pumping speed on a duplex pump, increase the air supply while the pump rate of flow increases. When the rate of flow no longer increases, throttle back the air supply until the pump rate of flow starts to decrease. This point is the optimum pump cycle rate achievable under these system conditions.
Drf Figure 2.3.3.2.3a – Enclosed impeller plain top shroud.
+F Downthrust
DS
For more information about air-operated pumps see ANSI/ HI 10.1-10.5 Air-Operated Pumps for Nomenclature, Definitions, Application, and Operation.
Drf Figure 2.3.3.2.3b – Semi-open impeller.
D rb Back ring Balance holes
Q. How does axial thrust compare amongst different impeller types for a rotodynamic vertical pump?
Figure 10.4.2.1 – Duplex pump operation.
www.worldpumps.com
A. The net axial downthrust force is carried by the pump shaft. The shaft will stretch, i.e., elongate, under this load.
+F Downthrust
DS Drf
Figure 2.3.3.2.3c – Enclosed impeller with back ring and balance holes.
WORLD PUMPS
Pump FAQs December 2014
pump enclosed impeller (Figure 2.3.3.2.3a) are due to the difference in pressure distributions on the upper and lower shrouds along with the force from the change in momentum of the flow through the impeller. The semi-open impeller (Figure 2.3.3.2.3b) has only an upper shroud. The difference in pressure distributions along both the backside and the vaned side of the shroud is
typically greater than between upper and lower shrouds of an enclosed impeller. Semiopen impeller axial thrust is higher than that of the enclosed impeller. The axial flow pump impeller (propeller) has no upper or lower shroud; vanes are attached directly to the hub. The axial thrust generated is primarily from dynamic forces created by interaction of the propeller vanes with liquid.
The impeller back ring with balance holes configuration (Figure 2.3.3.2.3c) reduces the axial thrust. Back rings may be cast integrally into impellers with a top shroud. They are used when pump total axial thrust requires reduction. The flow through balance holes in the impeller hub shroud, combined with the leakage past the balance ring, reduces efficiency. The exact efficiency reduction depends on the individual design and pump
size and specific speed. The effect of increased leakage through clearances due to wear of the back ring arrangement may be an increase in downthrust and should be considered in sizing the thrust bearing. For more information about axial thrust for vertical rotodynamic pumps, see ANSI/ HI 2.3 Rotodynamic (Vertical) Pumps for Design and Application.
Newly Published Power Plant Pumps Guideline Guidelines for Application & Operation to Maximize Uptime, Availability, & Reliability A one-of-a-kind guide exclusively for the application and operation of pumps in combined-cycle power plant service. This guide: t Identifies the styles of pumps used in combined-cycle power plants t Describes the basic operation of combined-cycle plants t Provides recommendations for operating pumps at the highest efficiency and greatest reliability t Places an emphasis on boiler feed, boiler feed boosting, condensate, condenser circulating, boiler circulating, and heater drain pump applications.
Now available for $150 at the HI eStore www.eStore.Pumps.org/PowerPlantPumps www.Pumps.org
www.worldpumps.com
43