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Pump FAQs
42
Pump FAQs
WORLD PUMPS
May 2016
Pump FAQs Q. How should pressuremeasuring instruments be configured during a rotary pump test? A. The following precautions are necessary when forming orifices for pressuremeasuring instruments and making connections: • The opening in the pipe should be perpendicular to the wall of the liquid passage. • The wall of the liquid passage should be smooth and of unvarying cross section for a distance of at
Figure 3.6.9a - Pressure tap opening with corrosion-related plug
least the larger of two pipe diameters or 12 inches (300 millimeters) preceding the orifice. All tubercles and roughness should be removed with a file or emery cloth. • The opening must be of a diameter d from 0.125 to 0.25 inches (3 to 6 millimeters) and a length equal to twice the diameter. • The edges of the opening should be provided with a suitable radius tangential to the wall of the liquid passage and should be free from burrs or irregularities. Figures 3.6.9a and 3.6.9b show two suggested arrangements of taps or orifices in conformance with the above. • Manometers, when used on very viscous products with wet lines interfacing directly or indirectly, require larger inlet taps into the pipeline to avoid excessive lag time for fluid to stabilize in the measuring system. Where more than one tap or orifice is required at a given measuring section, separate connections, properly valved, should be made. As an alternative, separate instruments should be provided.
Figure 3.6.9b - Welded-on pressure tap opening
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Multiple orifices can be connected to an instrument, except on those metering devices such as enture meters, where proper calibrations have been made.
The multiple orifices can be connected to a manifold and then to an instrument. With four or more pressure readings, there will be no more than 1% pressure variance between readings (see Figure 3.6.9c). All connections or leads from the orifice tap should be tight. These leads should be as short and direct as possible. For dry-tube leads, suitable drain pots should be provided and a loop should be formed high enough to keep the pumped liquid from entering the leads. For wet-tube leads, vent cocks for flushing should be provided at any high point or loop crest to ensure that there are no leaks. For additional information regarding rotary pump test procedures, see ANSI/HI 3.6 Rotary Pump Tests. Q. What are some differences between specific speed and suction specific speed for a rotodynamic pump? A. Specific speed is an index used to describe the rate of flow per head developed by an impeller at given rate of flow and speed and generally describes the geometry of an impeller as shown in Figure 1.1.4.1. Suction specific speed is an index that describes the Net Positive Suction Head (NPSH) requirements of an impeller at a given flow and speed. It is an assessment of a pump’s inlet design, including both the stationary casing and the rotating
impeller design. Higher numerical values are associated with better NPSH capabilities; however, higher suction specific speed designs generally require additional NPSH margin and can have a limited range of operation due to the onset of suction recirculation. The Hydraulic Institute defines specific speed as an index of pump performance (developed total head) at the pump’s best efficiency point (BEP) rate of flow, with the maximum diameter impeller and at a given rotative speed. Specific speed is expressed by the following equation:
ns=
n(Q) 0.5 (H) 0.75
Where: ns = specific speed (Ns in US customary units) n = rotative speed measured in revolutions per minute Q = total pump flow rate measured in U.S. gallons per minute (cubic meters per second) H = head per stage measured in feet (meters) The Hydraulic Institute defines suction specific speed as an index of pump suction operating characteristics. It is determined at the BEP rate of flow with the maximum diameter impeller. (Suction specific speed is an indicator of the net positive
0262 1762/16 © 2016 Elsevier Ltd. All rights reserved
WORLD PUMPS
Pump FAQs May 2016
suction head required [NPSH3] for given values of capacity and also provides an assessment of a pump's susceptibility to internal recirculation.) Suction specific speed is expressed by the following equation:
S=
n(Q) 0.5 (NPSH3)
0.75
n = rotative speed, in revolutions per minute Q = flow rate per impeller eye measured in U.S. gallons per minute (cubic meters per second) = total flow rate for single suction impellers
Where:
= one half total flow rate for double suction impellers
S = suction specific speed (Nss in US customary units)
NPSH3 = net positive suction head required in feet (meters) that will cause the total head
(or first-stage head of multistage pumps) to be reduced by 3%. For more information about specific speed and suction specific speed, see ANSI/HI 1.1-1.2 Rotodynamic (Centrifugal) Pumps for Nomenclature and Definitions, ANSI/HI 1.3 Rotodynamic Centrifugal Pumps for Design & Application, ANSI/HI 9.6.1 Rotodynamic Pumps Guideline for NPSH Margin and ANSI/HI 9.6.3 Rotodynamic Pumps Guideline for Allowable Operating Region.
Figure 3.6.9c – Loop manifold connecting pressure taps
www.worldpumps.com
43
Pump FAQs
WORLD PUMPS
May 2016
Pump FAQs Q. How should pressuremeasuring instruments be configured during a rotary pump test? A. The following precautions are necessary when forming orifices for pressuremeasuring instruments and making connections: • The opening in the pipe should be perpendicular to the wall of the liquid passage. • The wall of the liquid passage should be smooth and of unvarying cross section for a distance of at
Figure 3.6.9a - Pressure tap opening with corrosion-related plug
least the larger of two pipe diameters or 12 inches (300 millimeters) preceding the orifice. All tubercles and roughness should be removed with a file or emery cloth. • The opening must be of a diameter d from 0.125 to 0.25 inches (3 to 6 millimeters) and a length equal to twice the diameter. • The edges of the opening should be provided with a suitable radius tangential to the wall of the liquid passage and should be free from burrs or irregularities. Figures 3.6.9a and 3.6.9b show two suggested arrangements of taps or orifices in conformance with the above. • Manometers, when used on very viscous products with wet lines interfacing directly or indirectly, require larger inlet taps into the pipeline to avoid excessive lag time for fluid to stabilize in the measuring system. Where more than one tap or orifice is required at a given measuring section, separate connections, properly valved, should be made. As an alternative, separate instruments should be provided.
Figure 3.6.9b - Welded-on pressure tap opening
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Multiple orifices can be connected to an instrument, except on those metering devices such as enture meters, where proper calibrations have been made.
The multiple orifices can be connected to a manifold and then to an instrument. With four or more pressure readings, there will be no more than 1% pressure variance between readings (see Figure 3.6.9c). All connections or leads from the orifice tap should be tight. These leads should be as short and direct as possible. For dry-tube leads, suitable drain pots should be provided and a loop should be formed high enough to keep the pumped liquid from entering the leads. For wet-tube leads, vent cocks for flushing should be provided at any high point or loop crest to ensure that there are no leaks. For additional information regarding rotary pump test procedures, see ANSI/HI 3.6 Rotary Pump Tests. Q. What are some differences between specific speed and suction specific speed for a rotodynamic pump? A. Specific speed is an index used to describe the rate of flow per head developed by an impeller at given rate of flow and speed and generally describes the geometry of an impeller as shown in Figure 1.1.4.1. Suction specific speed is an index that describes the Net Positive Suction Head (NPSH) requirements of an impeller at a given flow and speed. It is an assessment of a pump’s inlet design, including both the stationary casing and the rotating
impeller design. Higher numerical values are associated with better NPSH capabilities; however, higher suction specific speed designs generally require additional NPSH margin and can have a limited range of operation due to the onset of suction recirculation. The Hydraulic Institute defines specific speed as an index of pump performance (developed total head) at the pump’s best efficiency point (BEP) rate of flow, with the maximum diameter impeller and at a given rotative speed. Specific speed is expressed by the following equation:
ns=
n(Q) 0.5 (H) 0.75
Where: ns = specific speed (Ns in US customary units) n = rotative speed measured in revolutions per minute Q = total pump flow rate measured in U.S. gallons per minute (cubic meters per second) H = head per stage measured in feet (meters) The Hydraulic Institute defines suction specific speed as an index of pump suction operating characteristics. It is determined at the BEP rate of flow with the maximum diameter impeller. (Suction specific speed is an indicator of the net positive
0262 1762/16 © 2016 Elsevier Ltd. All rights reserved
WORLD PUMPS
Pump FAQs May 2016
suction head required [NPSH3] for given values of capacity and also provides an assessment of a pump's susceptibility to internal recirculation.) Suction specific speed is expressed by the following equation:
S=
n(Q) 0.5 (NPSH3)
0.75
n = rotative speed, in revolutions per minute Q = flow rate per impeller eye measured in U.S. gallons per minute (cubic meters per second) = total flow rate for single suction impellers
Where:
= one half total flow rate for double suction impellers
S = suction specific speed (Nss in US customary units)
NPSH3 = net positive suction head required in feet (meters) that will cause the total head
(or first-stage head of multistage pumps) to be reduced by 3%. For more information about specific speed and suction specific speed, see ANSI/HI 1.1-1.2 Rotodynamic (Centrifugal) Pumps for Nomenclature and Definitions, ANSI/HI 1.3 Rotodynamic Centrifugal Pumps for Design & Application, ANSI/HI 9.6.1 Rotodynamic Pumps Guideline for NPSH Margin and ANSI/HI 9.6.3 Rotodynamic Pumps Guideline for Allowable Operating Region.
Figure 3.6.9c – Loop manifold connecting pressure taps
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43