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A pump can be defined as “a mechanical device that adds energy to a fluid to increase its flow rate and static pressure.”<ref name="r1"/> This process can be accomplished with positive displacement pumps or kinetic-energy pumps.  
A pump can be defined as “a mechanical device that adds energy to a fluid to increase its flow rate and static pressure.”<ref name="r1">World LNG Source Book 2001. 2001. Des Plaines, Illinois: Gas Technology Inst.</ref> This process can be accomplished with positive displacement pumps or kinetic-energy pumps.
 
== Fluid principles and hydraulics ==
 
=== Types of fluids ===
 
Pumps are used to move fluids, which include:


==Fluid principles and hydraulics==
===Types of fluids===
Pumps are used to move fluids, which include:
*Liquids
*Liquids
*Dissolved gases - dissolved air and hydrocarbon vapors
*Dissolved gases - dissolved air and hydrocarbon vapors
*Solids - sand, clay, corrosion byproducts, and scale
*Solids - sand, clay, corrosion byproducts, and scale


The most common types of liquids pumped in upstream operations are:  
The most common types of liquids pumped in upstream operations are:
 
*Crude oil
*Crude oil
*Condensate
*Condensate
Line 16: Line 20:
*Water
*Water


Each fluid has different physical properties that must be taken into consideration when sizing and selecting a pump. The most important physical properties are suction pressure, specific gravity, viscosity, vapor pressure, solids content, and lubricity.  
Each fluid has different physical properties that must be taken into consideration when sizing and selecting a pump. The most important physical properties are suction pressure, specific gravity, viscosity, vapor pressure, solids content, and lubricity.
 
== Types of pumps ==
 
=== Positive displacement pumps ===
 
[[Positive_displacement_pumps|Positive displacement pumps]] add energy to a fluid by applying force to the fluid with a mechanical device such as a piston, plunger, or diaphragm. There are two types of positive-displacement pumps:


==Types of pumps==
===Positive displacement pumps===
[[Positive_displacement_pumps|Positive displacement pumps]] add energy to a fluid by applying force to the fluid with a mechanical device such as a piston, plunger, or diaphragm. There are two types of positive-displacement pumps:
*Reciprocating
*Reciprocating
*Rotary
*Rotary


Reciprocating pumps use pistons, plungers, or diaphragms to displace the fluid, while rotary pumps operate through the mating action of gears, lobes, or screw-type shafts.  
Reciprocating pumps use pistons, plungers, or diaphragms to displace the fluid, while rotary pumps operate through the mating action of gears, lobes, or screw-type shafts.
 
=== Kinetic energy pumps ===
 
[[Centrifugal_pumps|Kinetic Energy Pumps]] (energy associated with motion) is added to a liquid to increase its velocity and, indirectly, its pressure. Kinetic-energy pumps operate by drawing liquid into the center of a rapidly rotating impeller. Radial vanes on the impeller throw the liquid outward toward the impeller rim. As liquid leaves the impeller, it comes in contact with the pump casing or volute. The casing is shaped to direct liquid toward a discharge port. The casing slows the liquid and converts some of its velocity into pressure. There are three classes of kinetic-energy pumps:


===Kinetic energy pumps===
[[Centrifugal_pumps|Kinetic Energy Pumps]] (energy associated with motion) is added to a liquid to increase its velocity and, indirectly, its pressure. Kinetic-energy pumps operate by drawing liquid into the center of a rapidly rotating impeller. Radial vanes on the impeller throw the liquid outward toward the impeller rim. As liquid leaves the impeller, it comes in contact with the pump casing or volute. The casing is shaped to direct liquid toward a discharge port. The casing slows the liquid and converts some of its velocity into pressure. There are three classes of kinetic-energy pumps:
*Centrifugal - radial-, axial-, and mixed-flow designs
*Centrifugal - radial-, axial-, and mixed-flow designs
*Regenerative-turbine
*Regenerative-turbine
*Special-effects pumps
*Special-effects pumps


Centrifugal pumps account for more than 80% of pumps used in production operations because they exhibit uniform flow, are free of low-frequency pulsations, and are not subject to mechanical problems. '''Fig. 1''' illustrates pumps commonly used in upstream production operations.  
Centrifugal pumps account for more than 80% of pumps used in production operations because they exhibit uniform flow, are free of low-frequency pulsations, and are not subject to mechanical problems. '''Fig. 1''' illustrates pumps commonly used in upstream production operations.


<gallery widths=300px heights=200px>
<gallery widths="300px" heights="200px">
File:Vol3 Page 232 Image 0001.png|'''Fig. 1—Pumps commonly used in production operations.'''
File:Vol3 Page 232 Image 0001.png|'''Fig. 1—Pumps commonly used in production operations.'''
</gallery>
</gallery>




===Pumping system design===
Designing any pumping service involves three major activities: process design, mechanical design, and vendor selection.


Process Design. The first step in process design is to obtain a design flow rate. The design flow rate should be selected after considering all flow variations, such as:  
=== Pumping system design ===
 
Designing any pumping service involves three major activities: process design, mechanical design, and vendor selection.
 
Process Design. The first step in process design is to obtain a design flow rate. The design flow rate should be selected after considering all flow variations, such as:
 
*Startup conditions
*Startup conditions
*Future expansion
*Future expansion
*Maximum anticipated flow
*Maximum anticipated flow


The next step is to determine the liquid properties critical to pump design. These properties include:  
The next step is to determine the liquid properties critical to pump design. These properties include:
 
*Specific gravity
*Specific gravity
*Temperature
*Temperature
*Viscosity
*Viscosity
*Pour point
*Pour point
*etc.  
*etc.
 
Values are required at pumping conditions and, in some cases, at ambient conditions as well. The next step is to calculate available suction conditions such as rated suction pressure, maximum suction pressure, and net positive suction head available (NPSHA). (See Hydrodynamics for information on NPSHA.) Once the available suction conditions have been established, the effect of the selected control system on pump performance requirements must be determined (see [[Regulation_of_Flow_Rate|Regulation of Flow Rate]]). The next step is to calculate the minimum discharge-pressure requirements of the pump. The last step is to calculate the total dynamic head (TDH) at the specific gravity corresponding to rated pumping temperature.
 
==== Mechanical Design ====
 
The first step in mechanical design is to determine the design pressure and temperature required for the pump and its associated piping. Once this is done, a pump type and materials of construction are selected. The next step is to determine the sparing (backup) requirements, the need for parallel operation and control-system details. Then select a shaft-seal type and determine the requirements for an external flushing or sealing system and estimate the power requirements and choose a driver (motor, engine, or turbine) for the pump. Lastly, document the design by including calculations, studies, design specifications, utility requirements, and estimate summary.


Values are required at pumping conditions and, in some cases, at ambient conditions as well. The next step is to calculate available suction conditions such as rated suction pressure, maximum suction pressure, and net positive suction head available (NPSHA). (See Hydrodynamics for information on NPSHA.) Once the available suction conditions have been established, the effect of the selected control system on pump performance requirements must be determined (see [[Regulation of Flow Rate]]). The next step is to calculate the minimum discharge-pressure requirements of the pump. The last step is to calculate the total dynamic head (TDH) at the specific gravity corresponding to rated pumping temperature.
==== Vendor Selection ====


====Mechanical Design====
Factors that have the greatest influence on the selection of the most cost-effective pump type include:
The first step in mechanical design is to determine the design pressure and temperature required for the pump and its associated piping. Once this is done, a pump type and materials of construction are selected. The next step is to determine the sparing (backup) requirements, the need for parallel operation and control-system details. Then select a shaft-seal type and determine the requirements for an external flushing or sealing system and estimate the power requirements and choose a driver (motor, engine, or turbine) for the pump. Lastly, document the design by including calculations, studies, design specifications, utility requirements, and estimate summary.


====Vendor Selection====
Factors that have the greatest influence on the selection of the most cost-effective pump type include:
*Capacity
*Capacity
*TDH
*TDH
Line 67: Line 83:
*Capacity control
*Capacity control


Within the general type selections, a particular construction style is most influenced by:  
Within the general type selections, a particular construction style is most influenced by:
 
*Discharge pressure
*Discharge pressure
*NPSHA
*NPSHA
*Fluid temperature
*Fluid temperature
*Space and weight limitations
*Space and weight limitations
*Fluid shearing characteristics
== Hydraulic principles ==
Hydraulics deals with the mechanical properties of water and other liquids and the application of these properties to engineering. Hydraulics is divided into two areas:


==Hydraulic principles==
Hydraulics deals with the mechanical properties of water and other liquids and the application of these properties to engineering. Hydraulics is divided into two areas:
*Hydrostatics (fluids at rest)
*Hydrostatics (fluids at rest)
*Hydrodynamics (fluids in motion)
*Hydrodynamics (fluids in motion)


===Hydrostatics===
=== Hydrostatics ===
A liquid has a definite volume when compared to a gas, which tends to expand to fit its container. When unconfined, a liquid seeks the lowest possible level. Because of its fluidity, a liquid will conform to the shape of its container.  
 
A liquid has a definite volume when compared to a gas, which tends to expand to fit its container. When unconfined, a liquid seeks the lowest possible level. Because of its fluidity, a liquid will conform to the shape of its container.
 
==== Pressure ====
 
The pressure existing at any point in a liquid body at rest is caused by the atmospheric pressure exerted on the surface plus the weight of the liquid above that point. This pressure is equal in all directions.
 
==== Temperature ====


====Pressure====
For most liquids, an increase in temperature decreases viscosity, decreases specific gravity, and increases volume. Temperature affects:
The pressure existing at any point in a liquid body at rest is caused by the atmospheric pressure exerted on the surface plus the weight of the liquid above that point. This pressure is equal in all directions.  


====Temperature====
For most liquids, an increase in temperature decreases viscosity, decreases specific gravity, and increases volume. Temperature affects:
*The type of pump construction
*The type of pump construction
*Material selection
*Material selection
Line 91: Line 115:
*The pump’s flange pressure/temperature rating
*The pump’s flange pressure/temperature rating


====Air properties====
==== Air properties ====
Air is a mixture of oxygen, nitrogen, and other compounds. The standard pressure of air is defined at 60°F, 36% relative humidity, and sea level. The weight of a column of air above the Earth’s surface at 45° latitude and sea level is 14.696 psia (29.92 in. of mercury). Atmospheric pressure decreases by approximately 0.5 psi for each 1,000 ft of elevation above sea level.  
 
Air is a mixture of oxygen, nitrogen, and other compounds. The standard pressure of air is defined at 60°F, 36% relative humidity, and sea level. The weight of a column of air above the Earth’s surface at 45° latitude and sea level is 14.696 psia (29.92 in. of mercury). Atmospheric pressure decreases by approximately 0.5 psi for each 1,000 ft of elevation above sea level.
 
==== Head ====


====Head====
The relationship of head to pressure is expressed as
The relationship of head to pressure is expressed as  


[[File:Vol3_page_233_eq_001.PNG]]'''(Eq. 1)'''
[[File:Vol3 page 233 eq 001.PNG|RTENOTITLE]]'''(Eq. 1)'''


where  
where


h = height of the fluid column above a reference point
h = height of the fluid column above a reference point
p = pressure.


The head of liquid is not related to the area occupied by the liquid. '''Fig. 2''' illustrates types of head.  
p = pressure.
 
The head of liquid is not related to the area occupied by the liquid. '''Fig. 2''' illustrates types of head.


<gallery widths=300px heights=200px>
<gallery widths="300px" heights="200px">
File:Vol3 Page 234 Image 0001.png|'''Fig. 2—Types of head.'''
File:Vol3 Page 234 Image 0001.png|'''Fig. 2—Types of head.'''
</gallery>
</gallery>


[[File:Vol3_page_233_eq_002.PNG]]'''(Eq. 2)'''
[[File:Vol3 page 233 eq 002.PNG|RTENOTITLE]]'''(Eq. 2)'''


where  
where


γ = specific gravity of the liquid
γ = specific gravity of the liquid
Line 119: Line 145:
ρ<sub>f</sub> = density of the liquid being pumped
ρ<sub>f</sub> = density of the liquid being pumped


ρ<sub>w</sub> = density of water at standard conditions of temperature and pressure.  
ρ<sub>w</sub> = density of water at standard conditions of temperature and pressure.
 
It is important to realize that although the heads of different liquids are the same, their pressures are different because of the differences in specific gravities. For example, assume three 100-ft-tall tanks filled with gasoline, water, and molasses, respectively. The pressure measured at the bottom of each tank is different because of the differences of specific gravities of gasoline (0.75), water (1.0), and molasses (1.45).
 
==== Centrifugal pump considerations ====
 
Operating pressure is expressed in feet of the liquid that is being pumped. Suction and discharge pressures are expressed as suction head and discharge head, respectively. Pressures are expressed in feet of head, because it is more important to know how much a pump can raise the liquid it is pumping, rather than the amount of pressure the pump is adding to the liquid.


It is important to realize that although the heads of different liquids are the same, their pressures are different because of the differences in specific gravities. For example, assume three 100-ft-tall tanks filled with gasoline, water, and molasses, respectively. The pressure measured at the bottom of each tank is different because of the differences of specific gravities of gasoline (0.75), water (1.0), and molasses (1.45).
==== Positive displacement pump considerations ====


====Centrifugal pump considerations====
Operating pressures are almost always expressed in terms of pressure (psi).
Operating pressure is expressed in feet of the liquid that is being pumped. Suction and discharge pressures are expressed as suction head and discharge head, respectively. Pressures are expressed in feet of head, because it is more important to know how much a pump can raise the liquid it is pumping, rather than the amount of pressure the pump is adding to the liquid.  


====Positive displacement pump considerations====
==== Static head, static lift, and submergence terminology ====
Operating pressures are almost always expressed in terms of pressure (psi).


====Static head, static lift, and submergence terminology====
'''Fig. 3''' illustrates the relationship between static head, static lift, and submergence. Static head is the vertical distance between a liquid level and a datum line, when the supply is above the datum. Static lift is the vertical distance between a liquid level and a datum level, when the datum is above the liquid. Datum line is the centerline of the pump inlet connection, or the horizontal centerline of the first-stage impeller in vertical pumps.
'''Fig. 3''' illustrates the relationship between static head, static lift, and submergence. Static head is the vertical distance between a liquid level and a datum line, when the supply is above the datum. Static lift is the vertical distance between a liquid level and a datum level, when the datum is above the liquid. Datum line is the centerline of the pump inlet connection, or the horizontal centerline of the first-stage impeller in vertical pumps.  


<gallery widths=300px heights=200px>
<gallery widths="300px" heights="200px">
File:Vol3 Page 232 Image 0001.png|'''Fig. 3—Relationship between static head, static lift, and submergence.'''
File:Vol3 Page 232 Image 0001.png|'''Fig. 3—Relationship between static head, static lift, and submergence.'''
</gallery>
</gallery>




====Theoretical lift====
A pump that develops a perfect vacuum at its suction end can lift a column of water 34 ft. This vertical distance is called theoretical lift. The pressure to lift the liquid comes from atmosphere pressure. At sea level, atmosphere pressure is approximately 14.7 psia.


====Actual suction lift====
==== Theoretical lift ====
Because a perfect vacuum is never achieved and because some lift is lost to friction in the suction line, the maximum actual suction lift for a positive-displacement pump is approximately 22 ft. The maximum actual suction lift for a centrifugal pump is approximately 15 ft when pumping water from an open air tank. Positive-displacement pumps can operate with lower suction pressures or high suction lifts because they can create stronger vacuums. Suction lift will be greater if the pressure in a closed tank is greater than atmospheric pressure.  
 
A pump that develops a perfect vacuum at its suction end can lift a column of water 34 ft. This vertical distance is called theoretical lift. The pressure to lift the liquid comes from atmosphere pressure. At sea level, atmosphere pressure is approximately 14.7 psia.
 
==== Actual suction lift ====
 
Because a perfect vacuum is never achieved and because some lift is lost to friction in the suction line, the maximum actual suction lift for a positive-displacement pump is approximately 22 ft. The maximum actual suction lift for a centrifugal pump is approximately 15 ft when pumping water from an open air tank. Positive-displacement pumps can operate with lower suction pressures or high suction lifts because they can create stronger vacuums. Suction lift will be greater if the pressure in a closed tank is greater than atmospheric pressure.
 
==== Submergence ====
 
Submergence is often confused with either suction static head or static lift. For vertical pumps, submergence relates the liquid level to the setting of the pump. For horizontal pumps, submergence relates to the height of liquid level necessary in the source vessel or tank to prevent the formation of vortexing and the resulting flashing of vapors in the pump suction.


====Submergence====
==== Vapor pressure ====
Submergence is often confused with either suction static head or static lift. For vertical pumps, submergence relates the liquid level to the setting of the pump. For horizontal pumps, submergence relates to the height of liquid level necessary in the source vessel or tank to prevent the formation of vortexing and the resulting flashing of vapors in the pump suction.


====Vapor pressure====
As the pressure on a liquid is decreased, there is a tendency for the bubbles of vapor to be liberated. The vapor pressure of a liquid is the pressure at which the first bubble of vapor appears at a given temperature. At 60°F, the vapor pressure of water is 0.3 psia (0.7 ft). At 212°F, the vapor pressure of water is 14.7 psia (34 ft). '''Fig. 4''' illustrates the vapor pressure of water for various temperatures. For other fluids, refer to standard references (e.g., Hydraulic Institute Engineering Data Book1).
As the pressure on a liquid is decreased, there is a tendency for the bubbles of vapor to be liberated. The vapor pressure of a liquid is the pressure at which the first bubble of vapor appears at a given temperature. At 60°F, the vapor pressure of water is 0.3 psia (0.7 ft). At 212°F, the vapor pressure of water is 14.7 psia (34 ft). '''Fig. 4''' illustrates the vapor pressure of water for various temperatures. For other fluids, refer to standard references (e.g., Hydraulic Institute Engineering Data Book1).  


<gallery widths=300px heights=200px>
<gallery widths="300px" heights="200px">
File:Vol3 Page 236 Image 0001.png|'''Fig. 4—Vapor pressure of water for various temperatures.'''
File:Vol3 Page 236 Image 0001.png|'''Fig. 4—Vapor pressure of water for various temperatures.'''
</gallery>
</gallery>




====Suspended solids====
The amount and type of suspended solids entrained in the liquid can affect the characteristics and behavior of that liquid. Increased concentrations of solids increase the specific gravity, viscosity, and abrasiveness of a liquid. The type and concentration of suspended solids can affect the style of pump selected and the materials of construction. Suspended solids also affect the selection of impeller design in centrifugal pumps, which in turn affects the wear rate, efficiency, and power consumption.


====Dissolved gases====
==== Suspended solids ====
Small amounts of dissolved gases have little effect on flow rate or other pumping requirements. If large amounts of gas enter the liquid through piping leaks or as a result of vortexing in vessels, the specific gravity of the liquid will decrease. Dissolved gases can also reduce the amount of NPSHA at the pump suction. (See Hydrodynamics for a discussion of NPSHA).  
 
The amount and type of suspended solids entrained in the liquid can affect the characteristics and behavior of that liquid. Increased concentrations of solids increase the specific gravity, viscosity, and abrasiveness of a liquid. The type and concentration of suspended solids can affect the style of pump selected and the materials of construction. Suspended solids also affect the selection of impeller design in centrifugal pumps, which in turn affects the wear rate, efficiency, and power consumption.
 
==== Dissolved gases ====
 
Small amounts of dissolved gases have little effect on flow rate or other pumping requirements. If large amounts of gas enter the liquid through piping leaks or as a result of vortexing in vessels, the specific gravity of the liquid will decrease. Dissolved gases can also reduce the amount of NPSHA at the pump suction. (See Hydrodynamics for a discussion of NPSHA).
 
==== Viscosity ====
 
Viscosity offers resistance to flow because of friction within the fluid. Viscosity levels have a significant impact on pump type selection, efficiency, head capacity, and warm-up. High-viscosity liquids decrease a centrifugal pump’s efficiency and head performance, while increasing the power requirements. The viscosity of all liquids varies with temperature. For viscosities of liquids, refer to standard industry references (e.g., Hydraulic Institute Engineering Data Book<ref name="r1">World LNG Source Book 2001. 2001. Des Plaines, Illinois: Gas Technology Inst.</ref>).
 
==== Corrosivity ====


====Viscosity====
The corrosive nature of the fluid being pumped has a bearing on pump type selection, materials of construction, and corrosion allowance. Special mechanical seals and flushing arrangements may be required.
Viscosity offers resistance to flow because of friction within the fluid. Viscosity levels have a significant impact on pump type selection, efficiency, head capacity, and warm-up. High-viscosity liquids decrease a centrifugal pump’s efficiency and head performance, while increasing the power requirements. The viscosity of all liquids varies with temperature. For viscosities of liquids, refer to standard industry references (e.g., Hydraulic Institute Engineering Data Book<ref name="r1"/>).  


====Corrosivity====
=== Hydrodynamics ===
The corrosive nature of the fluid being pumped has a bearing on pump type selection, materials of construction, and corrosion allowance. Special mechanical seals and flushing arrangements may be required.


===Hydrodynamics===
Hydrodynamics is the study of fluids in motion. Bernoulli’s equation states that
Hydrodynamics is the study of fluids in motion. Bernoulli’s equation states that


[[File:Vol3_page_237_eq_001.PNG]]'''(Eq. 3)'''  
[[File:Vol3 page 237 eq 001.PNG|RTENOTITLE]]'''(Eq. 3)'''


where  
where


v = average velocity of the liquid in the pipe
v = average velocity of the liquid in the pipe
Line 181: Line 221:
Z = height above a datum
Z = height above a datum


h<sub>f</sub> = friction loss between points 1 and 2. Subscripts 1 and 2 refer to locations along a pipe. An examination of each of the terms in '''Eq. 3''' provides a better understanding of the general equation for modeling a pumping system.  
h<sub>f</sub> = friction loss between points 1 and 2. Subscripts 1 and 2 refer to locations along a pipe. An examination of each of the terms in '''Eq. 3''' provides a better understanding of the general equation for modeling a pumping system.


Velocity Head. Velocity head is the potential energy that has been converted to kinetic energy. Velocity head can be expressed as  
Velocity Head. Velocity head is the potential energy that has been converted to kinetic energy. Velocity head can be expressed as


[[File:Vol3_page_237_eq_002.PNG]]'''(Eq. 4)'''
[[File:Vol3 page 237 eq 002.PNG|RTENOTITLE]]'''(Eq. 4)'''


[[File:Vol3_page_237_eq_003.PNG]]'''(Eq. 5)'''
[[File:Vol3 page 237 eq 003.PNG|RTENOTITLE]]'''(Eq. 5)''' where
where  


Q = flow rate
Q = flow rate


d = inside pipe diameter.  
d = inside pipe diameter.
 
The velocity head increases the amount of work required of a pump. The velocity head is usually not included in actual system calculations when piping velocities are kept within the prescribed limits of 3 to 15 ft/sec. The velocity head is included in the total dynamic head on the centrifugal-pump curves.
 
==== Pressure head ====
 
The energy contained in the liquid is expressed as pressure head and expressed as p/ρ in '''Eq. 3'''.
 
==== Elevation head ====
 
The energy contained in the liquid as a result of its elevation relative to a datum is called the elevation head and is expressed as Z in '''Eq. 3'''.
 
==== Head losses ====
 
Head losses are potential energy that has been lost because of frictional resistance of the piping system (pipe, valves, fittings, and entrance and exit losses). Unlike velocity head, friction head cannot be ignored in system calculations. Head loss values vary as the square of the flow rate. Head losses can be a significant portion of the total head.


The velocity head increases the amount of work required of a pump. The velocity head is usually not included in actual system calculations when piping velocities are kept within the prescribed limits of 3 to 15 ft/sec. The velocity head is included in the total dynamic head on the centrifugal-pump curves.
==== Control losses ====


====Pressure head====
Control losses occur on the discharge side of a centrifugal pump that has been equipped with a backpressure valve to control flow rate. As the liquid flows through the control valve, energy is lost. Next to static head, control losses are frequently the most important factor in calculating the pump’s total dynamic head. For pump applications, control losses are treated separately from head losses, even though they are included in the hf term in '''Eq. 3'''.
The energy contained in the liquid is expressed as pressure head and expressed as p/ρ in '''Eq. 3'''.  


====Elevation head====
==== Acceleration head ====
The energy contained in the liquid as a result of its elevation relative to a datum is called the elevation head and is expressed as Z in '''Eq. 3'''.


====Head losses====
Acceleration head is used to describe the losses associated with the pulsating flow of reciprocating pumps. Theoretically, acceleration head should be included in the hf term of '''Eq. 3'''. The Hydraulic Institute Engineering Data Book<ref name="r1">World LNG Source Book 2001. 2001. Des Plaines, Illinois: Gas Technology Inst.</ref> discusses the calculation of acceleration head.
Head losses are potential energy that has been lost because of frictional resistance of the piping system (pipe, valves, fittings, and entrance and exit losses). Unlike velocity head, friction head cannot be ignored in system calculations. Head loss values vary as the square of the flow rate. Head losses can be a significant portion of the total head.  


====Control losses====
==== Total dynamic head ====
Control losses occur on the discharge side of a centrifugal pump that has been equipped with a backpressure valve to control flow rate. As the liquid flows through the control valve, energy is lost. Next to static head, control losses are frequently the most important factor in calculating the pump’s total dynamic head. For pump applications, control losses are treated separately from head losses, even though they are included in the hf term in '''Eq. 3'''.


====Acceleration head====
TDH is the difference between the pumping system’s discharge head and suction head. It is also equal to the difference in pressure-gauge readings (converted to feet) across an existing operating pump (discounting velocity head).
Acceleration head is used to describe the losses associated with the pulsating flow of reciprocating pumps. Theoretically, acceleration head should be included in the hf term of '''Eq. 3'''. The Hydraulic Institute Engineering Data Book<ref name="r1"/> discusses the calculation of acceleration head.  


====Total dynamic head====
==== Suction head ====
TDH is the difference between the pumping system’s discharge head and suction head. It is also equal to the difference in pressure-gauge readings (converted to feet) across an existing operating pump (discounting velocity head).


====Suction head====
Suction head is defined as the sum of the suction-vessel operating gauge pressure (converted to feet), the vertical distance between the suction-vessel liquid level and the pump reference point, less head losses in the suction piping [discounting change in velocity,
Suction head is defined as the sum of the suction-vessel operating gauge pressure (converted to feet), the vertical distance between the suction-vessel liquid level and the pump reference point, less head losses in the suction piping [discounting change in velocity,  


[[File:Vol3_page_237_eq_004.PNG]]'''(Eq. 6)'''
[[File:Vol3 page 237 eq 004.PNG|RTENOTITLE]]'''(Eq. 6)'''


and acceleration head]. Suction head can be expressed as  
and acceleration head]. Suction head can be expressed as


[[File:Vol3_page_238_eq_001.PNG]]'''(Eq. 7)'''
[[File:Vol3 page 238 eq 001.PNG|RTENOTITLE]]'''(Eq. 7)'''


which can be reduced to  
which can be reduced to


[[File:Vol3_page_238_eq_002.PNG]]'''(Eq. 8)'''   
[[File:Vol3 page 238 eq 002.PNG|RTENOTITLE]]'''(Eq. 8)''' 


where  
where


H<sub>s</sub> = suction head of liquid being pumped
H<sub>s</sub> = suction head of liquid being pumped
Line 235: Line 281:
H<sub>1</sub> = height of liquid suction vessel above pump reference point
H<sub>1</sub> = height of liquid suction vessel above pump reference point


p<sub>f1</sub> = pressure drop resulting from friction in the suction piping.  
p<sub>f1</sub> = pressure drop resulting from friction in the suction piping.
 
==== Discharge head ====


====Discharge head====
Discharge head is defined as the sum of the discharge-vessel operating gauge pressure (converted to feet), the liquid level in the discharge vessel above the pump reference point, pressure drop because of friction in the discharge piping, and control losses (discounting velocity head). It can be expressed as
Discharge head is defined as the sum of the discharge-vessel operating gauge pressure (converted to feet), the liquid level in the discharge vessel above the pump reference point, pressure drop because of friction in the discharge piping, and control losses (discounting velocity head). It can be expressed as  


[[File:Vol3_page_238_eq_003.PNG]]'''(Eq. 9)'''
[[File:Vol3 page 238 eq 003.PNG|RTENOTITLE]]'''(Eq. 9)'''


which can be reduced to  
which can be reduced to


[[File:Vol3_page_238_eq_004.PNG]]'''(Eq. 10)'''  
[[File:Vol3 page 238 eq 004.PNG|RTENOTITLE]]'''(Eq. 10)'''


where  
where


H<sub>d</sub> = discharge head of liquid being pumped
H<sub>d</sub> = discharge head of liquid being pumped
Line 256: Line 303:
p<sub>f2</sub> = pressure drop resulting from friction in the discharge piping
p<sub>f2</sub> = pressure drop resulting from friction in the discharge piping


P<sub>c</sub> = discharge flow-control-valve losses.  
P<sub>c</sub> = discharge flow-control-valve losses.
 
==== Calculating TDH ====
 
The pump TDH is the difference between the suction and discharge heads.
 
[[File:Vol3 page 238 eq 005.PNG|RTENOTITLE]]'''(Eq. 11)'''
 
which can be substituted as


====Calculating TDH====
[[File:Vol3 page 238 eq 006.PNG|RTENOTITLE]]'''(Eq. 12)'''
The pump TDH is the difference between the suction and discharge heads.  


[[File:Vol3_page_238_eq_005.PNG]]'''(Eq. 11)'''
where


which can be substituted as
H<sub>td</sub> = total dynamic head required of a pump.


[[File:Vol3_page_238_eq_006.PNG]]'''(Eq. 12)'''
==== Net positive suction head (NPSH) ====


where
NPSH is defined as the total suction head in feet of liquid (absolute at the pump centerline or impeller eye) less the vapor pressure (in feet) of the liquid being pumped.


H<sub>td</sub> = total dynamic head required of a pump.
==== Net positive suction head required ====


====Net positive suction head (NPSH)====
Net positive suction head required (NPSHR) is defined as the amount of NPSH required to move and accelerate the liquid from the pump suction into the pump itself. It is determined either by test or calculation by the pump manufacturer for the specific pump under consideration. NPSHR is a function of liquid geometry and the smoothness of the surface areas. For centrifugal pumps, other factors that control NPSHR are:
NPSH is defined as the total suction head in feet of liquid (absolute at the pump centerline or impeller eye) less the vapor pressure (in feet) of the liquid being pumped.  


====Net positive suction head required====
Net positive suction head required (NPSHR) is defined as the amount of NPSH required to move and accelerate the liquid from the pump suction into the pump itself. It is determined either by test or calculation by the pump manufacturer for the specific pump under consideration. NPSHR is a function of liquid geometry and the smoothness of the surface areas. For centrifugal pumps, other factors that control NPSHR are:
*The type of impeller
*The type of impeller
*Design of impeller eye
*Design of impeller eye
*Rotational speeds
*Rotational speeds


NPSHR is determined on the basis of handling cold water. Field experience coupled with laboratory testing have confirmed that centrifugal pumps handling gas-free hydrocarbon liquids and water at elevated temperatures will operate satisfactorily, with harmless cavitation and less NPSHR than would be required for cold water.  
NPSHR is determined on the basis of handling cold water. Field experience coupled with laboratory testing have confirmed that centrifugal pumps handling gas-free hydrocarbon liquids and water at elevated temperatures will operate satisfactorily, with harmless cavitation and less NPSHR than would be required for cold water.
 
==== Net positive suction head available ====
 
NPSHA must be equal to or greater than NPSHR. If this is not the case, cavitation or flashing may occur in the pump suction. Cavitation occurs when small vapor bubbles appear in the liquid because of a drop in pressure and then collapse rapidly with explosive force when the pressure is increased in the pump. Cavitation results in decreased efficiency, capacity, and head and can cause serious erosion of pump parts. Flashing causes the pump suction cavity to be filled with vapors and, as a result, the pump becomes vapor locked. This usually results in the pump freezing up, which is called pump seizure.
 
NPSHA is not a function of the pump itself but of the piping system for the pump. It can be calculated from


====Net positive suction head available====
[[File:Vol3 page 239 eq 001.PNG|RTENOTITLE]] '''(Eq. 13)'''
NPSHA must be equal to or greater than NPSHR. If this is not the case, cavitation or flashing may occur in the pump suction. Cavitation occurs when small vapor bubbles appear in the liquid because of a drop in pressure and then collapse rapidly with explosive force when the pressure is increased in the pump. Cavitation results in decreased efficiency, capacity, and head and can cause serious erosion of pump parts. Flashing causes the pump suction cavity to be filled with vapors and, as a result, the pump becomes vapor locked. This usually results in the pump freezing up, which is called pump seizure.  


NPSHA is not a function of the pump itself but of the piping system for the pump. It can be calculated from
where


[[File:Vol3_page_239_eq_001.PNG]] '''(Eq. 13)'''
p<sub>A</sub> = atmospheric pressure


where
p<sub>va</sub> = liquid vapor pressure at pumping temperature.


p<sub>A</sub> = atmospheric pressure  
NPSHA decreases with increases in liquid temperature and pipe friction losses. Because pipe friction losses vary as the square of the flow, NPSHA also varies as the square of the flow. Thus, NPSHA will be the lowest at the maximum flow requirement. Accordingly, it is important to recognize the need for calculating NPSHA (and NPSHR) at maximum flow conditions as well as maximum fluid temperature, not just at design conditions. Unless subcooled, a pure-component hydrocarbon liquid is typically in equilibrium with the vapors in a pressure vessel. Thus, increases in the vessel operating pressures are almost fully offset by a corresponding increase in the vapor pressure. When this occurs,


p<sub>va</sub> = liquid vapor pressure at pumping temperature.  
[[File:Vol3 page 239 eq 002.PNG|RTENOTITLE]]'''(Eq. 14)'''


NPSHA decreases with increases in liquid temperature and pipe friction losses. Because pipe friction losses vary as the square of the flow, NPSHA also varies as the square of the flow. Thus, NPSHA will be the lowest at the maximum flow requirement. Accordingly, it is important to recognize the need for calculating NPSHA (and NPSHR) at maximum flow conditions as well as maximum fluid temperature, not just at design conditions. Unless subcooled, a pure-component hydrocarbon liquid is typically in equilibrium with the vapors in a pressure vessel. Thus, increases in the vessel operating pressures are almost fully offset by a corresponding increase in the vapor pressure. When this occurs,
==== NPSH margin ====


[[File:Vol3_page_239_eq_002.PNG]]'''(Eq. 14)'''
The NPSH margin is NPSHA less the NPSHR. The Hydraulic Inst. recommends an NPSH margin of 3 to 5 ft.<ref name="r1">World LNG Source Book 2001. 2001. Des Plaines, Illinois: Gas Technology Inst.</ref>


====NPSH margin====
When a new system offers insufficient NPSH margin for optimum pump selection, either the NPSHA must be increased, the NPSHR must be decreased, or both. To increase the NPSHA, one can raise the liquid level, lower the elevation of the selected pump, change to a low-NPSHR pump, or cool the liquid. To reduce the NPSHR, one can use different design impellers or inducers or use several smaller pumps with lower NPSHRs in parallel.
The NPSH margin is NPSHA less the NPSHR. The Hydraulic Inst. recommends an NPSH margin of 3 to 5 ft.<ref name="r1"/>


When a new system offers insufficient NPSH margin for optimum pump selection, either the NPSHA must be increased, the NPSHR must be decreased, or both. To increase the NPSHA, one can raise the liquid level, lower the elevation of the selected pump, change to a low-NPSHR pump, or cool the liquid. To reduce the NPSHR, one can use different design impellers or inducers or use several smaller pumps with lower NPSHRs in parallel.  
When an existing pumping system exhibits insufficient NPSH margin, it is too late to use these solutions without going through an expensive change. Most of these problems can be traced to suction flow restrictions (orifice plates, plugged strainers, partially closed valves, etc.) and inadequate source-tank liquid levels.


When an existing pumping system exhibits insufficient NPSH margin, it is too late to use these solutions without going through an expensive change. Most of these problems can be traced to suction flow restrictions (orifice plates, plugged strainers, partially closed valves, etc.) and inadequate source-tank liquid levels.
==== Power requirements ====


====Power requirements====
Once the TDH has been calculated, the power requirements can be determined with
Once the TDH has been calculated, the power requirements can be determined with  


[[File:Vol3_page_239_eq_003.PNG]]'''(Eq. 15)'''
[[File:Vol3 page 239 eq 003.PNG|RTENOTITLE]]'''(Eq. 15)'''


For kinetic-energy pumps,  
For kinetic-energy pumps,


[[File:Vol3_page_239_eq_004.PNG]]'''(Eq. 16)'''
[[File:Vol3 page 239 eq 004.PNG|RTENOTITLE]]'''(Eq. 16)'''


For positive-displacement pumps,  
For positive-displacement pumps,


[[File:Vol3_page_239_eq_005.PNG]]'''(Eq. 17)'''
[[File:Vol3 page 239 eq 005.PNG|RTENOTITLE]]'''(Eq. 17)'''


where  
where


P<sub>B</sub> = brake horsepower
P<sub>B</sub> = brake horsepower


e = the pump efficiency factor obtained from the pump manufacturer.  
e = the pump efficiency factor obtained from the pump manufacturer.
 
For electric-motor-driven pumps, the energy consumption can be estimated with


For electric-motor-driven pumps, the energy consumption can be estimated with
[[File:Vol3 page 240 eq 001.PNG|RTENOTITLE]]'''(Eq. 18)'''


[[File:Vol3_page_240_eq_001.PNG]]'''(Eq. 18)'''
== Nomenclature ==


==Nomenclature==
{|
{|
|h
|=
|height of the fluid column above a reference point
|-
|p
|=
|pressure
|-
|-
|γ  
| h
|=  
| =
|specific gravity of the liquid
| height of the fluid column above a reference point
|-
| p
| =
| pressure
|-
| γ
| =
| specific gravity of the liquid
|-
|-
|ρ<sub>f</sub>  
| ρ<sub>f</sub>
|=  
| =
|density of the liquid being pumped
| density of the liquid being pumped
|-
|-
|ρ<sub>w</sub>  
| ρ<sub>w</sub>
|=  
| =
|density of water at standard conditions of temperature and pressure
| density of water at standard conditions of temperature and pressure
|-
|-
|v  
| v
|=  
| =
|average velocity of the liquid in the pipe
| average velocity of the liquid in the pipe
|-
|-
|g  
| g
|=  
| =
|acceleration of gravity
| acceleration of gravity
|-
|-
|p  
| p
|=  
| =
|pressure
| pressure
|-
|-
|ρ  
| ρ
|=  
| =
|density
| density
|-
|-
|Z  
| Z
|=  
| =
|height above a datum
| height above a datum
|-
|-
|h<sub>f</sub>  
| h<sub>f</sub>
|=  
| =
|friction loss between points 1 and 2.
| friction loss between points 1 and 2.
|-
|-
|Q  
| Q
|=  
| =
|flow rate
| flow rate
|-
|-
|d  
| d
|=  
| =
|inside pipe diameter
| inside pipe diameter
|-
|-
|H<sub>s</sub>  
| H<sub>s</sub>
|=  
| =
|suction head of liquid being pumped
| suction head of liquid being pumped
|-
|-
|p<sub>1</sub>  
| p<sub>1</sub>
|=  
| =
|suction-vessel operating pressure
| suction-vessel operating pressure
|-
|-
|H<sub>1</sub>  
| H<sub>1</sub>
|=  
| =
|height of liquid suction vessel above pump reference point
| height of liquid suction vessel above pump reference point
|-
|-
|p<sub>f1</sub>  
| p<sub>f1</sub>
|=  
| =
|pressure drop resulting from friction in the suction piping
| pressure drop resulting from friction in the suction piping
|}
|}


Line 403: Line 459:


{|
{|
|1,2  
|-
|=  
| 1,2
|locations along a pipe
| =
| locations along a pipe
|}
|}


==References==
== References ==
<references>
 
<ref name="r1">''World LNG Source Book 2001''. 2001. Des Plaines, Illinois: Gas Technology Inst.</ref>
<references />
</references>
 
== Noteworthy papers in OnePetro ==


==Noteworthy papers in OnePetro==
Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read
Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read


==External links==
== External links ==
 
Use this section to provide links to relevant material on websites other than PetroWiki and OnePetro
Use this section to provide links to relevant material on websites other than PetroWiki and OnePetro


==See also==
== See also ==
[[PEH%3APumps|PEH:Pumps]]
 
[[PEH:Pumps|PEH:Pumps]]


[[Positive_displacement_pumps|Positive displacement pumps]]
[[Positive_displacement_pumps|Positive displacement pumps]]


[[Centrifugal_pumps|Centrifugal pumps]]
[[Centrifugal_pumps|Centrifugal pumps]]
[[Low_shear_pumps]]


[[Pump_drivers|Pump drivers]]
[[Pump_drivers|Pump drivers]]
[[Category:4.1.6 Compressors, engines, and turbines]]

Latest revision as of 06:11, 16 February 2017

A pump can be defined as “a mechanical device that adds energy to a fluid to increase its flow rate and static pressure.”[1] This process can be accomplished with positive displacement pumps or kinetic-energy pumps.

Fluid principles and hydraulics

Types of fluids

Pumps are used to move fluids, which include:

  • Liquids
  • Dissolved gases - dissolved air and hydrocarbon vapors
  • Solids - sand, clay, corrosion byproducts, and scale

The most common types of liquids pumped in upstream operations are:

  • Crude oil
  • Condensate
  • Lube oils
  • Glycols
  • Amines
  • Water

Each fluid has different physical properties that must be taken into consideration when sizing and selecting a pump. The most important physical properties are suction pressure, specific gravity, viscosity, vapor pressure, solids content, and lubricity.

Types of pumps

Positive displacement pumps

Positive displacement pumps add energy to a fluid by applying force to the fluid with a mechanical device such as a piston, plunger, or diaphragm. There are two types of positive-displacement pumps:

  • Reciprocating
  • Rotary

Reciprocating pumps use pistons, plungers, or diaphragms to displace the fluid, while rotary pumps operate through the mating action of gears, lobes, or screw-type shafts.

Kinetic energy pumps

Kinetic Energy Pumps (energy associated with motion) is added to a liquid to increase its velocity and, indirectly, its pressure. Kinetic-energy pumps operate by drawing liquid into the center of a rapidly rotating impeller. Radial vanes on the impeller throw the liquid outward toward the impeller rim. As liquid leaves the impeller, it comes in contact with the pump casing or volute. The casing is shaped to direct liquid toward a discharge port. The casing slows the liquid and converts some of its velocity into pressure. There are three classes of kinetic-energy pumps:

  • Centrifugal - radial-, axial-, and mixed-flow designs
  • Regenerative-turbine
  • Special-effects pumps

Centrifugal pumps account for more than 80% of pumps used in production operations because they exhibit uniform flow, are free of low-frequency pulsations, and are not subject to mechanical problems. Fig. 1 illustrates pumps commonly used in upstream production operations.


Pumping system design

Designing any pumping service involves three major activities: process design, mechanical design, and vendor selection.

Process Design. The first step in process design is to obtain a design flow rate. The design flow rate should be selected after considering all flow variations, such as:

  • Startup conditions
  • Future expansion
  • Maximum anticipated flow

The next step is to determine the liquid properties critical to pump design. These properties include:

  • Specific gravity
  • Temperature
  • Viscosity
  • Pour point
  • etc.

Values are required at pumping conditions and, in some cases, at ambient conditions as well. The next step is to calculate available suction conditions such as rated suction pressure, maximum suction pressure, and net positive suction head available (NPSHA). (See Hydrodynamics for information on NPSHA.) Once the available suction conditions have been established, the effect of the selected control system on pump performance requirements must be determined (see Regulation of Flow Rate). The next step is to calculate the minimum discharge-pressure requirements of the pump. The last step is to calculate the total dynamic head (TDH) at the specific gravity corresponding to rated pumping temperature.

Mechanical Design

The first step in mechanical design is to determine the design pressure and temperature required for the pump and its associated piping. Once this is done, a pump type and materials of construction are selected. The next step is to determine the sparing (backup) requirements, the need for parallel operation and control-system details. Then select a shaft-seal type and determine the requirements for an external flushing or sealing system and estimate the power requirements and choose a driver (motor, engine, or turbine) for the pump. Lastly, document the design by including calculations, studies, design specifications, utility requirements, and estimate summary.

Vendor Selection

Factors that have the greatest influence on the selection of the most cost-effective pump type include:

  • Capacity
  • TDH
  • Maintenance
  • Viscosity
  • Capacity control

Within the general type selections, a particular construction style is most influenced by:

  • Discharge pressure
  • NPSHA
  • Fluid temperature
  • Space and weight limitations
  • Fluid shearing characteristics

Hydraulic principles

Hydraulics deals with the mechanical properties of water and other liquids and the application of these properties to engineering. Hydraulics is divided into two areas:

  • Hydrostatics (fluids at rest)
  • Hydrodynamics (fluids in motion)

Hydrostatics

A liquid has a definite volume when compared to a gas, which tends to expand to fit its container. When unconfined, a liquid seeks the lowest possible level. Because of its fluidity, a liquid will conform to the shape of its container.

Pressure

The pressure existing at any point in a liquid body at rest is caused by the atmospheric pressure exerted on the surface plus the weight of the liquid above that point. This pressure is equal in all directions.

Temperature

For most liquids, an increase in temperature decreases viscosity, decreases specific gravity, and increases volume. Temperature affects:

  • The type of pump construction
  • Material selection
  • Corrosive properties of the fluid
  • The pump’s flange pressure/temperature rating

Air properties

Air is a mixture of oxygen, nitrogen, and other compounds. The standard pressure of air is defined at 60°F, 36% relative humidity, and sea level. The weight of a column of air above the Earth’s surface at 45° latitude and sea level is 14.696 psia (29.92 in. of mercury). Atmospheric pressure decreases by approximately 0.5 psi for each 1,000 ft of elevation above sea level.

Head

The relationship of head to pressure is expressed as

RTENOTITLE(Eq. 1)

where

h = height of the fluid column above a reference point

p = pressure.

The head of liquid is not related to the area occupied by the liquid. Fig. 2 illustrates types of head.

RTENOTITLE(Eq. 2)

where

γ = specific gravity of the liquid

ρf = density of the liquid being pumped

ρw = density of water at standard conditions of temperature and pressure.

It is important to realize that although the heads of different liquids are the same, their pressures are different because of the differences in specific gravities. For example, assume three 100-ft-tall tanks filled with gasoline, water, and molasses, respectively. The pressure measured at the bottom of each tank is different because of the differences of specific gravities of gasoline (0.75), water (1.0), and molasses (1.45).

Centrifugal pump considerations

Operating pressure is expressed in feet of the liquid that is being pumped. Suction and discharge pressures are expressed as suction head and discharge head, respectively. Pressures are expressed in feet of head, because it is more important to know how much a pump can raise the liquid it is pumping, rather than the amount of pressure the pump is adding to the liquid.

Positive displacement pump considerations

Operating pressures are almost always expressed in terms of pressure (psi).

Static head, static lift, and submergence terminology

Fig. 3 illustrates the relationship between static head, static lift, and submergence. Static head is the vertical distance between a liquid level and a datum line, when the supply is above the datum. Static lift is the vertical distance between a liquid level and a datum level, when the datum is above the liquid. Datum line is the centerline of the pump inlet connection, or the horizontal centerline of the first-stage impeller in vertical pumps.


Theoretical lift

A pump that develops a perfect vacuum at its suction end can lift a column of water 34 ft. This vertical distance is called theoretical lift. The pressure to lift the liquid comes from atmosphere pressure. At sea level, atmosphere pressure is approximately 14.7 psia.

Actual suction lift

Because a perfect vacuum is never achieved and because some lift is lost to friction in the suction line, the maximum actual suction lift for a positive-displacement pump is approximately 22 ft. The maximum actual suction lift for a centrifugal pump is approximately 15 ft when pumping water from an open air tank. Positive-displacement pumps can operate with lower suction pressures or high suction lifts because they can create stronger vacuums. Suction lift will be greater if the pressure in a closed tank is greater than atmospheric pressure.

Submergence

Submergence is often confused with either suction static head or static lift. For vertical pumps, submergence relates the liquid level to the setting of the pump. For horizontal pumps, submergence relates to the height of liquid level necessary in the source vessel or tank to prevent the formation of vortexing and the resulting flashing of vapors in the pump suction.

Vapor pressure

As the pressure on a liquid is decreased, there is a tendency for the bubbles of vapor to be liberated. The vapor pressure of a liquid is the pressure at which the first bubble of vapor appears at a given temperature. At 60°F, the vapor pressure of water is 0.3 psia (0.7 ft). At 212°F, the vapor pressure of water is 14.7 psia (34 ft). Fig. 4 illustrates the vapor pressure of water for various temperatures. For other fluids, refer to standard references (e.g., Hydraulic Institute Engineering Data Book1).


Suspended solids

The amount and type of suspended solids entrained in the liquid can affect the characteristics and behavior of that liquid. Increased concentrations of solids increase the specific gravity, viscosity, and abrasiveness of a liquid. The type and concentration of suspended solids can affect the style of pump selected and the materials of construction. Suspended solids also affect the selection of impeller design in centrifugal pumps, which in turn affects the wear rate, efficiency, and power consumption.

Dissolved gases

Small amounts of dissolved gases have little effect on flow rate or other pumping requirements. If large amounts of gas enter the liquid through piping leaks or as a result of vortexing in vessels, the specific gravity of the liquid will decrease. Dissolved gases can also reduce the amount of NPSHA at the pump suction. (See Hydrodynamics for a discussion of NPSHA).

Viscosity

Viscosity offers resistance to flow because of friction within the fluid. Viscosity levels have a significant impact on pump type selection, efficiency, head capacity, and warm-up. High-viscosity liquids decrease a centrifugal pump’s efficiency and head performance, while increasing the power requirements. The viscosity of all liquids varies with temperature. For viscosities of liquids, refer to standard industry references (e.g., Hydraulic Institute Engineering Data Book[1]).

Corrosivity

The corrosive nature of the fluid being pumped has a bearing on pump type selection, materials of construction, and corrosion allowance. Special mechanical seals and flushing arrangements may be required.

Hydrodynamics

Hydrodynamics is the study of fluids in motion. Bernoulli’s equation states that

RTENOTITLE(Eq. 3)

where

v = average velocity of the liquid in the pipe

g = acceleration of gravity

p = pressure, ρ = density

Z = height above a datum

hf = friction loss between points 1 and 2. Subscripts 1 and 2 refer to locations along a pipe. An examination of each of the terms in Eq. 3 provides a better understanding of the general equation for modeling a pumping system.

Velocity Head. Velocity head is the potential energy that has been converted to kinetic energy. Velocity head can be expressed as

RTENOTITLE(Eq. 4)

RTENOTITLE(Eq. 5) where

Q = flow rate

d = inside pipe diameter.

The velocity head increases the amount of work required of a pump. The velocity head is usually not included in actual system calculations when piping velocities are kept within the prescribed limits of 3 to 15 ft/sec. The velocity head is included in the total dynamic head on the centrifugal-pump curves.

Pressure head

The energy contained in the liquid is expressed as pressure head and expressed as p/ρ in Eq. 3.

Elevation head

The energy contained in the liquid as a result of its elevation relative to a datum is called the elevation head and is expressed as Z in Eq. 3.

Head losses

Head losses are potential energy that has been lost because of frictional resistance of the piping system (pipe, valves, fittings, and entrance and exit losses). Unlike velocity head, friction head cannot be ignored in system calculations. Head loss values vary as the square of the flow rate. Head losses can be a significant portion of the total head.

Control losses

Control losses occur on the discharge side of a centrifugal pump that has been equipped with a backpressure valve to control flow rate. As the liquid flows through the control valve, energy is lost. Next to static head, control losses are frequently the most important factor in calculating the pump’s total dynamic head. For pump applications, control losses are treated separately from head losses, even though they are included in the hf term in Eq. 3.

Acceleration head

Acceleration head is used to describe the losses associated with the pulsating flow of reciprocating pumps. Theoretically, acceleration head should be included in the hf term of Eq. 3. The Hydraulic Institute Engineering Data Book[1] discusses the calculation of acceleration head.

Total dynamic head

TDH is the difference between the pumping system’s discharge head and suction head. It is also equal to the difference in pressure-gauge readings (converted to feet) across an existing operating pump (discounting velocity head).

Suction head

Suction head is defined as the sum of the suction-vessel operating gauge pressure (converted to feet), the vertical distance between the suction-vessel liquid level and the pump reference point, less head losses in the suction piping [discounting change in velocity,

RTENOTITLE(Eq. 6)

and acceleration head]. Suction head can be expressed as

RTENOTITLE(Eq. 7)

which can be reduced to

RTENOTITLE(Eq. 8)

where

Hs = suction head of liquid being pumped

p1 = suction-vessel operating pressure

H1 = height of liquid suction vessel above pump reference point

pf1 = pressure drop resulting from friction in the suction piping.

Discharge head

Discharge head is defined as the sum of the discharge-vessel operating gauge pressure (converted to feet), the liquid level in the discharge vessel above the pump reference point, pressure drop because of friction in the discharge piping, and control losses (discounting velocity head). It can be expressed as

RTENOTITLE(Eq. 9)

which can be reduced to

RTENOTITLE(Eq. 10)

where

Hd = discharge head of liquid being pumped

p2 = discharge-vessel operating pressure

H2 = operating or normal height of liquid in the discharge vessel above the pump reference

pf2 = pressure drop resulting from friction in the discharge piping

Pc = discharge flow-control-valve losses.

Calculating TDH

The pump TDH is the difference between the suction and discharge heads.

RTENOTITLE(Eq. 11)

which can be substituted as

RTENOTITLE(Eq. 12)

where

Htd = total dynamic head required of a pump.

Net positive suction head (NPSH)

NPSH is defined as the total suction head in feet of liquid (absolute at the pump centerline or impeller eye) less the vapor pressure (in feet) of the liquid being pumped.

Net positive suction head required

Net positive suction head required (NPSHR) is defined as the amount of NPSH required to move and accelerate the liquid from the pump suction into the pump itself. It is determined either by test or calculation by the pump manufacturer for the specific pump under consideration. NPSHR is a function of liquid geometry and the smoothness of the surface areas. For centrifugal pumps, other factors that control NPSHR are:

  • The type of impeller
  • Design of impeller eye
  • Rotational speeds

NPSHR is determined on the basis of handling cold water. Field experience coupled with laboratory testing have confirmed that centrifugal pumps handling gas-free hydrocarbon liquids and water at elevated temperatures will operate satisfactorily, with harmless cavitation and less NPSHR than would be required for cold water.

Net positive suction head available

NPSHA must be equal to or greater than NPSHR. If this is not the case, cavitation or flashing may occur in the pump suction. Cavitation occurs when small vapor bubbles appear in the liquid because of a drop in pressure and then collapse rapidly with explosive force when the pressure is increased in the pump. Cavitation results in decreased efficiency, capacity, and head and can cause serious erosion of pump parts. Flashing causes the pump suction cavity to be filled with vapors and, as a result, the pump becomes vapor locked. This usually results in the pump freezing up, which is called pump seizure.

NPSHA is not a function of the pump itself but of the piping system for the pump. It can be calculated from

RTENOTITLE(Eq. 13)

where

pA = atmospheric pressure

pva = liquid vapor pressure at pumping temperature.

NPSHA decreases with increases in liquid temperature and pipe friction losses. Because pipe friction losses vary as the square of the flow, NPSHA also varies as the square of the flow. Thus, NPSHA will be the lowest at the maximum flow requirement. Accordingly, it is important to recognize the need for calculating NPSHA (and NPSHR) at maximum flow conditions as well as maximum fluid temperature, not just at design conditions. Unless subcooled, a pure-component hydrocarbon liquid is typically in equilibrium with the vapors in a pressure vessel. Thus, increases in the vessel operating pressures are almost fully offset by a corresponding increase in the vapor pressure. When this occurs,

RTENOTITLE(Eq. 14)

NPSH margin

The NPSH margin is NPSHA less the NPSHR. The Hydraulic Inst. recommends an NPSH margin of 3 to 5 ft.[1]

When a new system offers insufficient NPSH margin for optimum pump selection, either the NPSHA must be increased, the NPSHR must be decreased, or both. To increase the NPSHA, one can raise the liquid level, lower the elevation of the selected pump, change to a low-NPSHR pump, or cool the liquid. To reduce the NPSHR, one can use different design impellers or inducers or use several smaller pumps with lower NPSHRs in parallel.

When an existing pumping system exhibits insufficient NPSH margin, it is too late to use these solutions without going through an expensive change. Most of these problems can be traced to suction flow restrictions (orifice plates, plugged strainers, partially closed valves, etc.) and inadequate source-tank liquid levels.

Power requirements

Once the TDH has been calculated, the power requirements can be determined with

RTENOTITLE(Eq. 15)

For kinetic-energy pumps,

RTENOTITLE(Eq. 16)

For positive-displacement pumps,

RTENOTITLE(Eq. 17)

where

PB = brake horsepower

e = the pump efficiency factor obtained from the pump manufacturer.

For electric-motor-driven pumps, the energy consumption can be estimated with

RTENOTITLE(Eq. 18)

Nomenclature

h = height of the fluid column above a reference point
p = pressure
γ = specific gravity of the liquid
ρf = density of the liquid being pumped
ρw = density of water at standard conditions of temperature and pressure
v = average velocity of the liquid in the pipe
g = acceleration of gravity
p = pressure
ρ = density
Z = height above a datum
hf = friction loss between points 1 and 2.
Q = flow rate
d = inside pipe diameter
Hs = suction head of liquid being pumped
p1 = suction-vessel operating pressure
H1 = height of liquid suction vessel above pump reference point
pf1 = pressure drop resulting from friction in the suction piping

Subscripts

1,2 = locations along a pipe

References

  1. 1.0 1.1 1.2 1.3 World LNG Source Book 2001. 2001. Des Plaines, Illinois: Gas Technology Inst.

Noteworthy papers in OnePetro

Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read

External links

Use this section to provide links to relevant material on websites other than PetroWiki and OnePetro

See also

PEH:Pumps

Positive displacement pumps

Centrifugal pumps

Low_shear_pumps

Pump drivers