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Pumps
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
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.
where
γ = specific gravity of the liquid
ρ_{f} = density of the liquid being pumped
ρ_{w} = 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
where
v = average velocity of the liquid in the pipe
g = acceleration of gravity
p = pressure, ρ = density
Z = height above a datum
h_{f} = 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
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,
and acceleration head]. Suction head can be expressed as
which can be reduced to
where
H_{s} = suction head of liquid being pumped
p_{1} = suction-vessel operating pressure
H_{1} = height of liquid suction vessel above pump reference point
p_{f1} = 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
which can be reduced to
where
H_{d} = discharge head of liquid being pumped
p_{2} = discharge-vessel operating pressure
H_{2} = operating or normal height of liquid in the discharge vessel above the pump reference
p_{f2} = pressure drop resulting from friction in the discharge piping
P_{c} = discharge flow-control-valve losses.
Calculating TDH
The pump TDH is the difference between the suction and discharge heads.
which can be substituted as
where
H_{td} = 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
where
p_{A} = atmospheric pressure
p_{va} = 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,
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
For kinetic-energy pumps,
For positive-displacement pumps,
where
P_{B} = brake horsepower
e = the pump efficiency factor obtained from the pump manufacturer.
For electric-motor-driven pumps, the energy consumption can be estimated with
Nomenclature
h | = | height of the fluid column above a reference point |
p | = | pressure |
γ | = | specific gravity of the liquid |
ρ_{f} | = | density of the liquid being pumped |
ρ_{w} | = | 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 |
h_{f} | = | friction loss between points 1 and 2. |
Q | = | flow rate |
d | = | inside pipe diameter |
H_{s} | = | suction head of liquid being pumped |
p_{1} | = | suction-vessel operating pressure |
H_{1} | = | height of liquid suction vessel above pump reference point |
p_{f1} | = | pressure drop resulting from friction in the suction piping |
Subscripts
1,2 | = | locations along a pipe |
References
- ↑ ^{1.0} ^{1.1} ^{1.2} ^{1.3} World LNG Source Book 2001. 2001. Des Plaines, Illinois: Gas Technology Inst.
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