Centrifugal Pump

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Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi

Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not

already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering

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Saudi Aramco DeskTop Standards

CONTENTS PAGES

INFORMATION

Principle of Operation 1

Head Produced by a Centrifugal Pump 2

Application of Centrifugal Pump 3

Mechanical Components 5

Head vs. Flow Characteristic 10

System Resistance 11

Pump Calculations 17

Pump Horsepower 18

Driver Power, Motors 19

Actual Volumes 20

Calculate Pump ÆP Required From Process Data 21

Head Produced by an Operating Pump 22

Net Positive Suction Head 23

Cavitation 26

Performance Curves 27

Impeller Diameter Changes 29

Characteristics of Pumps in Series 32

Characteristics of Parallel Pumps 33

Control Systems 36

Typical Centrifugal Pump Installation 39 Operating Problems with Centrifugal Pumps 42

Standards 44

WORK AIDS 45

GLOSSARY 58

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PRINCIPLE OF OPERATION

A pump converts mechanical energy into pressure in a flowing liquid. A centrifugal pump does this by centrifugal action, in two steps. Refer to Figure 1. (1) A centrifugal pump has two major components: the internal impeller and the outer casing. The liquid enters the suction of the pump at A. It then flows to B and outward through the channels of the impeller marked C. As the liquid flows outward in the impeller, the impeller imparts a very high spinning or tangential velocity to the liquid. (2) The liquid then enters the volute of the pump, area D. Here the velocity energy is converted to pressure.

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Saudi Aramco DeskTop Standards 2 HEAD PRODUCED BY A CENTRIFUGAL PUMP

Head is the term used to describe the energy imparted to the liquid. The units of head are foot-pounds (ft-lb) of force per pound of mass.

Head produced ft-lb = V2 1b 2g where:

V = Velocity of impeller tip, ft/sec g = gravitational constant, 32.2 ft/sec2

Note that the important velocity is the tangential velocity at the tip of the impeller. This velocity is proportional to the diameter of the impeller and the rotational speed. Therefore, the equation for head can be written in terms of pump characteristics as follows:

Head (ft) =

1840

 

where:

D = Impeller diameter, inches N = Pump speed, rpm

The precise units of head are ft-lb (force) per lb (mass). However, it is conventional practice to cancel the lb units and to speak of head in terms of feet. Note that the pump vendor designs the impeller to produce the head required at the design point.

The pressure differential produced by a pump is equivalent to a column of the pumped liquid, where the height of the column is equal to the head produced by the pump. See Figure 2. For a given flow and speed, head produced is constant, assuming no wear and fouling.

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APPLICATION OF CENTRIFUGAL PUMPS

Centrifugal pumps are the type of pump most commonly used in the process industries. They are the first choice because they have very few moving parts, are simple to maintain, and are available for a wide range of flow rates and differential pressures.

There are a few exceptions where other types of pumps are more appropriate. These are services with a very high differential pressure, above about 2000 psi; very high viscosities, above 500 cSt; or very low flow rates, below 10 gpm. However, in most industries, more than 90% of the pumping applications will be covered by centrifugal pumps.

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Saudi Aramco DeskTop Standards 4 APPLICATION OF CENTRIFUGAL PUMPS (CONT’D)

∆∆

H

==

100 Ft

∆∆

p

==

43 . 3 psi

∆∆

p psi

( )

==

Head ( Feet )

××

0. 433

××

S . G .

43 . 3 psig

0psig

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MECHANICAL COMPONENTS

Figure 3 illustrates the major components of a centrifugal pump. This is a diagram of a horizontal single-stage, overhung pump, the most common type. Horizontal refers to the orientation of the shaft; single-stage means there is one impeller. Overhung means that the impeller is outside of the two supporting bearings, not between the bearings.

The shaft runs through the center of the pump and holds the impeller at the left end. The drive motor is connected to the right end of the shaft through a flexible coupling. The liquid enters the suction nozzle, passes through the enclosed sections of the spinning impeller, and exits through the discharge nozzle at the top of the pump. The right end of the pump is the bearing housing. This housing contains two sets of ball bearings that support the weight of the shaft. They also absorb the axial thrust on the shaft.

The casing contains the liquid under pressure. A seal is required where the rotating shaft enters the casing. This area is called the stuffing box and may actually contain a stuffing or packing. However, most modern pumps have mechanical seals at this point. Sealing the shaft is very important to prevent leakage of the pumped fluid, which is frequently hazardous, flammable, or toxic. Therefore, careful attention must be paid to the design, installation, and maintenance of the seals. Many different types of seals are available for different process conditions.

Heat is generated by friction in seal area of the shaft, and sometimes cooling is required. A channel called the flushing connection is available for this purpose.

The amount of head that can be generated by a single impeller is limited to a maximum value. If more head is required, pump designs incorporate two or more impellers. These may be arranged in a horizontal multistage configuration or a vertical multistage configuration. These configurations are described later.

Impellers may be the open, semi-closed, or closed. These are shown in Figure 5. In the petroleum and gas process plants, most impellers are the closed type. Closed impellers can generate higher heads at greater efficiencies. Open and semi-closed impellers are used for liquids that contain solids. They will not clog as easily as closed impellers.

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Saudi Aramco DeskTop Standards 6 MECHANICAL COMPONENTS (CONT’D)

Casing Rings Front and Back Side

of Impeller Shaft_Sleeve Lantern Ring

Connection Stuffing Box forMechanical Seal or

Packing (Packing Illustrated)

Ball Bearing Thrust Bearing Cantilevered or Overhung Type Shaft Support Guide Bracket (Not for Structural Support)

Oil Lubrication System

Quenching Type Packing Gland Impeller

Balance Port

Close Tube Impeller

End Suction Casing _{Circular Casing} Joints with Confined Gasket

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Saudi Aramco DeskTop Standards 8 MECHANICAL COMPONENTS (CONT’D)

• Horizontal-Single Stage The most common type

- Used for moderate head, <500 ft - End suction top discharge

+ or top suction, top discharge • Vertical In-line

- Supported by piping or small foundation

- Motor is supported by pump; piping forces do not affect alignment

- Lower cost, simpler maintenance

- _{Slightly higher NPSHR than horizontal pump} • Horizontal Multistage

- Up to 8 impellers for higher head - Shaft supported between bearings • Vertical Can

- _{Used when low NPSHR is needed} • Vertical-Submerged Suction

- Like vertical can type, without the can - Used in sumps or shallow wells

- Used to pump water from the sea, or from reservoirs • Submersible

- Used in oil production wells

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MECHANICAL COMPONENTS (CONT’D)

Partially Open

(Semi-Closed)

Open

Enclosed

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HEAD VS. FLOW CHARACTERISTIC

The process performance of a centrifugal pump is described by a curve called the head versus flow characteristic. See Figure 6. Centrifugal pumps are constant-head devices. This means that they provide a nearly constant head, or pressure differential, even though the flow rate changes. As Figure 6 shows, the head produced by the pump does increase somewhat as the flow rate decreases from the design point. Conversely, the head decreases at flow rates above the design point. However, over the normal operating range of the pump, the head is relatively constant or, as we say, the curve is relatively flat. Normally, the head developed at zero flow is no more than 110 to 120% of the head at the design point. This is called the shutoff point, or shutoff head.

FIGURE 6. HEAD VS FLOW CHARACTERISTIC

Note that shutoff means that the flow is shut off, for example by closing a valve at the discharge of the pump. The pump itself continues to rotate and develop differential pressure. However, a pump should not be operated this way except for a short period. After a minute or two, the pump will overheat and damage will occur.

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SYSTEM RESISTANCE

The discussion has centered on the head produced by an operating pump. Another important concept is system resistance. This is the head required to move liquid from one point in the process to another.

The total head (or differential pressure) required for a circuit can be divided into three components: (See Fig. 7, 8 and 9).

• Static pressure differential, the difference in pressure between the two vessels, P2 - P1.

• Elevation differential, the head required to lift the liquid from its initial to its final elevation.

• Friction resistance in the flowing system.

Figure 10 shows a typical pump circuit. This circuit contains all three components of system resistance.

The magnitudes of the three components are illustrated in the lower half of Figure 10. Notice that pressure differential and elevation are constant values, independent of the flow rate through the circuit. However, the dynamic friction resistance depends on the flow. The dynamic friction resistance is proportional to the square of the flow rate. Thus, at zero flow rate, the friction resistance is zero, but it rises exponentially as the flow rate increases.

To understand the dynamics of a pumped circuit, it is sometimes useful to plot the pump curve and the system curve together. This has been done in Figure 11. The head can be expressed either as feet of fluid or differential pressure (psi), as long as the units are consistent. At zero flow rate, the head produced by the pump is much greater than the head required to overcome the resistances of the system. However, as the flow rate increases, the head required increases. At the same time, the head produced by the pump decreases somewhat. At the design flow rate, the head produced by the pump is still larger than the head required. The difference, or excess delta P, is taken up by a control valve.

The curve shows that if the flow rate is increased beyond the design value, the pressure drop available for the control valve becomes smaller and smaller. When the curves meet, the pressure drop available for control is zero, the control valve is wide open and the flow rate cannot increase further.

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SYSTEM RESISTANCE (CONT’D)

p

₁

==

100 psig

p

₂

==

200 psig

∆∆

p

==

(

p

₂

−−

p

₁

)

==

200 −−

100 ==

100 psi

Head

==

(

p

₂

−−

p

₁

)

××

2.31 S.G.

(Feet )

FIGURE 7. COMPONENTS OF SYSTEM RESISTANCE - STATIC PRESSURE DIFFERENTIAL

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∆∆

H

Head

== ∆∆

H

(Fe et )

∆∆

p

_{EL .}

== ∆∆

H

××

0. 433

××

S.G.

FIGURE 8. COMPONENTS OF SYSTEM RESISTANCE - ELEVATION DIFFERENTIAL

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1 4

Line Friction

Orifice

Heat

Exchanger

Filter

Head (Fe et )

== ∆∆

p psi

( )

××

2. 31

S.G.

Friction Resistance Is Dynamic

∆∆

p

==

k (Flow Rate )

2

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p1

Static Press Diff, P

₂

- P

₁

P

₂

Total

Circuit

²p

(Excluding

Control

Valve)

Flow Rate

Pressure Diff, P

₂

- P

₁

Static

Elev.

Diff

h

₂

-h

₁

Dynamic

Friction

Resistance

kx(Flow)

2

Elevation, h

₂

-h

₁

kx(Flow)

Friction

2

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1 6

Flow

gpm

Design

Flow

Max

Flow

Head ∆∆p

( )

Control Valve ²p

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PUMP CALCULATIONS

Equations 1, 2 and 3 are for calculating pump head.

Pump ÆP is the difference between discharge pressure and suction pressure in psi. Pump head is the same value, but expressed in terms of feet of liquid.

Usually, the system requirements are calculated in psi. The pump capability is known in feet. Equations 1, 2 and 3 are used to convert from one unit to the other. The end user or contractor calculates these values.

Equations for Calculation of Head Required

• _{Pump ÆP = P2 - P1 (psig or psia)} Eqn. (1)

P1 = Suction pressure P2 = Discharge pressure

• Head (feet) = _P (psi) x 2.31 Eqn. (2) S.G.

S.G. = Specific Gravity relative to water.

• ÆP = Head (ft) x 0.433 x S. G. Eqn. (3)

Density of water at standard temperature (60oF) S.G. = 1.0

Density = 8.33 lb/gal = 62.4 lb/ft3 = 350 lb/barrel = 2205 lb/metric ton

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PUMP HORSEPOWER

Brake horsepower is the power applied to the shaft between the pump and its driver. It is calculated as follows:

• bhp = (gpm) x (_P) Eqn. (4) 1715 x (Pump Eff.)

where:

bhp = Brake horsepower

gpm = Pump flow rate, actual gallons per minute. ÆP = Differential pressure, psi

Pump Eff. = Hydraulic efficiency of the pump, as a decimal fraction.

Pump efficiency is a characteristic of the pump. Typical values are 0.50 to 0.85. You can read the efficiency from the manufacturer's performance curve, at operating flow rate and head.

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DRIVER POWER, MOTORS

• Power (kW) = bhp x 0.746 Eqn. (5)

Motor Eff. where:

kW = Power input to motor, kilowatts

Motor eff. = Efficiency of the electric motor, as a decimal fraction. Typical values are 0.85 to 0.95. See "Electric Motors" section of this course.

Note one important point about Eqns. 1-5. For a particular centrifugal pump (at a given speed and flow rate), the head produced is a characteristic of the pump. It is a constant value. However, the delta P produced is not constant. The delta P varies directly with the specific gravity of the pumped fluid. Also, if the specific gravity increases, the brake horsepower increases. Therefore, a pump and driver set that has been designed for a liquid with a low specific gravity, such as a light hydrocarbon, may not have sufficient drive horsepower to pump water at the same flow rate. Because of the higher specific gravity, the horsepower requirement is greater and the driver may be overloaded.

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ACTUAL VOLUMES

When liquids are heated they expand. The volume that determines the pump performance is the actual volume at the pumping temperature. Frequently, the information about volume flows is expressed as volume at standard conditions, or 60 degrees F. These standard volumes must be converted to actual volumes before pump performance calculations can be made. Values for volume at standard conditions may be obtained from material balance calculations or from actual plant samples as measured by hydrometers.

These standard densities can be converted to density at actual temperature, using the charts in the GPSA Manual, Figure 23-17.

Expansion Factor = Specific Volume at actual temperature Specific Volume at 60°F

Plant Data

When an operating pump is evaluated, the flow rate through the pump is often determined from an orifice flowmeter. An orifice flowmeter does not measure volume flowing directly. It measures pressure drop across an orifice. The volume can then be calculated from this pressure drop and the specific gravity of the fluid. Standard charts or meter factors are used in the plant for convenience. However, these charts and factors have been calculated for one specific gravity. If the specific gravity at the time of the reading is different, the flowmeter factor must be corrected. In this case, ask an instrument specialist for help.

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CALCULATE PUMP ÆP REQUIRED FROM PROCESS DATA Procedure

First calculate the pump suction pressure by starting with the pressure in the suction vessel, adding and subtracting the relevant pressure differences from the suction vessel to the eye of the pump. In a similar way, start with the pressure in the downstream vessel and calculate all of the differences back to the pump discharge. Pump delta P is the difference between the discharge and suction pressures required. Delta P in psi can be converted to head in feet using Eqn. (3).

This calculation is performed to specify a new pump or to check an existing pump to see if it is suitable for an operation.

Frequently, different values may be obtained for one or more of the input variables. This would be due to different operations, or to pumping liquids with different specific gravities at different times. It is important to use the combination of variables that will result in the greatest head required for the pump.

Contingency Factors

It is common to add extra amounts as contingency factors to the calculated head and horsepower values. Once a motor-driven pump is installed, you cannot increase the head that it will produce without removing the pump from operation. The design flow rate can be exceeded if there is enough difference between the head produced and the head required: the pump can "run out on its curve" to some degree. However, operation at higher flow rates will increase the power required from the driver.

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HEAD PRODUCED BY AN OPERATING PUMP

It is frequently necessary to calculate the head produced by an operating pump, to determine whether the pump is in good mechanical condition.

Normally, there is a pressure gauge is installed on the vessel feeding the pump but not at the suction of the pump. There should be a drain or vent connection at the suction side. A pressure gauge can be installed there. If not, pump suction pressure must be calculated from the suction vessel and drops and gains to the pump. In this case, a pressure tap as close to the pump flange as possible should be installed.

Discharge pressure can be read from a pressure gauge at the pump discharge. It may be necessary to read the pressure further downstream and to calculate the discharge pressure. Once the delta P of the pump has been calculated, the head produced by the pump can be determined using Eqn. (3). Be sure to use actual specific gravity at the time of the test.

Note that it is necessary to have the best readings of suction and discharge pressure possible. Therefore, calibrated gauges must be substituted for the gauges installed for normal operation.

The calculated head can be compared to the head predicted by the manufacturer's performance curve. If the difference between these two values is greater than ±5, there is probably something mechanically wrong with the pump or the system.

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NET POSITIVE SUCTION HEAD

It is important that the fluid flowing through the centrifugal pump remain liquid at all points in the flow path. If even a small portion of the liquid vaporizes, two problems result. First, the density of the fluid in the pump decreases, and the pressure differential developed will decrease. Second, the presence of vapor bubbles in the pump can cause mechanical damage. Frequently, a centrifugal pump is handling a liquid that is at its boiling point at the surface of the suction drum. This pressure is also called the "bubble point." It is necessary during design to ensure that the actual pressure remains above the bubble point, at every point through the flow path. The mathematical term used to cover this procedure is called Net Positive Suction Head or NPSH.

NPSH is the actual pressure of the liquid at the suction flange of the pump minus the vapor pressure of the liquid. In other words, it is the positive pressure above boiling pressure (vapor pressure). This pressure difference is expressed in feet of the liquid being pumped.

• NPSH = (Actual Pressure) - (Vapor Pressure) Eqn. (6) - At pump inlet

- Calculated in feet of liquid

A positive NPSH is required by all centrifugal pumps. The reason is as follows:

As the liquid enters the pump, it is subjected to rapid acceleration by the spinning impeller. This acceleration results decreases the static pressure of the liquid. Vaporization occurs if the static pressure drops below the bubble point pressure.

NPSH Available vs. NPSH Required

There are two kinds of NPSH. One is NPSH available (NPSHA), which depends on the design of the system, particularly on the elevation of the suction vessel above the pump and the friction drop in the suction line. The other is NPSH required (NPSHR), the amount of net positive head required by the design of the pump.

The design and operation of the suction system to a centrifugal pump should be arranged so that NPSHA is always greater than NPSHR. If it is not, cavitation damage to the pump or loss of head and capacity may occur.

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2 4

NPSH Available vs. NPSH Required (Cont’d)

The calculation procedure for NPSHA is shown in the following example. Figure 12 accompanies the example.

NPSHR is a function of pump design and flow rate through the pump. It is always shown on the manufacturer's performance curve. For any pump, NPSHR increases as flow rate increases. A typical relationship is shown in Figure 13. Note that the NPSHR can rise steeply at flow rates higher than design. It is actually the pressure drop from the pump inlet flange to the impeller vanes.

NPSH_A = P

[

_s +(hx 0.433 xS. G.)− ∆P_f−P_V

]

x2.31 S. G.

Ps = Pressure in Vapor Space of Suction Vessel, psia

h = Height of Minimum Level Above Suction Flange of Pump, ft ÆPF = Friction Loss in Suction, Including Contraction, psi

Pv = Vapor Pressure of Pumped Fluid, psia

Note: Elevation head “h” is negative when the liquid level is below the centerline of the pump.

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NPSH Available vs. NPSH Required (Cont’d)

FIGURE 13. AVERAGE NPSHR AS A FUNCTION OF PUMP CAPACITY AT CONSTANT SPEED

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2 6

CAVITATION

Cavitation occurs when the NPSH available is less than that required. As the liquid flows through the pump and decreases in pressure, small bubbles of vapor form in the suction passages. As soon as these bubbles reach a higher pressure in the impeller, they can recondense and collapse so quickly that a violent force is imposed on the impeller. This makes a distinctive noise that sounds like the rattling of stones in the pump. If cavitation continues, pitting of the impeller can occur. The damage can be severe. Cavitation can also occur when low volume flow causes flow separation that vaporizes the liquid being pumped. Cavitation damage is most likely with single-component liquids such as water. Single-component liquids tend to recondense very suddenly. Multi-Single-component liquids recondense more gradually and therefore cause less damage. However, even with multi-component liquids, the presence of vapor in the impeller can decrease the head or flow capacity.

Dissolved Gases

In addition to vaporization of the major component of the pumped liquid, dissolved gases can also vaporize, for example, air in water or nitrogen in hydrocarbons. As the pressure drops in the suction passages, small bubbles of dissolved gas can form. However, these gases do not condense and collapse suddenly. They redissolve quite slowly. Because sudden collapse does not occur, the impeller damage does not occur. Furthermore, since the amount of gas released is small, the head produced by the pump is usually not affected significantly. Therefore, when you calculate the vapor pressure of a liquid to be pumped, you can usually ignore these dissolved components such as air, nitrogen, and hydrogen.

Sometimes, dissolved gases can even be beneficial. For example, if a pump operating on water has severe cavitation, one remedy is to inject a small amount of nitrogen or air into the pump suction. This gas remains as bubbles as the pressure increases. The bubbles cushion the imploding force of the condensing bubbles of water vapor.

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PERFORMANCE CURVES

Analysis of an existing pump and prediction of its performance are done by means of the manufacturers performance curves. For a typical example of this curve see Figure 14. The most important curve is head versus capacity. If you know the head that a pump will produce, you can calculate the differential pressure that it will develop.

Note that the head is shown for a range of impeller diameters. Most centrifugal pumps can be fitted with impellers of different diameter in the same casing. This flexibility is a way to adapt the pump to a changed future service. Pumps are normally purchased with an impeller somewhere near the middle of the possible size range of impellers. Therefore, if a head increase is required by changed operating conditions, a larger impeller can be installed.

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2 8

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PERFORMANCE CURVES (CONT'D)

Curves do not normally show the effect of a change in speed because most pumps are driven by constant-speed motors. If a pump is purchased with a turbine driver, a family of speed curves will be provided.

A curve of horsepower versus capacity is also shown for the range of possible impeller diameters. Note that this horsepower is valid only for the rated specific gravity. If the liquid being pumped has a different specific gravity, the horsepower will have to be corrected. The third major characteristic shown on the performance curves is NPSH required versus flow rate. This characteristic is independent of specific gravity, operating pressure, and impeller diameter. Impeller diameter changes do not affect the geometry on the suction side of the impeller.

A pump curve also shows the hydraulic efficiency of a pump for various flow rates and impeller diameters. The point of maximum efficiency is called the Best Efficiency Point. It should be somewhere near the design operating point for the pump but depends on how the pump was selected. Remember, pumps are not generally custom designed!

Viscosity

Performance curves are based on tests performed with water. When viscous fluids are pumped, head, capacity, and efficiency are all reduced. This effect becomes significant at about 5 cSt. Correction factors for the affected variables are shown in Figure 15. Viscosity corrections are significant in cold charge pump services and start up of lube system in cold weather.

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3 0

IMPELLER DIAMETER CHANGES

Occasionally, a plant engineer will be called upon to specify a change in the diameter of the impeller of an operating pump. The change may be required to increase the head available, either to expand the capacity of a plant or to use a pump in a new service.

Sometimes, the impeller diameter is reduced in order to decrease the head. This may be done to reduce the power consumption, to avoid overloading the motor, or to reduce the maximum discharge pressure, to avoid overpressuring downstream equipment.

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IMPELLER DIAMETER CHANGES (CONT’D)

1. Enter Chart at Design Capacity and Move Up To Design Head (For Multi-Stage Pumps, Use Head Per Stage).

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3 2

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IMPELLER DIAMETER CHANGES (CONT'D)

The relationships of impeller diameter to flow rate, head, and horsepower are commonly called the affinity laws, as follows:

bhp2 = bhp1 x D1     where: Q = Flow rate H = Head bhp = Brake horsepower D = Diameter

Estimated performance changes can be made using these relationships, but remember that they are approximate. It is better to use the manufacturer's performance curves whenever possible. See Figure 16.

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3 4

CHARACTERISTICS OF PUMPS IN SERIES

Sometimes two centrifugal pumps are connected in series, to increase the pumping capability of an installation. The calculations for this kind of operation are illustrated in Figure 17. You can construct a single head/capacity curve for the two pumps operating together. In the figure, the head/capacity curve for a single pump is shown. When two pumps operate in series, the heads produced are added. At any given capacity, the head can be plotted. Using the new pump curve for two pumps and the system resistance curve, you can determine the maximum capacity for the new system as shown in the figure.

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CHARACTERISTICS OF PARALLEL PUMPS

Another way to increase pumping capacity is to use two pumps in parallel. This arrangement is illustrated in Figure 18. When two pumps are installed in parallel, the head produced is the same as for a single pump. However, at any given value for head, the capacity for the two pumps is the addition of the capacity of each pump. Thus, a new head/capacity curve can be drawn for the two pumps in parallel. Again, using the new pump curve and the system head curve, you can determine the maximum capacity.

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3 6

Caution: Pumps in Series

Two pumps in series will generate much more discharge pressure than one pump alone. In some cases, this pressure might be greater than the design pressure of the downstream piping or other equipment. This condition must be checked before proceeding with an installation of two or more pumps in series. It is important to check the design pressure at the condition called "pump shutoff pressure." Shutoff pressure is obtained when the downstream control valve is closed and the pumps operate at zero capacity and maximum head. The shutoff pressure is equal to the pressure in the suction vessel plus the shutoff delta P of both pumps combined. See Figure 19 and the example table beneath it. For this example, the normal operating discharge pressure is satisfactory because it is less than the design pressure. However, at shutoff, the discharge pressure downstream of the second pump would be greater than the equipment design pressure. This situation is not allowed. One remedy is to install a safety valve at the discharge of the second pump. as shown.

FOR EXAMPLE: Suction Press psig Æ P1 Æ P2 Discharge Press psig Design Press* psig Norm 0 60 60 120 150 Max. (At Shutoff) 0 100 100 200 150

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3 8

Caution: Pumps in Parallel

A problem that can occur with pumps operating in parallel is shown in Figure 20. Two pumps are never exactly like. If two pumps are installed in parallel, one pump may take more than half of the total flow and the other pump less than half. The pump with the lower flow rate may be operating below its minimum acceptable flow rate. As the figure shows, the head produced by the two pumps will be identical because they are connected to the same process. If the head produced by pump B is lower than the head produced by pump A, the situation shown in the figure will occur. Pump B will decrease its flow rate until it can produce the same head as Pump A.

This situation is most dangerous when one pump is driven by a motor and the other by a turbine. It is impossible to set the two speeds exactly equal, and the difference in speed will cause a difference in head produced.

If two pumps are nominally identical and both driven by motors, the two head curves can be assumed to be within 3% of each other. If so, you can make the worst assumption, that is, the head of pump B is 3% lower than the head of pump A. Then, using the system operating conditions, plot the flow through both pumps. Make sure that the lowest flow rate is not below the pump minimum allowable flow rate.

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CONTROL SYSTEMS

The most common control device for a centrifugal pump is a control valve in the discharge line. This valve controls the amount of liquid delivered to the process. This valve takes a pressure drop equal to the difference between the pressure supplied by the pump and the pressure required by the process.

A control valve is almost never used in the suction line of a pump. A pressure drop in the suction line could cause vapor to form, which is always harmful to centrifugal pump operation.

Variable speed is an alternative method for controlling centrifugal pumps. The rotating speed is changed until the head generated by the pump exactly equals the head required. If the driver is a steam turbine or gas turbine, speed control is normally used. This is the case in many pipeline and production services in Saudi Aramco. It is always more efficient to control produced head than to control required head by throttling.

It is also necessary to control the minimum flow through a centrifugal pump. The minimum flow that can be tolerated is normally 25 to 30% of design flow to the pump. However, this value can be considerately higher for pumps with double suction impellers (40 to 60% of design flow). Below this flow rate, unstable operation can cause mechanical damage to the pump. If the flow rate required by the process is less than this minimum value, some excess flow is recycled from the discharge of the pump to the suction vessel. Recycle directly to the pump suction is normally not employed. This would increase the temperature of the recirculating fluid, leading to possible vaporization.

Recycle can be controlled in the three ways shown in Figure 21: • Manually controlled recycle

• Automatic recycle control with a control valve in the recycle line.

• An automatic minimum flow controller installed in the pump discharge line. It senses the net flow rate through the pump and opens a path to the recycle line when flow drops below a preset value.

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4 0

CONTROL SYSTEMS (CONT’D)

Methods

Manual Recycle

Automatic Recycle

Automatic Minimum Flow Controller

LC Fl Recycle Line Restriction Orifice Sized for Min Pump Flow LC Fl FC LC Fl

Senses Net Flow Rate. Bypass is Normally Closed, Opens When Flow Drops Below a Preset Value

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CONTROL SYSTEMS (CONT’D)

If Natural Flow Balancing Cannot be Guaranteed, Use Separate Flow Controllers

Or Separate Minimum Flow Recycle Controls

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4 2

TYPICAL CENTRIFUGAL PUMP INSTALLATION

Figure 23 shows a typical installation. Its elements are as follows: • Normal operating pump

• The spare pump. Pumps are normally spared so that the process can operate continuously even if maintenance is required on one pump.

• Suction line with block valve for isolation. • Discharge line with block valve for isolation.

• Check valve or non-return valve in the discharge line. This valve prevents reverse flow through the pump. Reverse flow would cause the impeller to spin backwards, which would damage the pump.

• Pressure gauge, PI, in the discharge line. This is to monitor the performance of the pump.

• Flushing connection to the seal. Normally a liquid is circulated through the seal to keep it clean and cool.

• Casing vent. Before a centrifugal pump is started, be sure to vent vapors from the casing. A centrifugal pump containing vapor will not develop differential pressure. The vent may be on the casing itself or on the discharge line.

• Kickback line or recycle line. This is the line used to keep the flow rate through the pump above the minimum value.

• Suction strainer. A suction strainer is installed upstream of the pump. It prevents solid material from entering the pump. Solid material could cause mechanical damage. Normally, the suction strainer is in place only during startup and is removed after an initial period that flushes construction debris from the suction system.

Note: If the strainer is not to be removed, a differential pressure gauge should be installed around the suction screen.

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TYPICAL CENTRIFUGAL PUMP INSTALLATION (CONT’D)

Vent to Suct. Vess.

(If Pump Self-Venting)

~

Suction Recycle (Kickback)

to Suction Vessel Discharge

Flow Controller Set for Minimum Pump Rate Pl Pl Casing Vent MAIN (Operating) SPARE (Standby)

Spool Piece for Suction Strainer (Strainer Installed During Startup)

Flush to Seal

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4 4

Starting a Centrifugal Pump

The normal method for starting a centrifugal pump is as follows. Before startup, close both the discharge and suction block valves. Close the casing vent. Open the valve in the line to the seal.

1. Open the suction block valve to allow liquid to enter the pump. 2. Open the casing vent to release trapped gases or vapors.

3. Close the casing vent.

4. Start the pump motor; observe the pressure rise in the discharge line as indicated by the PI.

5. When the discharge pressure reaches the normal value, start to open the discharge block valve.

6. Gradually open the discharge block valve until it is fully open. If the discharge pressure starts to fall, close the block valve a small amount to reestablish discharge pressure.

Optional Features

Cooling water to stuffing box. Sometimes cooling water is provided to the seal housing to prevent vaporization of the liquid at the surface of the seal.

Steam quench. If the pump fluid is very hot and also flammable, steam is injected between the seal and the outside atmosphere. If there is leakage through the seal, the steam quench cools and dilutes the material. This prevents solidification of flammable pump fluid, such as oil, and reduces the risk of fire.

Casing vent line. The vapors will be vented to atmosphere through a connection at the pump discharge if the material is not toxic or hazardous. For toxic or hazardous materials, a pipe is installed to vent the material back to the suction drum. This is especially necessary if a pump is handling cold liquids. The vent line is left open for five or ten minutes before the pump is started. During this period, cold liquid circulates from the suction line through the pump and back to the suction vessel. This cools the pump to operating temperature before startup. If this step is not carried out, vaporization can prevent successful starting of the pump.

Warm-up bypass. If the pump normally operates at high temperature, it must be heated before startup to avoid sudden heating and thermal shock. Gradual heating is done by

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circulating pumped liquid backwards through the idle pump. A small (1-in.) bypass around the check valve is used for this purpose.

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4 6

OPERATING PROBLEMS WITH CENTRIFUGAL PUMPS

A list of the most common process problems is given below. For a more complete list, including mechanical problems, see GPSA Fig. 12-9.

SYMPTOM CAUSE CURE

Pump loses suction High-point pockets Modify piping so flow when flow rate in suction line. is continuously

increases. (Figure 24) horizontal or down-ward.

Low head, motor High viscosity. Heat fluid. Replace

overload. pump and motor. Run

two pumps in parallel. Pump loses suction Insufficient Vent casing before at start. venting of vapor. starting.

Cavitation noise or Insufficient NPSH. Raise suction liquid

loss of capacity at level, reduce rate,

high flows. new impeller.

Failure of mechanical Low flow of seal Adequate cooling seal; leakage. flush liquid. In- and flush. Proper

sufficient cooling stuffing box pressure of seals. and temperature.

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OPERATING PROBLEMS WITH CENTRIFUGAL PUMPS (CONT’D)

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4 8

STANDARDS

The applicable industry standards and the areas they cover are as follows: API 610 Centrifugal Pumps, most recent edition.

ANSI B73.1 Specifications for horizontal, end suction centrifugal pumps for chemical process.

ANSI B73.2 Specifications for vertical in-line centrifugal pumps for chemical process.

NFPA-20 Centrifugal fire water pumps.

Saudi Aramco Design Practice, ADP-G-005

- Exceptions to industry standards

- Special mechanical design requirements - Special materials of construction

- Preferences for pump types

- Guidelines for max. working pressure, test pressure - Inspection and test requirements

Saudi Aramco Engineering Standard AES-G-005 - Hydraulic Performance Criteria - Casing Design Pressure Criteria - Mechanical Seal Selection Guide - Materials of Construction Guide

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5 0

WORK AID 1:

EQUATIONS FOR CALCULATION OF HEAD

• Pump ÆP _{= P2 - P1 (psig or psia)} P1 = Suction pressure P2 = Discharge pressure

• Head (feet) = _P (psi) x 2.31 S.G.

S.G. = Specific gravity relative to water

• ÆP = Head (ft) x 0.433 x S.G.

• Density of water at standard temperature (60°F)

S.G. = 1.0

Density = 8.33 lb/gal = 62.4 lb/ft3 = 350 lb/barrel = 2205 lb/metric ton

EQUATIONS FOR CALCULATING POWER

• bhp = (gpm) x ÆP 1715 x (Pump Eff.) ÆP = Differential Pressure, psi

• kW = bhp x 0.746

Motor Eff.

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Saudi Aramco, Centrifugal Pumps 52

WORK AID 2:

FIGURE 33. AVERAGE NPSHR AS A FUNCTION OF PUMP CAPACITY AT CONSTANT SPEED

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WORK AID 3:

1. Enter Chart at Design Capacity and Move Up To Design Head (For Multi-Stage Pumps, Use Head Per Stage).

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WORK AID 4: AFFINITY LAWS

bhp2 = bhp1 N1     where: Q = Flow Rate H = Head Developed bhp = Power Required D = Impeller Diameter

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WORK AID 5:

(Strainer Installed During Startup) FIGURE 35. TYPICAL CENTRIFUGAL PUMP INSTALLATION

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WORK AID 6: Pump Type and Construction Style Distinguishing Construction Characteristics Usual Orienta-tion Usual No. of Stages Relative Maint-enance Require-ment Comments

Capacity varies with head

CENTRIFUGAL Low to Medium Specific

Speed Horizontal

Single Stage Overhung,

Process Type Impeller CantileveredBeyond Bearings Horiz. 1 Low Most Common StyleUsed in Process Service Two Stage Overhung,

Process Type

2 Impellers Cantilevered Beyond Bearings

Horiz. 2 Low For Heads Above Single

Capacity Single Stage Impeller

Between Bearings Impeller BetweenBearings; Casing Radially or Axially Split

Horiz. 1 Low For High Flows to 1100

Feet Head

Slurry Large Flow Passages,

Erosion Control Features Horiz. 1 High Low Speed; AdjustableAxial Clearance

Canned Pump and Motor

Enclosed in Pressure Shell; no Stuffing Box

Horiz. 1 Low Low-Head Capacity

Limits for Models Used in Chemical Services Multistaged,

Horizontally Split Casing

Nozzles Usually in Bottom Half of Casing

Horiz. Multi Low For Moderate

Temperature-Pressure Ratings

Multistage Barrel Type Outer Casing Confines Inner Stack of Diaphragms

Horiz. Multi Low For High Temperature-Pressure Ratings Vertical

Single Stage Process

Type Vertical Orientation Vert. 1 Low Style Used Primarily toExploit Low NPSH Requirement

Multistage Process Type Many Stages, Low Head/Stage

Vert. Multi Medium High Head Capability, Low NPSH Requirement

In-Line Arranged for In-Line

Installation, Like a Valve

Vert. 1 Low Allows Low Cost

Installation, Simplified Piping Systems High Speed Speeds to 23,000 rpm,

Head to 5800 Feet Vert. 1 Medium Attractive Cost for HighHead/Low Flow

Sump Casing immersed in

Sump for Installation Convenience and Priming Ease

Vert. 1 Low Low Cost Installation

Multistage Deep Well Very Long Shafts Vert. Multi Medium Water Well Service with Driver at Grade

FIGURE 36. COMPARISON OF PUMP TYPES AND CONSTRUCTION STYLES: GENERAL CHARACTERISTICS

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WORK AID 7: Pump Type and Construction Style Capacity GPM Max. Head Ft Max P2 Psi Typical NPSH/ Req Ft. Max Viscos SSU Effic-iency % Solids Toler-ance Max. Pumping Temp. °F CENTRIFUGAL Horizontal

Single Stage Overhung 15-5,000 800 600 6-20 3000 20-80 Mod.

High

850

Two Stage Overhung 15-1,200 1400 600 6-22 2000 20-75 Mod.

High

850 Single Stage Impeller

Between Bearings 15-40,000 1100 980 6-25 3000 30-90 Mod. High 400-500 Slurry 1000 400 600 5-25 3000 20-80 High 850 Canned 1-20,000 5000 10,000 6-20 2000 20-70 Low 1000 Multistaged, Horiz. Split 20-11,000 5500 3000 6-20 2000 65-90 Medium 400-500

Multistage, Barrel Type 20-9,000 5500 6000 6-20 2000 40-75 Medium 850

Vertical

Single Stage Process Type

20-10,000 800 600 1-20 3000 20-85 Medium 650

Multistage 20-80,000 6000 700 1-20 2000 25-90 Medium 500

In-Line 20-12,000 700 500 6-20 2000 20-80 Medium 500

High Speed 5-400 5800 2000 4-40 500 10-65 Low 500

Sump 10-700 200 200 1-22 2000 40-75 Mod.

High

Multistage Deep Well 5-400 6000 2000 1-20 2000 30-75 Medium 400

Note: These data are typical only. Many exceptional cases exist.

FIGURE 37. COMPARISON OF PUMP TYPES AND CONSTRUCTION STYLES: PERFORMANCE CHARACTERISTICS

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WORK AID 8: SELECTION CHARTS

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WORK AID 9:

Trouble: Possible Causes: Trouble: Possible Causes: 1. Failure to deliver

liquid: a. Wrong direction of rotation.b. Pump not primed. c. Suction line not filled with liquid. d. Air or vapor pocket in suction line. e. Inlet to suction pipe not sufficiently

submerged.

f. Available NPSH not sufficient. g. Pump not up to rated speed. h. Total head required greater than head

for which pump is capable of delivering.

6. Vibration: a. Starved suction. (1) Gas or vapor in liquid (2) Available NPSH not sufficient (3) Inlet to suction line not

sufficiently submerged (4) Gas or vapor pockets in suction

line b. Misalignment. c. Worn or loose bearings. d. Rotor out of balance.

(1) Impeller plugged (2) Impeller damaged e. Shaft bent.

f. Improper location of control valve in discharge line.

g. Foundation not rigid. 2. Pump does not

deliver rated capacity:

a. Wrong direction of rotation. b. Suction line not filled with liquid. c. Air or vapor pocket in suction line. d. Air leaks in suction line or stuffing

boxes.

e. Inlet to suction pipe not sufficiently submerged.

f. Available NPSH not sufficient. g. Pump not up to rated speed. h. Total head greater than head for

which pump designed. j. Foot valve too small. k. Foot valve clogged with trash. m. Viscosity of liquid greater than that

which pump designed n. Mechanical defects ... (1) Wearing rings worn (2) Impeller damaged

(3) Internal leakage resulting from defective gaskets.

7. Stuffing boxes

overheat: a. Packing too tight.b. Packing not lubricated. c. Wrong grade of packing.

d. Insufficient cooling water to jackets. e. Box improperly packed.

4. Pump loses liquid

after starting: a. Suction line not filled with liquid.b. Air leaks in suction line or stuffing boxes.

c. Gas or vapor in liquid.

d. Air or vapor pockets in suction line. e. Inlet to suction line not sufficient. f. Available NPSH not sufficient. g. Liquid seal piping to lantern ring

plugged.

h. Lantern ring not properly located in stuffing box.

8. Bearings

overheat: a. Oil level too low.b. Improper or poor grade of oil. c. Dirt in bearings

d. Dirt in oil. e. Moisture in oil.

f. Oil cooler clogged or scaled. g. Failure of oiling system. h. Insufficient cooling water

circulation. j Bearings too tight.

k. Oil seals too close fit on shaft. m. Misalignment.

5. Pump overloads

driver: a. Speed too high.b. Total head lower than rated head. c. Either or both the specific gravity

and viscosity of liquid different from that for which pump is rated.

9. Bearings wear

rapidly: a. Misalignment.b. Shaft bent. c. Vibration.

d. Excessive thrust resulting from mechanical failure inside the pump.

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WORK AID 10: PUMP HEAD AND HORSEPOWER

50 psig NLL LLL EL=20 Ft. Drum D-1 EL=100 Ft. 160 psig Column C-1 3 Ft. Grade E-1 E-2 600 gpm S.G. = 0.72 Pump Eff. = 0.69 FIGURE 40 Line Lengths: Suction - 100 equivalent ft Discharge - 500 equivalent ft Pressure drops:

Suction line- 0.2 psi/100 ft Discharge line - 2.2 psi/100 ft

E-1 - 22 psi

E-2 - 17 psi

Control valve - 20 psi minimum orifice - 1 psi

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WORK AID 10: PUMP HEAD AND HORSEPOWER (CONT’D) Calculate pump head and brake horsepower (required)

Solution:

1. Calculate P₂ Discharge pressure (Max)

a. Vessel pressure 160 psig

b. Static head

(100-3) ft x 0.433 x 0.72 + 30.2 psi c. Friction pressure drops

E-2 + 17 psi

E-1 + 22 psi

Control valve + 20

Flow orifice + 1 psi

Line = 500 x 2.2 + 11 psi

100

P₂ = 261.2 psi

2. Calculate P₁, suction pressure (Min.)

a. Vessel pressure 50 psig

b. Static head

(20-3) ft x 0.433 x 0.72 +5.3 psi c. Friction drops

Line = 100 x 0.2 -0.2

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WORK AID 10: PUMP HEAD AND HORSEPOWER (CONT’D) Problem CP-1, page 3

3. Calculate ÆP

ÆP = P₂ - P1

= 261.2 - 55.1 = 206.1 psi

4. Calculate head required

Head = ÆP (2.31)

S.G. = 206.1 (2.31) 0.72 = 661 ft

5. Calculate brake horsepower

bhp = gpm X ÆP

1715 X Eff. = 600 x 206.1 1715 x 0.69

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WORK AID 10: PUMP HEAD AND HORSEPOWER (CONT’D) Problem CP-1, page 4

6. Check pump head

Total head = Æ Vessel pressure + Æ Elevation + Total friction drop

a. Æ Vessel pressure

(160-50) x 2.31 = 353 ft 0.72

b. Æ Elevation

100 - 20 = 80 ft c. Total friction drop

(0.2 x 100 2.2 x 500 100 100 1201722 x 2.31 = 228 ft

0.72 __________ Total head required = 661 ft

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GLOSSARY

Actual Volume The volume of a given mass of liquid at actual temperature in a process.

Brake Horsepower The quantity of power required to turn the shaft of a pump. The power loading on the shaft between the pump and its driver. Best Efficiency Point The point on the map of head, capacity, and impeller (BEP)

diameter where hydraulic efficiency is maximum. Bearings The parts that support the rotating shaft.

Casing The outer housing of a centrifugal pump. The pressure-containing component.

Circuit A section of plant containing a pump, piping, and heat exchangers. A flow path between two points.

Cavitation The implosion of vapor bubbles in a liquid inside a pump on the impeller vanes. Potentially damaging.

Delta P (ÆP) The pressure difference from pump suction to pump discharge. Diffuser An area of some pumps containing vanes where velocity energy

is converted to pressure. Used instead of a volute in some pumps.

Driver A motor or turbine which provides the power for the pump.

Discharge Pump outlet.

Design Point The specified condition of volume and head for selection of a pump. Also called "rated point."

Eye The center of the impeller where liquid enters the impeller. Efficiency The hydraulic (pressure) energy added to the liquid, divided by

the power input to the shaft.

Flushing A small flow of liquid which keeps solids away from the seal and also cools the seal.

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Head The energy added to a liquid by a pump, ft-lb force/lb. mass. Also referred to as simply "feet."

Hydrometer A float type device that measures specific gravity of a liquid. Impeller The rotating element of a centrifugal pump.

Kickback A recycle stream that increases the flow rate through a pump, independent of process requirements.

Net Positive Suction Actual pressure at the pump suction minus vapor pressure of the Head Available liquid. The amount of pressure drop that can occur before

(NPSH)A vaporization begins.

Orifice Flowmeter A device for measuring fluid flow rate in a pipe. It consists of a restriction orifice in the pipe, pressure taps upstream and downstream of the orifice, and a gauge to measure the ÆP. Performance Curve Graphs that show head produced, power required, NPSH

required, and efficiency; all as functions of flow rate. Pitting Mechanical damage; pits or holes in a metal surface.

Recycle A return flow of some liquid from the discharge side to the suction side. Also called "kickback."

Stage A section of a pump containing one impeller and one diffuser. Pumps may have one or more stages.

Suction Pump inlet.

Shutoff The condition when a pump is rotating but flow is blocked at the discharge. (i.e., pump is acting as a mixer.)

Shutoff Head The head produced by a pump when the discharge is blocked and flow is zero. Usually maximum head produced.

Standard Volume The volume of a given mass of liquid at 60°F. Specific Volume The volume of one pound of liquid.

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Safety Valve A valve that protects a pipe or vessel from overpressure. It opens automatically at a set pressure.

Seal A device that prevents leakage at the point where the rotating shaft enters the casing.

Volute The annular area between the impeller and casing. The place where liquid velocity energy is converted to pressure.

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REFERENCES

Saudi Aramco Standards

AES-G-005 Centrifugal Pumps Saudi Aramco Design Practices

ADP-G-005 Centrifugal Pumps

Exxon Basic Practices

BP10-1-1 Heavy Duty Centrifugal Pumps BP10-1-2 Medium Duty Centrifugal Pumps

Industry Standards

API Standard 610, American Petroleum Institute

Other References

Engineering Data Book, Gas Processors Suppliers Assn., Vol. 1, Section 12 - Pumps and Hydraulic Turbines