INTRODUCTION TO CENTRIFUGAL PUMPPrepared by: Muhammad Ridhwan Abdul Rasid, Mechanical Trainee. Date: Rev 1, 4thMarch2010

1. DEFINITION & DESCRIPTION

Centrifugal pump is a machine that imparts energy to a fluid. This energy infusion can cause a liquid to flow, rise to a higher level, or both. It is a member of a family known as rotary machines and consists of two basic parts:

a) The rotary element or impellerb) The stationary element or casing (volute)

The Figure 1 is a cross section of a centrifugal pump and shows the two basic parts.

Figure 1

The centrifugal pump’s function is as simple as its design. It is filled with liquid and the impeller is rotated by motor. Rotation imparts energy to the liquid causing it to exit the impeller’s vanes at greater velocity than it possessed when it entered. This outward flow reduces the pressure at the impeller eye, allowing more liquid to enter. The liquid that exits the impeller is collected in the casing (volute) where its velocity is converted to pressure before it leaves the pump’s discharge.

2. THEORY

Centrifugal Force

In operation, a centrifugal pump ‘slings’ liquid out of the impeller via centrifugal force. A classic example of the action of centrifugal force is shown in Figure 2. We see a pail of water swinging in a circle. The swinging pail generates a centrifugal force that holds the water in the pail.

Figure 2

Then, if a hole is bored in the bottom of the pail, water will be thrown out. The distance the stream (from the hole and tangent to the circle) and the volume that flows out (per unit time) depend upon the velocity of the rotating pail. The faster the pail rotates the greater the centrifugal force. Therefore the greater the volume of water discharged and distance it carries.

Peripheral Velocity & Head

The description above demonstrates that the flow and head (pressure) developed by a centrifugal pump depends upon the rotational speed and more precisely, the peripheral velocity of its impeller.

Gravity is one of the more important forces that a centrifugal must overcome. A simple example of gravitational action is a stone dropped from the top of a building. Its velocity will increase every second due to gravitational acceleration. When the stone is thrown upward to a certain height, the same concept used to determine the initial velocity required to counter the potential energy because the final velocity of a falling body happens to be equal to the initial velocity required to launch it to height from which it fell.

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v2=2gh

The same equation and concept applies when pumping water or oil with centrifugal pump. The velocity of the water/ oil as it leaves the impeller determines the head developed. In other words the water is ‘thrown’ to a certain height. To reach this height it must start with same velocity it would attain if it fell from that height.

3. SUCTION CONDITIONS

Suction conditions are some of the most important factors affecting centrifugal pump operation.

Suction Lift

A pump cannot pull or ‘suck’ a liquid up its suction pipe because liquids do not exhibit tensile strength to transmit tension or be pulled. It is simply reducing local pressure by creating a partial vacuum. Atmospheric pressure pushes the liquid up the suction pipe into the pump.

Atmospheric pressure at sea level is called absolute pressure (PSIA) because it is a measurement using absolute zero as base. If pressure is measured using atmospheric pressure as a base it is called gauge pressure (PSIG or PSI).

In addition to pump design and suction piping, there are two physical properties of the liquid being pumped that affected suction lift.

1) Maximum suction lift is dependant upon the pressure applied to the surface of the liquid at the suction source. Maximum suction lift decreases as pressure decreases.

2) Maximum suction lift is dependant upon the liquid being pumped. The vapor pressure of a liquid is the pressure necessary to keep the liquid from vaporizing (boiling) at a given temperature. Vapor pressure increases as liquid temperature increases. Maximum suction lift decreases as vapor pressure rises.

The maximum suction lift of a centrifugal pump varies inversely with altitude. Conversely, maximum suction lift will increase as the atmosphere pressure increases. However, higher altitude will provide lower atmospheric pressure.

Besides, the maximum suction lift of a liquid varies inversely with the temperature of the liquid. The higher the temperature, the higher the vapor pressure and thus suction lift is decreased. If centrifugal pump is used to pump a liquid that is too hot the liquid will boil or vaporize in the pump suction. This condition is called cavitations.

Capacity and Suction Lift

The suction lift of a centrifugal pump also varies inversely with pump capacity. Illustration below shows how the head – capacity curve falls off quickly at various suction lifts. The maximum suction lift increases as pump capacity decreases. For this reason pumps used in high suction lift applications are selected to operate in a range considerably to the left of their peak efficiency.

Figure 3

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Net Positive Suction Head (NPSH)

1) Net Positive Suction Head Required (NPSHR) – is a function of specific pump design. In simple terms it is the pressure, measured at the centerline of the pump suction, necessary for the pump to function satisfactorily at a given flow. NPSHR varies with flow; however temperature and altitude have no effect.

2) Net Positive Suction Head Available (NPSHA) – is a characteristic of the system in which the pump operates. It depends upon the elevation or pressure of the suction supply, friction in the suction line, altitude of the installation, and the vapor pressure of the liquid being pumped.

Both required and available NPSH vary with the capacity of a given pump and suction system. NPSHA is decreased as the capacity increased due to the increased friction losses in the suction piping. NPSHR increases approximately as the square of capacity since it is a function of the velocities and friction in the pump inlet. NPSHA must always greater than the NPSHR with safety margin about 2 feet or 1 meter to cover unforeseen circumstances.

The calculation as follows:

NPSHA = Ha ± Hs – Hvp – Hfs

Where,

Ha=Atmospheric pressure in feetHs=Total suction head or lift headHvp=Vapor pressure in feetHfs=All suction line losses (entrance losses and friction losses through pipe, valves & fittings).

4. READING PUMP CURVE

Centrifugal pump performance is represented by multiple curves indicating either:

Various impeller diameters at a constant speed. Various speeds with a constant impeller diameter.

The curve consists of a line starting at "shut head"(zero flow on bottom scale / maximum head on left scale). The line continues to the right, with head reducing and flow increasing until the "end of curve" is reached, (this is often outside the recommended operating range of the pump).

Flow and head are linked; one can not be changed without varying the other. The relationship between them is locked until wear or blockages change the pump characteristics.

The pump can not develop pressure unless the system creates backpressure (i.e.: Static (vertical height), and /or friction loss). Therefore the performance of a pump can not be estimated without knowing full details of the system in which it will be operating.

Refer to Figure 4 below for a sample curve showing:

i. Three performance curves (various impellers or speed).ii. Curves showing power absorbed by pump (read power at operating point, see note 1).

iii. Best efficiency point (BEP).iv. Recommended operating range (operation outside this range reduces pump life).v. Nett positive suction head required by the pump (NPSH [R]).

vi. The circled numbers indicate the following for bottom curve (i.e.: smallest diameter impeller or slowest speed curve shown):

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Figure 4

The points referred to as “shut head” and “end of curve”.

Note 1:Power absorbed by pump is read at point where power curve crosses pump curve at operating point. However this does not indicate motor / engine size required. Various methods are used to determine driver size.

i. Select motor or engine to suit specific engine speed or operating range - most cost effective method where operating conditions will not vary greatly.

ii. Read power at end of curve - most common way that ensures adequate power at most operating conditions.

iii. Read power at operating point plus 10% - usually only used in refinery or other applications where there is no variation in system characteristics.

IV. By using system curves all operating conditions can be considered - best method where filling of long pipelines, large variations in static head, or siphon effect exist.

5. OPERATING RANGE

All types of pumps have operational limitations. This is a consideration with any pump whether it is positive displacement or centrifugal. The single volute centrifugal pump (the most common pump used worldwide) has additional limitations in operating range which, if not considered, can drastically reduce the service life of pump components.

"BEP" - Best Efficiency Point (Figure 4) is not only the operating point of highest efficiency but also the point where velocity and therefore pressure is equal around the impeller and volute. As the operating point moves away from the Best Efficiency Point, the velocity changes, which changes the pressure acting on one side of the impeller. This uneven pressure on the impeller results in radial thrust which deflects the shaft causing:

Excess load on bearings. Excess deflection of mechanical seal. or: Uneven wear of gland packing or shaft / sleeve.

The resulting damage can include shortened bearing / seal life or a damaged shaft. The radial load is greatest at shut head.

Outside the recommended operating range damage to pump is also sustained due to excess velocity and turbulence. The resulting vortexes can create cavitations damage capable of destroying the pump casing, back plate, and impeller in a short period of operation. Refer to Figure 4 which indicates range of operation (between approximately 50% and 120% of Best Efficiency Point)

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When selecting or specifying a pump, it is important not to add safety margins or base selection on inaccurate information. The actual system curve may cross the pump curve outside the recommended operating range. In extreme cases the operating point may not allow sufficient cooling of pump, with serious ramifications!

The best practice is to confirm the actual operating point of the pump during operation (using flow measurement and / or a pressure gauge) to allow adjustment (throttling of discharge or fitting of bypass line) to ensure correct operation and long service life.

6. CAVITATION IN PUMP

There are two main causes leading to cavitations of the pumps:

i. NPSHR exceeds NPSHADue to low pressure the water vaporizes (boils) and higher pressure implodes into the vapor bubbles as they pass through the pump causing reduced performance and potentially major damage.

ii. Suction or discharge recirculationThe pump is designed for a certain flow range, if there is not enough or too much flow going through the pump, the resulting turbulence and vortexes can reduce performance and damage the pump.

7. PERFORMANCE CURVE

Once a pump is ready for production, it is given a complete and thorough test to gather accurate data on flow, head, horsepower and net positive suction head required which plotted versus capacity. The data is recorded at shut off (no flow), full flow and 5 to 10 points between. The Characteristic Curve is drawn through the points. It shows the entire operating characteristic of a given pump. The curve (Figure 5) below is an example.

Figure 5

For publication purposes, several curves are drawn to show Head-Capacity curves for one speed and several impeller diameters or one impeller diameter and several different speed of the same pump. This type of curve is called an Iso-Efficiency or Composite Characteristic Curve. The curve (Figure 6) below is an example.

Figure 6

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Calculating Brake Horsepower (BHP)

Usually, if a specific Head-Capacity point falls between two horsepower contour lines, the higher horsepower motor is selected. So it is important to understand a composite curve well enough to interpolate and find the approximate values. Often, the middle one third of the curve is suitable for application use. However, we need to know the exact horsepower requirement for that point of operation.

BHP for centrifugal pump can be calculated as follows:

BHP = GPM (flow rate) x Head / 3960 x Efficiency

Operation is series (Booster Service)

When a centrifugal pump is operated with a positive suction pressure, the resulting discharge pressure will be the sum of the suction pressure. The pressure normally developed by the pump. This quality makes it ideally suited for use as a booster pump which boosts the existing pressure to some higher value. This quality also makes it practical to build multi-stage (multiple impellers) pumps. Capacity however remains the same as that of either one of the pump.

Figure 7

From the figure, it shows that the capacity remains same, the increment only at head of either series or single pump.

Parallel Operation

Two or more pumps may also be operated in parallel. The curves developed during parallel different with series operation and illustrated below.

Figure 8

As figure illustrates, the flows and horsepower are additive, while head does not change. The flow is almost doubled at any given point.

8. THE AFFINITY LAW

The centrifugal pump is a flexible machine. The performance of a centrifugal pump can be varied by changing the impeller diameter or its rotational speed. Because of this it is unnecessary to design a separate pump for each job. Reducing impeller diameter is probably the most common change and usually the most economical.

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The speed can be altered by changing pullet diameters or by changing the speed of driver. In some cases both speed and impeller diameter is changed to obtain the desired results.

When the driven speed or impeller diameter of a centrifugal pump changes, operation of the pump changes in accordance with three fundamental laws. These laws are known as the ‘Laws of Affinity’. They state that:

1) Capacity varies directly as the change in speed or impeller diameter2) Head varies as the square of the change in speed or impeller diameter3) Brake horsepower varies as the cube of the change in speed or impeller diameter.

9. SPECIFIC GRAVITY AND VISCOSITY

Specific Gravity

The specific gravity of a substance is the ratio of the weight of a given volume of the substance to that of an equal volume of water at standard temperature and pressure (STP). The following statements will always be true regardless of the specific gravity:

1) A centrifugal pump will always develop the same head in feet regardless of a liquid’s specific gravity.2) Pressure will increase or decrease in direct proportion to a liquid’s specific gravity.3) Brake HP required will vary directly with a liquid’s specific gravity.

The preceding discussion of SG illustrates why centrifugal pump head (or pressure) is expressed in feet or meter. Since pump specialist work with many liquids of varying SG, head in feet or meter is the most convenient system of designating head.

The Effect of Viscosity

Viscosity is the internal friction of a fluid. Fluids having a high viscosity are sluggish in flow. However viscosity varies greatly with temperature with viscosity decreasing as temperature rises.

High viscosity can gum up the internals of a centrifugal pump. Viscous liquids tend to reduce capacity, head and efficiency while increasing the BHP. Normally, small and medium sized centrifugal pump can be used to handle liquids with viscosities up to 2000 SSU. SSU or Seconds Saybolt Universal is the unit measurement of viscosity. Viscosities over 2000 SSU are usually better suited for positive displacement pumps.

10. FRICTION & FRICTION HEAD

Friction occurs when a fluid flows in or around a stationary object or when an object moves through a fluid. Fluids create friction as they move through a closed pipe. Thus, the design shall reduce the friction because friction produces heat and where there is heat there is energy, wasted energy that is. Reducing the friction therefore will reduce the energy required to get the fluid flow from point A to point B. The higher the flow, the greater the friction will be.

Under conditions of laminar flow in smooth pipe, the fluid nearest the pipe wall moves more slowly than that in the center (refer Figure 9). Relate to that, the smaller the pipe diameter the greater the contact between the liquid and the wall thus the greater the friction.

Figure 9

There is an optimum flow for each size, after which friction can use up a disproportionate amount of the pump output. It is evident from the tables that are universally available for wide range of pipe sizes and materials. These tables show the friction loss per 100 feet of a specific pipe size at various flow rates.

*G.P.M. = Gallon per minute (flow rate).

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They are also available friction for various pipe fittings and valves. The friction losses for valves and fittings can also add up. 90 degree turns and restrictive valves add the most friction. All these make take into account in calculating the required pump power. The calculation will refer to as table (Figure 10) below.

*Figure for enlargement, contraction and tapers apply to smaller diameter Figure 10

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