Download - Performance of Centrifugal Pumps

PERFORMANCE OF CENTRIFUGAL PUMPS

Performance of Centrifugal Pumps

Capacity

HEAD

Q

HH

Q

The curve is used by engineers to determine the suitability of a given pump for a particular duty.

Relationship between Head and Capacity


Capacity

HEAD

Q

H

Also be useful to pump operators to check if pumps are performing correctly to their design specifications.


H

Q



Capacity

HEAD

Q

HH

Q

The head capacity curve can be used to illustrate two important properties of a centrifugal pump:

1. The discharge from a centrifugal pump may be throttled without causing

damage to the pump.



Capacity

HEAD


1. The discharge from a centrifugal pump may be throttled without causing

damage to the pump.

shut-off



Capacity

HEAD


2. The total head developed is not affected by the specific

gravity of the liquid being pumped.

Brain Water Oil


Pump Characteristic Curves

PERCENT OF DESIGN FLOW

PE

RC

EN

T O

F H

EA

D A

T D

ES

IGN

PO

INT

PE

RC

EN

T O

F B

ES

T E

FF

ICIE

NC

Y

PE

RC

EN

T O

F B

HP

AT

DE

SIG

N P

OIN

T

HEAD

BHP

EFF

Radial Flow Pump



Mixed Flow Pump

PERCENT OF DESIGN FLOW

PE

RC

EN

T O

F H

EA

D A

T D

ES

IGN

PO

INT

PE

RC

EN

T O

F B

HP

AT

DE

SIG

N P

OIN

T

HEAD

BHP

EFF

PE

RC

EN

T O

F B

ES

T E

FF

ICIE

NC

Y



Axial Flow Pump


System Curve

It is the relationship between flow and hydraulic losses in a system.

The point where the pump operates on its curve is dependent upon the characteristics of the system In

which it is operating.

By plotting the system head curve and pump curve together, it can be determined:

1. Where the pump will operate on its curve.

2. What changes will occur if the system head curve or the pump performance curve changes.


System Curve


System CurveNO STATIC HEAD - ALL FRICTION


System CurvePOSITIVE STATIC HEAD


System CurveNEGATIVE (GRAVITY) HEAD


System CurveMOSTLY LIFT- LITTLE FRICTION HEAD


Specific Speed and Pump Type (NS)

It is defined as the speed in revolutions per minute at which a geometrically similar impeller would operate if it were of such a size as to

deliver one gallon per minute against one foot head.

Specific speed (Ns) is a non-dimensional design index used to classify pump impellers as to their type and proportions.

Where:

N = Pump speed in RPM.

Q = Capacity in gpm at the best efficiency point.

H = Total head per stage at the best efficiency point .


Specific Speed and Pump Type (NS)The specific speed determines the general shape or class of the impeller .

Values of Specific Speed, Ns


Specific Speed and Pump Type (NS)Pumps of higher specific speeds develop head partly by centrifugal

force and partly by axial force.


A higher specific speed indicates a pump design with head generation more by axial forces and less by centrifugal forces.


Specific Speed and Pump Type (NS)An axial flow or propeller pump with a specific speed of 10,000 or

greater generates it's head exclusively through axial forces.


Radial impellers are generally low flow high head designs whereas axial flow impellers are high flow low head designs.


Net Positive Section Head and Cavitation (NPSH)it is an analysis of energy conditions on the suction side of a pump to determine if the liquid will vaporize at the lowest pressure point in the

pump.

The Hydraulic Institute defines NPSH as the total suction head in feet absolute, determined at the suction nozzle and corrected to datum, less

the vapor pressure of the liquid in feet absolute.


Net Positive Section Head and Cavitation (NPSH)NPSH Required

The NPSH Required is the positive head in feet absolute required at the pump suction to overcome all pressure drops in the pump and maintain

the majority of the liquid above its vapor pressure.

The NPSH Required varies with speed and capacity within any particular pump. Pump manufacturer's curves normally provide this

information.


Net Positive Section Head and Cavitation (NPSH)NPSH Available

It is a function of the system in which the pump operates.

It is the excess pressure of the liquid in feet absolute over its vapor pressure as it arrives at the pump suction.



Suction supply open to atmosphere with section lift.

NPSHA =PB – (VP +LS +hf)

Where

PB= Barometric pressure in feet absolute.

VP= Vapor pressure of the liquid at maximum pumping temperature, in feet absolute.

Ls = Maximum static suction lift in feet.

hf = Friction loss in feet in suction pipe at required capacity.



Suction supply open to atmosphere with section head.

NPSHA =PB + LH - (VP +hf)

Where



LH = Minimum static suction head in feet.




Closed suction supply with suction head.

NPSHA =P + LH - (VP +hf)

Where

P = Pressure on surface of liquid in closed suction tank, in feet absolute.


LH = Minimum static suction head in feet.




Closed suction supply with suction lift.

NPSHA = P - (VP - LS +hf)

Where

P = Pressure on surface of liquid in closed suction tank, in feet absolute.



Ls = Maximum static suction lift in feet.



In an existing system, the NPSH Available can be determined by a gauge on the pump suction. The following formula applies:

NPSHA = PB - VP - (Gr +hv)

Where



Gr = Gauge reading at the pump suction expressed in feet (plus if above atmospheric, minus if below atmospheric) corrected to the pump centerline.



Cavitation

Cavitation means different things to different people. It has been described as:

• A reduction in pump capacity.

• A reduction in the head of the pump.

• The formation of bubbles in a low pressure area of the pump volute.

• A noise that can be heard when the pump is running.

• Damaged that can be seen on the pump impeller and volute.

Just what then is this thing called cavitation?

Actually it is all of the above.


Cavitation

It takes place as the following:

1. The pressure of the liquid is reduced to a value equal to or below its vapor pressure.

2. The liquid begins to boil and small vapor bubbles or pockets begin to form.

3. As these vapor bubbles move along the impeller vanes to a higher pressure area above the vapor pressure, they rapidly collapse.

4. As these vapor bubbles move along the impeller vanes to a higher pressure area above the vapor pressure, they rapidly collapse.

5. In high suction energy pumps, the collapses are generally high enough to cause minute pockets of fatigue failure on the impeller

vane surfaces.


Cavitation

The capacity of the pump is reduced:

Now we will go back to clear up some of the confusion:

This happens because bubbles take up space and you cannot have bubbles and liquid in the same place at the same time.

If the bubble gets big enough at the eye of the impeller, the pump will lose its suction and will require priming.


Cavitation

The discharge head is often reduced Bubbles, unlike liquid, are compressible. It is this compression that can

change the head.

You should keep in mind that as the velocity of a fluid increases, the pressure of the fluid decreases. This means that high velocity liquid is by definition a

lower pressure area. This can be a problem any time a liquid flows through a restriction in the piping, volute, or changes direction suddenly. The fluid will

accelerate as it changes direction. The same acceleration takes place as the fluid flows in the small area between the tip of the impeller and the volute cut

water.

The bubbles form in a lower pressure area because they cannot form in a high pressure area.


Cavitation

A noise is heard Any time a fluid moves faster than the speed of sound in the medium you are

pumping, a sonic boom will be heard. The speed of sound in water is 4800 feet per second (1480 meters/sec) or 3,273 miles per hour (5,267 kilometers

per hour).


Cavitation

Pump parts show damage

The bubble tries to collapse on its self. This is called imploding, the opposite of exploding. The bubble is trying to collapse from

all sides.

If the bubble is laying against a piece of metal such as the impeller or volute it cannot collapse from that side, so the fluid

comes in from the opposite side at this high velocity proceeded by a shock wave that can cause all kinds of damage. There is a very characteristic round shape to the liquid as it bangs against the metal creating the impression that the metal was hit with a

"ball peen hammer".


Cavitation

Pump parts show damage

This damage would normally occur at right angles to the metal, but experience shows that the high velocity liquid seems to come

at the metal from a variety of angles.

This can be explained by the fact that dirt particles get stuck on the surface of the bubble and are held there by the surface

tension of the fluid. Since the dirt particle has weakened the surface tension of the bubble, it becomes the weakest part, and

the section where the collapse will probably take place.


Cavitation

The higher the capacity of the pump the more likely cavitation will occur. High specific speed pumps have a different impeller shape that allows

them to run at high capacity with less power and less chance of cavitating.

This impeller is normally found in a pipe shaped casing rather than the volute type of casing that you commonly see.


Cavitation

The cavities form for five basic reasons and it is common practice to lump all of them into the general classification of cavitation.

This is an error because to correct each of these conditions, you must understand why they occur, and how to fix them.

In no particular order they are :

• Vaporization cavitation

• Air ingestion cavitation.

• Internal recirculation cavitation.

• Flow turbulence cavitation.

• Vane Passing Syndrome cavitation.


Cavitation

The way to prevent the undesirable effects of Vaporization cavitation in standard low suction energy pumps is to insure

that:

NPSHA > NPSHR

High suction energy pumps require an additional NPSH margin, above the NPSH Required. Hydraulic Institute Standard (ANSI/HI 9.6.1) suggests NPSH margin ratios of from 1.2 to 2.5 times the NPSH

Required, for high and very high suction energy pumps, when operating in the allowable operating range.


NPSH and Suction Specific Speed

In designing a pumping system, it is essential to provide adequate NPSH available for proper pump operation.

Insufficient NPSH available may seriously restrict pump selection, or even force an expensive system redesign.

On the other hand, providing excessive NPSH available may needlessly increase system cost.

Suction specific speed may provide help in this situation.

Suction specific speed (S) is defined as:



Suction specific speed (S) is defined as:

Where:

GPM = Pump flow at best efficiency point at impeller inlet (for double suction impellers divide total pump flow by

two).

NPSHR = Pump NPSH required at best efficiency point.

N = Pump speed RPM.

For a given pump, the suction specific speed is generally a constant - it does not change when the pump speed is

changed.



Experience has shown that 9000 is a reasonable value of suction specific speed.

Example:

Flow 2,000 GPM; head 600 ft. What NPSHA will be required?

Assume: at 600 ft., 3500 RPM operation will be required.

43

R

21

NPSH

200035509000=

17.7NPSH 43R

46ftNPSHR )Pump (

NPSHA =(NPSHR) (NPSH Margin Ratio)= 46 x 1.5 = 69 ft



According to the Hydraulic Institute, NPSH margin is required above the NPSHR of the pump to suppress incipient cavitation. The amount of margin is a

function of Suction Energy and the critical nature of the application as follows:

Section Energy NPSHMargin Ratio (NPSHA/NPSHR)

Low 1.1 - 1.3

High 1.2 - 1.7

Very High 1.7 - 2.5


How to stop vaporization Cavitation

By increasing the suction head

• Raise the liquid level in the tank • Elevate the supply tank. • Put the pump in a pit. • Reduce the piping losses.

• Retrofit the pump with a higher specific speed impeller. • Install a booster pump or inducer.

• Pressurize the tank. • Be sure the tank vent is open and not obstructed. Some vents can

freeze in cold weather.



By lowering the fluid inlet temperature

• Injecting a small amount of cooler fluid at the suction is often practical.

• Insulate the suction piping from the sun's rays.

• Be careful of discharge re-circulation and vent lines re-circulated to the pump suction; they can heat up the suction fluid.



By decrease the fluid velocity

• Remove obstructions in the suction piping.

• Do not run the impeller too close to the pump cutwater.

• Reduce the speed of the pump.

• Reduce the capacity of the pump.

• Do not install an elbow too close to the pump suction.



By reducing the net positive suction head required (NPSHR)

• Use a double suction pump. Double suction designs can reduce the net positive suction head required (NPSHR) by as much as 27%, or in some

cases it will allow you to raise the pump speed by 41%.

• Use a lower speed pump.

• Use a pump with a larger impeller eye opening. If possible install an inducer. These inducers can cut net positive suction head required

(NPSHR) by almost 50%.

• Use several smaller pumps. Three half-capacity pumps can be cheaper than one large pump plus a spare. This will also conserve energy at lighter

loads.


Affinity Laws

The affinity laws express the mathematical relationship between the several variables involved in pump performance.

They apply to all types of centrifugal and axial flow pumps.


Affinity Laws

They are as follows:

With impeller diameter D held constant:


Affinity Laws

With speed N held constant:

When the performance (Q1, H1, & BHP1) is known at some particular speed (N1) or diameter (D1), the formulas can be used to estimate the performance

(Q2, H2, & BHP2) at some other speed (N2) or diameter (D2).

The efficiency remains nearly constant for speed changes and for small changes in impeller diameter.


Series Operation of Centrifugal Pumps


Parallel Operation of Centrifugal Pumps