Performance of Centrifugal Pumps

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PERFORMANCE OF CENTRIFUGAL PUMPS

Performance of Centrifugal Pumps

Relationship between Head and Capacity The curve is used by engineers to determine the suitability of a given pump for a particular duty.H H

HEADQ Q Capacity

Performance of Centrifugal Pumps

Relationship between Head and Capacity Also be useful to pump operators to check if pumps are performing correctly to their design specifications.H H

HEADQ Q Capacity

Performance of Centrifugal Pumps

Relationship between Head and Capacity The head capacity curve can be used to illustrate two important properties of a centrifugal pump:H H

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

HEADQ Q Capacity

Performance of Centrifugal Pumps

Relationship between Head and Capacity The head capacity curve can be used to illustrate two important properties of a centrifugal pump:shut-off

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

HEADCapacity

Performance of Centrifugal Pumps

Relationship between Head and Capacity The head capacity curve can be used to illustrate two important properties of a centrifugal pump:Water Brain Oil

2. The total head developed is not affected by the specific gravity of the liquid being pumped.

HEADCapacity

Performance of Centrifugal Pumps

Pump Characteristic CurvesRadial Flow PumpPERCENT OF HEAD AT DESIGN POINT PERCENT OF BHP AT DESIGN POINT PERCENT OF BEST EFFICIENCY

HEAD

BHP

EFF

PERCENT OF DESIGN FLOW

Performance of Centrifugal Pumps

Pump Characteristic CurvesMixed Flow PumpPERCENT OF HEAD AT DESIGN POINT PERCENT OF BEST EFFICIENCYHEAD

BHP

EFF

PERCENT OF DESIGN FLOW

PERCENT OF BHP AT DESIGN POINT

Performance of Centrifugal Pumps

Pump Characteristic CurvesAxial Flow Pump

Performance of Centrifugal Pumps

Pump Characteristic Curves

Performance of Centrifugal Pumps

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.

Performance of Centrifugal Pumps

System Curve

Performance of Centrifugal Pumps

System CurveNO STATIC HEAD - ALL FRICTION

Performance of Centrifugal Pumps

System CurvePOSITIVE STATIC HEAD

Performance of Centrifugal Pumps

System CurveNEGATIVE (GRAVITY) HEAD

Performance of Centrifugal Pumps

System CurveMOSTLY LIFT- LITTLE FRICTION HEAD

Performance of Centrifugal Pumps

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

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.

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 .

Performance of Centrifugal Pumps

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

Performance of Centrifugal Pumps

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.Values of Specific Speed, Ns

Performance of Centrifugal Pumps

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.Values of Specific Speed, Ns

Performance of Centrifugal Pumps

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.

Performance of Centrifugal Pumps

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.

Performance of Centrifugal Pumps

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.

Performance of Centrifugal Pumps

Net Positive Section Head and Cavitation (NPSH)NPSH Available

Performance of Centrifugal Pumps

Net Positive Section Head and Cavitation (NPSH)NPSH AvailableSuction 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.

Performance of Centrifugal Pumps

Net Positive Section Head and Cavitation (NPSH)NPSH AvailableSuction supply open to atmosphere with section head. NPSHA =PB + LH - (VP +hf) Where PB= Barometric pressure in feet absolute. VP= Vapor pressure of the liquid at maximum pumping temperature, in feet absolute. LH = Minimum static suction head in feet. hf = Friction loss in feet in suction pipe at required capacity.

Performance of Centrifugal Pumps

Net Positive Section Head and Cavitation (NPSH)NPSH AvailableClosed suction supply with suction head. NPSHA =P + LH - (VP +hf) Where P = Pressure on surface of liquid in closed suction tank, in feet absolute. VP= Vapor pressure of the liquid at maximum pumping temperature, in feet absolute. LH = Minimum static suction head in feet. hf = Friction loss in feet in suction pipe at required capacity.

Performance of Centrifugal Pumps

Net Positive Section Head and Cavitation (NPSH)NPSH AvailableClosed suction supply with suction lift. NPSHA = P - (VP - LS +hf) Where P = Pressure on surface of liquid in closed suction tank, 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.

Performance of Centrifugal Pumps

Net Positive Section Head and Cavitation (NPSH)NPSH AvailableIn an existing system, the NPSH Available can be determined by a gauge on the pump suction. The following formula applies: NPSHA = PB - VP Where PB= Barometric pressure in feet absolute. VP= Vapor pressure of the liquid at maximum pumping temperature, in feet absolute. Gr = Gauge reading at the pump suction expressed in feet (plus if above atmospheric, minus if below atmospheric) corrected to the pump centerline. hf = Friction loss in feet in suction pipe at required capacity.

(Gr +hv)

Performance of Centrifugal Pumps

CavitationCavitation 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.

Performance of Centrifugal Pumps

CavitationIt 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.

Performance of Centrifugal Pumps

CavitationNow we will go back to clear up some of the confusion:The capacity of the pump is reduced: 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.

Performance of Centrifugal Pumps

CavitationThe discharge head is often reduced Bubbles, unlike liquid, are compressible. It is this compression that can change the head. The bubbles form in a lower pressure area because they cannot form in a high pressure area. You should keep in mind that as the velocity of a fluid increases, the pressure of the fluid decreases. This means that high velocity l