Atmospheric Pressure
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Atmospheric Pressure @ Sea Level
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Absolute
Pressure
The sum of the available atmospheric pressure and the gage pressure in the
pumping system
Absolute Pressure
(PSIA) = Gauge Pressure + Atmospheric Pressure
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Absolute P. = 150
PSIG (Gauge P.) + 14.7 PSI (Atmospheric P.) = 164.7 PSIA
Vacuum
The full or partial
elimination of Atmospheric Pressure
Atmospheric Pressure
on the Moon = 0 = Full Vacuum
1 Inch Hg Vacuum =
1.13 Ft of Water
Specific
Gravity
Specific Gravity is the ratio of the weight of
anything to the weight of water.
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Specific Gravity of
HCl = (Weight of HCl)/(Weight of Water) = (10.0)/(8.34) = 1.2
Pressure
and Liquid Height Relationship (Head)
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1 PSI = 2.31 Ft of
Water
Pressure, Liquid
Height, & Specific Gravity Relationship
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Pressure (PSI) = Head (FT) x Specific Gravity (SG) / 2.31
Example - Water -
231Ft x 1.0 / 2.31 = 100 PSI
Example - HCL - 231
Ft x 1.2 / 2.31 = 120 PSI
Example - Gas - 231
Ft x .80 / 2.31 = 80 PSI
Vapor
Pressure
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The pressure pushing
against atmospheric pressure on liquids at elevated temperatures.
Suction
Head
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A Suction Head
exists when the liquid is taken from an open to atmosphere tank where the
liquid level is above the centerline of the pump suction, commonly known as a
Flooded Suction.
Total
Dynamic Head
Total Dynamic Head
(TDH) = Elevation(ft) + Friction(ft)
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Centrifugal
Pump Components
The two main
components of a centrifugal pump are the impeller and the volute. The impeller
produces liquid velocity and the volute forces the liquid to discharge from the
pump converting velocity to pressure. This is accomplished by offsetting the
impeller in the volute and by maintaining a close clearance between the
impeller and the volute at the cut-water. Please note the impeller rotation. A
centrifugal pump impeller slings the liquid
out of the volute. It does not cup the
liquid.
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Pump
Performance Curve
A Pump Performance
Curve is produced by a pump manufacturer from actual tests performed and shows
the relationship between Flow and Total Dynamic Head, the Efficiency, the NPSH
Required, and the BHP Required.
Higher Head = Lower
Flow Lower Head = Higher
Flow
Lower Flow = Lower
Horsepower Higher Flow =
Higher Horsepower
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Based on Water SG
1.0
Capacity
A Centrifugal Pump
is a variable displacement pump. The actual flow rate achieved is directly
dependent on the Total Dynamic Head it must work against.
The flow capacity of
a centrifugal pump also depends on three (3) other factors:
1
Pump Design
2
Impeller Diameter
3
Pump Speed
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Affinity Laws
The performance of a
centrifugal pump is affected by a change in speed or impeller diameter.
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Q = Capacity (GPM) D = Impeller Diameter N= Speed(RPM)
H = Total Dynamic
Head(Feet) BHP
= Brake Horsepower
The affinity law for
a centrifugal pump with the impeller diameter held constant and the speed
changed:
Flow: Q1 / Q2 = N1 / N2
Example: 100 / Q2 = 1750/3500 Q2 = 200 GPM
Head: H1/H2 = (N1) x (N1) / (N2) x (N2)
Example: 100 /H2 = 1750 x 1750 / 3500 x 3500 H2 = 400 Ft
Horsepower (BHP):
BHP1 / BHP2 = (N1) x
(N1) x (N1) / (N2) x (N2) x (N2)
Example: 5/BHP2 = 1750 x 1750 x 1750 / 3500 x 3500 x
3500 BHP2 = 40
The affinity law for
a centrifugal pump with the speed held constant and the impeller diameter changed:
Flow: Q1 / Q2 = D1 / D2
Example: 100 / Q2 =
8/6 Q2 = 75 GPM
Head: H1/H2 = (D1) x (D1) / (D2) x (D2)
Example: 100 /H2 = 8
x 8 / 6 x 6 H2 = 56.25 Ft
Horsepower (BHP):
BHP1 / BHP2 = (D1) x
(D1) x (D1) / (D2) x (D2) x (D2)
Example: 5/BHP2 = 8
x 8 x 8 / 6 x 6 x 6 BHP2 = 2.1
Brake
Horsepower
BHP = Flow(GPM) X
TDH(FT) x SG /3960xEFFICIENCY(%)
Example: BHP = (100
GPM) x (95 Ft) x (1.0) / 3960 x .6 BHP
= 4.0
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Calculating
Total Dynamic Head (TDH)
Flooded Suction
Application
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TDH = Total
Discharge Head - Total Suction Head
Total Suction Head =
Static - Friction
Total Discharge Head
= Static + Friction
Suction Lift
Application
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TDH = Total
Discharge Head + Total Suction Lift
Total Suction Lift=
Static + Friction
Total Discharge Head
= Static + Friction
Total Dynamic Head =
Total Discharge Head + Total Suction Head
System
Head Curve
To Calculate a
System Head Curve several points must be chosen to calculate friction losses on
both the suction and discharge sides of the pump at various flow rates. The
static suction head/lift and the static discharge head remain constant.
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Net
Positive Suction Head
Net Positive Suction
Head Required (NPSHR)
The net positive
suction head required is a function of the pump design at the operating point
on the pump performance curve.
Net Positive Suction
Head Available (NPSHA)
The net positive
suction head available is a function of the pump suction system.
The Net Positive
Suction Head is the absolute total suction head in feet.
The NPSH available
in a flooded suction system is:
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Atmospheric Pressure (-) Vapor
Pressure (+) Liquid Height (-) Friction in the Suction Line.
The NPSH available
in a suction lift system is:
Atmospheric Pressure
(-) Vapor Pressure (-) Liquid Ht. (-) Friction in the Suction Line.
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If
the NPSHA < NPSHR the Pump will cavitate
Cavitation
Cavitation may occur
in two different forms:
Suction Cavitation
Suction Cavitation
occurs when the pump suction is under a low pressure/high vacuum condition
where the liquid turns into a vapor at the eye of the pump impeller. This vapor
is carried over to the discharge side of the pump where it no longer sees
vacuum and is compressed back into a liquid by the discharge pressure. This
imploding action occurs violently and attacks the face of the impeller. An
impeller that has been operating under a suction cavitation condition has large
chunks of material removed from its face causing premature failure of the pump.
Discharge Cavitation
Discharge Cavitation
occurs when the pump discharge is extremely high. It normally occurs in a pump
that is running at less than 10% of its best efficiency point. The high
discharge pressure causes the majority of the fluid to circulate inside the
pump instead of being allowed to flow out the discharge. As the liquid flows
around the impeller it must pass through the small clearance between the impeller
and the pump cutwater at extremely high velocity. This velocity causes a vacuum
to develop at the cutwater similar to what occurs in a venturi and turns the
liquid into a vapor. A pump that has been operating under these conditions
shows premature wear of the impeller vane tips and the pump cutwater. In
addition due to the high pressure condition premature failure of the pump
mechanical seal and bearings can be expected and under extreme conditions will
break the impeller shaft.
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Suction
Cavitation & Discharge Cavitation are extremely damaging to pump
components.