NPSH.pdf

Engineering Solutions . . . Delivering Results
Introduction
• Pumps are a mechanical device that forces a fluid to move from one
position to another.
• This is achieved by supplying energy or head to a flowing liquid in
order to overcome head losses due to friction, pipe work and also, if
necessary, to raise the liquid to a higher level.

Classification of Pumps

Centrifugal pump
• Centrifugal pump
• Its purpose
• The energy changes

Centrifugal pump

Reciprocating pump
 Piston plunger pump is simplest
form
 Prime movers
 The capacity of a pump is
determined by the number of plungers
or pistons and the size of these
elements (bore and stroke).

Reciprocating pump

Features of different pump

Pump Types-Comparison and Selection Guide
Yes
Yes
No
Self-priming
Constant
Constant
Variable
Variable or
Constant Flow
Smooth
Pulsating
Smooth
Smooth or
Pulsating Flow
Yes
Yes
No
Requires Relief
Valve
2.81 X 106 + kg/m2
7.03 X 106 + kg/m2
4.23 X 104 + kg/m2
Maximum Pressure
Yes
Yes
No
Low Flow Rate
Capability
2271.2 + m3/hr
2271.2 + m3/hr
27276.6 + m3/hr
Maximum Flow
Rate
Low/Medium
Capacity,
Low/Medium
Pressure
Low Capacity,
High Pressure
Medium/High
Capacity,
Low/Medium
Pressure
Optimum Flow and
Pressure
Applications
Rotary Pumps
Reciprocating Pumps
Centrifugal Pumps
Parameter

Calculation
• Suction pressure
Suction pressure is the pressure at the upstream of the pump. This is given
by
Suction pressure = source press. + static head – del p in pump suction
BTL
Pump centre line
Vessel
Source pressure Source pressure – pressure
above the liquid level
Static head – static pressure
difference due to the liquid
between pump centre line
and vessel BTL
Del P – losses in the pump
suction line
Pump

NPSH – Net Positive Suction Head
• NPSHa is the net remaining pressure at the suction flange of the
pump after subtracting all negative forces that restrict liquid from
getting into the pump.
• A similar term NPSHr is used by pump manufactures to describe the
energy losses that occur within many pumps as the fluid volume is
allowed to expand within the pump body. This energy loss is
expressed as a head of fluid.

NPSHa Calculation
The NPSHa is calculated from:
Fluid surface pressure + positive
head – pipework friction loss –
fluid vapour pressure
Or
Fluid surface pressure – negative
head – pipework friction loss –
fluid vapour pressure

Cavitation
• Vapour pressure
• Local boiling
• Gas bubbles collapse
• Reduce the flow of delivered fluid.
• Vibrations, damage to the pipework system or the pump. This
effect is known as cavitation.

Understanding NPSHa and NPSHr

In a system where the fluid needs to be lifted to the pump inlet , the negative
head reduces the motive force to move the fluid to the pump.

Discharge pressure
• Discharge pressure is pressure at the downstream of the pump
• This is given by
Discharge pressure = Destination pressure + static head + del p in
pump discharge circuit + contingency
Where,
Destination pressure – Operating pressure of the destination
vessel/column, etc.
Static head – static pressure difference due to the liquid head
between final destination point and grade.
Del p – line losses in the discharge line.
Contingency – This is kept to take care of any unforeseen
additional requirement of del p in the discharge
circuit.

• Differential pressure = Discharge pressure – Suction pressure
• Differential head = [(Diff. pressure in Kg/cm2) / (Density in Kg/m3)]
X 104
• Shut off pressure = Max suction pressure + Max diff. pressure
where, Max suction press. = Max source press + Static head
Max diff. pressure:
for centrifugal pump = 1.2 X Diff. pressure
for positive displacement pump = 1.1 X Diff. pressure
• Pump power consumption (kw) = (Flow rate in m3/hr X Diff.
pressure in Kg/cm2) / (36 X efficiency)

Typical characteristic curve for centrifugal pump

Proportioning pump efficiency

Example

• Required flow rate Q = 200 l/s……(given)
• Pressure at the Suction side of pump
Source pressure = 3.0 m……given
Pressure loss in suction line = 50 kpa
= 50 X 10.33 = 5.10 m of water
therefore, pressure at pump suction
= 3.0 – 5.10 = -2.10 m of water
• Required Discharge pressure
Static head to be overcome = 25.0 m of water
Line losses to be overcome = 250 kpa
= 250 X 10.33 = 25.5 m of water
• Total pressure required at the discharge of pump
= 25.0 + 25.5 = 50.5 m of water
• Thus, head to be added by pump = 50.5 – (-2.1)
= 52.6 m of water

• Operating point lies in the
operating Envelop as such this
pump can supply the required
head at the required flow rate.
• Pump will operate at around
1040 rpm
• Required NPSH is 6.5 m of
water
• Efficiency at the operating
point can be seen to be about
66 %

• NPSH Calculation
• NPSHa = Height of liquid in tank above pump suction + Atmospheric
pressure - Pressure losses in suction piping - Vapour pressure of liquid
• Atmospheric pressure = 10.33 m of water
• Vapour pressure of water @ 60 deg. C = 149.4 mm Hg
= (149.4 / 760) X 10.33 = 2.0 m of water
• NPSHa = 3.0 + 10.33 – 5.10 – 2.0
= 6.23 m of water
• From this we can see NPSHr > NPSHa which is not feasible and hence
cavitation will occur.
Therefore this pump is not suitable for this application
• We will calculate the power requirements, as an example of how to do this.
Pump power consumption kw = (Flow rate in m3/hr X Diff. pressure in
Kg/cm2) / (36 X efficiency)
= (720 X 5.26) / (36 X 0.66)
= 160 kw

Excel Sheet
Pump Calculation.xls

“THANK YOU”

RP

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