Introduction
A water pump is a mechanical rotodynamic machine that delivers useful energy to the fluid on pumpage
largely through velocity changes that occur as this fluid flows through the impeller and the associated fixed
passage ways of the pump. It is converting of mechanical energy to hydraulic energy of the handling fluid
to get it to a required place or height by the centrifugal force of the impeller blade.
The centrifugal pump is a single stage pump designed for the following factors
o Pump cavitation
o Pump sizing, weight and Noise
o Operating pressure
o Delivery
o Control Type
o Pump Speed
o Performance Rating
SN FACTORS SPECIFICATIONS
o Matching the flow and pressure rating of a pump with the flow
rate and pressure required for the process.
o System Head is the hydraulic resistance needed to overcome
the mass flow rate of the system process diagram by the mass
1 Pump Sizing balance The pump must be able to generate a pressure high
enough to overcome the hydraulic resistance of the system of
pipes and valves.
o The system head is the amount of pressure required to achieve
a given flow rate in the system downstream of the pump.
Pump Cavitation: o Keep suction line velocities below 1.2 m/s.
o Keep the pump inlet lines as short as possible.
This occurs when the pressure in the o Minimize the number of fittings in the inlet line.
2
pump inlet drops below the vapor o Mount the pump as close as possible to the reservoir.
pressure of the liquid. o Use low-pressure drop inlet filters.
Higher working pressures is the reduction in fluid flow rates for
3 Maximum operating pressure. a given system power, resulting in smaller pumps, smaller bore
pipes and smaller components.
o If circuit demand is reasonably constant, a fixed displacement
pump is chosen.
o When the demand is at a series of fixed levels, a multi-pump
system is used.
o For demands which vary within a relatively narrow band, a
4 Maximum delivery. variable displacement pump is used.
o If there is a wide variance in system demand, an accumulator
circuit may best satisfy the requirements.
o It is usual to select a pump with a capacity about 10% higher
than that required to make an allowance for the reduction in
volumetric efficiency with wear.
The choice of control is dependent upon the circuit requirement
such as complexity, accuracy of control, cost, type of machining
Type of control.
5 operation from the following manual servo control, pressure
compensated control, constant power control and constant flow
control.
The fluid delivery rate is proportional to the speed of rotation.
Pump drive speed.
6 Each design has a minimum and maximum operating speed: the
faster the pump runs, the shorter its life.
The following aspects affect the pump noise:
Pump noise. port plate design, bearings, flow passages, pressure controls,
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materials and methods of mounting and sound generated
increases with speed and pressure
o To reduce the weight of the water pump system by increasing
Size and weight of a pump. the operating pressure, reducing the size of the reservoir
8
o The best power-to-weight ratios is achieved in the 200–300 bar
operating pressure range.
Performance curves to see if a specific pump has the pressure and
volume flow rate to operate a given set of actuators. In a second
instance, the system designer may be computing the noise,
9 Performance Rating
vibration, cavitation and flow characteristics of a specific pump
before or after installation to determine if the pump and existing
system are compatible.
SN FACTORS SPECIFICATIONS
Pump Cavitation: o Keep suction line velocities below 1.2 m/s.
o Keep the pump inlet lines as short as possible.
1 o Too much elevation head between the o Minimize the number of fittings in the inlet line.
reservoir and the pump inlet. o Mount the pump as close as possible to the reservoir.
o High fluid viscosity. o Use low-pressure drop inlet filters.
Higher working pressures is the reduction in fluid flow rates for
2 Maximum operating pressure. a given system power, resulting in smaller pumps, smaller bore
pipes and smaller components.
o If circuit demand is reasonably constant, a fixed displacement
pump is chosen.
o When the demand is at a series of fixed levels, a multi-pump
system is used.
o For demands which vary within a relatively narrow band, a
Maximum delivery. variable displacement pump is used.
o If there is a wide variance in system demand, an accumulator
circuit may best satisfy the requirements.
o It is usual to select a pump with a capacity about 10% higher
than that required to make an allowance for the reduction in
volumetric efficiency with wear.
The choice of control is dependent upon the circuit requirement
such as complexity, accuracy of control, cost, type of machining
Type of control.
3 operation from the following manual servo control, pressure
compensated control, constant power control and constant flow
control.
The fluid delivery rate is proportional to the speed of rotation.
Pump drive speed.
4 Each design has a minimum and maximum operating speed: the
faster the pump runs, the shorter its life.
The following aspects affect the pump noise:
Pump noise. port plate design, bearings, flow passages, pressure controls,
5
materials and methods of mounting and sound generated
increases with speed and pressure
o To reduce the weight of the water pump system by increasing
Size and weight of a pump. the operating pressure, reducing the size of the reservoir
6
o The best power-to-weight ratios is achieved in the 200–300 bar
operating pressure range.
SN Factors Specifications
Identify the amount of flow you require from the For an estimated amount of flow of 300 gpm as shown by
1
pump line 2
Follow 100ft across the curve as shown by line 3 were it
Identify the specific head total needed to overcome
2 intersects with the flow line which indicates your
at the specified flow
performance point.
Impeller Performance Trim The impeller diameters are listed on the left side of the
To accommodate different performance points, curve and the performance for each trim is shown across
centrifugal pumps have the capability of trimming as a bold line. Our selection is between 10” and 11” so a
3
impellers. By reducing impeller size, the pump can trim of 10.5” is appropriate as shown by the BOLD LINE
be limited to your specific performance labeled 4
requirement.
Horsepower is indicated across the curve as a dotted line
in this case at a downward angle. Our performance point
4 Amount of horsepower required.
is between the 10hp and 15 hp lines, we estimate this
selection to require 12 hp.
Net Positive Suction Head If sufficient NPSH is not met the pump will cavitate
This is the minimum amount of pressure on the which will affect performance and pump life.
5
suction side of the pump to overcome pump entrance
losses.
Efficiency The higher the efficiency, the less energy required to
6
operate for a specific performance point.
Minimum Flow On the left side of the curve, minimum flow is indicated
7
by a vertical bold line; operation to the left of this line is
Amount of flow to be moving through the pump to not recommended and can significantly decrease the life
dissipate heat created of the pump.
H-Q Curve: For a Pump user Shut off pressure of the centrifugal pump is an important tool to identify the
priming loss/Cavitation of the pump. Head curve will fall for increased flow rates
Energy Curve: Brake Horse power is plotted against the Flow to obtain energy curve in a centrifugal pump
performance curve. It is a simple straight line. There will be minimum power consumed by the pump even
at zero flow which is used to develop shut off head of the pump. Brake horse power rises with the flow.
NPSHR Curve: Net Positive Suction Head Required is plotted against the Flow rate. NPSHR curve is a flat
curve till the BEP of the pump and then it rises sharply beyond the Best efficiency point. Net Positive
Suction Head Available must be greater than the Net Positive Suction Head Required to avoid Cavitation
of the pump
Performance Parameters
Net Positive Suction Head
Were the PT is inlet total pressure, Pv is vapor pressure of the fluid, r is density, and g is acceleration of
gravity
The Available NPSH (NPSHA): a measure of how close the fluid at a given point is to flashing, and so to
cavitation. Technically it is the absolute pressure head minus the vapor pressure of the liquid.
The Required NPSH (NPSHR): the head value at the suction side (e.g. the inlet of a pump) required to keep
the fluid from cavitating (provided by the manufacturer).
Static Head
The static head is created by any vertical columns of liquid attached to the pump and any pressurized
systems attached to the pump outlet. The static head exists under static conditions, with the pump switched
off, and does not change based on flow.
Dynamic Head
The dynamic head varies dynamically with flow rate (and also with the degree of opening of valves). The
dynamic head represents the inefficiency of the system — losses of energy as a result of friction within
pipes and fittings and changes of direction. This ineffiency increases with the square of the average velocity
of the fluid.
Dynamic head can be further split into two parts. The frictional loss as the liquid moves along lengths of
straight pipe is called the straight-run head loss, and the loss as a result of fluid passing through pipe fittings
such as bends, valves, and so on is called the fittings head loss.
Superficial Velocity
Superficial velocity is the same as average velocity and is the volumetric flow rate (in m3/sec, for example)
divided by the pipe’s internal cross-sectional area (e.g., in m2). A very quick way to start the hydraulic
calculations is to use the following superficial velocities:
o pumped water-like fluids: <1.5 m/sec
o gravity-fed water-like fluids: <1 m/sec
o water-like fluids with settle able solids: >1, <1.5 m/sec
o air-like gases: 20 m/sec
Frictional Losses Through Fittings
To use this method, count the number of valves on the piping and instrumentation diagram (P&ID), and
the fittings, bends, and tees on the plant layout drawing for the relevant suction or delivery line. Multiply
the number of each type of fitting by the corresponding k value, and add the k values for the various types
of fittings to get the total k value. Use the total k value to calculate the head loss due to fittings:
Straight-Run Head loss
To use the nomogram, use a ruler to draw a straight line through any pair of known quantities to determine
unknown quantities. For example, for a 25-mm nominal-bore pipe with a flow velocity of 1 m/sec, the
straight-run headloss is about 6 m per 100 m of pipe. So the headloss through 10 m of this pipe is around
0.6 mwg
Suction Head and Net Positive Suction Head
The pump’s required net positive suction head takes into consideration the liquid’s vapor pressure to avoid
cavitation in the pump. We use the Antoine equation to estimate the vapor pressure of the liquid at the
pump inlet and then calculate the NPSH at that vapor pressure.The Antoine equation may be expressed as:
where P v is vapor pressure of the liquid at the pump inlet, T is temperature, and A, B, and C are coefficients
that can be obtained from the NIST database (http://webbook.nist.gov) among other places. Table 3 shows
an example for water.
The net positive suction head is:
where P o is the absolute pressure at the suction reservoir, h o is the reservoir liquid level relative to the
pump centerline, and hsf is the head loss due to friction on the suction side of the pump.
Determining pump power
After the system head has been calculated, it can be used to calculate an approximate pump power rating
for a centrifugal pump:
where P is the pump power (kW), Q is the flowrate (m3/hr), H is the total pump head (m of fluid), and η is
the pump efficiency (if you do not know the efficiency, use η = 0.7).
Supply Network
Avoid manifold arrangements that provide a straight-through path from the feed line to a branch. Entry
perpendicular to branch direction is preferred.
Size manifolds such that the superficial velocity never exceeds 1 m/sec at the highest anticipated
flowrate.
Specify progressively smaller manifold diameters to accommodate lower flows to downstream
branches.
Include a small hydraulic restriction in the branch so the branch headloss is 10–100 times the headloss
across the manifold.
Design-in passive flow equalization throughout the piping system wherever possible by making
branches hydraulically equivalent.
Pump Curves
A pump curve is a plot of outlet pressure as a function of flow.
