ENERGY_EFFICIENCY_IN_ELECTRICAL_UTILITIES
(Chapter 6:Pumps & Pumping System)
Pump Types
Pumps come in a variety of sizes for a wide range of applications. They can be classified according to their basic operating principle as dynamic or displacement pumps. Dynamic pumps can be sub-classified as centrifugal and special effect pumps. Displacement pumps can be sub-classified as rotary or reciprocating pumps. In principle, any liquid can be and led by any of the pump designs. Where different pump designs could be used, the centrifugal pump is generally the most economical followed by rotary and reciprocating pumps. Although, positive displacement pumps are generally more efficient than centrifugal pumps, the benefit of higher efficiency tends to be offset by increased maintenance costs. Since, worldwide, centrifugal pumps account for the majority of electricity used by pumps, the focus of this chapter is on centrifugal pump.
Centrifugal Pumps
A centrifugal pump (Figure 6.1) is of a very simple design. The two main parts of the pump are the impeller and the diffuser. Impeller, which is the only moving part, is attached to a shaft and driven by a motor. Impellers are generally made of bronze, polycarbonate, cast iron, stainless steel as well as other materials. The diffuser (also called as volute) houses the impeller and captures and directs the water off the impeller. Water enters the center (eye) of the impeller and exits the impeller with the help of centrifugal force. As water leaves the eye of the impeller a low-pressure area is created, causing more water to flow into the eye. Atmospheric pressure and centrifugal force cause this to happen. Velocity is developed as the water flows through the impeller spinning at high speed. The water velocity is collected by the diffuser and converted to pressure by specially designed passageways that direct the flow to the discharge of the pump, or to the next impeller should the pump have a multi-stage configuration.
A centrifugal pump is not positive acting, it will not pump the same volume always. The greater the
depth of the water, the lesser is the flow from the pump. Also, when it pumps against increasing pressure, the less it will pump. For these reasons it is important to select a centrifugal pump that is designed to do a particular job.
Since the pump is a dynamic device, it is convenient to consider the pressure in terms of head i.e. meters of liquid column. The pump generates the same head of liquid whatever the density of the liquid
being pumped. The actual contours of the hydraulic passages of the impeller and the casing are extremely
important, in order to attain the highest efficiency possible. The standard convention for centrifugal
pump is to draw the pump performance curves showing Flow on the horizontal axis and Head generated
on the vertical axis. Efficiency, Power & NPSH Required (described later), are also all conventionally
shown on the vertical axis, plotted against Flow, as illustrated in Figure 6.2.
Given the significant amount of electricity attributed to pumping systems, even small improvements
in pumping efficiency could yield very significant savings of electricity. The pump is among the most
inefficient of the components that comprise a pumping system, including the motor, transmission drive,
piping and valves.
Hydraulic Power, Pump Shaft Power and Motor Input Power
System Characteristics
In a pumping system, the objective, in most cases, is either to transfer a liquid from a source to a required destination, e.g. filling a high level reservoir or to circulate liquid around a system e.g. as a means of heat transfer in heat exchanger.
A pressure is needed to make the liquid flow at the required rate and this must overcome head ‘losses’ in the system. Losses are of two types: static and friction head.
Static head is simply the difference in height of the supply and destination reservoirs, as in Figure 6.3. In this illustration, flow velocity in the pipe is assumed to be very small. Another example of a system
with only static head is pumping into a pressurized vessel with short pipe runs. Static head is independent of flow and graphically would be shown as in Figure 6.4.
Friction head (sometimes called dynamic head loss) is the friction loss, on the liquid being moved, in
pipes, valves and equipment in the system. Friction tables are universally available for various pipe
fittings and valves. These tables show friction loss per 100 feet (or meters) of a specific pipe size at
various flow rates. In case of fittings, friction is stated as an equivalent length of pipe of the same size.
The friction losses are proportional to the square of the flow rate. A closed loop circulating system
without a surface open to atmospheric pressure, would exhibit only friction losses and would have a
system friction head loss vs. flow curve as Figure 6.5.
Most systems have a combination of static and friction head and the system curves for two cases are
shown in Figures 6.6 and 6.7. The ratio of static to friction head over the operating range influences
the benefits achievable from variable speed drives which shall be discussed later.
Static head is a characteristic of the specific installation and reducing this head where this is possible
generally helps both the cost of the installation and the cost of pumping the liquid. Friction head losses
must be minimized to reduce pumping cost, but after eliminating unnecessary pipe fittings and length,
further reduction in friction head will require larger diameter pipe, which adds to capital cost.
Pump Curves
The performance of a pump can be expressed graphically as head against flow rate. The centrifugal
pump has a curve where the head falls gradually with increasing flow. This is called the pump characteristic curve (Head — Flow curve). See Figure 6.8.
Pump operating point
When a pump is installed in a system the effect can be illustrated graphically by superimposing
pump and system curves. The operating point will always be where the two curves intersect.
Figure: 6.9.
If the actual system curve is different in reality to that calculated, the pump will operate at a flow and
head different to that expected.
For a centrifugal pump, an increasing system resistance will reduce the flow, eventually to zero, but
the maximum head is limited as shown. Even so, this condition is only acceptable for a short period without causing problems. An error in the system curve calculation is also likely to lead to a centrifugal
pump selection, which is less than optimal for the actual system head losses. Adding safety margins to the calculated system curve to ensure that a sufficiently large pump is selected will generally result in installing an oversized pump, which will operate at an excessive flow rate or in a throttled condition, which increases energy usage and reduces pump life.
Factors Affecting Pump Performance
Matching Pump and System Head-flow Characteristics
Centrifugal pumps are characterized by the relationship between the flow rate (Q) they produce and the pressure (H) at which the flow is delivered. Pump efficiency varies with flow and pressure, and it is highest at one particular flow rate.
The Figure 6.10 below shows a typical vendor-supplied head-flow curve for a centrifugal pump. Pump
head-flow curves are typically given for clear water. The choice of pump for a given application depends largely on how the pump head-flow characteristics match the requirement of the system downstream of the pump.
Effect of over sizing the pump
As mentioned earlier, pressure losses to be overcome by the pumps are functions of flow — the system characteristics — are also quantified in the form of head-flow curves. The system curve is basically a plot of system resistance i.e. head to be overcome by the pump versus various flow rates. The system curves change with the physical configuration of the system; for example, the system curves depends upon height or elevation, diameter and length of piping, number and type of fittings and pressure drops across various equipment - say a heat exchanger.
A pump is selected based on how well the pump curve and system head-flow curves match. The pump operating point is identified as the point, where the system curve crosses the pump curve when they are superimposed on each other. The Figure 6.11 shows the effect on system curve with throttling. In the system under consideration, water has to be first lifted to a height — this represents the static head. Then, we make a system curve, considering the friction and pressure drops in the system- this is shown
as the green curve.
Suppose, we have estimated our operating conditions as 500 m°/hr flow and 50 m head, we will chose
a pump curve which intersects the system curve (Point A) at the pump’s best efficiency point (BEP).
But, in actual operation, we find that 300 m3/hr is sufficient. The reduction in flow rate has to be effected by a throttle valve. In other words, we are introducing an artificial resistance in the system. Due to this additional resistance, the frictional part of the system curve increases and thus the new system curve will shift to the left - this is shown as the red curve.
So the pump has to overcome additional pressure in order to deliver the reduced flow. Now, the new
system curve will intersect the pump curve at point B. The revised parameters are 300 m3/hr at 70 m
head. The red double arrow line shows the additional pressure drop due to throttling.
It may be noted that the best efficiency point has shifted from 82% to 77% efficiency. So it is actually
needed to operate at point C, which is 300 m*/hr on the original system curve. The head required at this point is only 42 meters.
Hence a new pump is needed, which will operate with its best efficiency point at C. But there are other
simpler options rather than replacing the pump. The speed of the pump can be reduced or the existing
impeller can be trimmed (or new lower size impeller). The blue pump curve represents either of these
options.
Energy loss in throttling
Consider a case (see Figure 6.12) where we need to pump 68 m°/hr of water at 47 m head. The pump
characteristic curves (A...E) for a range of pumps are given in the Figure 6.12.
If we select pump E, then the efficiency is 60%
Obviously, this is an oversize pump. Hence, the pump has to be throttled to achieve the desired flow.
Throttling increases the head to be overcome by the pump. In this case, head is 76 meters.
Extra energy used = 8000 hrs/yr x 14.9 = 1,19,200 kWh/annum = =Rs. 6,55,600/annum
(Rs. 5.50 per kWh)
In this example, the extra cost of the electricity is more than the cost of a new pump.
Efficient Pumping System Operation
To understand a pumping system, one must realize that all of its components are interdependent. When
examining or designing a pump system, the process demands must first be established and most energy
efficiency solution introduced. For example, does the flow rate have to be regulated continuously or
in steps? Can on-off batch pumping be used? What is the flow rates needed and how are they distributed
in time?
The first step to achieve energy efficiency in pumping system is to target the end-use. A plant water
balance would establish usage pattern and highlight areas where water consumption can be reduced
or optimized. Good water conservation measures, alone, may eliminate the need for some pumps.
Once flow requirements are optimized, then the pumping system can be analyzed for energy conservation
opportunities. Basically this means matching the pump to requirements by adopting proper flow control
strategies. Common symptoms that indicate opportunities for energy efficiency in pumps are given in
the Table 6.1.
Effect of speed variation
As stated above, a centrifugal pump is a dynamic device with the head generated from a rotating
impeller. There is therefore a relationship between impeller peripheral velocity and generated head.
Peripheral velocity is directly related to shaft rotational speed, for a fixed impeller diameter and so
varying the rotational speed has a direct effect on the performance of the pump. All the parameters
shown in figure 6.2 will change if the speed is varied and it is important to have an appreciation of
how these parameters vary in order to safely control a pump at different speeds. The equations relating
rotodynamic pump performance parameters of flow, head and power absorbed, to speed are known as
the Affinity Laws:
O = Flow rate
H = Head
P= Power absorbed
N= Rotating speed
Efficiency is essentially independent of speed
Flow:
Flow is proportional to the speed
Q1/Q2=N1/N2
Example: 100/Q2 = 3000/1500
Q2= 50 m3/hr
Head: Head is proportional to the square of speed
Head: Head is proportional to the square of speed
Power (kW): Power is proportional to the cube of speed
As can be seen from the above laws, reduction in speed will result in considerable reduction in power
consumption. This forms the basis for energy conservation in centrifugal pumps with varying flow
requirements. The implication of this can be better understood as shown in an example of a centrifugal
pump in Figure 6.13 below.
Points of equal efficiency on the curves for three different speeds are joined to make the iso-efficiency
lines, showing that efficiency remains constant over small changes of speed providing the pump
continues to operate at the same position related to its best efficiency point (BEP).
The affinity laws give a good approximation of how pump performance curves change with speed but
in order to obtain the actual performance of the pump in a system, the system curve also has to be
taken into account.
Effects of impeller diameter change
Changing the impeller diameter gives a proportional change in peripheral velocity, so it follows that
there are equations, similar to the affinity laws, for the variation of performance with impeller
diameter D:
Efficiency varies when the diameter is changed within a particular casing. Note the difference in isoefficiency lines in Figure 6.14 compared with Figure 6.13. The relationships shown here apply to the
case for changing only the diameter of an impeller within a fixed casing geometry, which is a common
practice for making small permanent adjustments to the performance of a centrifugal pump. Diameter
changes are generally limited to reducing the diameter to about 75% of the maximum, i.e. a head
reduction to about 50%. Beyond this, efficiency and NPSH are badly affected. However speed change
can be used over a wider range without seriously reducing efficiency. For example reducing the speed
by 50% typically results in a reduction of efficiency by 1 or 2 percentage points. The reason for the
small loss of efficiency with the lower speed is that mechanical losses in seals and bearings, which
generally represent <5% of total power, are proportional to speed, rather than speed cubed. It should
be noted that if the change in diameter is more than about 5%, the accuracy of the squared and cubic
relationships can fall off and for precise calculations, the pump manufacturer’s performance curves
should be referred to.
The illustrated curves are typical of most centrifugal pump types. Certain high flow, low head pumps
have performance curve shapes somewhat different and have a reduced operating region of flows. This
requires additional care in matching the pump to the system when changing speed and diameter.
Pump suction performance
Liquid entering the impeller eye turns and is split into separate streams by the leading edges of the
impeller vanes, an action which locally drops the pressure below that in the inlet pipe to the pump.
If the incoming liquid is at a pressure with insufficient margin above its vapour pressure then vapour
cavities or bubbles appear along the impeller vanes just behind the inlet edges. This phenomenon is
known as cavitation and has three undesirable effects:
1) The collapsing cavitation bubbles can erode the vane surface, especially when pumping
water-based liquids.
2) Noise and vibration are increased, with possible shortened seal and bearing life.
3) The cavity areas will initially partially choke the impeller passages and reduce the pump performance. In extreme cases, total loss of pump developed head occurs.
The value, by which the liquid pressure at the eye of pump exceeds the liquid vapour pressure, is
expressed as a head of liquid and referred to as Net Positive Suction Head Available — (NPSHA). This
is a characteristic of the system design. The value of NPSH needed at the pump suction to prevent the
pump from cavitation is known as NPSH Required — (NPSHR). This is a characteristic of the pump
design.
The three undesirable effects of cavitation described above begin at different values of NPSHA and
generally there will be cavitation erosion before there is a noticeable loss of pump head. However for a consistent approach, manufacturers and industry standards, usually define the onset of cavitation as
the value of NPSHR when there is a head drop of 3% compared with the head with cavitation free
performance. At this point cavitation is present and prolonged operation at this point will usually lead
to damage. It is usual therefore to apply a margin by which NPSHA should exceed NPSHR.
As would be expected, the NPSHR increases as the flow through the pump increases, see fig 6.2. In
addition, as flow increases in the suction pipework, friction losses also increase, giving a lower NPSHA
at the pump suction, both of which give a greater chance that cavitation will occur. NPSHR also varies
approximately with the square of speed in the same way as pump head and conversion of NPSHR
from one speed to another can be made using the following equations.
It should be noted however that at very low speeds there is a minimum NPSHR plateau, NPSHR does
not tend to zero at zero speed. It is therefore essential to carefully consider NPSH in variable speed
pumping.
Flow Control Strategies
Pump control by varying speed
To understand how the speed variation changes the duty point, the pump and system curves are over laid. Two systems are considered, one with only friction loss and another where static head is high in
relation to friction head. It will be seen that the benefits are different.
In Figure 6.15, reducing speed in the friction loss system moves the intersection point on the system
curve along a line of constant efficiency. The operating point of the pump, relative to its best efficiency
point, remains constant and the pump continues to operate in its ideal region. The affinity laws are
obeyed which means that there is a substantial reduction in power absorbed accompanying the reduction
in flow and head, making variable speed the ideal control method for systems with friction loss.
In a system where static head is high, as illustrated in Figure 6.16, the operating point for the pump
moves relative to the lines of constant pump efficiency when the speed is changed. The reduction in
flow is no longer proportional to speed. A small turn down in speed could give a big reduction in flow
rate and pump efficiency, which could result in the pump operating in a region where it could be
damaged if it ran for an extended period of time even at the lower speed. At the lowest speed illustrated,
(1184 rpm), the pump does not generate sufficient head to pump any liquid into the system, i.e. pump
efficiency and flow rate are zero and with energy still being input to the liquid, the pump becomes a
water heater and damaging temperatures can quickly be reached.
It is relevant to note that flow control by speed regulation is always more efficient than by control
valve. In addition to energy savings there could be other benefits of lower speed. The hydraulic forces
on the impeller, created by the pressure profile inside the pump casing, reduce approximately with the
square of speed. These forces are carried by the pump bearings and so reducing speed increases bearing
life. It can be shown that for a centrifugal pump, bearing life is inversely proportional to the 7" power
of speed. In addition, vibration and noise are reduced and seal life is increased providing the duty point
remains within the allowable operating range.
The corollary to this is that small increases in the speed of a pump significantly increase power absorbed,
shaft stress and bearing loads. It should be remembered that the pump and motor must be sized for the
maximum speed at which the pump set will operate. At higher speed the noise and vibration from both
pump and motor will increase, although for small increases the change will be small. If the liquid
contains abrasive particles, increasing speed will give a corresponding increase in surface wear in the
pump and pipework.
The effect on the mechanical seal of the change in seal chamber pressure should be reviewed with the
pump or seal manufacturer, if the speed increase is large. Conventional mechanical seals operate
satisfactorily at very low speeds and generally there is no requirement for a minimum speed to be specified,
however due to their method of operation, gas seals require a minimum peripheral speed of 5 m/s.
Pumps in parallel switched to meet demand
Another energy efficient method of flow control, particularly for systems where static head is a high
proportion of the total, is to install two or more pumps to operate in parallel. Variation of flow rate is
achieved by switching on and off additional pumps to meet demand. The combined pump curve is
obtained by adding the flow rates at a specific head. The head/flow rate curves for two and three pumps
are shown in Figure 6.17.
It is possible to run pumps of different sizes in parallel provided their closed valve heads are similar.
By arranging different combinations of pumps running together, a larger number of different flow rates
can be provided into the system.
Care must be taken when running pumps in parallel to ensure that the operating point of the pump is
controlled within the region deemed as acceptable by the manufacturer. It can be seen from Figure
6.18 that if 1 or 2 pumps were stopped then the remaining pump(s) would operate well out along the
curve where NPSH is higher and vibration level increased, giving an increased risk of operating
problems.
In this control method, the flow is controlled by switching pumps on or off. It is necessary to have a
storage capacity in the system e.g. a reservoir, a wet well, an elevated tank or an accumulator type
pressure vessel. The storage can provide a steady flow to the system with an intermittent operating
pump. When the pump runs, it does so at the chosen (presumably optimum) duty point and when it is
off, there is no energy consumption. If intermittent flow, stop/start operation and the storage facility
are acceptable, this is an effective approach to minimize energy consumption.
The stop/start operation causes additional loads on the power transmission components and increased
heating in the motor. The frequency of the stop/start cycle should be within the motor design criteria
and checked with the pump manufacturer.
It may also be used to benefit from “off peak” energy tariffs by arranging the run times during the low
tariff periods.
To minimize energy consumption with stop/start control, it is better to pump at as low flow rate as the
process permits. This minimizes friction losses in the pipe and an appropriately small pump can be
installed. For example, pumping at half the flow rate for twice as long can reduce energy consumption
to a quarter. It means it is beneficial to run one pump at full capacity continuously rather than running
two pumps at a time with a stop/start control.
Flow control valve
With this control method, the pump runs continuously and a valve in the pump discharge line is opened
or closed to adjust the flow to the required value.
To understand how the flow rate is controlled, see Figure 6.19. With the valve fully open, the pump
operates at “Flow 1”. When the valve is partially closed, it introduces an additional friction loss in the
system which is proportional to flow squared. The new system curve cuts the pump curve at “Flow 2”
which is the new operating point. The head difference between the two curves is the pressure drop
across the valve.
It is usual practice with valve control to have the valve 10% shut even at maximum flow. Energy is
therefore wasted overcoming the resistance through the valve at all flow conditions. There is some
reduction in pump power absorbed at the lower flow rate (see Figure 6.19), but the flow multiplied by
the head drop across the valve, is wasted energy. It should also be noted that while the pump will
accommodate changes in its operating point as far as it is able within its performance range, it can be
forced to operate high on the curve where its efficiency is low and its reliability is affected.
Maintenance cost of control valves can be high, particularly on corrosive and solids-containing liquids.
Therefore, the lifetime cost could be unnecessarily high.
By-pass control
With this control approach, the pump runs continuously at the maximum process demand duty with a
permanent by-pass line attached to the outlet. When a lower flow is required the surplus liquid is
bypassed and returned to the supply source.
An alternative configuration may have a tank supplying a varying process demand, which is kept full
by a fixed duty pump running at the peak flow rate. Most of the time, the tank overflows and recycles
back to the pump suction. This is even less energy efficient than a control valve because there is no
reduction in power consumption with reduced process demand.
The small by-pass line sometimes installed to prevent a pump running at zero flow is not a means of
flow control, but required for the safe operation of the pump.
Fixed Flow reduction
Impeller trimming
Impeller trimming refers to the process of machining
the diameter of an impeller to reduce the energy added
to the system liquid.
Impeller trimming offers a useful correction to pumps
that, through overly conservative design practices or
changes in system loads are oversized for their
application.
Trimming an impeller provides a level of correction
below buying a smaller impeller from the pump
manufacturer. But in many cases, the next smaller size
impeller is too small for the pump load. Also, smaller
impellers may not be available for the pump size in
question and impeller trimming is the only practical
alternative short of replacing the entire pump/motor
assembly. (See Figures 6.20 & 6.21 for before and
after impeller trimming).
Impeller trimming reduces tip speed which in turn
directly lowers the amount of energy imparted to the
system liquid and lowers both the flow and pressure
generated by the pump.
The Affinity Laws, which describe centrifugal pump
performance, provide a theoretical relationship
between impeller size and pump output (assuming
constant pump speed):
Q = flow
H = head
P = power
D = diameter of impeller
Subscript1 = original pump,
Subscript 2 = pump after impeller trimming
Trimming an impeller changes its operating efficiency and the non-linearities of the Affinity Laws
with respect to impeller machining complicate the prediction of pump performance. Consequently,
impeller diameters are rarely reduced below 75 percent of their original size.
Meeting variable flow reduction
Variable Speed Drives (VSDs)
In contrast, pump speed adjustments provide the most efficient means of controlling pump flow. By
reducing pump speed, less energy is imparted to the fluid and less energy needs to be throttled or
bypassed. There are two primary methods of reducing pump speed: multiple-speed pump motors and
variable speed drives (VSDs).
Although both directly control pump output, multiplespeed motors and VSDs serve entirely separate
applications. Multiple-speed motors contain a
different set of windings for each motor speed
consequently they are more expensive and less
efficient than single speed motors. Multiple speed
motors also lack subtle speed changing capabilities
within discrete speeds.
VSDs allow pump speed adjustments over a continuous
range, avoiding the need to jump from speed to speed as with multiple-speed pumps. VSDs control pump speeds using several different types of mechanical
and electrical systems. Mechanical VSDs include hydraulic clutches, fluid couplings and adjustable
belts and pulleys. Electrical VSDs include eddy current clutches, wound-rotor motor controllers and
variable frequency drives (VFDs). VFDs adjust the electrical frequency of the power supplied to a motor
to change the motor’s rotational speed. VFDs are by far the most popular type of VSD.
However, pump speed adjustment is not appropriate for all systems. In applications with high static
head, slowing a pump risks inducing vibrations and creating performance problems that are similar to
those found when a pump operates against its shutoff head. For systems in which the static head
represents a large portion of the total head, caution should be used in deciding whether to use VFDs.
Operators should review the performance of VFDs in similar applications and consult VFD
manufacturers to avoid the damage that can result when a pump operates too slowly against high static
head.
For many systems, VFDs offer a means to improve pump operating efficiency despite changes in
operating conditions. The effect of slowing pump speed on pump operation is illustrated by the three
curves in Figure 6.22. When a VED slows a pump, its head/flow and power curves drop down and to
the left and its efficiency curve shifts to the left. This efficiency response provides an essential cost
advantage by keeping the operating efficiency as high as possible across variations in the system’s
flow demand, the energy and maintenance costs of the pump can be significantly reduced.
VFDs may offer operating cost reductions by allowing higher pump operating efficiency but the
principal savings derive from the reduction in frictional or bypass flow losses. Using a system perspective
to identify areas in which fluid energy is dissipated in non-useful work often reveals opportunities for
operating cost reductions.
For example, in many systems, increasing flow through bypass lines does not noticeably impact the
backpressure on a pump. Consequently, in these applications pump efficiency does not necessarily
decline during periods of low flow demand. By analyzing the entire system, however, the energy lost
in pushing liquid through bypass lines and across throttle valves can be identified.
Another system benefit of VFDs is a soft start capability. During startup, most motors experience inrush currents that are 5 to 6 times higher than normal operating currents. This high current fades when
the motor spins up to normal speed. VFDs allow the motor to be started with a lower startup current
(usually only about 1.5 times the normal operating current). This reduces wear on the motor and its
controller. VFDs will consume 4 to 6% power as a running cost apart from its initial cost.
Boiler Feed Water Pumps (BFP)
The feed water pumps are normally multi stage centrifugal pumps, sized based on boiler design pressure.
The operation of a multistage pump is similar to the operation of several single stage pumps, of identical
capacity, in series. Since most boilers operate below design pressure, the feed water pump head is
often higher than required. This excessive pump head is dropped across pressure reducing valves and
manual valves. Installing a VFD on the feed water pump in such cases can decrease pump power
consumption and improve control performance. Trimming the impeller, reducing number of stages or
changing the feed water pumps may also be feasible depending on variation in operating load of the
boiler.
Boiler Feed Pump Control with VFD
There are several ways of controlling the pump
(1) One pump, one boiler, no feed water regulating valve: In this the pump speed is varied
according to the level of water in the boiler. The level control system used for the feed water
admission valve transmits its signal directly to the pump VFD controller. With this system it is
possible to eliminate not only the feed pump constant discharge but also the boiler feed water
regulating control valve and, thereby, cut initial capital investment. The inherent efficiency loss
due to throttling is eliminated.
(2)Constant discharge pressure control: The feed pump is controlled to a predetermined pressure
setting irrespective of plant load. The advantage of this system is that the pump will not be
required to operate near shutoff pressures, due to the shifting of the operating point on the curve.
(3)Constant differential pressure control: Feed pump pressure is controlled to produce a
predetermined pressure drop across the feed water regulating valve, usually approximately 3.5
to 5.5 kg/cm’, thus allowing the boiler feed pump to follow plant demand.
Optimizing Boiler Feed Water Pump Capacity - Case
A waste heat boiler has two feed water pumps, each of 6 stages and having a capacity of 35 m3/hr. The
pumps are designed to generate a head of 276 m, normally one pump is operated
The actual steam demand is 28 TPH at 15 kg/c2. The capacity of the feed water pumps is far in excess
of the requirement. This results in throttling of pump discharge leading to energy loss. To save energy
in the BFW pumps, it was suggested to remove two impeller stages of the pump to effectively regulate
the pressure developed in the pumps.
The impact of this measure on power consumption was then evaluated and the results are given in Table 6.2:
Municipal Water Pumping System
Municipal water pumps are predominantly centrifugal pumps and vertical turbine pumps. The capacity
of a water pumping station is normally specified in Million Liters per Day (MLD) of water handled.
Municipal water system consists of the following sub systems:
1. Raw water pump house, intake pumps at water source/river
2. Pure water pump house and filtration plant
3. Booster station as per the requirement
4. Elevated Storage Reservoirs in the distribution system
Vertical Turbine Pumps
Vertical turbine pump (Figure 6.23) [deep well turbine pump] is vertical axis centrifugal or mixed flow
type pump comprising of stages which accommodate rotating impellers and stationary bowls possessing
guide vanes.
These pumps are used where the pumping water level is
below the limits of Volute centrifugal pump. They have higher initial cost and are more difficult to install and repair. The pressure head developed depends on the
diameter of impeller and the speed at which it is rotated. The pressure head developed by single impeller is not great. Additional head is obtained by adding more bowl
assemblies or stage
Construction: It has three parts:
1. Pump Element:
The pump element is made up of one or more bowls or stages. Each bowl consists of an impeller and diffuser.
2. Discharge Column:
It connects the bowl assembly and pump head and
conducts water from former to later.
Discharge head:
It consists of base from which the discharge column, bowl assembly and shaft assembly are suspended.
Submersible Pumps:
A vertical turbine pump close coupled to a small diameter submersible electric motor is termed as
“submersible pump”. The motor is fixed directly below the intake of the pump. The pump element
and the motor operate under submerged condition. It can be used in very deep tube well where a long
shaft would not be practical.
The fundamental difference between a centrifugal sewage pump
impeller and those of clear water pumps is its ability to pass solid
material that would normally clog the latter. What differentiates
various sewage pump impellers is the method by which they
accomplish this. All sewage handling pumps comprise single
suction impellers. This is to avoid the necessity of locating the
pump shaft in the intake.
The Figure 6.24 is that of a typical radial flow impeller of a pump used to handle coarse solids or
fibrous matters. These pumps use volutes because diffuser is prone to clogging. The design of sewage
pumps is largely determined by the size of foreign matter that must pass through the pump without
clogging.
Agricultural Pumping System
The pumps used in agriculture sector are normally installed by individual farmers based on the guidelines
provided by agriculture department/state utilities and feedback from other users. Most of the pumps
used are locally manufactured keeping initial investment as the selection criteria rather than efficiency
and energy conservation.
The pump sets used are generally inefficient with operating efficiency ranging from 30 - 55%. The
wide variation is due to changing water levels in the intake thus forcing the pump to operate away
from the best efficinecy point. The pump sets are more often oversized so as to draw water from
increasingly declining depths and also to withstand large voltage fluctuations.
Mostly centrifugal pumps are used and the capacity of the pumps vary from 1 Hp to 25 HP. The
rating of the pumps is decided based on water table levels. High rating pumps above 25 HP are also
used in several areas. Large capacity cerifugal pumps of 75 HP to 500 HP ratings are also used by
Irrgation departments to povide water to agricultural consumers. Diesel engine driven pumps are also
common in areas where there is erratic or no power supply.
The following energy conservation opportunites have been demonstrated for energy savings in
agricltural pumping.
¢ Installation of low friction foot valves
e Installation of low friction HDPE suction and delivery pipes
¢ Installation of long bends
¢ Installation of high efficency pumps and motors
Energy Conservation Opportunities in Pumping Systems
1.Ensure adequate NPSH at site of installation.
e¢ Ensure availability of basic instruments at pumps like pressure gauges, flow meters.
2.Operate pumps near Best Efficiency Point.
3.Modify pumping system and pumps losses to minimize throttling.
4.Adapt to wide load variation with variable speed drives or sequenced control of multiple units.
5.Stop running multiple pumps - add an auto-start for an on-line spare or add a booster pump in
the problem area.
6.Use booster pumps for small loads requiring higher pressures.
7.Increase liquid temperature differentials to reduce pumping rates in case of heat exchangers.
8.Decrease outlet cold water temperature of cooling tower in order to reduce the pumping flow
rates in case of mixing.
9.Separate High Pressure and Low Pressure systems
10.Repair seals and packing to minimize water loss by dripping.
11.Balance the system to minimize flows and reduce pump power requirements.
12.Avoid pumping head with a free-fall return (gravity); Use siphon effect to advantage:
13.Conduct water balance to minimize water consumption.
14.Avoid cooling water re-circulation in DG sets, air compressors, refrigeration systems, cooling
towers feed water pumps, condenser pumps and process pumps.
15.In multiple pump operations, carefully combine the operation of pumps to avoid throttling.
16.Provide booster pump for few areas of higher head.
17.Replace old pumps by energy efficient pumps.
18.Inthe case of over designed pump, provide variable speed drive, or downsize / replace impeller
or replace with correct sized pump for efficient operation.
19.Optimize number of stages in multi-stage pump in case of head margins.
20.Reduce system resistance by pressure drop assessment and pipe size optimization.
Solved Example:
The cooling water circuit of a process industry is depicted in the figure below. Cooling water is pumped
to three heat exchangers via pipes A, B and C where flow is throttled depending upon the requirement.
The diameter of pipes and measured velocities with non-contact ultrasonic flow meter in each pipe
are indicated in the figure.
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