Pump Report

Types of Pumps used in Thermal Power Plants:

-Boiler Feed Pump (BFP):

A boiler feedwater pump is a specific type of pump used to pump feedwater into a

steam boiler. The water may be freshly supplied or returning condensate produced

as a result of the condensation of the steam produced by the boiler. These pumps

are normally high pressure units that use suction from a condensate return system

and can be of the centrifugal pump type or positive displacement type.

Cross section of boiler feedwater pump

Construction and operation:

Feedwater pumps range in size up to many horsepower and the electric motor is

usually separated from the pump body by some form of mechanical coupling.

Large industrial condensate pumps may also serve as the feedwater pump. In

either case, to force the water into the boiler, the pump must generate sufficient

pressure to overcome the steam pressure developed by the boiler. This is usually

accomplished through the use of a centrifugal pump.

Feedwater pumps sometimes run intermittently and are controlled by a float

switch or other similar level-sensing device energizing the pump when it detects a

lowered liquid level in the boiler. The pump then runs until the level of liquid in

the boiler is substantially increased. Some pumps contain a two-stage switch. As

liquid lowers to the trigger point of the first stage, the pump is activated. If the

liquid continues to drop (perhaps because the pump has failed, its supply has been

cut off or exhausted, or its discharge is blocked), the second stage will be

triggered. This stage may switch off the boiler equipment (preventing the boiler

from running dry and overheating), trigger an alarm, or both.

-Condensate Pumps:

A condensate pump is a specific type of pump used to pump the condensate

(water) produced in an HVAC (heating or cooling), refrigeration, condensing

boiler furnace or steam system.

Construction and operation:

Condensate pumps as used in hydronic systems are usually electrically powered

centrifugal pumps. As used in homes and individual heat exchangers, they are

often small and rated at a fraction of a horsepower, but in commercial applications

they range in size up to many horsepower and the electric motor is usually

separated from the pump body by some form of mechanical coupling. Large

industrial pumps may also serve as the feedwater pump for returning the

condensate under pressure to a boiler.

Condensate pumps usually run

intermittently and have a tank in

which condensate can

accumulate. Eventually, the

accumulating liquid raises a

float switch energizing the

pump. The pump then runs until

the level of liquid in the tank is

substantially lowered. Some

pumps contain a two-stage

switch. As liquid rises to the

trigger point of the first stage,

the pump is activated. If the

liquid continues to rise (perhaps

because the pump has failed or its discharge is blocked), the second stage will be

triggered. This stage may switch off the HVAC equipment (preventing the

production of further condensate), trigger an alarm, or both.

Some systems may include two pumps to service the tank. In this case, the two

pumps often alternate operation, and a two-stage switch serves to energize the on-

duty pump at the first stage and then energize the remaining pump at the second

stage. This second stage action is in addition to any triggering of other system

changes as noted for a single pump installation. In this way pump runtime is

shared between the two, and a backup pump is provided in case one pump fails to

function as designed.

Steam condensate pump

Small pumps have tanks that range from 2 to 4 litres (0.5 to 1 gallon) and are

usually supported using the flanges on their tanks or simply placed upon the floor.

A plastic impeller in a molded volute at the bottom of the pump provides the

pumping action; this impeller is connected to the motor via a metal shaft that

extends downwards from the motor mounted above the tank's top. Large pumps

are usually pad-mounted drawing liquid from a tank (sump) below the floor. The

smallest pumps may have no tank at all and are simply placed within a container

such as the drip pan of a dehumidifier appliance.

In industrial steam systems the condensate pump is used to collect and return

condensate from remote areas of the plant. The steam produced in the boiler can

heat equipment and processes a considerable distance away. Once steam is used it

turns to hot water or condensate. This pump and possibly many more around the

plant returns this hot water back to a make-up tank closer to the boiler, where it

can be reclaimed, chemically treated, and reused, in the boiler, consequently it can

sometimes be referred to as a condensate return pump.

In a steam power plant, particularly shipboard ones, the condensate pump is

normally located adjacent to the main condenser hotwell often directly below it.

This pump sends the water to a make-up tank closer to the steam generator or

boiler. If the tank is also designed to remove dissolved oxygen from the

condensate, it is known as a Deareating feed tank (DFT). The output of the DFT

supplies the feed booster pump which, in turn, supplies the feed pump (feedwater

pump) which returns the feedwater to the boiler so the cycle can start over. Two

pumps in succession are used to provide sufficient Net Positive Suction Head to

prevent cavitation and the subsequent damage associated with it.

This pump is usually associated with a much larger tank, float switch, and an

electric motor than the example above. Some systems are so remote that steam

power is used to return the condensate where electricity is impractical to provide.

-Circulating Pumps:

A circulator pump is a specific type of pump used to circulate gases, liquids, or

slurries in a closed circuit. They are commonly found circulating water in a

hydronic heating or cooling system. Because they only circulate liquid within a

closed circuit, they only need to overcome the friction of a piping system (as

opposed to lifting a fluid from a point of lower potential energy to a point of

higher potential energy).

Circulator pumps as used in hydronic systems are usually electrically powered

centrifugal pumps. As used in homes, they are often small, sealed, and rated at a

fraction of a horsepower, but in commercial applications they range in size up to

many horsepower and the electric motor is usually separated from the pump body

by some form of mechanical coupling. The sealed units used in home applications

often have the motor rotor, pump impeller, and support bearings combined and

sealed within the water circuit. This avoids one of the principal challenges faced

by the larger, two-part pumps: maintaining a water-tight seal at the point where the

pump drive shaft enters the pump body.

Small- to medium-sized circulator pumps are

usually supported entirely by the pipe flanges

that join them to the rest of the hydronic

plumbing. Large pumps are usually pad-

mounted.

Pumps that are used solely for closed hydronic

systems can be made with cast iron

components as the water in the loop will either

become de-oxygenated or be treated with

chemicals to inhibit corrosion. But pumps that

have a steady stream of oxygenated, potable

water flowing through them must be made of

more expensive materials such as bronze.

Use with domestic hot water:

Bronze pumps are often used to circulate domestic hot water so that a faucet will

provide hot water instantly upon demand. In regions where water conservation

issues are rising in importance with a rapidly expanding and urbanizing

population, and with more consumers seeking comfort, but since water and energy

resources are finite, so-called Energy-saving Hot Water Recirculation (HWR)

pumps can aid in water conservation at a relatively small expense in added energy

use. In typical one-way plumbing without a circulation pump, water is simply

piped from the water heater through the pipes to the tap. Once the tap is shut off,

the water remaining in the pipes cools producing the familiar wait for hot water

the next time the tap is opened. By adding a circulator pump and constantly

circulating a small amount of hot water through the pipes from the heater to the

furthest fixture and back to the heater, the water in the pipes is always hot, and no

water is wasted during the wait. The tradeoff is the energy wasted in operating the

pump and the heat lost from the constantly-hot pipes. While the majority of these

pumps mount nearest to the hot water heater and have no adjustable temperature

capabilities, a significant reduction in energy can be achieved by utilizing a

temperature adjustable thermostatically controlled circulation pump mounted just

after the last fixture on the loop. Thermostatically controlled circulation pumps

allow owners to choose the desired temperature of hot water to be maintained

within the hot water pipes since most homes do not require 120 °F (49 °C) degree

water instantly out of their taps. Thermostatically controlled circulation pumps

cycle on and off to maintain a user's chosen temperature and consume less energy

than a continuously operating pump. By installing a thermostatically controlled

pump just after the farthest fixture on the loop, cyclic pumping maintains ready

hot water up to the last fixture on the loop instead of wasting energy heating the

piping from the last fixture to the water heater. Often installing a circulation pump

at the farthest fixture on hot water circulation loop due to limited available space,

cosmetics, noise restrictions or lack of available power. Recent advancements in

hot water circulation technology allow for benefiting from temperature controlled

pumping without having to install the pump at the last fixture on the hot water

loop. These advanced hot water circulation systems utilize a water contacting

temperature probe strategically installed at the last fixture on the loop to minimize

the energy wasted heating lengthy return pipes. Thermal insulation applied to the

pipes helps mitigate this second loss and minimize the amount of water that must

be pumped to keep hot water constantly available.

The traditional hot water

recirculation system uses a

dedicated return line from the

point of use located farthest from

the hot water tank back to the hot

water tank. In homes where this

return line was not installed the

cold water line is used as a return

line. The first of two system types

has a pump mounted at the hot

water heater while a "normally-

open" thermostatic control valve

gets installed at the furthest fixture

from the water heater and closes

once hot water contacts the valve

to control crossover flow between the hot and cold lines. A second type of system

utilizes a thermostatically controlled pump which gets installed at the furthest

fixture from the water heater. These thermostatically controlled pumps often have

a built-in "normally closed" check-valve which prevents water in the cold water

line from entering into the hot water line. Compared to a dedicated return line,

using the cold water line as a return has the disadvantage of heating the cold water

pipe (and the contained water). Accurate temperature monitoring and active flow

control can minimize loss of cold water within the cold water line. Technological

advancements within the industry allowed for incorporating timers to limit the

operations during specific hours of the day to reduce energy waste by only

operating when occupants were likely to use hot water. Additional advancements

in technology include pumps which cycle on and off to maintain hot water

temperature versus a continuously operating pump which consumes more

electrical energy. Reduced energy waste and discomfort is possible by preventing

occurrences of hot water line siphoning in open-loop hot water circulation systems

which utilize the cold water line to return water back to the water heater. Hot

Water Line Siphoning occurs when water from within the hot water line siphons

or is forced into the cold water line due to differences in water pressure between

the hot and cold water lines. Utilizing "normally closed" solenoid valve

significantly reduces energy consumption by preventing siphoning of non-hot

water out of hot water lines during cold water use. Using cold water instantly

lowers the water pressure in the cold water lines, the higher water pressure in the

hot water lines force water through "normally open" thermostatic crossover valves

and backflow check valves (which only prevent cold water from flowing into hot

water line), increasing the energy demand on the water heater.

-Ash Handling Pumps:

Boilers that use solid fossil fuels produce refuse that is generally classified as ash

and includes bottom ash or slag, fly ash, and mill rejects such as pyrites and so on.

This refuse must be removed and disposed of, either by hydraulic or pneumatic

conveying. The latter, of course, does not involve the use of pumps and need not

be discussed here. Fly ash and bottom ash is removed from the boiler system

through a series of water ejectors. Centrifugal pumps provide the water, taken

from either the circulating water pump header or ash-settling pond. These pumps

are subjected to severe duty as pump flows vary significantly and the pumpage is

often contaminated with suspended silt or fly ash.

Hydraulic conveying is generally restricted to bottom ash and mill rejects. There

are a great number of different systems in use today, and the reader should consult

boiler design and operation literature to become acquainted with this subject. What

is common to most hydraulic conveying systems is that they use centrifugal pumps

to handle concentrated ash slurries that may contain considerable amounts of

coarse, heavy pieces of stone, slate, and iron pyrites.

It‟s noticed that the most common pumps used in these types are the positive

displacement and the centrifugal pumps:

Positive Displacement pumps:

A positive displacement pump causes a fluid to move by trapping a fixed amount

of it then forcing (displacing) that trapped volume into the discharge pipe. A

positive displacement pump can be further classified according to the mechanism

used to move the fluid:

Rotary-type, for example, the lobe, external gear, internal gear, screw,

shuttle block, flexible vane or sliding vane, helical twisted roots (e.g. the

Wendelkolben pump) or liquid ring vacuum pumps.

Positive displacement rotary pumps are pumps that move fluid using the principles

of rotation. The vacuum created by the rotation of the pump captures and draws in

the liquid. Rotary pumps are very efficient because they naturally remove air from

the lines, eliminating the need to bleed the air from the lines manually. Positive

displacement rotary pumps also have their weaknesses. Because of the nature of

the pump, the clearance between the rotating pump and the outer edge must be

very close, requiring that the pumps rotate at a slow, steady speed. If rotary pumps

are operated at high speeds, the fluids will cause erosion, much as ocean waves

polish stones or erode rock into sand. Rotary pumps that experience such erosion

eventually show signs of enlarged clearances, which allow liquid to slip through

and detract from the efficiency of the pump. Positive displacement rotary pumps

can be grouped into three main types. Gear pumps are the simplest type of rotary

pumps, consisting of two gears laid out side-by-side with their teeth enmeshed.

The gears turn away from each other, creating a current that traps fluid between

the teeth on the gears and the outer casing, eventually releasing the fluid on the

discharge side of the pump as the teeth mesh and go around again. Many small

teeth maintain a constant flow of fluid, while fewer, larger teeth create a tendency

for the pump to discharge fluids in short, pulsing gushes. Screw pumps are a more

complicated type of rotary pumps, featuring two screws with opposing thread —-

that is, one screw turns clockwise, and the other counterclockwise. The screws are

each mounted on shafts that run parallel to each other; the shafts also have gears

on them that mesh with each other in order to turn the shafts together and keep

everything in place. The turning of the screws, and consequently the shafts to

which they are mounted, draws the fluid through the pump. As with other forms of

rotary pumps, the clearance between moving parts and the pump's casing is

minimal. Moving vane pumps are the third type of rotary pumps, consisting of a

cylindrical rotor encased in a similarly shaped housing. As the rotor turns, the

vanes trap fluid between the rotor and the casing, drawing the fluid through the

pump.

Reciprocating-type, for example, piston or diaphragm pumps.

Positive Displacement Pumps has an expanding cavity on the suction side and a

decreasing cavity on the discharge side. Liquid flows into the pumps as the cavity

on the suction side expands and the liquid flows out of the discharge as the cavity

collapses. The volume is constant given each cycle of operation.

A Positive Displacement Pump must not be operated against a closed valve on the

discharge side of the pump because it has no shut-off head like Centrifugal Pumps.

A Positive Displacement Pump operating against a closed discharge valve, will

continue to produce flow until the pressure in the discharge line are increased until

the line bursts or the pump is severely damaged - or both.

A relief or safety valve on the discharge side of the Positive Displacement Pump is

therefore absolutely necessary. The relief valve can be internal or external. The

pump manufacturer normally has the option to supply internal relief or safety

valves. The internal valve should in general only be used as a safety precaution, an

external relief valve installed in the discharge line with a return line back to the

suction line or supply tank is recommended.

Centrifugal Pump:

A centrifugal pump is a rotodynamic pump that uses a rotating impeller to increase

the pressure and flow rate of a fluid. Centrifugal pumps are the most common type

of pump used to move liquids through a piping system. The fluid enters the pump

impeller along or near to the rotating axis and is accelerated by the impeller,

flowing radially outward or axially into a diffuser or volute chamber, from where

it exits into the downstream piping system. Centrifugal pumps are typically used

for large discharge through smaller heads.

The screw centrifugal impeller was

invented in 1960 by the late Martin

Stähle, the founder of Hidrostal AG. He

had received an order from the Amial

S.A. fish processing factory in

Chimbote (Peru) for the development of

a system for transporting fish from the

nets into a boat, and from the boat into

the fish processing plant. The pump was

to work reliably without damaging the

fish. The result was the pump with the

characteristic screw centrifugal

impeller. This invention was a great

success. It has since been used in many

ways throughout the world in countless

other fluid handling systems.

The screw centrifugal pump is a popular choice for handling delicate products

such as food and crystals. Its low shear characteristic reduces emulsification when

pumping mixtures making it ideal for pumping oily water and Return Activated

Sludge [RAS] as it does not damage the floc. The pump's ability to pass long

fibrous materials such as rope without clogging makes it a frequent choice for

municipal waste water applications. A screw centrifugal pump typically has an

operating efficiency of 70% to 85%. It has a relatively steeply rising head/capacity

curve shape giving it good flow control capability over its allowable operating

range

The impeller has a single blade, axially extended at the inlet and developed around

its axis much like a corkscrew. Linking this to a centrifugal outlet allows pumping

with the minimum of agitation and shear, essential factors when product bruising,

liquid emulsification or clogging is to be avoided.

Centrifugal Pumps

Centrifugal Pump – Working

Components of Centrifugal pump

A rotating component comprising of an impeller and a shaft.

A stationery component comprising a volute (casing), suction and delivery

pipe.

Working Principle of Centrifugal pump

Principle:

When a certain mass of fluid is rotated by an external source, it is thrown

away from the central axis of rotation and a centrifugal head is impressed

which enables it to rise to a higher level.

Working:

The delivery valve is closed and the pump is primed, so that no air pocket

is left.

Keeping the delivery valve still closed the electric motor is started to rotate

the impeller.

The rotation of the impeller is gradually increased till the impeller rotates

at its normal speed.

After the impeller attains the normal speed the delivery valve is opened

when the liquid is sucked continuously up to the suction pipe.

Working Principle of Centrifugal pump

It passes through the eye of the casing and enters the impeller at its centre.

The liquid is impelled out by the rotating vanes and it comes out at the

outlet tips of the vanes into the casing.

Due to the impeller action the pressure head as well as the velocity heads

are increased.

From the casing the liquid passes into the pipe and lifted to the required

height.

When pump is to be stopped the delivery valve is to be first closed,

otherwise there may be some backflow of water into the reservoir.

Volute and Vortex Casing

Volute Casing Vortex Casing

Volute and Diffuser casing

Volute Casing:

In this type of casing the area of flow gradually increases from the impeller

outlet to the delivery pipe.

Vortex Casing:

If a circular chamber is provided between the impeller and volute chamber the

casing is known as Vortex Chamber.

Diffuser C:

The impeller is surrounded by a diffuser.

The guide vanes are designed in such a way that the water from the

impeller enters the guide vanes without shock.

It reduces the vibration of the pump.

Diffuser casing, the diffuser and the outer casing are stationery parts.

Priming of a centrifugal Pump

The operation of filling the suction pipe, casing and a portion of delivery

pipe with the liquid to be raised, before starting the pump is known as

Priming.

It is done to remove any air, gas or vapour from these parts of pump.

If a Centrifugal pump is not primed before starting air pockets inside

impeller may give rise to vortices and causes discontinuity of flow.

Losses in Centrifugal pump

Hydraulic Losses:

Shock or eddy losses at the entrance to and exit from the impeller

Losses due to friction in the impeller

Friction and eddy losses in the guide vanes/diffuser and casing

Mechanical Losses:

Losses due to disc friction between the impeller and the liquid which fills

the clearance spaces between the impeller and casing

Losses pertaining to friction of the main bearing and glands.

Sizing a centrifugal pump:

In order to correctly size a pump for a particular application it is necessary to

understand the system in which it is installed. One selects a pump based on its

ability to supply the required flow rate for the system. The operating point of a

pump is set by the intersection of the PUMP curve (specific to the pump) with

the SYSTEM curve (defined by the piping system, tank elevation, and over-

pressures .etc)

Steps involved in selecting and sizing a pump:

1. Determine flow rate

2. Obtain fluid property information

3. Design piping system

4. Determine the System Head Curve

5. Decide on duty point

6. Calculate Power required and Specific speed values

7. Calculate Net Positive Suction Head available (NPSHA)

8. Develop pump specification sheet

9. Select a pump

10. Evaluate pump selection

Step 1: Determine flow rate/pump:

• Determined by process the flow rate.

• Determine number of pumps required (function of criticality and

reliability)

• Define realistic maximum and minimum flow rates

Step 2: Determine Fluid property information:

• Density, specific gravity, Dynamic or Absolute viscosity, Kinematics viscosity.

• Water density at 20 deg C = 1000 kg/m3

• Specific weight γ = ρ g , Weight of fluid per unit volume.

• Specific gravity S= ρ/ ρH20

.

Why is viscosity important?

• Increase viscosity - increase losses - less head generated - lower efficiency

• Effect is greater on smaller pumps due to smaller internal passage dimensions

• Used to calculate the Reynolds Number which determines the set of pump

equations to use

Step 3: Design piping system

Select pipe sizes

• This is a compromise between installation costs and running costs.

• Small diameter pipes lead to high line velocities and friction losses.

• Elbows and fittings also result in friction losses

• Suction piping design is critical to avoid creating swirl/uneven flow at the

pump suction

Friction losses in pipes:

Resistance to flow as liquid moves through pipe results in loss of head. This

friction loss hL

is measured in m. Resistance is due to viscous shear stresses within

the liquid and friction losses at contact of moving fluid and pipe wall

Step 4: Determine System Head Curve

For a new pump installation you will need

• P&ID (Piping and Instrumentation Drawing) Symbols needed

• GA (General arrangement) plan and elevation,

• Flow sheet,

• Isometric drawings

This is often done by considering the suction and discharge sides separately as

Total Head (Hp)

Hp

= h(d) - h(s)

Total Suction Head

h(s)= p(s)/ρ g z(s)+V2

s/2g - h

L(s) - h(i)

z(s) = static suction head, hL(s) = total friction loss in suction line, h(i) = entrance

loss, p(s) = pressure other than atmospheric in suction tank in m, h(s) = total

suction head.

Total Discharge Head

h(d)= p(d)/ρ g + z(s) +V2

d/2g+ z(d) + h

L(d) + h(e)

z(d) = static discharge head, hL(d) = friction loss in discharge line, h(e) = exit loss,

hP(d) = overpressure in discharge tank in m, h(d) = total discharge head

Calculation of the friction loss terms (hL)

There are TWO separate friction calculations, one for the pipes and one for the

fittings.

1. Darcy’s Formula for friction loss in pipes

For turbulent flow

hL

= pressure drop or friction loss in m, f=friction factor, L=length of pipe (m),

V=line velocity (m/s), D= pipe ID (m), Darcy‟s formula is valid for turbulent and

laminar flow only if line pressure >> vapor pressure of the liquid. I.e. no

cavitations.

For laminar flow

Friction Factor f:

The friction factor is determined experimentally. For laminar flow f=64/Re.

For turbulent flow f depends on Re also the relative roughness ε/d. ε = roughness

of pipe wall, d = pipe diameter

2. Darcy’s Formula for friction loss valves and fittings

Summary for Friction Loss calculation

The pressure P on a gauge located close to the flange of the suction of the pump

will measure

The conversion to „head‟ and addition of the suction velocity head will give a value

for the total suction head.

A pressure gauge placed on the discharge of the pump will read the following terms.

It can be seen that the Total Head HP

on the pump

The values VD

and VS

are the velocity in the pipe at the pump suction and

discharge. This is a function of the flow rate Q and the line diameter D. V = Q/A =

4Q/(πD2

), V = line velocity m/s, Q = flow rate m3

/s, A =

Inside pipe area m2

, D = inside diameter m. If the line size is the same for suction

and discharge, this term can be ignored.

Draw System Curve

System curve determined by Total Head (m) at different flow rates (below, design

and above design)

Flow (m3/hr) Total Head (m)

0 (Static head) 18

60 25

100 48

110 56

Superimpose System Curve on a „suitable‟ Pump Curve

Pump Curve

Step 5: Decide on a Duty Point

Duty Point is expressed as the calculated Head for the desired Flow rate

Determine high and low operating flow points.

Step 6: Calculate Power required, Efficiency and Specific Speed

Power : Hydraulic power is work done by a pump in moving the liquid.

Pump Efficiency:

ηP

= Hydraulic Power (W) / Power input to the pump shaft from the motor (W)

Pump Efficiency depends on energy loss due to:

• leakage (recirculation around the impeller outlet to inlet, internal to the pump)

• hydraulic losses (viscosity and non-uniform flow)

• mechanical losses (friction losses in the bearings and seals)

Motor Efficiency: ηM

WM is the power from the motor to the shaft = Power supplied to the motor x

motor efficiency

Calculation of the motor power requires

1. Measurement of the power delivered to the motor from the MCC (Motor

Control Centre). This is available as a kW reading.

2. Knowledge of the efficiency of the motor. This information is often available on

the motor nameplate or from the manufacturer. The motor efficiency is dependent

on the load on the motor and the speed, if it is a variable speed drive.

Step 7: Calculate NPSH available

An acceptable margin of NPSHA - NPSHR must be maintained over the entire

operating range to prevent CAVITATION. Cavitation is caused by the local

vaporisation of a fluid when the static pressure drops below the vapour pressure.

The small bubbles filled with vapour that form in the low pressure region (suction

eye of the pump) will collapse on moving into high pressure regions (inside the

impeller). This "implosion" causes pitting on the metal surface, vibration and a

drop in efficiency.

NPSH available

• Net positive suction head is the absolute suction head at suction nozzle corrected

to datum less the vapor pressure of the liquid at operating temperature. Determines

at what point liquid will vaporize at the lowest pressure point of the pump

(cavitations) and is characteristic of the system. NPSHA varies with capacity and

is always positive.

• hvp(abs)

= head in m corresponding to the ABSOLUTE vapour pressure of the

liquid at the temperature being pumped. This is determined from Tables of vapour

pressure (usually given in bar)

NPSH required The NPSHR is characteristic of pump design and represents the minimum margin

required between suction head and vapour pressure. NPSHR varies with capacity.

It is determined by manufacturer and verified by NPSH pump test. NPSHR

depends on impeller design, flow rate, rpm, liquid and other factors. As a rule of

thumb there should be a margin of at least 1m, though depends on application

between NPSHA and NPSHR)

Step 8: Develop Pump Specification

Step 9: Select a short-list of suitable Pumps from different manufacturers

Step 10: Evaluate Pump Selection

• Match Pump and System curve

• Determine Efficiency and NPSH margin

• Compare efficiency, NPSH margin, and off design performance of different

pumps

• Determine materials to be used based on fluid properties

• Consider vendor technical support and spare parts issues

• Consider preferred vendor supply contracts

system curve

The capacity and pressure needs of any system can be defined with the help of a

graph called a system curve. Similarly the capacity vs. pressure variation graph for

a particular pump defines its characteristic pump performance curve.

The pump suppliers try to match the system curve supplied by the user with a

pump curve that satisfies these needs as closely as possible. A pumping system

operates where the pump curve and the system resistance curve intersect. The

intersection of the two curves defines the operating point of both pump and

process. However, it is impossible for one operating point to meet all desired

operating conditions. For example, when the discharge valve is throttled, the

system resistance curve shift left and so does the operating point.

Typical system and pump performance curves

Nuclear Power Plants: The primary difference between the pumps for nuclear service and conventional steam

generation is the stringent requirements for reliability and safety. The continuous functioning

of nuclear pumps is critical to the safe operation of the plant. In addition, many of these units

maintain the responsibility of ensuring safe shutdown of the facility during emergency

conditions. Because the release of radioactive fluids could result in potential personnel and

environmental hazards along with associated costly cleanup, nuclear pumps must be self-

contained. The rules and regulatory standards for nuclear pumps are dedicated to the

promotion of public safety through reliable operation.

PWR Plants:

PWR plants employ two separate main systems to generate steam. In the primary system, the

water is circulated by reactor coolant pumps through the nuclear reactor and large steam

generators. Overpressure is provided to prevent vapor formation. The secondary side of the

steam generator provides nonradioactive steam to the turbogenerator, Typical primary water

conditions are 2250 lb/ina gauge (15.51 MPa) and 550°F (288°C).

The figure shown below is a simplified flow diagram of a PWR reactor coolant system. Only

one primary loop is shown, but in practice, plants use two, three, or four loops in parallel,

each loop consisting of its own reactor coolant pump, steam generator, and optional stop

valves. A single reactor and pressurizer supply all loops. The pressurizer provides the

overpressure referred to by maintaining a body of water at an elevated temperature such that

its vapor pressure satisfies the primary loop pressure requirements.

Flow diagram of pressurized water reactor coolant system

In the PWR plant, high-pressure water circulates through the reactor core to remove the heat

generated by the nuclear chain reaction. The heated water exits from the reactor vessel and

passes via the coolant loop piping to the primary side of the steam generators. Here it gives up

its heat to the feedwater to produce steam for the turbogenerator. The primary-side cycle is

completed when the reactor water is pumped back to the reactor by the reactor coolant pumps.

The secondary-side cycle, isolated from the primary loops, is completed when the feedwater

pumps return water to the secondary side of the steam generator.

Because a reactor, after it has been critical, continues to generate heat even when shutdown,

residual heat removal (RHR) pumps are provided to circulate reactor water through coolers

any time the reactor is inoperable and at low pressure, even during refueling.

When the system is hot, the pumps are used to maintain the water level in the pressurizer and

to replenish any fluid drawn from the primary loops by other systems. Additionally, the

pumps supply clean water to the reactor coolant pump seals and are used to adjust the boric

acid concentration in the reactor coolant water, which provides an auxiliary means of reactor

power regulation. If positive displacement pumps are included in the chemical and volume

control system, they are also used to hydrostatically test the reactor primary coolant system.

Where all the charging pumps are centrifugal, it is customary to provide a small positive

displacement pump exclusively for this hydrostatic testing.

For cooling essential components and for supplying a variety of heat exchangers, a

component cooling water system is provided. Component cooling water pumps circulate

clean water at low system pressures for the purpose of cooling: (1) primary water, which is

continuously bled for purification, (2) main and auxiliary pump bearings and seals, (3)

primary pump thermal barriers, (4) large motors, (5) the containment vessel, and (6) the spent

fuel pit water.

When the primary pressure boundary is breached, elements of the emergency core cooling

system (ECCS) are immediately activated. The primary function of the ECCS following a

loss-of-coolant accident is to remove the stored and fission product decay heat from the

reactor core. The safety injection system does most of this. Upon actuation of the safety

injection signal, the charging pumps inject boric acid solution, which is stored in special tanks

and continuously circulated by the boron injection recirculation pumps, into the reactor

coolant system. At the same time, the residual heat removal pumps are started. These pumps

take suction from a large refueling water storage tank and inject cold water into the reactor

coolant circuit. To provide additional capacity, the safety injection pumps are started, taking

suction from the cold water in the refueling water storage tank and pumping this water into

the reactor coolant system. If the large storage tank should run dry, these pumps will take

suction from the containment sump.

Operated by a pressure signal, containment spray pumps condense any steam in the

containment in order to lower the temperature and pressure in that environment. By taking

suction from the containment sump, these pumps continue to circulate water though spray

nozzles located near the top of the containment until the pressure has been reduced to an

acceptable level.

All of the pumps and vital equipment associated with pipe rupture (that is, the emergency

core cooling system) is supplied with diesel generator electrical power backup. If needed, the

diesel generators will accept the various pump loads sequentially at intervals of a few seconds

until all needed equipment is on line.

Other pumps serve other systems. Spent fuel pit pumps provide the necessary cooling of the

fuel elements that have been removed from the reactor. Resin beds, which are a part of the

water purification system, are flushed to a storage tank by spent resin sluicing pumps. An

evaporator package, partly for removing boron from the primary water, is supplied by recycle

evaporator feed pumps. Chilled water pumps supply the boron thermal regeneration system.

BWR Plants:

In BWR plants, active boiling takes place in the nuclear core and steam is piped to the

turbogenerator (Figure 5). Typical reactor water conditions are 1000 lb/in2 gauge (6.895

MPa) and 550°F (288°C). In the United States, the plant is usually arranged with a low-

leakage containment vessel completely surrounding a dry well and a pressure suppression

pool (Figure 6). The containment vessel is a cylindrical steel or concrete structure with an

ellipsoidal dome and a flat bottom supported by a reinforced concrete mat. The containment

forms a security barrier and prevents the escape of radioactive products to the atmosphere if

an accident should occur.

To assure a high reactor flow rate and to avoid local areas of core overheating, internal jet

pumps have been used in all but the earliest US. BWR plants. These jet pumps are driven by

large-volume, medium-head recirculation pumps. Variable flow rate is achieved either by

flow-control valves or by variable-speed motors driven by motor generator sets. The latest

designs employ a flow-control valve. The use ofjet pumps decreases the size of the external

loop piping and pumps and provides a core reflood capability in the event of pipe rupture.

The main recirculation pumps are not required for emergency cooling. In normal operation,

the recirculation pumps operate in conjunction with the jet pumps, which are wholly

contained in the reactor vessel. The purpose of the jet pumps is to increase the flow from the

recirculation pumps at reduced head for reactor cooling. The jet pumps have no moving parts.

In addition to their normal service, they also play a role in the natural circulation of the

reactor water during emergency cooling.

Reactor cooling pump for BWR

Several subsystems operate in support of the recirculation system, and each contains one or

more pumps. Reactor water cleanup pumps are used in a filter-demineralizer system to

remove particulate and dissolved impurities from the reactor coolant. “his system also

removes excess water from the reactor. The control rods are operated hydraulically with water

pressure provided by the control rod drive pumps. The pumps are located in an auxiliary

building, and the fluid is piped to the control rod drive units, which are positioned directly

under the reactor vessel. For those components that require constant cooling, such as the

recirculation pump motors and other equipment located in the containment, auxiliary, fuel, or

radwaste buildings, closed cooling waterpumps furnish the necessary flow. These pumps are

located in an auxiliary building to permit ready access for servicing if needed. The system is

closed so it can be isolated from ultimate, usually raw water, heat sink, such as a river, lake,

or ocean.

To cool the fuel stored under water in the fuel building and the water in the upper

containment, separate fuel pool cooling pumps are provided in an independent system.

The emergency core cooling system is in reality an array of subsystems providing the

necessary features, including redundancy, to protect the core in case of a significant

malfunction. The high-pressure core spray system uses a vertical high-pressure core spray

pump, motor-driven but backed by a diesel generator in event of loss of electric power. This

pump, a single unit, provides the initial response when a small pipe breaks or an equivalent

malfunction occurs. Should this system be inadequate to maintain reactor water level, the

reactor vessel is automatically depressurized and the low-pressure core spray pumps supply

additional capacity. As an added safeguard, the RHR pumps are used in a secondary-mode

operation to inject cooling water directly into the reactor vessel. If steam should enter the

containment region, the RHR pumps operate in another mode-as containment spray pumps-

and are manually operated to condense the steam and thus reduce any potential pressure build

up in the containment. The RHR pumps function when needed to limit the temperature of the

water in the suppression pool. The turbinedriven reactor core isolation coolingpumps, in a

redundant and independent system, inject cool water into the reactor vessel. The standby

liquid control system pumps inject boron solution into the reactor for alternative shutdown.

Principle Types of Pumps:

Most of the pumps in nuclear service are one- or two-stage centrifugal motor-driven pumps.

Both vertical and horizontal types are used. Charging, safety injection, feedwater, and other

high-head pumps are usually multistage motor-driven centrifugal units. Some requiring high

power are turbine driven. Double-suction designs are frequently used for RHR pumps, where

service requires operation at low available NPSH. Reciprocating pumps find limited service

in nuclear plants for makeup flow, seal injection flow, or chemical mixing service. Canned

pumps are frequently used in subsystems where their zero-leakage capabilities can be

exploited.

.

References:

http://en.wikipedia.org/wiki/Condensate_pump

http://en.wikipedia.org/wiki/Circulation_pump

http://en.wikipedia.org/wiki/Pump#Centrifugal_pump

http://en.wikipedia.org/wiki/Boiler_feedwater_pump

http://www.brighthub.com/engineering/mechanical/articles/20241.aspx?p=2

http://www.britannica.com/EBchecked/topic/483365/pump/5993/Kinetic-

pumps#ref=ref17067

P.L.Ballaney, “Thermal Engineering”, Dhanpat Rai & sons, 2008

R.K. Bansal, “Fluid Mechanics”, Dhanpat Rai & Sons, 2003

Rangasamy & Sundaramoorthy, “ Thermal Engineering”.

http://www.liquid-dynamics.com/animations