PUMP BASICS.doc

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PUMP BASICS

Q - What is a pump?

A - A pump is an machine that expends energy to increasethe pressure of a fluid (liquid or gas) and move it from one

point to another. In practice the term pump refers to a

machine that pumps liquid while a machine that pumps gas

is specifically referred to as vacuum pump, compressor,

blower, or fan.

A pump's performance is shown in its characteristics

performance curve where its capacity (in GPM) is plotted

against its developed head (in FT), efficiency (in %),

required input power (in BHP), and NPSHR (in FT.) The

pump curve also shows its speed (in RPM) and other

information such as pump size and type, impeller size, etc.

[ For simplicity the discussions are based on the US system

of units and on 60 cycles power supply.]

The term head refers to the differential head developed by a

pump expressed in feet of liquid:H = [Pd-Ps] x 2.31 / SG

where:

H = pump head, FT of liquid

Pd = pump discharge pressure, PSIG

Ps = pump suction pressure, PSIG

SG = liquid specific gravity

If a pump were an ideal machine the required input power to

drive the pump, called hydraulic horsepower (HHP), iscalculated from:

S.K.Gupta Maint Dept / Rotary Cell



HHP = [Q x H x SG] / [3960]

where Q = capacity in GPM

[ANSI/HI 1.1-1.5-1994, section 1.2.6.3 refers to this term

as water horsepower which is a misnomer. If the specificgravity were omitted (SG=1) then the term is correctly

referred to as water horsepower , WHP]

But since a pump is not an ideal machine the required input

power, called brake horsepower (BHP), is calculated from:

BHP = [Q x H x SG] / [3960 x E]

where E = pump efficiency in decimal

Centrifugal pumps

Q - What is a centrifugal pump?

A - There are various classification of pumps. One main

classification is according to the method energy is imparted

to the liquid: kinetic energy pump, or displacement pump.

A centrifugal pump is of kinetic energy type - it imparts

energy to a liquid by means of centrifugal force produced bya rotating impeller.

A displacement pump imparts energy by mechanical

displacement. Piston, diaphragm, plunger, screw, vane, and

gear pumps are some examples.

Centrifugal pumps are widely-used because of its design

simplicity, high efficiency, wide range of capacity and head,

smooth flow rate, and ease of operation and maintenance.S.K.Gupta Maint Dept / Rotary Cell



(Displacement pumps are of lower flow range and have

pulsating flow rate).

Types of centrifugal pumps

Q - What are the different types of centrifugal pumps? A - Centrifugal pumps can be grouped into several types

using different criteria such as its design, construction,

application, service, etc. Thus one specific pump can belong

to different groups and oftentimes this becomes descriptive

of the pump itself. Some of these groups are:

•Based on compliance with industry standards:

ANSI pump - ASME B73.1 specifications API pump - API 610 specifications DIN pump - DIN 24256 specifications

ISO pump - ISO 2858, 5199 specifications Nuclear pump - ASME specificationsUL/FM fire pump - NFPA specifications

• Based on number of impeller/s in the pump:

Single Stage - pump has one impeller only; for low

head service

Two-Stage - pump has two impellers in series; for

medium head service

Multi-Stage - pump has three or more impellers in

series; for high head service

The number of impellers, not the number of volutes,

determines the number of stages. Thus a pump with 4

volutes but only 3 impellers is normally referred to as

a 4-stage pump destaged to 3-stage, or 4/3-stage.

•

Based on impeller suction:




Single Suction - pump with single suction impeller

(impeller has suction cavity on one side only); simple

design but impeller is subjected to higher axial thrust

imbalance due to flow coming in on one side of impeller only.

Double Suction - pump with double suction impeller

(impeller has suction cavities on both sides); has

lower NPSHR than single suction impeller. Pump is

considered hydraulically balanced but is susceptibleto uneven flow on both sides of impeller if suction

piping is not done properly.

In a pump with more than one impeller the design of

the first stage impeller determines if the pump is

considered single or double suction type.

• Based on type of volute:

Single Volute - pump volute has single lip which is

very easy to cast. Is usually used in small low

capacity pumps where a double volute design is

impractical due to relatively small size of volute

passageway which make obtaining good quality

commercial casting difficult. Pumps with singlevolute design have higher radial loads.

Double Volute - pump volute has dual lips located

180 degrees apart resulting in balanced radial loads;

most centrifugal pumps are of double volute design.

•

Based on nozzle location:




End Suction/Top Discharge - the suction nozzle is

located at the end of, and concentric to, the shaft

while the discharge nozzle is located at the top of the

case perpendicular to the shaft. Pump is always of anoverhung type and typically has lower NPSHR

because the liquid feeds directly into the impeller eye.

Top/Top Nozzles -the suction and discharge nozzles

are located at the top of the case perpendicular to theshaft. Pump can either be overhung type or between-

bearing type but is always a radially-split case pump.

Side/Side Nozzles - the suction and discharge nozzles

are located at the sides of the case perpendicular to

the shaft. Pump can either be an axially or radially

split case type.

• Based on shaft orientation:

Horizontal - pump with shaft in horizontal plane;

popular due to ease of servicing and maintenance.

Vertical - pump with shaft in vertical plane; ideal

when space is limited or of a premium, or when pumping from a pit or underground barrel to increase

the available NPSH.

• Based on orientation of case-split:

Horizontal Or Axial Split - pump case is split

horizontally or axially; the upper half is called the

upper or top case, the lower half is called the lower or bottom case. The case cannot be supported at shaft




centerline because of the case split; is usually limited

to temperature up to 450 degrees F to avoid

misalignment because of uneven thermal expansion

from shaft centerline. The flat case gasket andirregular bolting pattern makes containing the bolt

stress difficult hence it is limited in its hydrotest and

allowable working pressure.

Vertical Or Radial Split - pump case is split

vertically or radially; the split parts are usuallyreferred to as case and cover; can be supported at

shaft centerline for even thermal expansion and is the

preferred construction for high temperature

application. The confined case gasket and circular

bolting pattern makes containing the bolt stress more

manageable hence it can be designed for higher

hydrotest and allowable working pressure.

• Based on bearing support:

Overhung - the impeller overhungs on one end of

shaft which is unsupported by a bearing; usually has

lower NPSHR because there is no shaft blockage at

the impeller eye. The trade-off is that pump has

higher shaft deflection.

Between-Bearing - the shaft has bearing support on

both ends, thus impeller is located in between-

bearings. Pump has lower shaft deflection than

overhung pump but usually has higher NPSHR

because shaft is blocking the impeller eye and shaft

diameter at the impeller is usually of larger size.




• Based on shaft connection to driver:

Close-Coupled - the impeller is mounted on the

driver shaft which is of special design. This is also

known as integral shaft design. Typically used for

light service. The pump-driver assembly is very

compact, lightweight, and inexpensive.

Direct-Coupled - the pump and driver have separate

shafts connected by a flexible coupling. Usually a

spacer coupling is used to allow the removal of sealswithout disturbing the driver.

Self-priming pumps

Q - What is a self-priming pump?

A - Priming is the addition of liquid inside the pump casing

to displace or evacuate the entrained air, usually through a

vent, and create a liquid seal inside the casing.

A self-priming pump is one that will develop a vacuum

sufficiently enough for the atmospheric pressure to force the

liquid to flow through the suction pipe into the pump casing

without priming the pump. Strictly speaking only positive

displacement pumps are truly self-priming but the term has been loosely used to include self-priming centrifugal pumps.

A self-priming centrifugal pump is especially designed with

a large chamber at its discharge side that acts both as an air

separator that separates the air from the liquid, and a

reservoir that holds residual liquid used for priming or re-

priming the pump. The pump has to be primed during the




initial start-up but re-priming is done automatically without

outside attention. Although self-priming appears to be a

desirable feature for centrifugal pumps the trade-off is that

pump efficiency is slightly compromised due to certaindesign constraints. This type of pump is popular with

intermittent service such as in contractors, drainage, sewage,

and similar applications, but is less popular in continuous

service where optimum efficiency is desirable and re-

priming is very seldom needed due to continuous operation.

Specific Speed

Q - What is specific speed ?

A - The old definition of specific speed ( NS ) is simply a

restatement of its equation. It states that NS is the speed in

RPM at which a pump, if sufficiently reduced in size, would

deliver 1 GPM at a head of 1 FT. This definition is

meaningless and has no practical application. In fact, because its equation has inconsistent units, NS is considered

dimensionless.

PumpLine.Com defines NS as a dimensionless number or

index that identifies the geometric similarity of pumps.

Pumps of the same NS but of different size are considered to

be geometrically similar, one pump being a size-factor of

the other.

Specific speed (NS) is calculated from:S.K.Gupta Maint Dept / Rotary Cell



NS = [N x Q^0.50] / [H^0.75]

where:

N = pump speed, RPM

Q = capacity at best efficiency point (BEP) at maximumimpeller diameter, GPM

H = head per stage at BEP at maximum impeller

diameter , FT

Example: What is the specific speed of a two-stage pump

whose capacity and total head at BEP is 400 GPM, and 200

FT, respectively when the pump runs at 1780 RPM?Solution: NS = [1780 x (400)^0.50 / (200/2)^0.75] = 1126

Q - Why is the term specific speed very important?

A - Specific speed has several practical applications:Specific speed identifies the type of pump according to its

design and flow pattern. According to this criteria a pump

can be classified as radial flow, mixed flow, or axial flow

type. A radial flow pump is one where the impeller discharges the liquid in the radial direction from the pump

shaft centerline, an axial flow pump discharges the liquid in

the axial direction and a mixed flow pump is one that is a

cross between a radial and an axial flow pump design.

Specific speed identifies the approximate acceptable ratio of

the impeller eye diameter (D1) to the impeller maximumdiameter (D2) in designing a good impeller.

NS: 500 to 5000; D1/D2 > 1.5 - radial flow pump

NS: 5000 to 10000; D1/D2 < 1.5 - mixed flow pump

NS:10000 to 15000; D1/D2 = 1 - axial flow pump

Specific speed is also used in designing a new pump by size-

factoring a smaller pump of the same specific speed . The

performance and construction of the smaller pump are used




to predict the performance and model the construction of the

new pump.

Rule-of-Thumb: For similar pumps with about the same

capacity at BEP, the pump with higher specific speed willtypically have a higher efficiency also.

Suction specific speed

Q - What is suction specific speed ?

A - Suction specific speed (NSS) is a dimensionless number or index that defines the suction characteristics of a pump. It

is calculated from:

NSS = [(N x Q^0.5) / (NPSHR)^0.75]

where:

N = speed, RPM

Q = capacity at BEP, GPM (For double suction impeller,

Q=GPM/2) NPSHR = NPSHR at BEP based on 3% head loss, FT

In multi-stage pump the NPSHR is based on the first stage

impeller NPSHR.

Example: What is the suction specific speed of a double

suction pump operating at 3560 RPM if it required 18 FT of NPSH at its BEP of 800 GPM?

Solution: NSS = [3560 x (800/2)^0.50 / (18)^0.75] = 8148

A practical application of NSS is that it is commonly used as

a basis for estimating the safe operating range of capacity

for a pump. The higher the NSS is, the narrower is its safe

operating range from its BEP. Most users prefer that their

pumps have NSS in the range of 8000 to 11000 for optimum

and trouble-free operation.S.K.Gupta Maint Dept / Rotary Cell



Best efficiency point

Q - What is referred to as Best Efficiency Point ?

A - Best Efficiency Point (BEP) is the capacity at themaximum impeller diameter performance curve at which the

efficiency is highest. BEP is an important parameter in that

many parametric calculations such as specific speed, suction

specific speed, hydrodynamic size, viscosity correction,

headrise to shut-off, etc. are based on the capacity at BEP.

Many users prefer that pumps operate within 80% to 110%of its BEP for optimum performance.

Affinity Laws

Q - What are the Affinity Laws?

A - The Affinity Laws refer to the mathematical expressions

that define the changes in pump capacity, head, and required

BHP when a change is made to pump speed, impeller diameter, or both. According to Affinity Laws:

The capacity (Q) changes in direct proportion to the

impeller diameter (D) ratio, or to the speed (N) ratio:

Q2 = Q1 x [D2/D1]

Q2 = Q1 x [N2/N1]The head (H) changes in direct proportion to the square

of impeller diameter (D) ratio, or to the square of the

speed (N) ratio:

H2 = H1 x [D2/D1]^2

H2 = H1 x [N2/N1]^2

The BHP changes in direct proportion to the cube of

impeller diameter ratio, or to the cube of the speed

ratio:S.K.Gupta Maint Dept / Rotary Cell



BHP2 = BHP1 x [D2/D1]^3

BHP2 = BHP1 x [N2/N1]^3

where the number:

1 refers to initial condition2 refers to new condition

If changes are made to both impeller diameter and

pump speed the above basic equations can be combined

to obtain the following extended equations:

Q2 = Q1 x [(D2xN2)/(D1xN1)]

H2 = H1 x [(D2xN2)/(D1xN1)]^2

BHP2 = BHP1 x [(D2xN2)/(D1xN1)]^3

The equation below is use to hand-calculate the

impeller trim diameter from a given pump performance

curve at a bigger diameter:

H2 = H1 x [Q2/Q1]^2

(Note: ^ is an exponential symbol)It is important to understand that Affinity Laws are valid

only under conditions of constant efficiency.


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PUMP BASICS.doc