Lecture 05

054410 PLANT DESIGNLECTURE FIVE

Alon Goldis, Technion1

Pump DesignPLANT DESIGN - Alon Goldis1 5-

054410 Plant Design

LECTURE 5: PUMP DESIGN

Alon GoldisDepartment of Chemical Engineering

Technion, Haifa, Israel


ObjectivesAfter reviewing this lecture you should:

1. Be familiar with the nomenclature used in the design of centrifugal pumps.

2. Be able to use pump characteristic curves.

3. Be able to compute the total dynamic head (TDH) of a pump, given its suction and discharge lines and installed fittings.




Heado The pressure at any point in a liquid can be

thought of as being caused by a vertical column of the liquid

o The height of the column is called the Static Head and is expressed in terms of feet of liquid.

o The Static Head corresponding to any specific pressure is dependent upon the weight of the liquid according to the following formula:

2.31 Pressure [psi]Head [ft] Specific gravity

⋅=


Centrifugal Pumps Fundamentalso A Centrifugal pump imparts velocity to a liquid. o This velocity energy is transformed largely into

pressure energy as the liquid leaves the pump.o The relationship between the head developed in

pump and it’s velocity is expressed by:

H - Total head developed (feet) V - Velocity of impeller (feet/sec) g - 32.2 feet/sec2

=2

2VH

g




Centrifugal Pumps Fundamentals

The approximate head of any centrifugal pump can predicted by calculating the velocity of the impeller. Use the next equation to calculate the impeller velocity in case the impeller diameter is given:

D - Impeller diameter (inch)V - Velocity (ft/sec)

229RPM DV ⋅

=


Centrifugal Pumps – Exercise 1Find the pressure in the inlet (impeller line) of centrifugal pump for Gasoline (S.G.=0.75), Water (S.G.=1), Brine (S.G.=1.2)

Gasoline Water Brine




Centrifugal Pumps - SUCTION LIFT

SUCTION LIFT exists when the source of supply is belowthe center line of the pump.

Thus the STATIC SUCTION LIFT is the vertical distance in feet from the centerline of the pump to the free level of the liquid to be pumped.

Source of supply

Pump


Centrifugal Pumps - SUCTION HEADSUCTION HEAD exists when the source of supply is abovethe centerline of the pump.

Thus the STATIC SUCTION HEAD is the vertical distance in feet from the centerline of the pump to the free level of the liquid to be pumped. Source of supply Pump




Centrifugal Pumps - CAPACITYCapacity (Q) is normally expressed in gallons per minute (GPM). Since liquids are essentially incompressible, there is a direct relationship between the capacity in a pipe and the velocity of flow. This relationship is as follows:

449Q A V= ⋅ ⋅

whereA = area of pipe cross section (ft2) V = velocity of flow (ft/sec) Q = Capacity (GPM=gallons per minute)


Centrifugal Pumps - Power & Efficiency

.3960 Pump Efficiency

Q TDH S GBHP ⋅ ⋅=

⋅

• The work performed by a pump is a function of the total head and the weight of the liquid pumped in a given time period.

• Pump input or brake horsepower (BHP) is the actual horsepower delivered to the pump shaft.

.3960

Q TDH S GWHP ⋅ ⋅=

• Pump output or hydraulic horsepower (WHP) is the liquid horsepower delivered by the pump.




Centrifugal Pumps – Power & Efficiency

The brake horsepower or input to a pump is greater than the hydraulic horsepower or output due to the mechanical and hydraulic losses incurred in the pump.

Therefore, the pump efficiency is the ratio of these two values.

.Pump Efficiency3960

WHP Q TDH S GBHP BHP

⋅ ⋅= =

⋅


NPSH & Cavitation

The definition of NET POSITIVE SUCTION HEAD:

NPSH - is defined as the total suction head determined at the suction nozzle, less the vapor pressure of the liquid in feet absolute. Simply stated, it is an analysis of energy conditions on the suction side of a pump to determine if the liquid will vaporize at the lowest pressure point in the pump.




NPSH & Cavitation• NPSH required is a function of the pump design.

• As the liquid passes from the pump suction to the impeller region, the pressure decreases.

• The centrifugal force of the impeller vanes further increases the velocity and decreases the pressure of the liquid.

• The NPSH Required is the positive head in feet required at the pump suction to overcome these pressure drops in the pump and maintain the majority of the liquid above its vapor pressure.


NPSH & CavitationTo overcome the phenomenon called “Cavitation”, NPSH available must be greater than available –NPSH required

NPSHR > NPSHA

NO CAVITATION




Pump Characteristic CurvesA performance of a pump can be shown graphically on a characteristic curve. A typical pump curve shows:

• Total Dynamic Head (TDH)

• Brake horse-power

• Efficiency

• NPSH


Pump Characteristic Curves

Operating Lines

Efficiency Line

BHP

NPSH




Class Exercise 1100 GPM of HCl is to be pumped by a centrifugal pump from a reservoir located 50 ft below a storage reservoir.

Find the TDH of the system.


Class Exercise 1 - SolutionLet’s divide the system in to two sub-systems:

1. Suction Area

2. Discharge Area




Class Exercise 1 - Solution Suction Area – in GENERAL:• The total Pipe length:

5+7+5 = 17ft• The Static Head – 10ft• One 3” 90o Knee – 7.7ft• One fully opened 3” Gate

Valve - 1.6 ft

• Lummus Rules: ∆P allowed in case of non-boiling liquidsentering pump - 1psi/100ft


Class Exercise 1 - SolutionHence the pressure drop in the pipe and its fittings is: (17+7.7+1.6)ftx1psi/100 ft = 0.26 psi

The static pressure in reservoir is calculated from:2.31 Pressure( )Head(ft)

. .10 1.1Head(ft) .Pressure(psi) 4.762.31 2.31

psiS G

ftS G psi

⋅= ⇒

⋅⋅= = =

Hence, the suction head is:⋅

=

⋅ −= =

2.31 Pressure(psi)Head(ft). .

2.31 (4.76 0.26) 9.451.1

S G

ft




Class Exercise 1 - Solution Discharge Area – in GENERAL:• The total Pipe length: 172ft• The Static Head – 60ft• Two 2” 90o Elbow – 5ft each• One fully opened 2” Gate

Valve - 1.25 ft• One 2” Check Valve – 15ft

• Lummus Rules: ∆P allowedin case of 0-250 GPM is6psi/100ft


Class Exercise 1 - SolutionHence the pressure drop in the pipe and its fittings is:(172+1.25+2×5+15)ft x 6psi/100 ft = 11.9 psi. The static pressure in reservoir is calculated from:

2.31 Pressure(psi)Head(ft). .

60 1.1Head(ft) .Pressure(psi) 28.572.31 2.31

S GftS G psi

⋅= ⇒

⋅⋅= = =

Hence, the discharge head is:2.31 Pressure(psi) 2.31 (28.57 11.9)Head(ft) 84.98

. . 1.1ft

S G⋅ ⋅ +

= = =

Note – the reason for the sum of the static head and the head loss is due to the fact that the pump must overcome the static pressure of the storage reservoir as well as that of the piping




Class Exercise 1 - Solution

Finally, the total dynamic head of this system is:

TDH = Disc.Head – Suct.Head

= 85.72 - 9.45 = 75.27 ft


Class Exercise 2

Compute the pump required to deliver a liquid stream from the bottom of distillation column to filter.

Given:

1. Dist. Column bottoms pressure: 1.72 bar

2. Filter inlet pressure: 1.5 bar

3. Heat Exchanger pressure drop: 0.77bar

4. Control Valve pressure drop is equal to 30% ofpump suction pressure.

5. Process data table




Class Exercise 2

0.947Spec. Gravity8Bottoms pipe diameter, inch6Pump discharge diameter, inch

1Viscosity, cP117Temperature, deg C69.6Capacity ( Normal ), m3/hr

Process Data Table


Class Exercise 2 – Pump Detailed Design

Solution ( Excel )

Data-Sheet

Request Form




Summary

After reviewing this lecture you should:

1. Be familiar with the nomenclature used in the design of centrifugal pumps.

2. Be able to use pump characteristic curves.

3. Be able to compute the total dynamic head (TDH) of a pump, given its suction and discharge lines and installed fittings.

Lecture 05

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