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Low pressure at the suction side of a pump can encounter the fluid to start boiling with

reduced efficiency

cavitation

damage

of the pump as a result. Boiling starts when the pressure in the liquid is reduced to the vapor

pressure of the fluid at the actual temperature.

To characterize the potential for boiling and cavitation, the difference between the total head on

the suction side of the pump - close to the impeller, and the liquid vapor pressure at the actual

temperature, can be used.

1 Suction Head

Based on the Energy Equation - the suction head in the fluid close to the impeller can be

expressed as the sum of the static and the velocity head:

hs = ps / γ + v s2 / 2 g (1)

where

hs = suction head close to the impeller

http://www.engineeringtoolbox.com/mechanical-energy-equation-d_614.html



http://www.engineeringtoolbox.com/static-pressure-head-d_610.html









The available NPSHa can be calculated with the Energy Equation. For a common application -

where the pump lifts a fluid from an open tank at one level to an other, the energy or head at the

surface of the tank is the same as the energy or head before the pump impeller and can beexpressed as:

h0 = hs + hl (4)

where

h0 = head at surface

hs = head before the impeller

hl = head loss from the surface to impeller - major and minor loss in the suction pipe

In an open tank the head at surface can be expressed as:

h0 = p0 / γ = patm / γ (4b)

For a closed pressurized tank the absolute static pressure inside the tank must be used.

The head before the impeller can be expressed as:

hs = ps / γ + v s2

/ 2 g + he (4c)

where

he = elevation from surface to pump - positive if pump is above the tank, negative if the pump is

below the tank

Transforming (4) with (4b) and (4c):













http://www.engineeringtoolbox.com/total-pressure-loss-ducts-pipes-d_625.html










patm / γ = ps / γ + v s2

/ 2 g + he + hl (4d)

The head available before the impeller can be expressed as:

ps / γ + v s2 / 2 g = patm / γ - he - hl (4e)

or as the available NPSHa:

NPSHa = patm / γ - he - hl - pv / γ (4f)

5 Available NPSHa - the Pump is above the Tank

If the pump is positioned above the tank, the elevation - he - is positive and the NPSHa decreases

when the elevation of the pump increases.

At some level the NPSHa will be reduced to zero and the fluid starts to evaporate.

6 Available NPSHa - the Pump is below the Tank

If the pump is positioned below the tank, the elevation - he - is negative and the NPSHa increases

when the elevation of the pump decreases (lowering the pump).

It's always possible to increase the NPSHa by lowering the pump (as long as the major and minor

head loss due to a longer pipe don't increase it more). This is important and it is common to lower

the pump when pumping fluids close to evaporation temperature.

7 Required NPSH - NPSHr or NPSHR

The NPSHr, called as the Net Suction Head as required by the pump in order to prevent cavitationfor safe and reliable operation of the pump.

The required NPSHr for a particular pump is in general determined experimentally by the pump

manufacturer and a part of the documentation of the pump.

http://www.engineeringtoolbox.com/npsh-net-positive-suction-head-d_634.html













The available NPSHa of the system should always exceeded the required NPSHr of the pump to

avoid vaporization and cavitation of the impellers eye. The available NPSHa should in general be

significant higher than the required NPSHr to avoid that head loss in the suction pipe and in the

pump casing, local velocity accelerations and pressure decreases, start boiling the fluid on the

impeller surface.

Note that the required NPSHr increases with the square capacity.

Pumps with double-suction impellers has lower NPSHr than pumps with single-suction impellers. A

pump with a double-suction impeller is considered hydraulically balanced but is susceptible to an

uneven flow on both sides with improper pipe-work.

8 Example - Pumping Water from an Open Tank

When increasing the the elevation for a pump located above a tank, the fluid will start to

evaporate at a maximum level for the actual temperature.

At the maximum elevation NPSHa is zero. The maximum elevation can therefore be expressed by

(4f):

NPSHa = patm / γ - he - hl - pv / γ = 0

For optimal theoretical conditions we neglect the major and minor head loss. The elevation head

can then be expressed as:

he = patm / γ - pv / γ (5)

The maximum elevation or suction head for an open tank depends on the atmospheric pressure -

which in general can be regarded as constant, and the vapor pressure of the fluid - which ingeneral vary with temperature, especially for water.

The absolute vapor pressure of water at temperature 20 oC is 2.3 kN/m2. The maximum theoretical

elevation height is therefore:

he = (101.33 kN/m2 ) / (9.80 kN/m

3 ) - (2.3 kN/m

2 ) / (9.80 kN/m

3 )

= 10.1 m

Due to the head loss in the suction pipe and the local conditions inside the pump - the theoretical

maximum elevation is significantly decreased.

The maximum theoretical elevation of a pump above an open water tank at different

temperatures can be found from the table below.

9 Suction Head as Affected by Temperature

http://www.engineeringtoolbox.com/standard-atmosphere-d_604.html



http://www.engineeringtoolbox.com/water-thermal-properties-d_162.html











TemperatureVapor

PressureMax. elevation

( oC) ( oF) (kN/m2 ) (m) (ft)

0 32 0.6 10.3 33.8

5 41 0.9 10.2 33.5

10 50 1.2 10.2 33.5

15 59 1.7 10.2 33.5

20 68 2.3 10.1 33.1

25 77 3.2 10.0 32.8

30 86 4.3 9.9 32.5

35 95 5.6 9.8 32.2

40 104 7.7 9.5 31.2

45 113 9.6 9.4 30.8

50 122 12.5 9.1 29.9

55 131 15.7 8.7 28.5

60 140 20 8.3 27.2

65 149 25 7.8 25.6

70 158 32.1 7.1 23.3





TemperatureVapor

PressureMax. elevation

( oC) ( oF) (kN/m2 ) (m) (ft)

75 167 38.6 6.4 21

80 176 47.5 5.5 18

85 185 57.8 4.4 14.4

90 194 70 3.2 10.5

95 203 84.5 1.7 5.6

100 212 101.33 0.0 0

10 Pumping Hydrocarbons

Be aware that the NPSH specification provided by the manufacturer in general is for use withcold

water. For hydrocarbons these values must be lowered to account for the vapor release properties

of complex organic liquids.

FluidTemperature

(oC)

Vapor

Pressure

(kPa abs)

Ethanol

20 5.9

65 58.2

Methyl

Acetate

20 22.8

55 93.9

Note that the head developed by a pump is independent of the liquid, and that the performance

curves for water from the manufacturer can be used for Newtonian liquids like gasoline, diesel or

similar. Be aware that required power depends on liquid density and must be adjusted.





SUBJECT : Calculating the total system head in USCS units 7-1:

USCS stands for "United States Customary System Units" as opposed to the SI(Le Syst`eme International d`Units) or metric units that have been adopted by

the International standards Organization (ISO).

It turn out that "head" is a very convenient term in the pumping business.

Capacity is measured in gallons per minute, and each gallon of liquid has weight,

so we can easily calculate the pounds per minute being pumped. Head or height

is measured in feet, so if we multiply these two together we get foot- pounds

per minute which converts directly to work at the rate of 33,000 foot pounds

per minute equals one horsepower.

Pressure is not as convenient a term because the amount of pressure that the

pump will deliver depends upon the weight (specific gravity) of the liquid being

pumped and the specific gravity changes with temperature, type of fluid, and

fluid concentration.

If you will refer to FIG 1, you should get a clear picture of what is meant by static

head. Note that we always measure from the center line of the pump to the

highest liquid level

To calculate head accurately we must calculate the total head on both the

suction and discharge sides of the pump. In addition to the static head we will



learn that there is a head caused by resistance in the piping, fittings and valves

called friction head, and a head caused by any pressure that might be acting on

the liquid in the tanks including atmospheric pressure, called " surface pressure

head".

Once we know these heads, we will then subtract the suction head from the

discharge head and the amount remaining will be the amount of head that the

pump must be able to generate at the rated flow. Here is how it looks in a

formula:

System head = total discharge head - total suction head

H = hd - hs

The total discharge head is made from three separate heads:

hd = hsd + hpd + hfd

hd = total discharge head

hsd = discharge static head

hpd = discharge surface pressure head

hfd = discharge friction head

The total suction head also consists of three separate heads

hs = hss + hps - hfs

hs = total suction head

hss = suction static head

hps = suction surface pressure head

hfs = suction friction head

As we make these calculations, you must sure that all calculations are made in

either "feet of liquid gauge" or "feet of liquid absolute". In case you have

forgotten "absolute means that you have added atmospheric pressure (head) tothe gauge reading.

Now we will make some actual calculations:

Figure #2 demonstrates that the discharge head is still measured to the liquid

level, but you will note that it is below the maximum height of the piping.



Although the pump must deliver enough head to get up to this maximum piping

height, it will not have to continue to deliver this head when the pump is

running because of the "siphon effect". There is of course a maximum siphon

effect. It is derived from: 14.7 psi (atmospheric pressure) x 2.31 feet / psi = 33.4

feet maximum siphon effect.

We will begin with the total suction head calculation

1. The suction head is negative because the liquid level in the suction tank is

below the centerline of the pump:

hss = - 6 feet

2. The suction tank is open, so the suction surface pressure equals atmospheric

pressure :

hps = 0 feet gauge

3. You will not have to calculate the suction friction head, I will tell you it is:

hfs = 4 feet at rated flow

4. The total suction head is a gauge value because atmosphere was given as 0,

hs = hss + hps - hfs = -6 + 0 - 4 = -10 feet of liquid gauge at rated flow

The total discharge head calculation



1. The static discharge head is:

hsd = 125 feet

2. The discharge tank is also open to atmospheric pressure, thus:

hpd = 0 feet, gauge

3. I will give you the discharge friction head as:

hfd = 25 feet at rated flow

4. The total discharge head is:

hd = hsd + hpd + hfd = 125 + 0 + 25 = 150 feet of liquid gauge at rated flow

The total system head calculation:

H = hd - hs = 150 - (-10)= 160 feet of liquid at rated flow

Note: did you notice that when we subtracted a minus number (-10) from a

positive number (150) we ended up with a positive 160 because whenever you

subtract minus numbers it is the same as adding them? If you have trouble with

this concept you can learn more about it from a mathematics book.

Our next example involves a few more calculations, but you should be able to

handle them. In this example we are going to learn how to handle a vacuum

application. Pipe friction numbers are taken from the Hydraulic Institute

Engineering Data Book. You can get a copy of this publication from your library if

you want to see the actual charts. I have some of this information in the chart

section of this web site.

http://www.mcnallyinstitute.com/Charts/friction_6.html








Specifications:

1. Transferring 1000 gpm. weak acid from the vacuum receiver to the storage

tank

2. Specific Gravity - 0.98

3. Viscosity - equal to water

4. Piping - All 6" Schedule 40 steel pipe

5. Discharge piping rises 40 feet vertically above the pump centerline and then

runs 400 feet horizontally. There is one 90° flanged elbow in this line

6. Suction piping has a square edge inlet, four feet of pipe, one gate valve, and

one 90° flanged elbow all of which are 6" in diameter.

7. The minimum level in the vacuum receiver is 5 feet above the pump

centerline.

8. The pressure on top of the liquid in the vacuum receiver is 20 inches of

mercury, vacuum.



To calculate suction surface pressure use one of the following formulas:

inches of mercury x 1.133/ specific gravity = feet of liquid

pounds per square inch x 2.31/specific gravity = feet of liquid

Millimeters of mercury / (22.4 x specific gravity) = feet of liquid

Now that you have all of the necessary information we will begin by dividing the

system into two different sections, using the pump as the dividing line.

Total suction head calculation

1. The suction side of the system shows a minimum static head of 5 feet above

suction centerline. Therefore, the static suction head is:

hss = 5 feet

2. Using the first conversion formula, the suction surface pressure is:

hps = -20 Hg x 1.133/ 0.98 = -23.12 feet gauge

3. The suction friction head, hfs, equals the sum of all the friction losses in the

suction line. Friction loss in 6" pipe at 1000 gpm from table 15 of the Hydraulic

Institute Engineering Data Book, is 6.17 feet per 100 feet of pipe.

in 4 feet of pipe friction loss = 4/100 x 6.17 = 0.3 feet

Friction loss coefficients (K factors) for the inlet, elbow and valve can be added

together and multiplied by the velocity head:

FITTING K FROM TABLE

6" Square edge inlet 0.50 32 (a)

6" 90 flanged elbow 0.29 32 (a)

6" Gate valve 0.11 32 (b)

Total coefficient, K = 0.90

Total friction loss on the suction side is:

hfs = 0.3 + 1.7 = 2.0 feet at 1000 gpm.



4. The total suction head then becomes:

hs = hss + hps - hfs = 5 + (-23.12) - 2.0 = -20.12 feet, gauge at 1000 gpm.

Total discharge head calculation

1. Static discharge head = hsd = 40 feet

2. Discharge surface pressure = hpd = 0 feet gauge

3. Discharge friction head = hfd = sum of the following losses :

Friction loss in 6" pipe at 1000 gpm. from table 15, is 6.17 feet per hundred feet

of pipe.

In 440 feet of pipe the friction loss = 440/100 x 6.17 = 27.2 feet

Friction loss in 6" elbow:

from table 32 (a), K = 0,29

from table 15, V2/2g = 1.92 at 1000 gpm.

Friction loss = K V2/2g = 0.29 x 1.92 = 0.6 feet

The friction loss in the sudden enlargement at the end of the discharge line iscalled the exit loss. In systems of this type where the area of the discharge tank

is very large in comparison to the area of the discharge pipe, the loss equals

V2/2g, as shown in table 32 (b).

Friction loss at exit = V2/2g = 1.9 feet

The discharge friction head is the sum of the above losses, that is:

hfd = 27.2 + 0.6 + 1.9 = 29.7 feet at 1000 gpm.

4. The total discharge head then becomes:

hd = hsd + hpd + hfd = 40 + 0 + 29.7 = 69.7 feet, gauge at 1000 gpm.

c. Total system head calculation:



H = hd - hs = 69.7 - (-20.2) = 89.9 feet at 1000 gpm.

Our next example will be the same as the one we just finished except. that there

is an additional 10 feet of pipe and another 90° flanged elbow in the vertical leg.

The total suction head will be the same as in the previous example. Take a look

at figure # 4

Nothing has changed on the suction side of the pump so the total suction head

will remain the same:

hs = -20.12 feet, gauge at 100 gpm.

Total discharge head calculation

1. The static discharge head "hsd" will change from 40 feet to 30 feet, since the

highest liquid surface in the discharge is now only 30 feet above the pump

centerline.(This value is based on the assumption that the vertical leg in the

discharge tank is full of liquid and that as this liquid falls it will tend to pull the

liquid up and over the loop in the pipe line. This arrangement is called a siphon

leg).

2. The discharge surface pressure is unchanged:

hpd = 0 feet



3. The friction loss in the discharge pipe will be increased by the additional 10

feet of pipe and the additional elbow.

In 10 feet of pipe the friction loss = 10/100 x 6.17 = 0.6 feet

The friction loss in the additional elbow = 0.6 feet

The friction head will then increase as follows:

hfd = 29.7 + 0.6 + 0.6 = 30.9 feet at 1000 gpm.

The total discharge head becomes:

hd = hsd + hpd + hfd

= 30 + 0 + 30.9

= 60.9 feet, gauge at 1000 gpm.

5. Total system head calculation

H = hd - hs = 60.9 - (-20.12) = 81 feet at 1000 gpm.

For our last example we will look at gauges. Take a look at FIG 5:

Specifications:

Capacity - 300 gpm.

Specific gravity - 1.3



Viscosity - Similar to water

Piping - 3 inch suction, 2 inch discharge

Atmospheric pressure - 14.7 psi.

Divide the heads into two sections again:

The discharge gauge head corrected to the centerline of the pump, in feet of

liquid absolute is found by adding the atmospheric pressure to the gauge

reading to get absolute pressure, and then converting to absolute head:

hgd = (130 + 14.7) x 2.31 / (1.3 Specific Gravity) + 4 = 261.1 feet, absolute

Note the 4 foot head correction to the pump centerline.

The discharge velocity head at 300 gpm. is found in table 9 of the Hydraulic

Institute Engineering Data Book

hvd = 12.8 feet at 300 gpm.

The suction gauge reading is in absolute terms so it needs only to be converted

to feet of liquid, absolute.

hgs = 40 x 2.3 / 1.3 +2 = 73.08 feet absolute

Note the 2 foot head correction to the pump centerline.

The suction velocity head at 300 gpm. is found in table 11 of the Pipe Friction

Manual:

hvs = 2.63 feet at 300 gpm.

The total system head developed by the pump =:

H = (hgd + hvd ) - ( hgs + hvs ) = (261.1 + 12.8) - (73.08 + 2.6)= 198.22 feet

absolute at 300 gpm.

For information about my CD with over 600 Seal & Pump

Subjects explained, click

http://www.mcnallyinstitute.com/Book.html/cd_rom_adv.htm





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