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Pump develops same head in metres of liquid (MLC)
independent of SG, and therefore SG is proportional
to pressure |
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Pump delivers same quantity by volume independent
of SG, but quantity by weight will be proportional
to SG. |
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Efficiency is unaffected by SG, but power absorbed
is proportional to SG |
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Permissible suction lift varies with SG. The height
of a column of a liquid corresponding to atmosphere
is inversely proportional to SG and suction lift
will vary accordingly. |
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| Can
a centrifugal pump be operated against a closed discharge
? |
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Generally should be avoided. There are special cases
when no significant harm can occur |
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BHP - WHP = Power losses within the pump |
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Except a small portion, the power converted to heat,
transferred to liquid, passing through the pumps |
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At closed discharge, no useful power, entire BHP
converter to heat |
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I HP - 42.4 Btu/Min. Heat. 100 HP - 4240 Btu/min.
for 100 lb of water - temperature rise - 42.4 F/min
Serious damage can result if pump allowed to operate
appreciable period of time |
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In
large capacity pumps, volume of water in casing
is high and temperature rise will be slow.
100
HP - 500 lbs of water - temp. rise - 0.85 F/min. |
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Pump
can operate quite a long time, say 15 - 30 minutes
without causing undue difficulty |
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If
condition of installation impose such operation,
a small bypass to pick up the heat to be provided |
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| Example
I - Open Tank - Suction Lift Condition |
Source
Pressure: 1 Kg/Cm2 ;Suction List: 3 m;Friction
loss in pipeline;
Foot Valve, Strainer, Bend, Valve,1 m, Temperature
of water: 21º C |
Method
Ha
= 10 x Pa/SG = 10 x 1/0.998 = 10.02 Mts.
Vapour pressure of water at 21º C = 0.0253
Kg/Cm2
hvp = 10 x 0.0253 / 0.0998 = 0.25 m
NPSH (A) = 10.02 - 3 - 0.25 - 1 = 5.77 mts |
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| Example
II - Open Tank Suction Head Condition |
| Data
: Suction Head: 4 m, Friction Loss: 1.2
m,Temperature: 21º C, Altitude: 1500
m,Pressure at 1500m Altitude is 0.9 Kg/Cm2
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Method
So,
ha = 10 x 0.9/0.998 = 9.02 m
NPSH (A) = 9.02 + 4 -1.2 - 0.25 = 11.57 mts. |
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| Example
III - Closed Tank under Vacuum Condition |
| Data
: Vacuum in Vessel: 600 mm of Hg, Temperature:
40º C, Friction Loss: 1m, Vapour Pressure
and specific gravity are 0.49 Kg/Cm2 and 0.72
respectively at 40º C, SG of Mercury:
13.6 |
Method
ha
= (10 x 1/0.72 - 600 x 13.6/1000 x 0.72) -
13.88 -11.33 = 2.55
hvp = 10 x 0.49/0.72 = 6.81 mts.
NPSH (A) = 2.55 +10.2 -1 -6.81 = 4.94 mts |
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| Example
IV - Closed Tank under Pressure |
Data
: Pressure in closed vessel: 0.5 Kg/Cm2,
Atmospheric Pressure at the installation:
0.9 Kg/Cm2, Friction Loss: 1.5
m vapour pressure; 0.45 Kg/Cm2
Specific Gravity: 0.8
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Method
ha
= (10 x 0.9/0.8 +10 x 0.5/0.8) = 11.25 + 6.25
= 17.5
hvp = (10 x 0.45/0.8) = 5.625
NPSH (A) = 17.25 +0.2 -5.625 - 1.5 = 10.325
mts. |
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| B-10 |
Life |
90%
of Bearing should still be serviceable after |
| ANSI |
24000 Hrs. |
3 years |
| API |
40000 Hrs. |
5 years |
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In practice, this is not what happens.
Life expectancy falls short by as much as 50% or more.
Not because, pump manufacturers do not know how to size
bearings or are too optimistic and cut corners.There
are two main reasons for the same:
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Actual
load exceeds predicted load
-
A pump misalignment
- Excessive
forces and moments exerted on pump by piping,
pump cavitation, operation below recommended
min. flow, poor suction piping
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The
load, the bearings can carry falls short of the
basic bearing load, because of bearing environment
conditions |
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- Lack
of lubrication
- Use
of wrong lubricants
- Water
contamination* of the lubricant
-
Inadequate cooling
- Excessive
cooling
*Water
contamination is the greatest and most frequent
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