This document discusses Net Positive Suction Head (NPSH) as it relates to centrifugal and positive displacement pumps. It defines NPSH Required (NPSHR) and NPSH Available (NPSHA), explaining that cavitation can occur if NPSHA is less than NPSHR. Methods to improve NPSHA include shortening suction pipe length, increasing pipe size, decreasing suction liquid temperature, and decreasing suction negative altitude. Equations are provided to calculate NPSHA based on factors like suction head, liquid velocity, vapor pressure, and specific gravity. Examples of NPSHA calculations are also given.

1. NPSH

2. YOU CAN GET FROM PUMP MANUAL
1- NET POSITIVE SUCTION HEAD
REQUIRED
NPSH
2- NET POSITIVE SUCTION HEAD
AVAILABLE
YOU CAN CALCULATE FROM PUMP SITE
TO AVOID SUCTION CAVITATION
AND FOR SAFE OPERATION
NPSHA > NPSHR

3. CAVITATION CAN OCCUR
FOR
CENTRIFUGAL
PUMPS
POSITIVE DISPLACEMENT
PUMPS
AND

4. HOW TO IMPROVE
NPSHA
SHORTENTHE SUCTIONPIPELENGTH
INCREASE SUCTIONPIPESIZE
DECREASE SUCTIONLIQUID TEMP.
DECREASE SUCTIONNEGATIVEALTITUDE
INCREASE SUCTIONPOSITIVEALTITUDE
STOP THE PIPING SUCTION LEAKS
RENEWTHE SUCTIONPIPE

5. liquid specific gravity
Sp.gr
Z liquid surface height ft
PS Pump suction pressure psig
V liquid velocity ft/sec
Pf Friction Pressure drop psi
Pa Atm. Pressure psi
Vp Vapor pressure psia
g 32.2 ft/sec.sec
h L Suction head loss ft
Vessel pressure psig
PSV
PVS
Z
PS v
NET POSITIVE (+)
SUCTION HEAD

6. 2
V
2g
NPSHA = Z +
{ ( P vs + Pa) – ( Pf + Vp )} 2.31
Sp.gr
+ ( ft )
2
V
2g
ft
2
sec
=
ft
2
sec
=
ft
2
sec 2
ft
2
sec
= ( ft )
P
Sp.g
Lb
Lb
=
2
ft
3
=
ft
ft
3
2
ft
= ( ft )
EQUATION & DIMENTIONS

7. Z
General Equation
+ V
2g
2 { ( P sv + Pa) – ( Pf + Vp ) } 2.31
+
Sp.gr
NPSHA = ( ft )
OR
Z + V
2g
2 { ( P sv + Pa) – Vp} 2.31
+
Sp.gr
- hL
OR
V
2g
2 { ( Ps + Pa) – Vp} 2.31
+
Sp.gr

8. OR
Z + V
2g
2 { ( P sv + Pa) – Vp} 2.31
+
Sp.gr
- hL
Z + V
2g
2 { ( P sv + Pa) – ( Pf + Vp ) } 2.31
+
Sp.gr
PVS
Z
V
1
NPSHA =

9. NPSHA =
( ft )
PS
v
2
V
2g
2 { ( Ps + Pa) – Vp} 2.31
+
Sp.gr

10. 1
T P
FIG-2
VAPOR PRESSURE
FIG-1
1
P
T
1- Heat up a little of water
in a pot up to boiling point
100 C ( valve 1 is opened)
2- Take off the heating source,
simultaneously close valve 1.

11. Closed
T P Gauge
3- During cooling down start to
record the P Gauge relevant to Temp.
4- Apply Absolute pressure Equation .
P Absolute P Gauge + 1
(bar)
Cool Down

12. 5- Record the Absolute Liquid vapor
pressure.
100
0
1
15
- 0.98
0.02
80
- 0.5
0.5
- 0.7
70
0.3
- 0.3
0.7
90
95
- 0.1
0.9
Vapor Pressure
P Gauge
Temp C
P Gauge + 1
( bar )
Vapor Pressure

13. Solution
NPSH Available
( Pa – Vp ) 2.31
hL
Sp gr
.
= + 12.8 ( ft )
= Z +
= - 12 +
( 14.7 – 3.7 ) 2.31
1
0.982
.
( ft )
Compare with NPSHR
Examples
If the liquid level Z = - 12 ft
, Friction loss is 1 ft of liquid
, Atmospheric pressure is 14.7 Psia. at150 F
, water sp gr. is 0 982
.
,Vp = 3.7 psia

14. Examples
If crude pump suction pressure is – 5 psig
Liquid velocity is 12 ft /sec
Vp. is 4 psia
Sp gr. is 0.8
Atmospheric pressure is 14.7 psia.
Solution
2
V
2 g
NPSHA = + ( ft )
S p gr
{ ( Pa + Ps ) – Vp 2.31
2
12
2 * 32.2
= + ( ft )
0.8
{ ( 14.7 - 5 ) – 4 ) 2.31
= 2 . 24 + 16 . 46
= + 18.7 ( ft )
Compare with NPSHR