This document discusses Net Positive Suction Head (NPSH) and cavitation in pumps. It defines key terms like NPSH, NPSHA, NPSHR. NPSH is the net positive pressure at the pump inlet to prevent vaporization. NPSHR increases with flow rate and is pump-specific. NPSHA must exceed NPSHR to avoid cavitation. The document provides a formula to calculate maximum pump elevation above a reservoir based on NPSHR, friction losses, and vapor pressure. It notes symptoms of low NPSH like noise, wear, and performance issues.

1. UNDERSTANDING NPSH
AND CAVITATION
ANKIT PANWAR
500063494
MTECH PLE

2. CONTENTS
 SYMPTOMS
 NPSH
 NPSHA
 NPSHR
 CASE STUDY

3. Do you have any of these symptom's ?

4. Symptoms
Growling Pumps
Worn out impellers
Broken shafts
Failed seals

5. All leading to change in performance

6. You might have low NPSH

7. NPSH (Net Positive Suction Head)
 NPSH is one of the most widely used and least understood terms associated
with pumps. Understanding the significance of NPSH is very much essential
during installation as well as operation of the pumps.
 NPSH is the head above the vapor pressure of the liquid existing at the pump
suction nozzle.
 Pumps can pump only liquids, not vapors.
 Rise in temperature and fall in pressure induces vaporization.
 NPSH as a measure to prevent liquid vaporization.

8. NPSHr is a function of pump design
 NPSH required is a function of the pump design and RPM is determined based on actual
pump test by the vendor.
 As the liquid passes from the pump suction to the eye of the impeller, the velocity
increases and the pressure decreases.
 There are also pressure losses due to shock and turbulence as the liquid strikes the
impeller.

10. NPSHr increases as capacity increases
 The NPSH required varies with speed and capacity within any particular pump.
 The NPSH required increase as the capacity is increasing because the velocity of
the liquid is increasing, and as anytime the velocity of a liquid goes up, the
pressure or head comes down.
 Pump manufacturer's curves normally provide this information.
 The NPSH is independent of the fluid density as are all head terms.

12. NPSHa in a nutshell
 NPSHa = Pressure head + Static head - Vapor pressure head of your
product – Friction head loss in the piping, valves and fittings.
 “All terms in feet absolute”
 In an existing system, the NPSHa can also be approximated by a gauge on
the pump suction using the formula:
 NPSH= Ha +/- Hz - Hf + Hv - Hvp
 Where:
 Ha is the atmospheric or absolute pressure
 Hz is the vertical distance from the surface of the water to the pump center
line.
 Hf is the friction formed in the suction piping
 Hv is the velocity head at the pump's suction
 Hvp is the vapor pressure of the water at its ambient temperature

13. NPSH and Cavitation
• Net Positive Suction Head Required (NPSHR): The
minimum value of NPSHA that is needed to
prevent cavitation in the pump, i.e., the value of
NPSHA that causes pmin to equal pvap.
• NPSHR is determined experimentally by pump
manufacturers and reported as a function of pump
flow rate (usually called ‘capacity’).
• To avoid cavitation, always operate with NPSHA ≥
NPSHR.

14. The Maximum Allowable Elevation of a
Pump
2
,
ANPSH
2
vaps abs s
pp V
g 
  
2
, ,
2
s abs atm abs s
sl L
p p V
z h
g 
   
,
ANPSH vapatm abs
sl L
pp
z h
 
   
A RNPSH NPSH
,
RNPSHvapatm abs
sl L
pp
z h
 
   

15. ,
RNPSHvapatm abs
L sl
pp
h z
 
   
,
RNPSHvapatm abs
sl L
pp
z h
 
   
Suction lift, zsl, must be less than the expression
on the left to avoid cavitation, so that
expression indicates the maximum allowable
suction lift (i.e., maximum elevation of the
pump above the reservoir).

16. Case Study

17. THANK YOU