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NPSH (Net positive suction head)

NPSH (Net positive suction head)

 What is NPSH?

NPSH (net positive suction head) is the difference between suction pressure and lowest pressure inside the pump or system. By the use of the Bernoulli theorem, calculated if the suction pressure is greater than vapor pressure, then the risk of cavitation is negligible, but in other cases, if the suction head pressure is less than vapor pressure, the formation of bubbles started due to this reason cavitation take please in the pump casing and pipings. So always take NPSH is more than the vapor pressure. In  picture shows the net positive suction head to avoid the Cavitation in centrifugal pump. Inlet line pressure always must be little more than the vapor pressure.

Vapor pressure

The pressure of vapor is the pressure acting on the surface of the liquid.  It is formed by the vaporization of the liquid, this pressure is formed by in the thermodynamic equilibrium state of liquid in the container.

NPSH (Net positive suction head)


FMECA (failure mode effects and criticality analysis)

Types of NPSH

There are two types of NPSH in the system mentioned below.

  1. NPSHA (Net positive suction head available) 
  2. NPSHR (Net positive suction head required)

What is the NPSH margin?

Net positive suction head margin

It is the difference between Net positive suction head available and Net positive suction head required.
NPSH MARGIN = (Net positive suction head available – Net positive suction head required)

What is NPSHA (Net positive suction head available)?

head Available practical at site conditions total suction head minus the pressure of the system liquids at the site conditions

What is NPSHR (Net positive suction head required)?


That is quantitative value is provided by the manufacturer to maintain the minimum suction head. NPSH (net positive suction head )required is a design process in which the designer has decided the minimum suction head is required to operate the pump at the site conditions the net positive suction head available is always should be greater than net positive suction head required For the cavity-free operation of pumps.

What is cavitation?

Cavitation is a process in which in the inside surface of piping or equipment pump casing starts the formation of bubbles self and that bubbles start to collapse and blast on the surface of the system. That process is called Cavitation.

Types of cavitation

5 types of Cavitation are mentioned below. Vapor Cavitation, Turbulence Cavitation, Internal circulation, Suction Cavitation, etc

Why NPSH required for a pump?

The main purpose of the net positive suction head is to avoid the vaporization of the fluid and save from the cavitation to pump in the general terms called cavitation is a killer of the pump. It also helps to improve the pump performance because the cavitation formation starts then pump performance is going down that’s why during the design of pump NPSH focus as a main factors. 

What happens if the net positive suction head is negative?

If the NPSH is negative inside the piping systems then fluids starts boiling and a large number of bubble formation starts that bubbles are suck by the impeller eye of the pump inside and that bubbles start busting on the inside casing. Due to this process pump inside the casing’s getting start to erode. And this process is called a cavitation. It is very dangerous for pumps, piping’s because of this pump and piping life is goes down.

How to calculated net positive suction head?

The calculation of NPSH is the difference between suction pressure head to vapor pressure of the liquid. NPSH = (Pressure at the suction head – vapor pressure)

Why net positive suction head is important?

For the pump design and operation, we need the value of net positive suction head available and net positive suction head required, based on the NPSHA AND NPSHR plant engineer or designer calculate the pump characteristic and performance curve. So it is more important to consider the life of the equipment so it mainly depends on the two factors, net positive suction head available and net positive suction head required.

Factor affecting net positive suction head margin


There are many factors that affect the NPSH margin some of them mentioned below.

  1. Operating range
  2. Impeller eye
  3. Casing and impeller clearance
  4. Section and discharged line reducer size
  5. Larger impeller eyes
  6. Suction specific speed
  7. Pump size
  8. Impellers materials
  9. Suction material
  10. Pump RPM

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