NPSH (Net Positive Suction Head)

Design and Engineering

Net positive suction head, which is called in short form as NPSH and used in engineering pump designs and even in design engineering of various plants for location of pumps.

What is NPSH?

NPSH is a static head, which will be calculated from single datum or reference elevation (Normally datum elevation is considered as pump suction elevation as reference elevation) minus vapour pressure head of liquid.

It is expressed in metres (feet) of head of the pumped liquid.

Refer below sketch which elaborate, and ease in understanding the term NPSH.

As we defined above NPSH is Atmospheric pressure head + Static Head – Vapour pressure head at liquid.

In above sketch, static head is available head in meter, from pump suction nozzle to liquid high level.

Whereas Vapour pressure head h(vp) is liquid vapour pressure (p) divided by liquid density (ρ) and gravitational acceleration or gravitational constant (g).

h(vp) = p/(ρ*g)

Where,

                                h(vp) – Head in m

                                P – Pressure in N/m2

                                ρ – Density in kg/m3

                                g – Gravitational constant (9.81 m/s2)

Atmospheric pressure h (a)

Atm. pressure h (a) = 1.01 bar = 760mm Hg = 10.33m water

Vapour Pressure

It is the pressure at which a liquid and its vapour coexist in equilibrium at a given temperature. Vapour pressure represents the energy required to keep the fluid in liquid state. Rise in temperature and fall in pressure induces vaporisation.

In a summary,

NPSHA = h(a)+h(ss)-h(vp)-h(fl)

h(a)- Atmospheric pressure  or pressure head (m)

h(ss)- Static suction head/lift at min. supply level (m)

h(vp)- Head of vapour pressure at max. pumping temperature (m)

h(fl)- Friction head or Frictional loss in piping, valves and fittings (m)

Pump vendor prototype testing was done considering NPSH and same testing NPSH considered by vendor is called as NPSH required. Vendor will specify the same for his pump design.

The NPSH available must always be more than the NPSH required for the pump to operate suitably. It is common practice to have at least 1m of additional NPSH available at the suction nozzle to avoid any complications like cavitation issues at the duty point. 

Cavitation

Cavitation is the formation of vapour bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapour pressure.

Simple Example for calculating NPSH for pumps.

Temperature of Water = 30 DegC, Density (ρ) at 30 DegC = 995.7 kg / m3

Vapour Pressure ‘p’ at 30 DegC = 4.241 kPa (abs), 

Atmospheric pressure h(a) = 101.325 kPa

Gravitational acceleration g = 9.81 m/s2

Pipe Frictional Loss h(fl) = 0.9 m

Static Suction Head h(ss) = 2.6 m

With Above Parameters, first we will calculate Vapour Pressure Head h(vp), as below,

h(vp)     = p/(ρ*g)

= (4.241*1000)/ (995.7*9.81)

= 0.4341 m

h (a)       = p/(ρ*g)    

                = (101.325*1000) /(995.7*9.81)

                = 10.33 m

And now NPSHA = h(a)+h(ss)-h(vp)-h(fl)

  = 10.37+2.6-0.4341-(0.9-0.1-0.4-1)

  = 4.95 m

Add 10% safety factor for value specified to pump supplier:

The NPSH required is thus 4.95/1.1 = 4.5 m