Understanding NPSH in Centrifugal pumps

NPSH

Definition

Net Positive Suction Head

NPSH is what the pump needs, the minimum requirement to perform its duties. Therefore, NPSH is what happens in the suction side of the pump, including what goes on in the eye of the impeller.

Remember :The unit of measure for NPSH is head in meters in the pump suction.

Inadequate NPSH establishes favorable conditions for cavitation in the pump.

NPSH takes into consideration:

The suction piping and connections.

The elevation of the liquid in suction side.

The absolute pressure of the fluid in the suction piping.

The velocity of the fluid.

The temperature of the fluid.

NPSHRNet Positive Suction Head Required

NPSHANet Positive Suction Head Available

NPSHrNet Positive Suction Head Required

DefinitionIt is the energy in the liquid required to overcome the friction losses from the suction nozzle to the eye of the impeller without causing vaporization.

Remember : It is a characteristic of the pump and is indicated on the pump's curve. It

varies by design, size, and the operating conditions. An easy way to understand NPSHr is to call it the minimum suction

pressurenecessary to keep the pumped fluid in a liquid state.According to the Standards of the Hydraulic Institute, a suction lift testis performed on the pump and the pressure in the suction vessel islowered to the point where the pump suffers a 3% loss in total head.This point is called the NPSHr of the pump.

NPSHaNet Positive Suction Head Available

DefinitionThis is the energy in the fluid at the suction connection of the pump over and above the liquid’s vapor pressure. It is a characteristic of the system.

Remember :NPSHa only deals with the suction side of the pump.

NPSHa > NPSHr

During pump operation,NPSHa should be always greater than the NPSHr

As a general guide the NPSHa should be a minimum 10% or 1 meter above the NPSHr,

whichever is greater.

HOW ?

NPSHa is in your systemTherefore you have to calculate it

Formula of NPSHa is

NPSHa = Ha +/- Hs - Hvp - Hf

Pressure can be converted into head with the following equation:

10,2 x Pressure ( bar ) Specific gr

H (mtr) =

Head can be converted into pressure with the following equation:

H ( mtr ) x Specific gr 10,2P (bar) =

Before we proceed keep in mind following:

NPSHa = Ha +/- Hs - Hvp – Hf

Atmospheric head at sea level

Ha =

a ) Open tank to atmosphereHa = 10,33 mtr

b ) FW Tank pressurized with air

(4,5 + 1) x 10,2

1

Tank Pressure4,5 bar g

= 56,1 mtr

Ha :

NPSHa = Ha +/- Hs - Hvp – Hf

Hs : Static head in meters (positive or negative) of the fluid level in the suction vessel to the pump centerline.

+ Hs

- Hs

NPSHa = Ha +/- Hs - Hvp – Hf

Hvp : The Liquid Vapour Pressure head expressed in meters. It is a function of the temperature of the liquid. Pvapour of any liquid can be found in thermodynamic properties tables.Example:

a) Liquid: Water @ 350 C Pvapour = 0,056 bar a

b) Liquid: Propane @ - 350 C Pvapour = 1,37 bar a

Hvp =

0,056 x 10,21

0,57 mtrHvp =

Hvp =

1,37 x 10,20,573

24,39 mtrHvp =

NPSHa = Ha +/- Hs - Hvp – Hf

Hf : Friction head or friction losses expressed in meters in the suction piping and connections.The friction head can be calculated with the frictiontables for pipe and fittings.For submerged and deepwell pumps can be ignored. e.g Hf = 0

Hf = Suction line piping losses

NPSHa calculation of a Ballast pump

NPSHa = Ha +/- Hs - Hvp - Hf

Ha = 10,33 mtr

Hs = 1 mtr

Hf = 1,1 mtr

FW @ 320 C

NPSHa = 10,33 + 1 – 0,49 – 1,1

NPSHa = 9,74 mtr

Hvp = 0,048 x 10,2 = 0,49 mtr

P = 4,5 bar g

Propane @ 50 C

NPSHa calculation of LPG pump

NPSHa = Ha +/- Hs - Hvp - Hf

Hs = 1 mtr

Ha =

(4,5+1) x 10,20,522 =107,5 mtr

Hvp = 5,5 x 10,20,522 =107,5 mtr

NPSHa = 107,5 + 1 – 107,5 – 0NPSHa = 1 mtr

NPSHa calculation of LPG pump

NPSHa = Hs

Propane @ 50 C

Hs = 1 mtr

LPG pumps