tech npsh dl-data

8/11/2019 Tech Npsh Dl-data

1/4

NPSH - An introduction for pump users

TECH NPSH 2008-01-22.doc Page 1 of 4

1. General formulation

Net Positive Suction Head (NPSH) is a local liquid property and is defined as the excessmechanical energy of the liquid above that required to prevent vaporization. Also calledNPSH available (NPSHA), its usefulness is straightforward: when the NPSHA at anypoint reaches zero, the liquid vaporizes.

NPSHA = Total mechanical energy of liquid Vapor pressure energy of liquid

In the design of liquid handling systems, one often wishes to calculate the NPSHA atsome point of interest. This is normally done by calculating the energy relative to someknown reference point:

Total mechanical energy of liquid = HA+ HR+ HF

NPSHA = ( HA+ HR+ HNR) - HVP

Where: *

HA (m , ft) = A known reference energy at some point in the system.

HR (m , ft) = The calculated reversible energy changes between the reference pointand the point of interest. This term may be positive or negative depending on thesystem geometry.

HNR (m , ft) = The calculated non-reversible energy changes between the referencepoint and the point of interest. This term is always negative (or zero if neglectedas minor). For this reason, it is commonly referred to as the system loss.

HVP (m , ft) = Vapor pressure energy of the liquid being handled at the currenttemperature.

* Consistent SI and US units are given for all quantities. Other unit systems are possible.

It is customary to express the above energy terms as potential energies, (i.e. as feet ormeters of the liquid), since this simplifies the overall system calculations. Strictly speaking,NPSH has units of mechanical energy per unit mass: (ft-lbf/lbm) or (m-kgFORCE/kgMASS).These same units are used for total dynamic pump head. When dealing withincompressible fluids under earth gravity, the (lbf/lbm) or (kgF/kgM) terms may be ignored,as they always cancel out.

If a reference location is selected where the liquid has a free surface (such as the liquidlevel in a sump) then the reference energy can be expressed simply in terms of theambient pressure over the liquid, all other terms being zero:

HA= PA/g

Where

PA (Pa or N/m2, psf) = The ambient pressure at the liquid free surface.

(kg/m3, slug/ft3) = Liquid density

g (m/s2, ft/s2) = Acceleration of gravity


2/4



Regarding the reversible energy changes, a widely used formulation for liquid systems isthe Bernoulli equation:

HR= ( P/g + V2/2g + z )

Where:

P (Pa or N/m2, psf) = Change in static pressure relative to the reference point.

P/g (m , ft) = Change in static pressure energy relative to the referencepoint.

V (m/s , ft/s) = Change in liquid velocity relative to the reference point. Note:

normally VREF=0 and therefore V = V.

V2/2g (m , ft) = Change in kinetic energy relative to the reference point.

z (m , ft) = Change in potential energy (in the direction of gravity) relativeto the reference point. Note: If the reference point is above the

point of interest, then z is positive.

The non-reversible energy changes are often lumped together and called friction losses,although they include both pure friction and local turbulent (or shock) losses. In a pipingsystem, these are the system resistance losses. Since they are usually represented as apositive quantity, we have the following relationship:

HNR = - HF

Where:

HF (m , ft) = Friction and shock losses expressed in feet or meters of liquid. Note:Although this term includes entrance and exit pressure losses due to turbulencearound areas of rapid velocity change, it does NOT include static pressurechanges due to the acceleration or deceleration of the liquid (i.e. velocity head).

Regarding the vapor pressure energy, this is simply determined from the vapor pressure:

HVP= PVP/g

Where:

PVP (Pa or N/m2, psf) = The liquid vapor pressure at the current temperature.

Back-substituting into the original equation for NPSHA gives a general NPSH equation ofpractical application to liquid systems:

NPSHA = (PA+ P - PVP)/g + V2/2g + z - HF


3/4



2. Useful simplifications

Several simplifications of the above formula are useful to pump users.

A. If themass flow is constantbetween the reference point and the point of interest,

the termsP/g and V2/2g cancel out leaving:

NPSHA = (PA- PVP)/g + z - HF

This formula is often used to calculate the NPSHA for a pump during the system designphase. In this case:

PA (Pa or N/m2, psf) = Atmospheric pressure (in an open system) or tank pressure

over the liquid (in a sealed system)

z (m , ft) = Level difference from the free surface of the liquid to the pumpsuction centerline. Note: If the free surface is above the pumpsuction centerline, then z is positive.

HF (m , ft) = System friction losses from the free surface to pump suctioninlet.

The pump suction inlet is usually defined as a measurement section in the pipelineapproximately one pipe diameter from the pump suction flange.

B. In the case of asealed sump without external pressurization, PA= PVP. In thiscase our equation simplifies to:

NPSHA = z - HF

C. In the case of a pump test, where the pressure and velocity at the pump suctioninlet are known, a different formulation is useful. For this purpose, we recognize that the

absolute static suction pressure (PS) as measured at pressure taps flush with the suctionpiping wall can be written as follows:

PS/g = (PA+P )/g + z - HF

Back-substituting into the original general equation gives a formula that allows NPSHA tobe calculated directly from the measured pressure and velocity at the pump suction:

NPSHA = (PS - PVP)/g + V2/2g

Where:

PS (Pa or N/m2, psf) = Absolute static pressure at the pump suction inlet, (e.g. as

measured by pressure taps flush with the suction piping wall).

D. Alternatively, we can define this equation in terms of the suction head.

NPSHA = HS- PVP/g

Where:

HS (m , ft) = PS/g + V2/2g = Suction Head


4/4



3. Application

Theoretically, cavitation at the pump suction inlet will occur when the NPSHA there falls tozero. In practice, localized cavitation occurs elsewhere in the pump at some suction inletNPSHA value that is greater than zero. This is usually the result of areas of reducedpressure caused by turbulence around the leading edges of the impeller vanes, or by

other characteristics of the pump inlet geometry. The value of suction inlet NPSHAresulting in actual cavitation elsewhere in the pump is normally called the required NPSH(or NPSHR) and must be determined in the test lab. Three values of NPSHR areimportant:

NPSHI = The incipient NPSH, i.e. that suction inlet NPSHA at which vapor bubbles arefirst observed at some point in the pump, usually at the vane inlets. Thesebubbles signal the potential onset of cavitation damage, even though the pumpperformance may be unaffected. Since NPSHImust normally be determined byvisual observation, it is difficult to measure. It can, however, be an importantvalue for pumps requiring a high degree of reliability over long periods ofcontinuous operation (e.g. nuclear power plant cooling pumps).

NPSH0%= The minimum value of suction inlet NPSHA at which the pump total dynamichead exhibits no appreciable drop and the pump itself no appreciable vibration.In many clear fluid applications, this is the NPSHA at which the pump canoperate continuously without damage.

NPSH3%= The value of suction inlet NPSHA at which the pump total dynamic head dropsby 3%. NPSH3% is relatively easy to measure in the test lab and gives a goodindication of the onset of significant performance losses due to cavitation. Onemust recognize, however, that at this value of NPSH, some cavitation is alreadyoccurring and that continuous operation at this point is generally not advisable.

In the dredging industry, pumps are often operated well into the cavitation range on aregular basis. As a result, NPSH5%and even NPSH10%are often measured and taken intoconsideration during operation.

Dredgers also sometimes refer to the value of Decisive Vacuum, rather than NPSHR.Decisive Vacuum is defined as the static gauge pressure at the pump suction inlet, (asmeasured by pressure taps flush with the suction piping wall), at the point where the pumpdischarge head falls by some given amount, usually 5%. It is derived by rearranging the

pump test equation C above to solve for suction head, then multiplying through by gand adding atmospheric pressure to convert suction head into static gauge pressure:

VacM = PA - (NPSH5%*g) - PVP+ V2/2

Where:

VacM (Pa or N/m2, psf) = The Decisive Vacuum.

PA (Pa or N/m2, psf) = The ambient pressure at the liquid free surface.

PVP (Pa or N/m2, psf) = The liquid vapor pressure at the current temperature.

(kg/m3, slug/ft3) = Liquid density

g (m/s2, ft/s2) = Acceleration of gravity

V (m/s2, ft/s2) = Liquid velocity at the pump suction

tech npsh dl-data

Documents