NPSH - An introdu ction for pump users users 1. General General formul ation Net Positive Suction Head (NPSH) is a local liquid property and is defined as the excess mechanical energy energy of the liquid above above that required to prevent prevent vaporization. Also called “NPSH available” (NPSHA), it‘s usefulness is straightforward: when the NPSHA at any point reaches zero, the liquid vaporizes. NPSH NPSHA A = Total mechanical energy energy o f li quid – Vapor Vapor p ressure energy energy o f li quid In the design of liquid handling systems, one often wishes to calculate the NPSHA at some point of interest. This is normally done done by calculating the ener energy gy relative to some known reference point: Total mechanical energy energy of liquid = H A + ΔHR + ΔHF NPSHA = ( H A + HR + HNR ) - HVP Where: * H A
(m , ft) = A known reference energy at some point in the system.
ΔHR
(m , ft) = The calculated reversible energy changes between the reference point and the point of interest. This term may be positive or negative negative depending depending on the system geometry.
ΔHNR
(m , ft) = The calculated non-reversible non-reversible energy changes between the reference point and the point point of interest. This term is always negative negative (or zero if neglected neglected as minor). For this reason, reason, it is is commonly referred to as as the system system “loss”.
HVP
(m , ft) = Vapor pressure pressure energy energy of the the liquid liquid being handled at the current current temperature.
* 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 or meters 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-kg FORCE/kgMASS). These same units are used for total dynamic pump head. When dealing with incompressible fluids under earth gravity, the (lbf/lbm) or (kg F/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 liquid level in a sump) then the reference energy can be expressed simply in terms of the ambient pressure over the liquid, all other terms being zero: H A = P A /ρg Where P A
(Pa or N/m2 , psf)
= The ambient pressure at the liquid free surface.
ρ
(kg/m3 , slug/ft3)
= Liquid density
g
(m/s2 , ft/s2)
= Acceleration Acceleration of gravity
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NPSH - An introd uction for pump users Regarding the reversible energy changes, a widely used formulation for liquid systems is the Bernoulli equation: ΔHR = ( ΔP/ρg + ΔV2/2g + Δz )
Where: ΔP
(Pa or N/m2 , psf) = Change in static pressure relative to the reference point.
ΔP/ρg ΔV
= Change in static pressure energy relative to the reference point.
(m/s , ft/s)
2
ΔV /2g Δz
(m , ft)
= Change in liquid velocity relative to the reference point. Note: normally VREF=0 and therefore ΔV = V.
(m , ft)
= Change in kinetic energy relative to the reference point.
(m , ft)
= Change in potential energy (in the direction of gravity) relative to 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 piping system, these are the system resistance losses. Since they are usually represented as a positive quantity, we have the following relationship: ΔHNR = - 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 turbulence around areas of rapid velocity change, it does NOT include static pressure changes 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 /ρg 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 of practical application to liquid systems:
NPSHA = (P A + P - PVP)/ g + V2/2g + z - HF
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NPSH - An introd uction for pump users 2. Useful simplifications Several simplifications of the above formula are useful to pump users. A. If the mass flow is const ant between the reference point and the point of interest, the terms ΔP/ρg and ΔV2/2g cancel out leaving: NPSHA = (P A - PVP)/ g + z - HF This formula is often used to calculate the NPSHA for a pump during the system design phase. In this case: P A
(Pa or N/m2 , psf) = Atmospheric pressure (in an open system) or tank pressure over the liquid (in a sealed system)
Δz
(m , ft)
= Level difference from the free surface of the liquid to the pump suction centerline. Note: If the free surface is above the pump suction centerline, then z is positive.
ΔHF (m , ft)
= System friction losses from the free surface to pump suction inlet.
The pump suction inlet is usually defined as a measurement section in the pipeline approximately one pipe diameter from the pump suction flange. B. In the case of a sealed sump without external pressurization , P A = PVP. In this case our equation simplifies to: NPSHA =
z - HF
C. In the case of a pump test, where the pressure and velocity at the pump suction inlet are known, a different formulation is useful. For this purpose, we recognize that the absolute static suction pressure (P S) as measured at pressure taps flush with the suction piping wall can be written as follows: PS/ρg = (P A + ΔP )/ρg + Δz - HF Back-substituting into the original general equation gives a formula that allows NPSHA to be 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/ρg + ΔV2/2g = Suction Head
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NPSH - An introd uction for pump users 3. Application Theoretically, cavitation at the pump suction inlet will occur when the NPSHA there falls to zero. In practice, localized cavitation occurs elsewhere in the pump at some suction inlet NPSHA value that is greater than zero. This is usually the result of areas of reduced pressure 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 NPSHA resulting 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 are important: NPSHI
= The incipient NPSH, i.e. that suction inlet NPSHA at which vapor bubbles are first observed at some point in the pump, usually at the vane inlets. These bubbles signal the potential onset of cavitation damage, even though the pump performance may be unaffected. Since NPSHI must normally be determined by visual observation, it is difficult to measure. It can, however, be an important value for pumps requiring a high degree of reliability over long periods of continuous operation (e.g. nuclear power plant cooling pumps).
NPSH0% = The minimum value of suction inlet NPSHA at which the pump total dynamic head exhibits no appreciable drop and the pump itself no appreciable vibration. In many clear fluid applications, this is the NPSHA at which the pump can operate continuously without damage. NPSH3% = The value of suction inlet NPSHA at which the pump total dynamic head drops by 3%. NPSH3% is relatively easy to measure in the test lab and gives a good indication of the onset of significant performance losses due to cavitation. One must recognize, however, that at this value of NPSH, some cavitation is already occurring 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 a regular basis. As a result, NPSH5% and even NPSH 10% are often measured and taken into consideration 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, (as measured by pressure taps flush with the suction piping wall), at the point where the pump discharge 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 ρg and adding atmospheric pressure to convert suction head into static gauge pressure: Vac M = P A - (NPSH5%* g) - PVP + V2/2 Where: VacM
(Pa or N/m2 , psf)
= The “Decisive Vacuum”.
P A
(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.
ρ
(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
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