Net Positive Suction Head: NPSHR and NPSHA

Written by: Joe Evans, Ph.D.    

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In Pumps & Systems January 2007, I wrote an article about cavitation and how a collapsing water vapor bubble can damage an impeller. Since then, I have received a number of requests to address Net Positive Suction Head (NPSH) and its relationship to cavitation. Here it is in a very simple, Pump Ed 101 perspective.

The process of boiling is not as simple as it may seem. We tend to think that it is all about temperature and often forget that pressure has an equal role in the process. The point at which water boils is proportional to both its temperature and the pressure acting upon its surface. As pressure decreases, so does the temperature required to initiate boiling.

The onset of cavitation also follows this rule. When water-at some ambient temperature-travels through an area of low pressure, it can undergo a change of state from liquid to vapor (boiling). As it progresses into an area of higher pressure, it will return to the liquid state (cavitation). The bubbles that form and collapse during this process are those of water vapor-not air. Although dissolved or entrained air can affect pump performance, it produces a totally different kind of bubble than the one produced by boiling.

The fact that boiling is proportional to both temperature and pressure is the reason cavitation is such a persistent problem. Simply stated, water can boil at virtually any temperature. At sea level, where atmospheric pressure is about 14.7-psi (34-ft), it takes 212-deg F. Increase that elevation to 6,000-ft and it drops to around 200-deg F because the corresponding atmospheric pressure decreases to 11.7-psi (27-ft). If we introduce a vacuum and continue to reduce pressure to about 0.2-ft, it will boil at its freezing point. Well, so what? We don't usually operate a pump in a vacuum, and even at the top of Mt. Everest we still have almost 5.2-psi (12-ft) of atmospheric pressure!

Well, it turns out that all centrifugal pumps produce a partial vacuum.  If they did not, they would be unable to pump water from a lower level. During normal operation, the area of lowest pressure occurs near the impeller vane entrances, and if the pressure in this area drops to about 1-ft, water will boil at 75-deg F! For a pump to operate cavitation free, an excess of pressure energy is required of the water entering this area. We typically refer to this requirement as NPSHR, or the NPSH required. Where does this pressure energy come from? It is a combination of several different forms of energy that exist, at various levels, on the suction side of the pumping system. We refer to this available pressure energy as NPSHA, or the NPSH available.

NPSHAThe NPSH available to a centrifugal pump combines the effect of atmospheric pressure, water temperature, supply elevation and the dynamics of the suction piping. The following equation illustrates this relationship. All values are in feet of water, and the sum of these components represents the total pressure available at the pump suction.

NPSHA = Ha +/- Hz - Hf + Hv - Hvp

Where:

Ha is the atmospheric or absolute pressure

Hz is the vertical distance from the surface of the water to the pump centerline

Hf is the friction formed in the suction piping

Hv is the velocity head at the pump's suction

Hvp is the vapor pressure of the water at its ambient temperature

Ha is the atmospheric or absolute pressure exerted on the surface of the water supply. Atmospheric pressure is the pressure due to the density of the earth's atmosphere at some elevation. It develops its greatest pressure (14.7-psi) at sea level (where it is most dense) and approaches zero at its upper boundary. We seldom think about this pressure because, out of the box or on the work bench, the typical pressure gauge reads 0-psi. These gauges are calibrated to something we call "gauge" scale (PSIG) and totally ignore atmospheric pressure. Gauges calibrated to the "absolute" scale (PSIA) include atmospheric pressure and will read 14.7-psi at sea level. The figure below compares these two pressure scales.  On the absolute scale, 0-psi equates to a perfect vacuum, but on the gauge scale it equates to atmospheric pressure.

If the water source is a reservoir or an open (or vented) tank, Ha is simply the measured atmospheric pressure. It takes on another dimension if the supply is an enclosed, unvented tank. In this case, Ha becomes the absolute pressure or the sum of the measured atmospheric pressure plus or minus the actual gauge pressure of the air in the tank.

Hz takes into account the positive or negative pressure of the water source due to its elevation. If it is above the pump, Hz is a positive number and if it is below, Hz is negative. Hf is simply the friction generated due to flow in the suction piping and is always a negative number. It is a function of the pipe length and diameter plus the fittings and valves it incorporates.

Hv and Hvp may be a little less familiar to some of us. Hv, or velocity head, is the kinetic energy of a mass of water moving at some velocity V. It is equivalent to the distance that water would have to fall in order to reach that velocity. It can be calculated by determining the velocity in the suction piping from a velocity table and substituting that value for V in the equation "h = V2/2g" (where g is the universal gravitational constant, 32-ft/sec2). It is usually small-at a velocity of 7-fps, Hv is just 0.765-ft-and is often ignored if Ha and Hz are sufficiently large.

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Net Positive Suction Head: NPSHR and NPSHA

Written by: Joe Evans, Ph.D.    

TOPIC SPONSOR

RESOURCES

Pump Ed 101

Pump Repair

HI Pump FAQs

Hvp represents the pressure that is required to keep water in the liquid state at some ambient temperature and is obtained from a vapor pressure table. At 50 deg F, just 0.41-ft is required, but at 160-deg F that requirement increases to 11.2-ft. Since this pressure must be reserved for its stated purpose, Hvp is always a negative number.

At first glance, the equation for NPSHA looks pretty static, but it is actually quite dynamic. All of the variables can be in a continuous state of change. Velocity head and suction line friction vary as a function of flow. Likewise, atmospheric pressure can vary by several feet depending on weather conditions. Water supply elevation and temperature can vary seasonally. Usually the "worst case" values for each of these components are used when calculating NPSHA.

NPSHRAs mentioned earlier, NPSHR is the suction pressure necessary to ensure proper pump operation. It is purely a function of the pump design, and although it can be calculated, it is more accurately determined by actual testing. Why does a pump require a positive suction head? Quite simply, it is impossible to design a centrifugal pump that exhibits absolutely no pressure drop between the suction inlet and its minimum pressure point, which normally occurs at the entrance to the impeller vanes. Therefore, all pump systems must maintain a positive suction pressure that is sufficient to overcome this pressure drop. If the pressure is not sufficient, some of the water will change state (liquid to vapor) and cavitation is initiated. Like NPSHA, NPSHR is also a dynamic quantity and increases substantially with pump flow.

You would think that the NPSHR, measured by the pump manufacturer, would be the suction pressure required to prevent cavitation. That used to be the definition, but it is currently defined as the suction pressure at which a particular pump's hydraulic performance is degraded by 3 percent. This raises some concern since this degradation is actually due to cavitation, and at the 3 percent level, it has the potential to be damaging. The Hydraulic Institute's standards stipulate that each of the points on a pump manufacturer's NPSHR curve must reflect this 3 percent value. There are rumors that the term NPSHR will eventually be changed to NPSH3, which more accurately describes its true meaning.

Depending on the pump design, HI recommends an NPSHA / NPSHR margin of 1.1 to 2.5. Some pump experts recommend even more. It is a good idea to check with your pump manufacturer for its specific margin requirement as it relates to a particular pump model and its application.

A new term, NPSHI (inception), was recently developed to define the suction pressure required that will suppress all cavitation.  The cavitation that occurs between NPSHI and the point where damage occurs is called incipient cavitation. This form of cavitation appears to cause little, if any, damage in normal pumping applications. There is some ongoing debate as to whether the cavitation that occurs due to a 3 percent performance degradation should be regarded as incipient cavitation.

Pumps & Systems, May 2008

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Low pressure at the suction side of a pump can encounter the fluid to start boiling with

reduced efficiency cavitation damage

of the pump as a result. Boiling starts when the pressure in the liquid is reduced to the vapor pressure of the fluid at the actual temperature.

To characterize the potential for boiling and cavitation, the difference between the total head on the suction side of the pump - close to the impeller, and the liquid vapor pressure at the actual temperature, can be used.

Suction Head

Based on the Energy Equation - the suction head in the fluid close to the impeller can be expressed as the sum of the static and the velocity head:

hs = ps / γ + vs2 / 2 g         (1)

where

hs = suction head close to the impeller

ps = static pressure in the fluid close to the impeller

γ = specific weight of the fluid

vs = velocity of fluid

g = acceleration of gravity

Liquids Vapor Head

The liquids vapor head at the actual temperature can be expressed as:

hv = pv / γ         (2)

where

hv = vapor head

pv = vapor pressure

Note! The vapor pressure in fluids depends on temperature. Water, our most common fluid, starts boiling at 20 oC if the absolute pressure in the fluid is 2.3 kN/m2. For an absolute pressure of 47.5 kN/m2, the water starts boiling at 80 oC. At an absolute pressure of 101.3 kN/m2 (normal atmosphere), the boiling starts at 100 oC. 

Net Positive Suction Head - NPSH

The Net Positive Suction Head - NPSH - can be expressed as the difference between the Suction Head and the Liquids Vapor Head and expressed like

NPSH = hs - hv         (3)

or, by combining (1) and (2)

NPSH = ps / γ + vs2 / 2 g - pv / γ         (3b)

Available NPSH - NPSHa or NPSHA

The Net Positive Suction Head made available the suction system for the pump is often named NPSHa. The NPSHa can be determined during design and construction, or determined experimentally from the actual physical system.

The available NPSHa can be calculated with the Energy Equation. For a common application - where the pump lifts a fluid from an open tank at one level to an other, the energy or head at the surface of the tank is the same as the energy or head before the pump impeller and can be expressed as:

h0 = hs + hl         (4)

where

h0 = head at surface

hs = head before the impeller

hl = head loss from the surface to impeller - major and minor loss in the suction pipe

In an open tank the head at surface can be expressed as:

h0 = p0 / γ = patm / γ         (4b)

For a closed pressurized tank the absolute static pressure inside the tank must be used.

The head before the impeller can be expressed as:

hs = ps / γ + vs2 / 2 g + he         (4c)

where

he = elevation from surface to pump - positive if pump is above the tank, negative if the pump is below the tank

Transforming (4) with (4b) and (4c):

patm / γ = ps / γ + vs2 / 2 g + he + hl         (4d)

The head available before the impeller can be expressed as:

ps / γ + vs2 / 2 g = patm / γ - he - hl         (4e)

or as the available NPSHa:

NPSHa = patm / γ - he - hl - pv / γ         (4f)

Available NPSHa - the Pump is above the Tank

If the pump is positioned above the tank, the elevation - he - is positive and the NPSHa decreases when the elevation of the pump increases.

At some level the NPSHa will be reduced to zero and the fluid starts to evaporate.

Available NPSHa - the Pump is below the Tank

If the pump is positioned below the tank, the elevation - he - is negative and the NPSHa increases when the elevation of the pump decreases (lowering the pump).

It's always possible to increase the NPSHa by lowering the pump (as long as the major and minor head loss due to a longer pipe don't increase it more). This is important and it is common to lower the pump when pumping fluids close to evaporation temperature.

Required NPSH - NPSHr or NPSHR

The NPSHr, called as the Net Suction Head as required by the pump in order to prevent cavitation for safe and reliable operation of the pump.

The required NPSHr for a particular pump is in general determined experimentally by the pump manufacturer and a part of the documentation of the pump.

The available NPSHa of the system should always exceeded the required NPSHr of the pump to avoid vaporization and cavitation of the impellers eye. The available NPSHa should in general be significant higher than the required NPSHr to avoid that head loss in the suction pipe and in the pump casing, local velocity accelerations and pressure decreases, start boiling the fluid on the impeller surface.

Note that the required NPSHr increases with the square capacity.

Pumps with double-suction impellers has lower NPSHr than pumps with single-suction impellers. A pump with a double-suction impeller is considered hydraulically balanced but is susceptible to an uneven flow on both sides with improper pipe-work.

Example - Pumping Water from an Open Tank

When increasing the the elevation for a pump located above a tank, the fluid will start to evaporate at a maximum level for the actual temperature.

At the maximum elevation NPSHa is zero. The maximum elevation can therefore be expressed by (4f):

NPSHa = patm / γ - he - hl - pv / γ = 0

For optimal theoretical conditions we neglect the major and minor head loss. The elevation head can then be expressed as:

he = patm / γ - pv / γ         (5)

The maximum elevation or suction head for an open tank depends on the atmospheric pressure - which in general can be regarded as constant, and the vapor pressure of the fluid - which in general vary with temperature, especially for water.

The absolute vapor pressure of water at temperature 20 oC is 2.3 kN/m2. The maximum theoretical elevation height is therefore:

he = (101.33 kN/m2) / (9.80 kN/m3) - (2.3 kN/m2) / (9.80 kN/m3)

    = 10.1 m

Due to the head loss in the suction pipe and the local conditions inside the pump - the theoretical maximum elevation is significantly decreased.

The maximum theoretical elevation of a pump above an open water tank at different temperatures can be found from the table below.

Suction Head as Affected by Temperature

Temperature Vapor Pressure Max. elevation

(oC) (oF) (kN/m2) (m) (ft)

0 32 0.6 10.3 33.8

5 41 0.9 10.2 33.5

Temperature Vapor Pressure Max. elevation

(oC) (oF) (kN/m2) (m) (ft)

10 50 1.2 10.2 33.5

15 59 1.7 10.2 33.5

20 68 2.3 10.1 33.1

25 77 3.2 10.0 32.8

30 86 4.3 9.9 32.5

35 95 5.6 9.8 32.2

40 104 7.7 9.5 31.2

45 113 9.6 9.4 30.8

50 122 12.5 9.1 29.9

55 131 15.7 8.7 28.5

60 140 20 8.3 27.2

65 149 25 7.8 25.6

70 158 32.1 7.1 23.3

75 167 38.6 6.4 21

80 176 47.5 5.5 18

Temperature Vapor Pressure Max. elevation

(oC) (oF) (kN/m2) (m) (ft)

85 185 57.8 4.4 14.4

90 194 70 3.2 10.5

95 203 84.5 1.7 5.6

100 212 101.33 0.0 0

Pumping Hydrocarbons

Be aware that the NPSH specification provided by the manufacturer in general is for use with cold water. For hydrocarbons these values must be lowered to account for the vapor release properties of complex organic liquids.

Fluid Temperature (oC)Vapor Pressure

(kPa abs)

Ethanol

20 5.9

65 58.2

Methyl Acetate

20 22.8

55 93.9

Note that the head developed by a pump is independent of the liquid, and that the performance curves for water from the manufacturer can be used for Newtonian liquids like gasoline, diesel or similar. Be aware that required power depends on liquid density and must be adjusted.

NPSH and Liquids with Dissolved Gas

Be aware that NPSH calculations might have to be modified if there are significant amounts of dissolved gas in the liquid. The gas saturation pressure is often much higher than the liquid's vapor pressure

NPSH in a Pump[edit]

A hydraulic circuit

In a pump, cavitation will first occur at the inlet of the impeller.[1] Denoting the inlet by i, the NPSHA at

this point is defined as:

Applying Bernoulli's principle from the suction free surface 0 to the pump inlet i, under the

assumption that the kinetic energy at 0 is negligible, that the fluid is inviscid, and that the fluidy

density is constant:

Using the above application of Bernoulli to eliminate the velocity term and local pressure terms in the

definition of NPSHA:

This is the standard expression for the Available NPSH at point. Cavitation will occur at the

point i when the Available NPSH is less than the NPSH required to prevent cavitation (NPSHR). For

simple impeller systems, NPSHR can be derived theoretically,[2] but very often it is determined

empirically.[3]Note NPSHA and NPSHR are in absolute units and usually expressed in "ft abs" not

"psia".

Experimentally, NPSHR is often defined as the NPSH3, the point at which the head output of the

pump decreases by 3% at a given flow due to reduced hydraulic performance. On multi-stage

pumps this is limited to a 3% drop in the first stage head.[4]

NPSH in a Turbine[edit]

The calculation of NPSH in a reaction turbine is different to the calculation of NPSH in a pump,

because the point at which cavitation will first occur is in a different place. In a reaction turbine,

cavitation will first occur at the outlet of the impeller, at the entrance of the draft tube.[5] Denoting the

entrance of the draft tube by e, the NPSHA is defined in the same way as for pumps:

[6]

Applying Bernoulli's principle from the draft tube entrance e to the lower free surface 0, under the

assumption that the kinetic energy at 0 is negligible, that the fluid is inviscid, and that the fluid

density is constant:

Using the above application of Bernoulli to eliminate the velocity term and local pressure terms in the

definition of NPSHA:

Note that, in turbines minor losses ( ) alleviate the effect of cavitation - opposite to what happens

in pumps.

NPSH design considerations[edit]

Vapour pressure is strongly dependent on temperature, and thus so will both NPSHR and

NPSHA. Centrifugal pumps are particularly vulnerable especially when pumping heated solution near

the vapor pressure, whereas positive displacement pumps are less affected by cavitation, as they

are better able to pump two-phase flow (the mixture of gas and liquid), however, the resultant flow

rate of the pump will be diminished because of the gas volumetrically displacing a disproportion of

liquid. Careful design is required to pump high temperature liquids with a centrifugal pump when the

liquid is near its boiling point.

The violent collapse of the cavitation bubble creates a shock wave that can carve material from

internal pump components (usually the leading edge of the impeller) and creates noise often

described as "pumping gravel". Additionally, the inevitable increase in vibration can cause other

mechanical faults in the pump and associated equipment.

Relationship to other cavitation parameters[edit]

The NPSH appears in a number of other cavitation-relevant parameters. The suction head

coefficient is a dimensionless measure of NPSH:

Where   is the angular velocity (in rad/s) of the turbomachine shaft, and   is the turbomachine

impeller diameter. The Thoma cavitation number is defined as:

Where   is the head across the turbomachine.

Some general NPSH Examples[edit]

(based on sea level).

Example 1: A tank with a liquid level 2 metres above the pump intake, plus the atmospheric

pressure of 10 metres, minus a 2 metre friction loss into the pump (say for pipe & valve loss), minus

the NPSHR curve (say 2.5 metres) of the pre-designed pump (see the manufacturers curve) = an

NPSHA (available) of 7.5 metres. (not forgetting the flow duty). This equates to 3 times the NPSH

required. This pump will operate well so long as all other parameters are correct.

Remember that (+ or -) flow duty will change the reading on the pump manufacture NPSHR curve.

The lower the flow, the lower the NPSHR, and vice versa.

Lifting out of a well will also create negative NPSH; however remember that atmospheric pressure at

sea level is 10 metres! This helps us, as it gives us a bonus boost or “push” into the pump intake.

(Remember that you only have 10 metres of atmospheric pressure as a bonus and nothing more!).

Example 2: A well or bore with an operating level of 5 metres below the intake, minus a 2 metre

friction loss into pump (pipe loss), minus the NPSHR curve (say 2.4 metres) of the pre-designed

pump = an NPSHA (available) of (negative) -9.4 metres. NOW we add the atmospheric pressure of

10 metres. We have a positive NPSHA of 0.6 metres. (minimum requirement is 0.6 metres above

NPSHR), so the pump should lift from the well.

Now we will try the situation from example 2 above, but will pump 70 degrees Celsius (158F) water

from a hot spring, creating negative NPSH.

Example 3: A well or bore running at 70 degrees Celsius (158F) with an operating level of 5 metres

below the intake, minus a 2 metre friction loss into pump (pipe loss), minus the NPSHR curve (say

2.4 metres) of the pre-designed pump, minus a temperature loss of 3 metres/10 feet = an NPSHA

(available) of (negative) -12.4 metres. NOW we add the atmospheric pressure of 10 metres and we

have a negative NPSHA of -2.4 metres remaining.

Remembering that the minimum requirement is 600 mm above the NPSHR therefore this pump will

not be able to pump the 70 degree Celsius liquid and will cavitate and lose performance and cause

damage. To work efficiently, the pump must be buried in the ground at a depth of 2.4 metres plus the

required 600 mm minimum, totalling a total depth of 3 metres into the pit. (3.5 metres to be

completely safe).

A minimum of 600 mm (0.06 bar) and a recommended 1.5 metre (0.15 bar) head

pressure “higher” than the NPSHR pressure value required by the manufacturer is required to allow

the pump to operate properly.

Serious damage may occur if a large pump has been sited incorrectly with an incorrect NPSHR

value and this may result in a very expensive pump or installation repair.

NPSH problems may be able to be solved by changing the NPSHR or by re-siting the pump.

If an NPSHA is say 10 bar then the pump you are using will deliver exactly 10 bar more over the

entire operational curve of a pump than its listed operational curve.

Example: A pump with a max. pressure head of 8 bar (80 metres) will actually run at 18 bar if the

NPSHA is 10 bar.

i.e.: 8 bar (pump curve) plus 10 bar NPSHA = 18 bar.

This phenomenon is what manufacturers use when they design multistage pumps, (Pumps with

more than one impeller). Each multi stacked impeller boosts the previous impeller to raise the

pressure head. Some pumps can have up to 150 stages or more, in order to boost heads up to

hundreds of metres.

NPSHA vs. NPSHR

< Back to archives

Ron Lowe — Nov 12, 2007

Imagine a thick chocolate malt with a straw sticking out of the top of the cup. In order for the malt to flow

through the straw, several factors have to be considered since atmospheric pressure (14.7 psi) is the only

plus that we have in the equation-open system. If a low-pressure area is created on the end of the straw

exiting the cup, the liquid will start to flow up the straw. In order for this to happen, the following items

must be evaluated: 

1. Length of the straw.

2. How many bends in the straw.

3. Friction loss within the straw.

4. ID of the straw.

5. Velocity of the liquid.

6. Viscosity of the liquid.

7. Acceleration head -- acceleration and deceleration of the liquid.

8. Entrained gasses within the liquid.

The above items make up the entire system. It can be stated that net positive suction head available is a

function of the system. 

It is estimated that most pump problems originate on the suction side of the pump. Therefore, Net Positive

Suction Head Required (NPSHR) is critical to the cost of the operation.

Each reciprocating pump has a Net Positive Suction Head Available (NPSHA) and a NPSHR in a given

system. As long as the pump is operating at the same RPM and the viscosity stays the same, the NPSHR

remains the same. Any change in pump speed or viscosity of the liquid will change the NPSHR. A closed

system (charged suction) will not necessarily provide adequate NPSHR due to the factors listed above.

Due to constantly changing pump RPM and viscosity changes in a drilling operation, NPSHR can be all

over the chart. The one thing that remains the same is the suction line. Keep it big, short and straight.

Calculate the velocity of the liquid. Never exceed 3 ft per second and never fall below 1 ft per second.

Bubble Action 

The consequences of insufficient NPSHR results in the static pressure dropping below the vapor pressure

of the liquid. Bubbles or cavities will form within the liquid. The bubbles attach themselves to the inside of

the pumping chamber early during the suction stroke.

As atmospheric pressure increases during the discharge stroke, the bubbles collapse. As the bubbles are

collapsing, liquid rushes in at a high-velocity to fill the void. The imploding bubbles will exert up to 180,000

psi impact. Over time, this process will cause pitting within the pumping chamber.

If starvation becomes severe enough, shock load damage occurs in other parts of the pump (i.e.

crankshaft, crank bearings, valves, etc.). When a pump is in this stage of cavitation, you can see as well

as hear the problem within.

One prime reason that causes bubbles to form in the drilling fluid is a pressure drop as the liquid enters

and leaves the throat of the suction valve. If a pump is running too fast (liquid is no longer in contact with

the face of the piston) or the pumpage becomes too viscous (too thick to flow smoothly through the throat

of the seat). This action does not allow the valve spring to function properly.

When the piston reverses and starts to re-enter the pumping chamber, momentarily the liquid will flow

backward through the suction seat and the suction valve will be slammed into its seat. The reverse of this

happens to the discharge valve.The valve is slammed into its cage. This action sends shock waves

through the entire system. This reaction is detrimental to both the pump and associated equipment.

Every pump should have a pressure gauge installed on the inlet and a positive pressure should always be

present. The only way to know what that gauge should indicate is to calculate the NPSHR for that

particular system.

Due to the varying configurations of suction systems, there is no way to give a formula to address all

systems. The operator should work with their pump spoiler and calculate the NPSHR needed for his

application. 

Ron Lowe is a manufacturer's representative with Myers-Aplex, a Pentair Pump company. All Drillmaster

Reports are reviewed by the Drillmaster Advisory Board: Lowe, Mike Dvorak, The Charles Machine

Works; Frank Canon, Baroid Industrial Drilling Products; Dan Miller, American Augers and Mark van

Houwelingen, Vermeer Mfg.